Self-Shielded Arc Welding

SELF-SHIELDED ARC WELDING Dr. Tad Boniszewski

ABINGTON PUBLISHING Woodhead Publishing Limited in association with TIle Welding Institute

SELF-SHIELDED ARC WELDING Dr. Tad Boniszewski

ABINGTON PUBLISHING Woodhead Publishing Ltd in association with The Welding Institute Cambridge Eagland

Published by Abington Publishing Abington Hall, Abington Cambridge CB21 6AR First published 1992, Abington Publishing ©

Woodhead Publishing Limited

Conditions of sale All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. The views expressed in this book do not necessarily represent those of TWI. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 978-1-85573-063-2 Printed by Victoire Press Ltd, Cambridge, England

iii

PREFACE The purpose of this work is to put the self-shielded arc welding (SSAW) on the map as a distinct process in its own right. Despite some important and significant advances in the development and applications of the SSAW in certain areas, on the whole the process lacks a sufficiently high profile commensurate with its technical and economic potentials. Hitherto, in textbooks, handbooks and standard specifications for arcwelding consumables, the self-shielded welding has been treated as a variant of the flux-cored arc-welding (FCAW) , or cored-wire welding, but without the auxiliary gas-shield. It has been claimed that the shielding gases and vapours are generated by the flux inside the tubular sheath, at the point of metal transfer where they are most effective. However, because of the general awareness of the paucity of space within the tube, such 'explanations' have not engendered confidence in the metallurgical integrity of the process. I t is shown in this work that the self-shielding wires, designed for the welding of mild, C-Mn and low-alloy steels, work well and deposit sound and ductile weld metal first and foremost because of the "killing" of the contamination by air with the addition of about 1% Al which deoxidizes weld metal vigorously and fixes nitrogen in nitride particles. Shielding with gases and vapours from the core flux is hardly important, if at all, because it is possible to carry out self-shielded welding with solid wires containing AI, Ti and Zr in appropriate quantities. The understanding of how the self-shielded welding really works is achieved by comparing nitrogen contents in various weld metals deposited from different arc-welding consumables. Using the common "Nitrogen Scale", it can be seen that those weld metals which are deposited under voluminous shielding have low N-contents, but those whose shielding is low or none and which rely on the killing have high N-contents. The minimal gas-shielding combined wi th very strong killing makes the self-shielding wires resistant to the effects of wind as there is not much to be lost. Therefore, the SSAW is eminently suitable for outdoor welding, especially in the construction of large structures which are assembled and erected on site. Also, some self-shielding wires are excellent for root-pass welding where there are large and variable gaps and misalignments, and for single-sided welding where there is no access for back-gouging and re-welding. Fume capture at source and vigorous extraction is not a problem with the SSAW, as there is no auxiliary gasshield to be disturbed. In the majority of self-shielded weld metals, about 1% Al is recovered as a result of killing the contamination from air. Concerning its oxygen content, the self-shielded weld metal is the cleanest among all the weld metals and it contains only about 100 ppm [0]. Although its total N-content is usually at or above 250 ppm, compared to 50-150 ppm in other arc-weld metals, most of the nitrogen in the self-shielded weld metal is fixed in nitride particles. The free or mobile nitrogen is lower in the self-shielded weld metal than in the other weld metals, and consequently the self-shielded weld metal has low susceptibility to strain-ageing embrittlement in the roots of thick butt welds.

iv At about 1% Al in iron, gamma-alpha transformation is eliminated and such metal would have large grain size after solidification and would be brittle. To counteract this and to ensure the occurrence of phase transformations needed for microstructural control and grain refinement, appropriate additions of C, Mn and Ni are made to different types of self-shielding wires. There is a broad range of self-shielding wires of different classifications which are capable of replacing the broad range of fluxcovered electrodes in appropriate applications, ranging from sheet metal work, through earth moving equipment, buildings and bridges, to Arctic piping and offshore structures where very high toughness is required. The logistic convenience of the SSAW is similar to that of the MMA/SMAW, as there are no auxiliary gases or fluxes, and only the welding equipment and consumable electrodes are required at the work station. However, unlike in the welding with the stick electrode, there are no enforced stops and starts when welding semi-automatically with self-shielding wires, and hence there are gains in productivity and profitability where the SSAW is used. This work is addressed to a broad readership. A young graduate engineer and materials scientist/technologist (the former metallurgist) will find the understanding of the SSAW process related coherently to other arcwelding processes, thus. dispelling the aura of "black magic" so often attached to fusion welding technology. Materials engineers, practising design and Quality Assurance in construction, may now look at the SSAW with either less suspicion, or even more favour, and would be able to specify and control the fabrication practices with enhanced certainty. The welding engineers and managers in the fabricating industry will have a ready-to-hand package of information and sources which can be followed up with the view to improving productivity and profitability in their companies. Their confidence in replacing the ubiquitous flux-covered electrode with· the self-shielding wires rather than the gas-shielded ones should be enhanced. And last but not least, it is hoped that bringing the true workings of the self-shielded welding into the open will generate some synergy in collaboration between the process users and consumables manufacturers. Although this may not be always welcome initially, it is almost always to everyone's benefit when the users can question the producers from the position of the strength of knowledge. The result is likely to be further improvements in the development of self-shielding wires. T. Boniszewski Harrow Corner Pear Tree Lane Hempstead Gillingham Kent, ME7 3PR England February 1992

v

CONTENTS Page 1.

2.

INTRODUCTION

1

1.1. 1.2.

1

5

8

10-11 12-14

BASICS OF PROCESS METALLURGY 2.1. Weld Metal Steelmaking in Air 2.2. Nitrogen as the Contamination Gauging Medium 2.3. Positions of Different Welding Consumables on the Nitrogen Scale

15 15 17

2.4. 2.4.

Shielding Capacities of Different Processes Limitations on Gas-Shield Generation in Self-Shielding Wires

21

Core Ingredients in Self-Shielding Wires The Killing Practice Effect of Welding Parameters on Nitrogen Pick-Up Advantages of Relying on Killing 2.9.1. Welding in the wind 2.9.2. Spatter on guns 2.9.3. Fume extraction 2.9.4. Welding primed steel 2.9.5. Welding Zn-galvanized steel References Tables 2.1 - 2.7. Figs. 2.1. - 2.29.

26 28 30 32 32 36 37

2.6. 2.7. 2.8. 2.9.

3.

General Considerations Process Selection References Tables 1.1. - 1.2. Figs. 1.1. - 1.3.

SOME ASPECTS OF PHYSICAL METALLURGY 3.1. Consequences of Heavy Killing on Phase Transformations 3.2. Microstructure and Toughness 3.3. Nitrogen and Toughness 3.4. Non-Metallic Inclusions 3.5. Aluminium in Weld Metal and Corrosion References Figs. 3.1. - 3.22.

19

24

38 40 41

46-52 53-79 80 80

82 85

90

93 94 98-119

vi 4.

PROCEDURES FOR HIGH FRACTURE TOUGHNESS 4.1. Preamble 4.2. Making the Welds 4.3. The CTOD Test Results 4.4. Corroboration of the Fast Travel Speed Thin Layer Stringer Bead Technique 4.4.1. Fully automatic welding in flat position 4.4.2. Welding in overhead position 4.4.3. Vertical welding of aT-butt References Tables 4.1. - 4.6. Figs. 4.1. - 4.9.

Page 120 120 121 123 124 124 125 126 126 128-133 134-142

5.

SPECIFICATIONS FOR SELF-SHIELDING WIRES 143 5.1. Historical Background ·143 5.2. AWS A5.20 Specification for Carbon Steel Electrodes 144 5.3. AWS AS.29-80 Specification for Low Alloy Steel Electrodes 145 5.4. BS.7084:1989 Specification for Carbon and Carbon-Manganese Steel Electrodes 146 5.5. NF A81-350 Specification for Bare Flux-Cored Wires for Unalloyed Steel 148 5.6. IIW Draft Specification for Gas-Shielded and Self-Shielded Tubular Cored Wires 149 5.7. General Comment 150 152-155 Tables 5.1. - 5.4. 156-157 Figs. 5.1. - 5.2.

6.

SOME 6.1. 6.2. 6.3. 6.4. 6.5.

ASPECTS OF PRODUCTION WELDING Logistic Convenience of the SSAW Welding Equipment for the SSAW Welder Training for the SSAW Productivity Benefits Hydrogen Control 6.5.1. General considerations 6.5.2. Diffusible hydrogen levels 6.5.3. History of hydrogen cracking 6.6. Root Pass Welding 6.7. Limitations References· Tables 6.1. - 6.4. Figs. 6.1. - 6.14.

158 158 159 162 165 168 168 169 170 172 174 176 178-180 181-194

vii Page 7•

7.1.

7.2. 7.3. 7.4. 7.5. 7.6. 7.7. 7.8.

8.

OF PROVEN APPLICATIONS Market Share of the SSAW Sheet Metal Work Earth Moving Equipment High Rise Buildings, Plant and Bridges Shipbuilding and Dockyard Work Pipelines Offshore Structures Assembly and Erection on Site References Figs. 7.1 - 7.3.

SPECTR~

195 195 197 198 198 200 201

202 203 203 205-207

CONCLUSIONS

208

Acknowledgements

211

APPENDIX A

APPENDIX B

APPENDIX C

INDEX

Collation of some data on the types and contents of slag, gas and vapour forming ingredients and killing agents used in self-shielding tubular/cored wires and published between 1970-1980.

212

Typical chemical compositions of some all-weld-metals deposited from commercial self-shielded wires for different applications.

214

Welding consumables manufacturers and their generic brand names for self-shielding tubular wires.

216 217

SELF-SIDELDED ARC WELDING

1.

INTRODUCTION

1.1.

General Considerations

Self-shielded arc welding (SSAW)

is a fusion welding process employing

a

electrode

continuous

(wire)

consumable

which

requires

no

external

shielding whatsoever (1, 2), either with mineral flux as in submerged-arc welding,. or with shielding gases (CO 2, Ar-C02, Ar-02, etc.) as in gas-shielded

welding

(MIG-MAG/GMAW).

necessary to supply only

two~

To

work

with

as opposed to three,

the

SSAW,

it

is

items at the work

station:(i)

welding equipment (a power source plus a wire feed unit),

and

(ii)

a suitable welding consumable compatible with the material welded, joint type and positions u·sed.

Figure 1.1 shows schematically the difference in typical installations for self-shielded welding (often referred to as SS-FCAW) and the gas-shielded welding either with solid or tubular/cored wires.

With the

SSAW, there is no need for item (iii): the protective consumable material - the shielding gas. Thus, the logistic convenience of the SSAW is similar to that of manual welding with flux-covered stick electrodes (MMA/SMAW).

However, as the

SSAW employs a continuous wire electrode, this confers obvious productivity advantages in comparison with the stick electrode because there are no enforced stops and starts.

Like the gas-shielded arc welding, the self-

-shielded arc welding can be semi-automatic or fully mechanized. despite the current marketing trend

Therefore,

for. the flux-covered electrode to

be replaced by gas-shielded welding, either with solid or tubular/cored

2

wires, the first question the current user of flux-covered electrodes should consider is this:Can the job in hand be done more cost-effectively with a selfshielding consumable if access permits the use of semiautomatic welding ?"

If

Productivity benefits of continuous electrode welding are generally recognized, but there are also some published data (3-5) showing technical benefits arising from the elimination of the operational discontinuity of the stick electrode and the adoption of the continuous electrode processes, as considered below. (a)

Toughness control. This is important in pressure vessels, storage tanks and large structures, such as offshore platforms; e.g. in the construction of one recent platform about $iM was spent on toughness testing alone in weld procedure development (6). However, there is always a question of whether the level of toughness demonstrated in a procedure weld is reproduced consistently in production welds controlled by the appropriate Welding Procedure Specification (WPS). Table 1.1 compares Charpy toughness results obtained in procedure qualification (PQ) tests and production tests (3). The comparison shows that with mechanized processes employing continuous electrodes, the production tests achieved over 80% of the toughness level demonstrated by the PQ test results, but with the manual stick electrode the achievement level in production tests was only slightly above 60%.

(b)

Fatigue behaviour. This is also a very important property which accounts for numerous structural failures and economic burdens. There appears to be a consensus (7-9) that most failures in metal structures which occur in service, ranging from large welded constructions such as bridges (8) to aircraft (9), involve significant fatigue crack growth which precedes final collapse or rupture. Figure 1.2 shows higher fatigue lives for semi-automatic, albeit gas-shielded FCA welds, and for automatic submerged-arc welds compared to those made manually with some flux-covered electrodes (4). In 1991, results were published (5) for fatigue lives of single-sided closure butt welds which are accessible from one side only, and in

3 which flawed roots cannot be gouged out and rewelded.

Some welds

had been made with flux-covered electrodes (E7016 for the root and E7018-G for the fill-up),

whilst others had been deposited semi-

-automatically from a self-shielded wire E61T8-K6.

Figure 1.3 shows

that there is a clear tendency for the joints welded with the self-shielding wire to give longer fatigue

lives than those obtained

from the joints welded with the stick electrodes. It was observed (5) that both the MMA/SMAW and SS-FCAW processes were

susceptible

penetration and

to

root

porosity.

flaws,

mainly

However,

lack

of

fusion

and/or

in general the frequency and

magnitude of flaws were markedly less in the SS-FCAW joints than in the MMA/SMAW joints. -

This was attributed to:-

the use of a continuous wire reducing the number of interruptions to welding which are caused by stick electrode changes;

NB. The

stop/start locations are often associated with flaws; -

the narrowness of the wire compared to a flux-covered electrode facilitating

arc manipulation and

being conducive

to

improved

fusion at the root, particularly where misalignment existed; -

the diminished risk of porosity in the self-shielded welds.

Currently however,

the perception of self-shielded arc welding is poor

in comparison with other processes and sometimes the SSAW is viewed as a somewhat enigmatic process. self-shielding

electrodes

as

In 1970, D.C. Smith (10) referred to the "covered

electrodes

turned

inside

out",

implying that in the self-shielding electrodes all the shielding must come from the inside of the wire, whereas with the flux-covered electrode the shielding is provided by the flux on the outside of the rod.

This

contraposition was made in the context of the flux-cored arc welding (FCAW) in general, the majority of which is carried out with the aid of some shielding gas.

Consequently,

it would appear that from that time an

impression has arisen that the self-shielded welding is a somewhat inferior variant of the FCAW, for it lacks the application of an external gas shield. To this day, in handbooks (1, 2) and textbooks (11, 12), the self-shielded welding remains buried within the descriptions of either the FCAW (1, 2, 12) or cored-wire welding (11) depending on the terminology adopted,

4 and it has not been recognized as a distinct process in its own right. The poverty of understanding of how the self-shielded welding really works, to deposit sound metal, is epitomized by the 1985 Desk Edition of the ASM Metals Handbook which states (13):"Aside from the use or nonuse of auxiliary shielding gas, the self-shielding and auxiliary-gas-shielded methods differ mainly in the type of electrode holder used and in the length of electrode extension." However,

even when recent

are considered,

(1990)

developments in welding torch design

the self-shielded welding is not even mentioned (14).

Under such circumstances, industry is slow in adopting the most suitable welding

consumables

for

the

job in hand with the view

to increased

productivity (15). The situation is not helped by the specifications which classify welding consumables, and which will be considered in more detail in a separate SECTION.

The oldest one dating from 1969 and hence the best known and

the most widely used world-wide is the AWS Specification AS .20-79 (16). Like in the handbooks (1,

2) and textbooks (11, 12), the self-shielded

and gas-shielded wires are treated together under the common heading of flux-cored arc welding (FCAW).,

In the various tables and in the Guide,

those two different types of wire are intermixed indiscriminately and, being listed under consecutive numbers (Table 1.2), are not amenable to easy discrimination, unless their class designation numbers are memorized. Yet despite this handicap, because of the established status of AWS A5.20-79 Specification, its Classifications will be used perforce in this work. Continuing treatment of self-shielded welding within the flux-cored arc welding

(FCAW)

process is

misleading

narrowed by new developments.

Today,

because

the

term FCAW has

there are metal-cored

been

(flux-fr.~)

tubular wires which give essentially slag-free welding like that with solid gas-shielded wires.

Also, the fact that the self-shielding wires

currently on the market are all in a tubular form is a matter of the current manufacturing convenience, rather than process principles.

Between

1962-67, some Soviet researchers (17-19) and Kobayashi (20) demonstrated that arc welding of C-Mn steel is perfectly possible with bare solid wire, the self-shielding capability of which depends on its appropriate contents of AI, Ti and Zr added to the steel melt from which the wire is made.

5

Therefore, in principle, like the gas-shielded welding, the self-shielded welding is possible either with solid or tubular/cored wires. The purpose of this work is to put the self-shielded arc welding (SSAW) on the map as a distinct process in its own right and with its own special characteristics, and to bring its existence and advantages to the attention of owners/operators, designers, fabricators and Certifying Authorities of metal structures and equipment. To engender confidence where familiarity may be lacking, metallurgical principles of the self-shielded welding will be explained. Those principles apply to the welding of mild, C-Mn and low-alloy steels only and they cannot be extended to Cr-Ni austenitic stainless steels, or to various (hard-)surfacing alloys, for which self-shielding wires are also available.

1.2.

Process Selection

Clear perception of the overall capabilities of different welding processes, unclouded by obsolete terminology, is necessary because decisions on process selection are often made long before the invol vement of an experienced welding engineer in a project. For a coded construction, at the design stage an appropriate Fabrication Specification is written and this becomes a part of a legally binding contract. Once this has been signed and the proj ect is underway, any changes are difficult to make even where cost-benefits and time savings are achievable. Because of the lack of confidence resulting from poor literature and insufficient knowledge, designers have been known to ban or restrict the use of some welding processes, including self-shielded welding. This has happened in bridge construction and repairs, power generation projects and in offshore platform construction. Such cases and the associated disputes are not publicised for commercial reasons, but some papers make hints on the subject (21, 22). The worst economic penalties for not exploiting self-shielded welding have occurred in very large constructions, which are not always made from thick and heavy components, and which cannot be welded within a fabrication shop and then transported to site. In one case some months were wasted

6 in attempts to produce solid joints when welding in strong wind in a desert, until by chance the desperate fabricator learned about the existence of self-shielded welding. All the handbooks,

textbooks and expert systems known to the writer,

including the latest process selection guide "Which Process 1" (23), fail to give clear and readily accessible information to the effect that all the arc-welding processes fall into the two separate fields of application: (i)

Indoor welding in fabrication shops where air currents can be kept low and where any arc-welding process can be used almost at will. shops

However, even here there are limitations because in large and

especially

close

to

large

necessary for gas-shielded welding.

doors,

screening

is often

In very large buildings known

as "module construction shops" (22), air currents are always present and sound gas-shielded welding cannot be carried out without local screens. (ii)

Outdoor welding where it can be very windy and where exposure to the elements,

as in shipyards (24),

problems.

such conditions,

Under

can cause severe quality

gas-shielded

and submerged-arc

welding are normally impractical and, for the want of choice, slow manual welding with flux-covered electrodes must be used. cases where cost is not a

problem,

In limited

localized weather protection

can be applied to large structures, but this always requires extra time,

resources and manpower.

This has been done (25,

building Phillips Petroleum Co. r s North Sea.

26) when

Maureen Field platform for the

But this was done before the pressure came to reduce

the construction costs for the North Sea structures by at least 15% (27).

The potential of self-shielded welding as the

very best

process

for

outdoor use is not properly presented in the current books and consequently much welding in the field is still carried out with the inefficient and slow flux-covered stick electrodes.

Let us consider some examples of

how the self-shielded welding is presented and described in this respect.

7

(a)

In Vol.1 of the latest (1987) AWS Welding Handbook (1), p.1 states:"Self-shielded electrodes can be used in moderate cross ventilation with minimal disturbance of the gas shielding around the arc."

(b)

In "Which Process ?" (23) on p.56, the following is stated for self-shielding wires:"Such wires allow welding which is reasonably resistant to draughts."

(c)

The very latest (1991) Vol.2 of the AWS Welding Handbook (2) provides some improvement (pp.159-161) on (a) and (b) above, and states:"In the self-shielded method ••• , shielding is obtained from vaporized flux ingredients which displace the air, and by slag compositions that cover the molten metal droplets, to protect the mol ten weld pool during welding. Production of CO 2 and introduction of deoxidizing and denitriding agents from flux ingredients right at the surface of the weld pool explain why self-shielded electrodes can tolerate stronger air currents than gas shielded electrodes. Thus self-shielded FCAW is the usual choice for field work ••• ".

It will be shown later in this work that the self-shielded welding is not just "reasonably" resistant to "moderate" air currents, but that in principle

and

in

practice

it

is

very

resistant

to

strong

winds.

Potentially, by design, some self-shielding wires of single tube geometry, or solid wires, can be made almost totally immune from the effects of strong winds, which of course can "disturb" completely any gas shield around the arc.

However, such wires do not rely on the gas shielding

at all for the deposition of sound metal. The "explanation" given in Item (c) above does not quite stand up to scrutiny and does not do justice to the special character of the SSAW. In all the arc welding processes, shielding, deoxidation and denitriding always occur right at the molten metal, in the molten pool. remedies

could

be

auxiliary shielding, SSAW.

both in droplets-in-flight and

If the above "explanation" were true, applied

to

other

arc-welding

processes

the same utilizing

to make them even more resistant to wind than the

Therefore, that "explanation" does not really explain what it is

that makes the SSAW the process most resistant to wind among all the arc-welding processes.

It will be shown that the SSAW derives its strong

8

resistance to, if not immunity from, the effects of strong winds not because of air displacement by gases and vapours, but because even the highest degree of contamination, which can be caused by completely undiluted air, can be effectively killed by strong deoxidizers/denitriders.

References

1. 2. 3.

4.

5.

6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16.

AWS WELDING HANDBOOK, 8th Ed., Vol. 1 "Welding Technology". American Welding Society, Miami, FL 33135, USA, 1987, pp.8-9. AWS WELDING HANDBOOK, 8th Ed., Vol.2 "Welding Processes". American Welding Society, Miami, FL 33135, USA, 1991, pp.158-165. LAI, M.O. and VILPPONEN, K.O. "Welding Procedure Qualification Tests vs. Production Tests - A Systematic Study", Welding Journal, June 1987, Vol.66, No.6, pp.40-42. LASSEN, T. "The Effect of Welding Processes on the Fatigue Crack Growth", Welding Journal Res. Suppl., Feb. 1990, Vol.69, No.2, pp.75s-81s. JONES, R.L. , ANDREWS, R.M. and FORSHAW, M.E. "Single-Sided Welding of Closure Joints in Large Tubular Fabrications" - Final Summary Report. Department of Energy - Offshore Technology Report OTH 90 335, London: HMSO, 1991. . MARSHALL, P •W• "Fracture Control Procedures for Deep-Water Offshore Towers", Welding Journal, Jan. 1990, Vol.69, No.1, pp.33-42. BOOTH, G.S. (Tech. Ed.) "Improving the Fatigue Performance of Welded Joints". The Welding Institute, Cambridge, CBl 6AL, UK, 1983. GURNEY, T .R. "Cumulative damage of welded joints", Joining and Materials, July 1989, Vol.2, No.7, pp.320-323. PEEL, C.J. and JONES, A. "Analysis of failures in aircraft structures", Metals and Materials, August 1990, Vol.6, No.8, pp.496-502. SMITH, D.C. "Flux-Cored Electrodes - Their CompOSition and Use", Welding Journal, July 1970, Vol.49, No.7, pp.535-547. HOULDCROFT, P. and JOHN, R. "Welding and cutting". Publ. by Woodhead-Faulkner, London, i988, pp.102-116: Chapter 6. CARY, Howard B. "Modern Welding Technology". 2nd Ed., Prentice Hall, Englewood Cliffs, New Jersey 07623, USA, 1989, Chapter 6. ASM METALS HANDBOOK, Desk Edition, 1985, p.30.16: Section 30 "Joining", American Society for Metals, Metals Park, HO 44073, USA. HILGERS, A. and SCHMITT, J. "Some notes on the design of welding torches", Welding & Metal Fabrication, Oct. 1990, Vol.58, No.8, pp.453-459. ANON. "What's new in welding consumables?", Welding & Metal Fabrication, April 1991, Vol.59, No.3, pp.140-142. AWS A5.2Q-79 "Specification for Carbon Steel Electrodes for Flux Cored Arc Welding". American Welding Society, Miami, Florida 33135, USA.

9

17.

PATON, B.E. and SLUTSKAYA, T.M. "Arc welding with bare electrode wire and no shielding gas", Automatic Welding, 1962, No.6, pp.I-4.

18.

KRIVENKO, L.F. and SLUTSKAYA, T .M. "Effects of alloying elements on the residual nitrogen content of the weld metal after open arc welding", Automatic Welding 1967, VoL 20 , No.3, pp.12-14.

19.

KRIVENKO, L.F. et a1. "Research .into the nitrides in the weld metal when steel is welded by open arc process", Automatic Welding, 1967, VoL 20 , No .• 7, pp.6-12.

20.

KOBAYASHI, T. "Nonshielded arc welding of steel", Journal, 1967, Vol.14, No.3, pp.l01-106.

21.

RODGERS, K.J. and LOCHHEAD, J.C. "Self-Shielded Flux Cored Arc Welding - The Route to Good Fracture Toughness", Welding Journal, July 1987, Vol.66, No.7, pp.49~59.

22.

RODGERS, K.J. and LOCHHEAD, J. C. "The Use of Gas":'Shielded FCAW for Offshore Fabrication", Welding Journal, Feb. 1989, Vo1.68, No.2, pp.26-32.

23.

HOULDCROFT, P. "Which Process 1". CBl 6AH, England, 1990.

24.

CULLISON, A., IRVING, B. and JOHNSEN, M.R. "Controlling Weld Quality: It's One Tough Job", Welding Journal, June 1991, Vo1.70, No.6, p.37.

25.

WEBER, J. "Unique Structure to Tap North Sea Oil", Welding Journal, July 1982, Vol.61, No.7, pp.21-25.

26.

SAVAGE, G.L. "Advances in gas-shielded flux-cored wire welding". Paper 6 in "Developments and Innovations for Improved Welding Production". B:t:tmingham, September 1983, The Welding Institute, Abington, Cambridge, England.

27.

EIU Special Report "The North Sea and British industry: the new opportunities". The Economist Intelligence Unit, London. Also see abridged version published in April 1984 by Shell UK Ltd., Shell-Mex House, London.

Bri tish Welding

Abington Publishing, Cambridge,

Table 1.1.

Comparison of Charpy V-notch impact toughness results obtained for weld procedure qualification (WPQ) and production tests on similar joints made with three different processes.

CIIARPY V-NOTCII IMPACT TEST J

SUB-ARC. (SAW)

MANUAL WELDING SMAW AWS E7016

Automatic GMAW

% Procedure Product. Qualific .• Achieved Test Test

Procedure Product. % Qualific. Test Achieved Test

Procedure Product. Qualific. Test Test

%

Achieved !

117

68

58

103

84

82

186

148

80

91

73

80

98

90

92

158

117

74

FUSION LINE - Bottom

201

141

70

216

198

92

---

---

FUSION LINE - Top

159

174

109

212

196

92

307

193

FL + 2 mm - Bottom

275

209

76

240

194

81

---

---

FL +2 mm - Top

279

230

82

232

202

87

339

123

FL + 5 mm - Bottom

261

227

87

253

212

84

---

---

FL + 5 mm - Top

258

214

83

246

217

88

220

133

WELD METAL - Bottom WELD METAL - Top

AVERAGE ACHIEVEMENT M.O. LAI and K.O. VILPPONEN

"\~elding

(AWS)

81

AVERAGE ACHIEVEMENT

87

AVERAGE ACHIEVEMENT

Procedure Qualification Tests vs. Production Tests - A Systematic Study."

\~ELDING

JOURNAL, June 1987, Vol. 66, No.6, pp.40-42.

I

63

36

60 63

....

o

11

Table 1.2.

Listing of AWS Classifications for self-shielding and gas-shielded tubular / cored welding wires wi th the digits after the hyphen denoting wire usability and performance capabilities, e.g. as in Table 7 of AWS AS.2o-79 Specification. Classification

Shielding

EXXT-I

CO2-SHIELDED

EXX T-2

CO2-SHIELDED

EXX T-3

SELF-SHIELDING

EXX T-4

SELF-SHIELDING

EXX T-5

C02 or Ar-C02-SHIELDED

EXX T-6

SELF-SHIELDING

EXX T-7

SELF-SHIELDING

EXX T-8

SELF-SHIELDING

EXX T-IO

SELF-SHIELDING

EXX T-il

SELF-SHIELDING

12

Power source

r

Electrode cable

l

-

Gun

]

Wire feeder

(A)

Gas hose

,1======================;\ Power source

-

L==~~~

1.

________

-

Gun

r' 'J ~EI~ect~r~o~de~c~a~b~le~~::::::::~~~;)~____________.J Wire feeder

(8)

Fig. 1.1.

Typical

installations

continuous

wire

for

metal

arc

welding

with

consumable electrode which can

be

either solid or tubular/cored:(A)

self-shielded welding,

(B)

gas-shielded welding.

Based on Hobart Brothers Co.'s literature

13 1000r-------------------------------------------------------------~

900

•

x

300

O~----~----~--------------~------------L---~~------------~--~ Failure 0.1 0.5 2 Fatigue crack depth, mm and failure

Fig. 1.2.

Lives at different crack depths and failure for non-load carrying fillet welds (7 mm throat) in 25 mm steel plate welded with different processes. Stress amplitude 150 N/mm 2 and stress ratio 0.5. T. Lassen, Welding Journal, 1990

•

Mean - 2SD to data

o '"E

~

~ a.

o

E7016 root E7018-G fill-up

•

S8-FCAW E61 T-K6 throughout

• • •8 • • 00

'0 ~

•

:::J

~

iii

0

0

Q)

0

Ol

c: ~

• •

•

Ul Ul

~

100

Ul

90

8. 15

::c

0

0

/

•

·0 0

80

o

0

70

•

60 50 104 Number of cycles to failure

Fig. 1.3.

Fatigue test results for specimens obtained from single-sided closure welds and plotted using the fatigue failure stress. The solid lines are mean minus 2 standard deviations weld design classes from BS.5400: Part 10:1980, and the dashed line is mean minus 2 standard deviations for all the data. Data from Offshore Technology Report OTR 90 335, London: RMSO, 1991

I-'

~

15

2.

BASICS OF PROCESS METALLURGY

2.1.

Weld Metal Steelmaking in Air

In metallurgical processing, the handling of most molten metals in the air atmosphere requires protection from the detrimental effects of oxygen and nitrogen which can cause various flaws (e.g. porosity and non-metallic inclusions), and the absorption of which can lead to property degradation. This need for protection applies equally well to fusion welding as a miniature process, as it does in bulk alloy- or steelmaking. In welding, molten metal transferred from the tip of an electrode or filler wire into the molten pool must be protected from unlimited or unrestricted reaction with oxygen and nitrogen in the air, in order:(i)

to produce sound deposits, primarily free from nitrogen porosity, i.e. to achieve what is known as the radiographic soundness,

(ii)

and

to achieve at least a modicum of ductility and notch toughness -

the properties which still distinguish metals

from oxide and

nitride ceramics which are being developed and applied for some components made from metals in the past. In principle, there are two extreme measures [(A) or (Z)] which can be considered as means for molten metal protection in the arc welding of steel:(A)

An almost total exclusion of air from the arc environment can be arranged, by displacing it with a burden of mineral flux (submerged-arc welding) or shielding the arc with a shroud of inert gases: Ar, He or their mixtures (MIG/GMAW and TIG/GTAW).

NB. In electron

beam welding, the vacuum generated for maintaining the beam fulfils a similar protective role. (Z)

Without any auxiliary or external shielding, a totally unprotected bare wire or rod electrode, the self-shielding one, can be used in air,

but

the

electrode

must

incorporate

a

sufficient

amount of

elements (Mn, Si, AI, Ti, Zr, Ca, Mg and REM) * whose affinities for

*

REM

= rare

earth metals

16 oxygen and nitrogen are greater than those of iron.

As shown in

Fig. 2.1, the increasing affinity of an element for oxygen and/or nitrogen is

represented

by

the increasingly

negative

free

energy

of compound formation between a given element and 02 or N2. the elements,

Thus

the compound-formation lines of which lie below the

FeO and Fe 4 N lines, have stronger affinities to oxygen and nitrogen than that of iron. Those elements react with oxygen and nitrogen in steel, to form their own oxides and nitrides, thus removing these gases from liquid and solid solutions in iron.

Silicon, Al and Ti

will take up enough oxygen to "kill" CO-porosity, but only AI, Ti and Zr are the elements with a strong enough affinity for nitrogen (see

the

dashed

formation.

lines

in Fig.

2.1)

to prevent nitrogen-porosity

As shown in Fig. 2.1, the nitrides of AI, Ti and Zr are

more stable than those of Si,

Ca and Mg which are effective as

deoxidants only, and consequently only AI, Ti and Zr are known as denitriders.

By analogy with the CO-porosity in steelmaking,

denitriders are referred to here as the "killing" agents,

the

but it

should be borne in mind that the chemistry of "killing" the nitrogen porosity is different from that of "killing" the CO-porosity.

In practice, the total exclusion of air from the arc environment is not possible, and there are also some residual oxygen and nitrogen contents in filler metals and in the parent (base) metal, a portion of which is melted and diluted into the molten pool.

Therefore, all the arc-welding

consumables incorporate some combination of (i) protection/shielding and (ii) deoxidation/denitriding,

but the proportions of these two measures

differ considerably from one process to another, and for some types of consumables

(e.g.

E6010 vs.

E7018 electrodes)

with the inert gas shielding,

within a

process.

Thus

the TIG/GTAW filler wire need have only

a minimum of 0.20% Si to suppress the oxygen activity sufficiently to kill the CO-porosity. the

nitrogen of air,

. Some shielding gases, whilst designed to exclude are

oxidizing

and

hence

about

1. 5%Mn-o .7%Si

is

required in the filler wire for weld metal deoxidation, as in the C02-shielded welding.

Similar considerations apply to the MIG/GMAW utilizing

shielding with Ar-C02

and Ar-02

gas mixtures,

electrodes the coatings of which contain CaC03 the arc space.

and

to the

flux-covered

which evolves CO 2

into

To deposit sound and ductile metal, a given oxidation

potential must be balanced by an appropriate degree of deoxidation.

17

Until recently,

there has been no coherent perception of the different

shielding/killing combinations employed in different process consumables because

the

secrets.

formulations

of

flux-bearing

consumables

are

proprietary

To obtain some quantitative means for gauging the degree of

protection-vs.-killing

in the

design of different welding consumables,

it has been proposed (1) to use the "Nitrogen Scale",

i.e. the total

residual nitrogen content analysed in the weld metal.

2.2.

Nitrogen as the Contamination Gauging Medium

The reasons why the residual nitrogen, and not the residual oxygen in the weld metal is used to gauge the degree of shielding vs. killing is explained below. Firstly, the residual oxygen content in the weld metal can originate from non-metallic inclusion oxides

in

the filler

and parent

(base) metals,

from oxides in the flux, and from the oxidizing shielding gas and the air.

In contrast,

(i.e. C02

)

the extra nitrogen additional to the small

quantity (e.g. about 20-80 ppm) present in the materials welded can come only from the air.

Thus, the contamination of the molten weld metal by

air is the main and the most significant source of the residual nitrogen. Secondly, as can be seen from Fig. 2.1, stable oxides can form readily at temperatures above 1530°C in the still molten weld metal. MnO -

For instance,

the oxide of the weakest deoxidant (Mn) is markedly more stable

than ZrN - the nitride of the strongest denitrider (Zr).

Therefore, whilst

substantial quantities of different nitrides may still remain in liquid solution, oxide phases separate out from the molten metal and can float out into the top slag well before solidification.

This oxide separation

can occur already in the droplet on the electrode tip, at arc temperatures much in excess of the melting point of iron.

Consequently, a substantial

quantity of deoxidation products always separates out of the weld metal well before solidification and the residual oxygen, present in the fraction of the trapped oxides, gives no indication at all as to how much the metal was protected from air during deposition. Thirdly, paradoxically as it may seem at first sight, the higher the degree of the original gaseous oxidation balanced by appropriate deoxidation,

18

the lower the residual oxygen content in the weld metal (2, 3).

This

is because the more voluminous the deoxidation products, the more readily they can coagulate, and thus with the increasing buoyancy, the quicker and easier they can separate to the top slag in the short time available before solidification. has the

lowest

This explains why the self-shielded weld metal

oxygen content among all the weld metals (Table 2.1)

deposited from consumable electrodes [i. e •. only about 100 ppm (4, 5)], and therefore it is the cleanest one as regards its oxide-inclusion content. In contrast as can be deduced from Fig. 2.1, nitrides begin to form near, at and below, the solidification temperature of iron.

Consequently, there

is not much opportunity for the nitrides to separate to the slag, and by and large in all the arc-welding deposits, the bulk of nitrogen that has entered the molten metal from the air becomes trapped on solidification, not unlike hydrogen.

Whereas most of the hydrogen diffuses out of the

weld metal in time at room temperature, most of the nitrogen remains within the weld metal, and in the self-shielded weld metal it is fixed by strong denitriders:

AI,

Ti and Zr (Fig.

2.1).

Thus,

the residual nitrogen

content of the weld metal does reflect quite well the initial contamination of molten metal by air.

This is the basis for using the Nitrogen Scale

to gauge the degree of shielding-vs.-killing inherent in different welding consumables (Fig. 2.2). There is some indication that a small proportion of nitrogen can sometimes separate

from

the

liquid

metal

before

solidification.

Some nitrogen

degassing may occur in the TIG/GTAW when the initial N-content of the materials is high, vacuum.

for the inert gas atmosphere acts upon nitrogen as

Also, depending on the solidification rate within a certain range

of heat input (arc energy), some fraction of AIN may also separate out from the self-shielded weld metal into the top slag.

All such secondary

effects only contribute to scatter in the Nitrogen Scale which nevertheless remains the best means available so far for judging the degree of shielding from air afforded by the different processes and consumables.

19

2.3.

Positions

of

Different

Welding

Consumables

on

the

Nitrogen Scale The positions of different processes and consumables on the Nitrogen Scale, including those for self-shielding welding, are shown in Fig. 2.2 which summarizes the nitrogen contents reported for numerous mild and C-Mn steel weld metals, over a period of some 20 years.

Of course the picture is

imperfect and there is some 'noise' shown by the boxes on the graph. There are a number of reasons for the spread of the data:(i)

All filler metals contain some initial residual nitrogen,

e.g.

20-100 ppm, and the degree of contamination by air could only be established more precisely if the nitrogen analyses were always published for the filler metals and the reSUlting all-weld metals. (ii)

Welding

conditions have

an effect

on

the

N-content,

e.g.

the

increasing arc length as reflected by the increasing arc voltage increases the path for the ingress of air into the arc and thus raises the nitrogen content in the weld metal.

When a flux-covered

electrode is oscillated in a pendulum fashion,

the periphery of

the weld bead can absorb more nitrogen than the centre (6).

In

the self-shielded welding with a given wire, as different welding conditions change the degree of contamination by air, different amounts of deoxidant/denitrider are consumed, and as more denitrider is consumed

(and less recovered),

the higher tends to

be

the

residual nitrogen content (Fig. 2.3). (iii)

The different denitriders and their varying additions (7, 8) also have some effect on the residual nitrogen content.

Notwithstanding some inevitable variations with a given process/consumable, the Nitrogen Scale as used in Fig. 2.2 provides a basis for consideration and discussion of the degrees to which protection and killing are combined in the design of the various consumables.

Figure 2.2 also shows that

all the arc-welding processes can be placed on a common Nitrogen Scale lying between approximately 25-1000 ppm:(A)

At one extreme on the left, there are processes which rely on the maximum protection with flux

("SA"

submerged-arc welding)

shielding with gas (TIG, MIG and E6010/11 electrodes).

and

the cellulose-coated

Here, the N-content in the weld metal is usually below

or at around 100 ppm.

20 (Z)

At the other extreme on the right, the self-shielding solid wire relies almost solely on the killing because the displacement of air by

the

arc-generated

metallic

vapours

is negligible.

Therefore,

the residual nitrogen content fixed by the nitrides (Fig. 2.1) within the

weld

metal

tends

towards

the

1200

ppm

expected

from

the

thermodynamics of arc-welding in air (9) and ranges between 600-1030 ppm. In between the above two extremes, lie the various flux-covered

electrodes

and tubular/cored self -shielding wires, and the N-contents of their weld metals reflect the decreasing amount of shielding, as one moves from left to right in Fig. 2.2:(a)

For basic low-hydrogen flux-covered electrodes which rely on shielding by the CO 2 -gas evolved from calcium carbonate, CaC03, in the coatings, the relationship between the weld metal N-content and the amount of shielding is reasonably straightforward:- E7016 electrodes with about 50% CaC03 give the lowest N-contents of 80-120 ppm; E7018 electrodes with about 30% CaC03 give intermediate N-contents of

120-150 ppm,

with

some

recent

developments

reaching

below

100 ppm; E7028 electrodes with some 15-20% CaC03 give the highest N-contents of 170-200 ppm. (b)

Self-shielding wires of single-tube construction, which have been dominant on the market in free-enterprise countries in the last lOIS years, rely more on the killing agents for fixing their weld metal N-contents rather than on the shielding of their arcs from air. Here, depending on the wire and the welding conditions, the N-content can vary between 200-500 ppm.

In the (former) Soviet Union, there are self-shielded wires of double-tube construction (10, 11) which are made in the Cherepovets Plant (12) according to the scheme shown in Fig. 2.4.

According to some data whi c h

appear plausible (10, 13, 14), the N-contents of the weld metal deposited from such wires lie in the range of 100-220 ppm.

This range is similar

to that represented by basic flux-covered electrodes (Fig. 2.2). is understandable because the outer

ring of

This

the double-tube contains

21 CaCO 3 + CaF 2 like the fluxes of the covered electrodes, and such wires can have CO 2 -gas shielding similar to that achievable with the covered electrodes. The three different types of the self-shielding wire are conside.red separately on the Nitrogen Scale in Fig. 2.5. potential

for

shielding

decreases

in

the

It is clear that as their order

of

(i)

double-tube,

(ii) single-tube and (iii) solid, so the N-contents of their weld metals increase. The

Soviet

double-tube

self-shielding

wire

is

not

marketed

in

the

free-enterprise countries yet and the solid wires for self-shielded welding are

not

produced

knowledge.

anywhere

Therefore,

the

in

the

world to

the best of the author's

remainder of this work will deal primarily

with the self-shielding wires of single-tube construction.

2.4.

Shielding Capacities of Different Processes

In the plots of Figs. 2.2 and 2.5, it has not been possible to have a single, abscissa.

unifying

quantitative

parameter

for

all

the

processes on the

However, the consideration of the masses of protective materials

(fluxes and gases) applied in the various processes (Tables 2.2 and 2.3) shows a good relationship with the positions of those processes on the Nitrogen Scale (Fig. 2.2).

Submerged-arc welding Here the burden of flux,

which buries the arc,

displaces the air from

around the arc cavity, and at average arc voltages about 1 kg of flux is consumed per 1 kg of filler wire.

As the arc voltage is increased

(accompanied by the increasing arc length conducive to nitrogen pick-up) up to 2 kg of flux are consumed per 1 kg of wire. flux burden is deep enough to bury the arc fully,

Thus, as long as the the SAW process has

a self-regulating capacity to consume more flux with the increasing arc voltage

(length),

and

consequently

the

N-content

in

the submerged-arc

weld metal is relatively low, in the range of 60-110 ppm (Table 2.2). Where N-analyses are available for wires and their respective weld metals,

22 they indicate that with up to 50 ppm N in the wire, the extra-N pick-up is under 50 ppm, and even some N-losses may occur where the wire N-content approaches and exceeds 100 ppm.

Flux-coYered stick electrodes Here the mass of flux, which generates shielding gases and the protective slag,

is bonded on to the core wire and it can be about 40% (ranging

between 30-60%) of the total mass of the consumable.

On average, this

gives approximately 0.7 kg of flux per 1 kg of metal (core wire plus iron powder in the coating) for E7018 basic low-hydrogen electrodes.

As the

mass of flux decreased relative to the filler metal in comparison with the SAW,

so did the N-content in the weld metal increase as shown in

Table 2.2. The relationship between the CaCO 3

content in the fluxes of different

classes (E7016, E7018 and E7028) of basic electrodes and their weld metal N-contents has been already considered in Sub-Section 2.3 above.

Here,

Class E8010-G cellulose-coated electrodes are compared with E7018 basic electrodes

(Table 2.3)

because the

volumes of shielding gases evolved

by the coatings of these two electrode classes are available (15, 16):(a)

Cellulosic electrodes have the most voluminous gas-shield (5.4 l/min) among

all

shielding

the gas

flux-covered being

electrodes,

hydrogen

(15,

with

16).

about

Hence,

half the

of

the

cellulosic

electrodes yield weld metal with the lowest N-content among all the covered electrodes (Fig. 2.2). (b)

Basic low-hydrogen (E7018) electrodes have a much less voluminous gas-shield

(1.4 l/min)

than the cellulosic electrodes (Table 2.3)

and hence the E7018 weld metal N-content is correspondingly greater (Fig. 2.2).

Self-shielding tubular wires Unlike with all the processes employing external shielding media (fluxes and gases), there is a natural limitation on the space available in the core of a tube with a given wall thickness.

This wall thickness of the

sheath holding the core powders must be sufficient for the wire to be

23 drawn to size without splitting and rupture.

Hence, at the most only

30% of the total electrode mass can be accommodated in the core of tubular wire (17), and this is considerably less than the maximum (60%) mass of coating application in flux-covered electrodes. For different commercial self-shielding tubular wires,

the core masses

or the flux "fill ratios" of 15-28% (18), 18% (19) and 15-24% (20) have been reported,

wi th 20% being considered typical (21) •

This gives a

typical consumption of 0.25 kg of flux per 1 kg of metal (Table 2.2). Even if most of that flux mass were used to generate shielding gases, this mass is only a quarter (t) of that available in submerged-arc welding, and

just over

electrodes.

one-third

(i)

of

that

available

for

some

flux-covered

Consequently, the tubular self-shielding wires have a rather

low potential for protection from the air, and this is reflected in their high position on the Nitrogen Scale (Fig. 2.2). Although the gas-shield generated by self-shielding wires is low, it does vary from one electrode to another (10).

With double-tube wires, it is

possible to have more CaC0 3 in the core, before the onset of unacceptable spatter, than with Single-tube wires (see the next Sub-Section), by placing the CaCO 3 in the outer annulus (Fig. reported (10) •

2.4):

up to 17% CaC03 has been

As shown in Table 2.3 such wire can have the volume of

gas-shield only slightly below that of E7018 electrode, and consequently the weld metal N-content for the double-tube wire is only marginally higher compared with that of the electrode deposit. Shielding-gas volumes reported (10) for Single-tube self-shielding wires are about an order of magnitude lower than that reported for the double-tube wire (Table 2.3).

With the lowering of the gas-shield, the weld

metal N-content goes up (Table 2.3) and this is reflected in different positions of

the

different

types

of

the self-shielding

wires

on

the

Nitrogen Scale (Fig. 2.5).

All tubular/cored or FCAW electrodes Conforming with the current convention discussed in SECTION 1, all the tubular/cored jointly.

wires,

gas-shielded

and

With the gas-shielded wire,

self-shielding, a

are

considered

gas shroud is applied through

24 the gun nozzle at a rate of 15-20 l/min, and the gas volumes generated internally by the two self-shielding wires are given in Table 2.3.

Using

the gas-shielding volumes, the three types of wire are placed on the common Nitrogen Scale (Fig. 2.6) from which it is clear that the gas-shielded and single-tube self-shielding wires are metallurgically far apart.

This

reveals how wrong is the current 'consensus' of treating all the so-called FCAW consumables as belonging to a single welding process.

Gas-shielded

and self-shielding tubular wires may look similar on the spool, and the power sources and wire feeders used with both the wires may be the same, but the two processes are different in their metallurgical nature and resulting consequences. In Fig. 2.6, Box C covers the N-contents similar to those covered by Box "MIG" (GMAW) in Fig. 2.2 because the degree of shielding from air by the externally applied gases is similar regardless as to whether the wire is

solid

metallurgy

or

tubular.

indicate

Thus,

that

the

chemical

gas-shielded

characteristics

tubular wires

of

(whether

process flux- or

metal-cored) represent variants within the gas-shielded process, and the mechanistic consideration of all the tubular wires (gas-shielded and self-shielding) within the FCAW is based on superficial appearances.

2.5.

Limitations on Gas-Shield Generation in Self-Shielding Wires

Mineral flux ingredients used for gas-shield generation (e.g. CaC03) are on the outside of the core wire of a flux-covered electrode, but they are within the sheath of a tubular electrode. the carbonates used

commonly

As shown in Table 2.4,

in electrode coatings

(CaC03

and MgC03)

decompose and evolve C02 at relatively low temperatures (on average about 600°C), and the gas pressure generated within the tube tends to blow the melting sheath apart,

thus causing spatter.

It has been reported (10,

17, 18, 21) that the spatter becomes greater with the increasing reliance on

gas-shield

generation

by

self-shielding wires.

This

places severe

limitations on the quantities of carbonates used in self-shielding wires (see Appendix A), compared to about 30% CaCO 3 in the covering of E7018 electrode and up to 50% CaC03 in the covering of E7016 electrode.

25 Even if the spatter problem could be minimized by the use of carbonates with higher decomposition temperatures (BaC03 and Li2C03 - see Table 2.4), there

is another

problem caused by the gas shield.

The gas stream

generated in the core bounces off the surface of the molten pool, creating a cushion on which the molten droplet can hover, and even climb up the wire (Fig. 2.7).

This hinders the droplet detachment and tends to increase

the exposure of molten metal to air.

As early as 1968, Smith and Johannes

( 17) published a high-speed photograph showing metal droplets building up and climbing up the sheath wall.

This was confirmed (10) by high-speed

cine films showing that droplets suspended outside the axis of the wire (as in Fig. 2.7) usually do not remain within the gas shield.

Therefore,

even with similar rates of gas-shield generation (see Table 2.3), the droplets

from

a

self-shielding wire

can

become more

exposed

to

air

contamination than those from a flux-covered E7018 electrode. Data (22) in Table 2.5 (Items 1 and 2) show only a very marginal (less than 10%) fall in the weld metal N-content when comparing the results for two self-shielding wires: (1) - with no CO2-gas shield and (2) - having 8% MgCO] , the amount tolerable from the spatter viewpoint.

As the droplet

detachment is hindered by the build-up of the gas-shield cushion, hovering tiffie increases and the droplets grow in size. the

incre>asing droplet size with the

generatej

by

self-shielding

wires

the

Figure 2.8 shows

increasing volume of gas-shield

(10).

Thus,

the

measure which is

supposed to protect the molten metal from air is self-defeating:

the

production of C02 right at the surface of the weld pool is not the answer to air contamination in self-shielded welding. Some air displacement from around the arc can be achieved with metal vapour and Mg-metal powder addition is the favoured choice.

Magnesium is a strong

deoxidant (Fig. 2.1) and it has been used for deoxidation (23, 24) and other purposes (25) in electrode coatings.

But additionally,

as Table 2.4

shows, Mg has a relatively low (about 1100°C) boiling temperature and the Mg-vapour pressure at that temperature is several orders of magnitude higher than those of Mn, Al, Fe, Si, Ti and Zr (Fig. 2.9) •

Comparing

Items 1 and 3 in Table 2.5 (22) shows that increasing Mg-powder content in the core from 0.5 to 4.5% is associated with the fall in the N-content of the weld metal from 650 to 500 ppm.

Being a deoxidant, the Mg-addition

also increased the Al-content recovered in the weld metal, and in addition

26 to the air-displacement effect, this could also have contributed to the lowering of the N-content according to the relationship shown in

Fig. 2.3.

Joint consideration of Figs. 2.7 and 2.8 and Table 2.5 indicates that some shielding by gas and vapour that is feasible, barring the spatter, can have only a marginal effect on the N-content in the weld metal. Tubular self-shielding wires employing this marginal shielding give lower N-contents in their weld metal compared with the solid wires (Figs. 2.2 and 2.5), but nonetheless those N-contents are much higher than those obtained with processes and consumables where the shielding is substantial.

In

the self-shielded weld metal, because of this marginal shielding, there is an overwhelming need for strong killing, and Table 2.5 shows the Al-content of the order of 1-2% necessary in the weld metal for the prevention of nitrogen porosity and the removal of nitrogen from solid solution. As discussed in SECTION 1, handbooks and textbooks are reluctant to admit that in self-shielded welding the arc protection by air displacement is minimal, and that the killing of the high degree inevitable contamination is the maj or means of ensuring weld metal soundness and ductility.

Gas

formation by the core materials is emphasized and the "extra deoxidizing and denitriding" are considered as "additional" to shielding.

In truth,

the situation is the opposite: the shielding from the air is minimal and the killing of contamination is decisive in the self-shielded welding. This confers some great advantages which will be discussed later.

2.6.

Core Ingredients in Self-Shielding Wires

The limitations on the gas-shield generation from carbonates in single-tube self-shielding wires have led to the extensive use of fluorides as the major flux/slag ingredients.

As shown in Table 2.4, some fluorides have

boiling temperatures in excess of 2000°C which is well above the melting temperature of steel.

As in bulk steelmaking (26), fluorides and specially

calcium fluoride (CaF2 - the main component of the mineral fluospar) have been used in the various arc-welding consumables to minimize the contamination of molten metal by air and hydrogen (from moisture), and to effect some metal purification (27-31).

Independently from any slag basicity

27

effect (28), CaF2 in its own right acts upon the molten metal to cause:(a)

de-sulphurizing,

(b)

de-phosphorizing,

(c)

lowering of oxygen content,

(d)

lowering of nitrogen content,

(e)

lowering of hydrogen content,

(f)

lowering of silicon recovery.

Figure 2.10 shows how with the increasing CaF2 -additions to a welding flux, the weld metal contents of oxygen, nitrogen and hydrogen are lowered. Concomitant with those compositional effects, the weld metal ductility (the tensile elongation) is increased, as shown in Fig. 2.11 over a similar range of the CaF2-additions. Appendix A summarizes some historical data, published up to 1980, on the composition of core-fluxes in self-shielding wires, and Table 2.6 gives examples of such fluxes in some more popular wires used during the 1980s. It can be seen that, in comparison with carbonates, the role of fluorides is dominant.

In many cases, the fluorides reach the level of 50% or more,

and there are few cases where carbonate (CaC03, MgC03) contents reach 10% or more. In addition to providing the bulk ingredients for the slag, the fluorides become ionized at the high temperatures of the arc, thus:[ 1] .

In the arc atmosphere, the fluorine ions interact strongly with oxygen, nitrogen and hydrogen, with the bond between fluorine and hydrogen being especially strong (32). AS

S hown

This accounts for the results plotted in Fig. 2.10.

in Appendix A,some wire cores contain meta11 ic Ca, or Ca 2 + ions

can be liberated from CaF 2 according to Reaction [1] above, or even from the CaO after the decomposition of CaC0 3•

Where both, metallic Ca and

LiF

are present in the core flux (Table 2.6), the following reactions are thought (10) to occur in the arc environment:Ca + 2 LiF 6Li

+

N2

~ ~

CaF2 + 2 Li

[ 2],

2 Li3N

[3].

28 Thus, the use of Li-bearing flux ingredients is thought to be efficacious in moderating the N-content of the self-shielded weld metal.

Additionally,

some positive metal ions released by Reaction [1] will combine with oxygen aiding the effect of the deoxidants (i.e. Mg - see Fig. 2.1) and killing agents (e.g. AI) present in the wire core.

2.7.

The Killing Practice

The position of single-tube self-shielding wires on the Nitrogen Scale (Figs.

2.2 and 2.5) indicates that the degree of gaseous and vapour

shielding, and protection by the fluoride slag are low, and consequently the weld metal soundness must be ensured by effective killing.

Figure 2.1

indicates that AI, Ti and perhaps Zr are candidate elements for binding nitrogen, and all these three elements have been used in solid self-shielding wires (7, 8, 33, 34).

As can be seen in Appendix A, the same three

elements have been used as killing agents in tubular wires, but

Table 2.6

shows that aluminium (AI) is the preferred denitrider in the single-tube self-shielding wires currently dominant on the market.

This is confirmed

by chemical compositions of the all-weld-metal deposits given in Appendix B. Only in limited cases (see E70T-3 in Appendix B) of wires applied for structurally less-demanding joints in sheet metal work (35), where single-pass high-speed welds are made, denitrider.

titanium (Ti) is used currently as a

In the early days (1968) of self-shielding wire developments,

it was claimed (17) that Ti might be preferable to Al for obtaining weld metal with good toughness,

but subsequent studies (19, 36) vindicated

the preference for aluminium as embodied in the very original self-shielding wire (INNERSHIELD) patent (1959) of The Lincoln Electric Co. (37). There are a number of reasons why Al is preferred to Ti as the best overall killing agent for self-shielding wires:(a)

On the mass (formerly weight) percent basis, Ti· is about 5 times as powerful as Al in promoting bainite formation (36), but this is undesirable for good weld metal toughness.

(b)

Unlike Ti, Al does not form carbides in steel and therefore Al does not interfere with the function of carbon as an austenite former and hardenability agent.

29

(c)

Metallography and fractography showed (36) low non-metallic inclusion content in the AI-killed weld metal, whereas the Ti-killed weld metal was densely populated with titanium carbo-nitride and oxy-sulphide inclusions which promote fracture initiation.

(d)

Controlled experiments (36) with other factors being equal have shown that AI-killing, additional to that achieved by Si alone, can have a bulk desulphurizing effect (Table 2.7).

The formation of aluminium

sulphides has been observed (36), and as shown in Table 2.4 Al2S3 sublimates at about 1500°C and this would account for some removal of sulphur from metal (Table 2.7) during the droplet transfer through the arc. There has been a widespread distrust of Al in ferritic steel weld metal since the findings (38-40) that Al (0.1-0.2%) in C02-shielded weld metal can have an inconsistent and mainly detrimental effect on toughness. Later, the variable effect of Al on toughness was also found in the submerged-arc weld metal

(41-43),

with

the complicating effects of flux

basicity and weld metal oxygen content, where sometimes (42) small amounts of Al

(below 0.q8%) could be detrimental to toughness.

As shown in

Fig. 2.2, the "MIG" and "SA" weld metals are low in nitrogen (60-100 ppm) and in such weld metals even very small quantities of Al (about 0.020-0.06%) are not used up for denitriding.

Thus, the excess of AI, however

small, is able to modify deoxidation products and consequently the microstructure they nucleate, and is able to affect toughness. In the self-shielded weld metal,

the situation is almost diametrically

opposite (Fig. 2.2): the nitrogen contamination is high and an appropriate addition of Al to the weld metal, in quantities of 1-2 orders of magnitude higher than those in the SA weld metal, is necessary to balance the high N-content.

Naturally, the requisite AI-content in a given self-shielded

weld metal would vary somewhat with the marginal degree of self-generated gas-shield built into a given wire (Fig. 2.12): as the gas shield evolution rate (at 1500°C) was raised from 0.24 to 1.04 llmin, the weld metal AI-content was lowered from 0.85 to 0.36% (10). Fig.

However, as shown in

2.8, when the gas volume (at 1500°C) evolution rate approaches 1

llmin,

the

droplet

size

becomes

undesirably

characteristics of the wire must deteriorate.

large

and

the operating

Therefore, most self-shield-

ing wires currently on the market generate much less gas-shielding than

30

1 l/min.

Consequently, with the exception of a few wires used for sheet-

-metal work (35) and killed with Ti, most self-shielding wires employ killing with AI, and their weld metal AI-content is nearer 1% than 0.5% (see Appendix B). This is especially so with wires designed for mUltipass and all-positional use. Because

of the very marginal self-generated gas-shielding possible in

practice with self-shielding wires, to eliminate nitrogen-induced porosity when welding in undiluted air,

a minimum of nearly 0.9% Al must

be

recovered in the weld metal (Fig. 2.13), as shown by Kaplan and Hill (9). Naturally, when the partial pressure of nitrogen is somewhat decreased, e.g.

by dilution with Mg-vapour, somewhat lower amounts of AI-recovery

can ensure weld metal soundness.

With a given wire, variation in welding

conditions has some effect on the degree of molten metal contamination by nitrogen, -nitriding.

and

consequently on the amount

of Al consumed for

de-

Therefore as shown in Fig. 2.3, some inverse relationship

can be observed between the AI- and N-contents in different deposits made from the same wire.

2.B.

Effect of Welding Parameters on Nitrogen Pick-Up

Like with other arc-welding processes, the welding parameters, e.g. current and voltage, have some effect on the weld metal N-content obtained with one and the same batch of filler metal. on the Nitrogen Scale (Fig.

This is one of the reasons why

2.2) the N-contents are indicated by the

scatter-range boxes. Figure

2.14

increasing

(9)

shows

welding

decreasing

current

for

a

N-content

in

constant

wire

the

weld

diameter.

metal

with

With

the

increasing current (current density), the vapourization of flux ingredients is enhanced and somewhat

the displacement of air from the arc environment

increased.

Also,

is

with the increasing current the deposition

rate is increased and this means that an increasing mass of molten metal passes

through

contamination by

the

arc

in

unit

time.

Consequently,

the available nitrogen is diluted.

the

degree

of

However, even at

very high currents, e. g. 400 A (Fig. 2.14), the N-content in the self-shielded weld metal is still high relative to that obtained with other processes (Fig. 2.2).

31 Figures 2 .15 (9) and 2 .16 (44) show increasing N-con ten t with increasing arc voltage. the

The arc voltage is proportional to the arc length, and with

increasing

arc

length

the

contamination

path

becomes longer.

To

prevent undue arc length increases by welders, there are now power sources on the market in which the arc voltage can be pre-set and locked.

When

the arc length is increased to exceed the pre-set arc voltage, the power is switched off and the welding stops. Figures 2.17 (45) and 2.18 (46) show the variation in the N-content with the nominal heat input (arc energy).

In the first case, with constant

electrical parameters the heat input was changed with the travel speed (Fig. 2.17).

In the second case, the heat input was lowered by decreasing

the welding current and increasing the travel speed simultaneously.

As

shown in Fig. 2.18, a change in the heat input is accompanied by a change in the cooling rate, Despite

the latter affecting the rate of solidification.

the

two

sets

of

results

laboratories

for

two different

having

been

generated

grades of wire,

in

different

in both the figures a

minimum (about 200 ppm) in the N-content occurs at a similar heat input of about 1.25-1.5 kJ/mm, U-type

(or

C-type)

and the two U-type curves are similar.

curves

are

usually

competition between two opposing processes.

observed

where

there

Such

is

some

In this case, there is:-

(a)

the dissolution of nitrogen in the molten metal, and

(b)

the clearance of nitrogen from the metal by Al and some sublimation of AIN (Table 2.4).

With the decreasing heat input from 2.5/3 kJ/mm down to 1.25/1.5 kJ/mm, the molten pool becomes progressively smaller and solidifies more rapidly, thus

diminishing the opportunity for N-contamination.

However,

wi th a

further decrease in the heat input down to 0.711 kJ/mm, accompanied by the

increasing cooling and solidification rates, more nitrogen becomes

trapped in the weld metal. The electrode extension or electrical stick-out is an important welding parameter, especially in self-shielded welding (47).

With constant wire

feed speed (WFS) and voltage, when using some wire brands,

penetration

can be increased by pushing the gun slightly nearer the work (decreasing stick-out), or i t can be decreased by pulling the gun away (increasing stick-out).

The latter option is useful where there is poor fit-up because

32 by increasing the stick-out burn through can be avoided.

With the self-

-shielded welding, there is no gas nozzle to interfere at short electrode extensions and little gas-shield to lose at high electrode extensions. However, it is necessary to be aware that the electrode extension can have some effect on the weld metal nitrogen content as shown in Fig. 2.19 (4, 48).

In this case, two separate investigations (4, 48) carried out

with the same brand of wire have shown that the weld metal N-content increases with the stick-out in the range of 8-30 mm.

However, there

are other data (49), albeit for austenitic stainless steel self-shielding wires, showing a decrease in the N-content with the increasing stick-out. This vindicates the qualification made in the INTRODUCTION (p. 5) that the process principles of self-shielded welding for ferritic and austenitic stainless steels are different.

In the ferritic weld metal,

must be killed because of its low solubility in ferrite,

nitrogen

and in the

austenitic weld metal no killing need be used because up to 3000 ppm N can be retained in solid solution. The effect of stick-out shown in Fig. 2.19 may be rationalized in terms of Figs. 2.20 and 2.14. feed speed (WFS) ,

Fig. 2.20 shows that when at a constant wire

the electrode extension is increased (see "LONG" in

Fig. 2.20) the current value falls (47).

This is because the resistance

of the circuit increases with the lengthening of electrode extension. However as the current falls, so the nitrogen pick-up increases as shown in Fig. 2.14 and this may account for the relationship in Fig. 2.20.

2.9.

Advantages of Relying on Killing

2.9.1.

Welding in the wind

It follows from simple reasoning that the less a given welding consumable relies on shielding and the more it relies on killing for the depOSition of sound weld metal, the less that consumable should be affected by cross air currents in the shop and side wind in the open.

If by design there

is little or no reliance on shielding, it follows that there is not much to be lost in a cross wind.

As the composition of air flowing past the

arc is not affected by its velocity, it follows that the side wind should have little effect on the N-content of the self-shielded weld metal.

33 This indeed is the case (Fig. 2.21) as illustrated by Houldcroft in 1977 (50).

For side wind speeds of up to 6 m/s (21.6 km/h

~

13.4 mile/h),

two cored wires, A and B in Fig. 2.21, gave hardly any change in the nitrogen-content of their weld metals. Similar results have been published recently (1989) by Soviet workers (51) and are shown in Fig. 2.22 where very little change in the weld metal N-content occurs with wind speeds of up to 10 m/s (36

km/h~

23 miles/h).

Such winds are much stronger than just "moderate cross ventilation" and the

flat

curves

in Fig.

2.21

and 2.22 show not just a "reasonable

resistance to draughts", but a remarkably high resistance to strong winds, verging on near immunity. One core wire (C) in Fig. 2.21 showed an increase in the weld metal N-content from 410 ppm (still air) to 480 ppm at the cross-wind of 6 m/s, this being a 17% rise which is not much.

The two wires in Fig. 2.22 showed

slight upward drifts in the weld metal N-contents at wind speeds above 10 m/s.

As shown by Killing (10) and the Soviet workers (51), different

self-shielding wires can be made to evolve somewhat different amounts of gases (see Figs. 2.8 and 2.12) and vapours (Table 2.5), and therefore some small effect of wind velocity on nitrogen pick-up may sometimes be expected.

However, as discussed in connection with Figs. 2.7 and 2.8,

the self-generated gas shielding is clearly undesirable for an unhindered droplet transfer, and as evidenced by the case of the solid wires, it need not be used at all in the self-shielding consumables.

Therefore,

contrary to the AWS Welding Handbook (see INTRODUCTION), it is not the "production of C02... right at the surface of the weld pool", but the almost total reliance on the deoxidation and denitriding, after the near unhindered

contamination

by

air,

that

accounts

for

the

remarkable

resistance to wind in self-shielding wires. In 1970, Millington (52) examined critical wind velocity for gas-shielded welding and used surface porosity as a criterion. wind

speeds

respectively.

of

0.5-3

m/s

for

gas

shielding

Porosity occurred at rates

of

14-28

l/min

One commercial self-shielding wire was included in the

tests and its resistance to surface porosity exceeded the original capacity of the wind

tunnel

(4.25 m/s).

In subsequent tests that wire could

tolerate winds up to 6.5 m/s which is in agreement with Figs. 2.21 and 2.22.

34 Occurrence

of

wind

induced

porosity in gas-shielded

welding indicates

a disastrous degree of N-contamination and Yeo (53) has warned that gas-shielded welding should not be used even in slight cross-winds because prior

to

porosity

formation,

the

increased

ductility and toughness of the weld metal.

N-content

will

lower

the

A side flow of air at a speed

as low as 0.73 m/s (2.63 km/h ~ 1.63 miles/h - a slow walk) can remove

75% of the C02 -gas shield at a distance of 20 mm from the gun nozzle orifice

(54).

formulated

The compositions of wires for gas-shielded welding are

to be compatible with gas-shielding which excludes air from

the arc, and consequently such wires are not designed to have resistance to

N-contamination.

Thus,

with

side

wind

speeds

in

the

range

of

0.3-2.0 m/s (% 1.1-7.2 km/h) the ductility and toughness of gas-shielded weld metal fall rapidly (55).

The fall in ductility and toughness begins

to occur already at the side wind speed as low as 0.3-0.5 m/s.

Increased

flow of shielding gas can give protection at winds of up to 1 m/s only. However, this is not the case with the self-shielded weld metal (56) which is designed with high AI-content for balancing the high N-content:(a)

Figure 2.23 compares the elongation of C02-shielded C-Mn steel weld metal

(55)

with

that of the self-shielded weld metal

function of cross-wind speed.

(56)

as a

At 2 m/ s, the CO 2 weld metal became

almost fully embrittled in tension, whereas the self-shielded weld metal %

(b)

retained

its

original

elongation

up

to

5

m/s

(18

km/h

11.2 miles/h).

Figure 2.24 compares Similarly the Charpy V-notch toughness of the CO 2 -shielded weld metal (55) with that of the self-shielded weld metal (56).

Again, the former becomes severely embrittled at side-

-wind speed of 2 m/s, whereas the latter shows a remarkable absence of any deterioration in toughness up to 5 m/s. The flat elongation and toughness curves for the self-shielded weld metal up to 5 m/s in Figs. 2.23 and 2.24 are in harmony with the flat nitrogen-content curves in Fig. 2.21 and 2.22.

In all the four Figures the curves

remain relatively flat for similar ranges of wind speeds of up to 5 and 6 (or 8) m/ s respectively. with as high a ductility

The self-shielded weld metal may not start (Fig. 2.23) and Charpy toughness (Fig.

as the gas-shielded weld metal, those properties despite

the

2.24)

but it retains the original values of

increasing wind

velocity.

The levels of

35 those properties in the self-shielded weld metal are perfectly adequate for numerous structural applications. Unlike with gas-shielded welding above, it is more difficult to compare the wind resistance of self-shielded welding with that of flux-covered electrodes because the data are even more sparse.

However, flux-covered

electrodes have always been used outdoors quite extensively and their good performance in the field is widely accepted.

Houldcroft (50) gives

the following nitrogen values for C-Mn steel weld metal deposited from basic low-hydrogen electrodes:Condition

Nitrogen, ppm

Short arc

90

Normal arc

150

Long arc in still air

480

Long arc in windy conditions

690.

Two queries arise immediately: quantitatively and

(i) the "windy conditions" are not defined

(ii) no data are available for the effects of wind

on the short arc, and more importantly - on the normal arc. A limited study (57) on the effect of wind only on porosity in the weld metal deposited

from E7018

electrodes showed that porosity free welds

could be made at cross-wind speeds of up to 45 miles/h (20 m/s), provided that short arc lengths were maintained. mechanical tests were not carried out.

However,

nitrogen analysis and

No doubt, the flux cup developed

on the electrode tip can screen the arc mechanically from the air flowing past the arc,

but no data have been located for the efficacy of such

screening. Despite the scarcity of the hard benchmark data for

the operation of

MMA/SMAW electrodes in the wind, the basic logic of the process metallurgy, as reflected in the Nitrogen Scale (Fig. 2.2), suggests that, on the whole, the self-shielding wires must be more resistant to the effects of draughts than flux-covered electrodes. publications unique

(as

capability

mentioned of

the

Unfortunately, even the latest authoritative in

the INTRODUCTION)

self-shielded

welding

sufficiently definitive guidance to the users.

undervalue and

do

this quite not

give

a

36 Yet,

over a span of some 20 years, there have been various statements

in literature that self-shielded welding is resistant to the effects of draughts and wind in outdoor locations (17, 58-65). known

brand

of

self-shielding

wire,

welding

Wi th a certain well

in

offshore

platform

construction has been carried out at winds of 30-35 miles/h (13.4-15.5 m/s) , and it has been claimed (59) that:"At such wind velocities, semi-automatic self-shielded wires can still make good welds; world-wide experience of welding offshore platforms with self-shielded wires has not yet shown a point where weld quality is affected by the wind." When

welding

structural

steelwork

for

high-rise buildings in Chicago,

it has been claimed (64) that:"Welding continued routinely in winds up to 30 miles/hour, and with winds sometimes reaching 60 miles/hour, conditions became so severe that work was halted." Those claims based on sheer production experience can now be understood in terms of the principles of process metallurgy inherent in self-shielded arc welding, and described in the preceding Sub-Sections. advisable for

It would be

the manufacturers of self-shielding wires to indicate in

the data sheets for the individual products (which may vary somewhat in their formulation) the maximum wind velocities which individual wire brands can tolerate.

The future user requirements are likely to move in such

a direction, for as T.Lefever (executive vice president of Hobart Brothers Co.) said in 1991 (66):"I see growth in the development of self-shielded flux cored wires. More and more people are looking for products that work under all conditions."

2.9.2. Some

Spatter on guns

degree

consumable

of

spatter

electrodes.

tubular/cored wires,

is In

usually

unavoidable

gas-shielded

welding

arc

with

welding

both

solid

with and

the globules of spatter are deposited not only on

the workpiece edges adj acent to the weld bead, of the welding gun.

in

but also on the nozzle

Consequently, the annular orifice through which the

shroud of gas must pass to shield the arc and the molten metal becomes gradually diminishes

blocked. the

The

dimensions

build-up of

the

of

spatter

gas-shroud,

in but

the

nozzle

also

not

increases

only its

37

turbulence which is conducive to air entrainment into the shielding gas. Figure 2.25 shows how with the increasing spatter build-up in the gun nozzle,

the N-content increased in the weld metal deposited with C02-

-shielding (67).

With the clean nozzle the N-content was below 100 ppm

(see also the Nitrogen Scale in Fig. 2.2), but as the spatter encrustation exceeded 50%, the N-content shot up with the resulting porosity. Guns for self-shielded welding have no gas nozzles surrounding the wire contact/guide tubes,

and as shown in Fig. 2.26, the guide tube can be

seen unprotected and protruding free from the gun spout.

As there is

no gas shroud to be hindered or disturbed, the occurrence of spatter in self-shielded welding is not so critical as in gas-shielded welding.

2.9.3.

Fume extraction

In arc welding, ventilation of an indoor working space and fume extraction are important in the provision of a safe and healthy working environment (68). or

However, the air currents associated either with general ventilation

local

fume

removal

may

cause

disturbing the gas-shielding itself. there is no or

problems

in

gas-shielded

welding

Fortunately in self-shielded welding,

hardly any shielding which can be disturbed and

extraction however

by

fume

vigorous has not been known to have ever presented

any problems. As shown in Table 2.6 (see also Appendix A), some self-shielding wires contain barium compounds in the core flux.

The fume generated from such

wires is considered to contain soluble barium compounds which are toxic (69).

The best way of dealing with such fume is to capture it at source,

thus eliminating the problem at the point of origin (70), by using local exhaust equipment.

This prevents the fume ever reaching the breathing

zone of the welder. Guns for self-shielded welding,

being simpler and lighter for the lack

of gas nozzles, are ideally suited for the attachment of integral fume extractors which operate in the closest possible proximity to the fume source.

Shown in Fig. 2.27 is a gun with an exhaust nozzle mounted above

the guide tube where it can catch the fume plume rising on a current of

38

hot gases and air.

Figure 2.28 shows a specially designed gun with an

integral exhaust nozzle which is concentric with the wire guide tube. Here, the fume plume capture can occur all the way around the periphery of the arc. clearly,

Figure 2.29 shows welding being carried out with such a gun:

no fume plume whatsoever can be seen rising towards the face

and the breathing zone of the welder. Welding with guns incorporating integrated exhausts has an extra advantage in comparison with the use of portable fume extractors which are positioned close to the fume source.

For such extractors to be effective, the exhaust

hood must be always close to the fume source and as the welding location changes,

the

arm

supporting

the

flexible

repositioned from time to time.

duct

and

the

hood

must

be

This slows down the welding work and

does not ensure complete fume removal at source at all times. The type of self-shielded welding gun shown in Fig. 2.27 was found very effecti ve at Portsmouth Dockyard in improving

the working environment,

when speedy modifications had to be carried out to HMS Gloucester and other Type 42 destroyers (71).

A choice of any other welding process

and

not

fume

control

method

would

have

ensured

the

same

excellent

combination of engineering quality and welding rate under the windy and onerous dockside conditions.

2.9.4.

Welding primed steel

Before welding or other fabrication operations, steel is grit- or shot-blasted to remove the mill scale.

In such a clean condition, the steel

surface is very active and prone to quick rusting which occurs within a day.

To prevent this, the steel must be coated immediately with a paint

primer which usually provides primers applied

to

prevent

the

rusting

basis for during

subsequent

fabrication

painting.

The

and welding are

usually referred to as shop primers. The big and often intractable problem with the arc welding of shop-primed steel is the occurrence of weld metal porosity which can take the forms of both surface breaking porosity and blowholes/wormholes extending from the root to the top of a weld bead. joints are most prone to porosity.

Fillet welds in T-j oints and lap

As such joints are used extensively

in shipbuilding and as the dockyard weather conditions demand diligent

39 priming of steel workpieces, weld porosity can be a problem. primer can be removed from the surface to be welded,

Naturally,

but this is time

and labour consuming and hence costly. Primer-induced weld porosity is thought to be a result of the breakdown of the organic CO 2

water, H 2

,

binders

(72,

73)

which release gases and vapours (CO,

nitrous gases, etc.).

,

The bubbles of those gases and

vapours become trapped in the solidifying metal to cause porosity.

High

Zn-content primers are usually found to be more conducive to porosity formation than low Zn-content primers. The

search for

resistant

to

and

testing

of arc

welding consumables

primer-induced porosity,

and

that would

the quest for

-inducing primers has been going on for some time (74).

porosity

be

non-

However, as far

as it could be established there is no generally recognized and definitive solution to the problem in the publications of authoritative organizations dedicated to welding.

Yet it seems that the solution has been there for

over 10-years, hidden in literature and unrecognized, being buried along with the self-shielded welding itself. shown in Japan

(75)

In the late 1970s, it has been

that when welding with wires giving a minimum of

0.74% Al in the weld metal, surface breaking porosity can be eliminated, and with 1% Al recovery the blowholes at the root are much reduced. Recent

enquiries

according

to

with

the

consumables

experience

manufacturers

available

so

far

revealed

from

(76)

industry,

that

Lincoln

INNER SHIELD NR-232 wire (see Appendix B) gives the best performance on primed

steel,

with

freedom

from

surface

breaking

porosity.

This

is

followed by INNER SHIELD NR-2llMP and NR-207 wires. No doubt, the primer-induced porosity is caused by a complex combination of gases, most of which contain oxygen and nitrogen, or both. killing

by Al

employed

in self-shielding

wires would

The heavy

reduce/decompose

those gases with the resulting formation of Al 2 0 3 and AIN, thus eliminating the sources of bubble formation. In addition to porosity formation, to welding fume evolution.

shop primer decomposition contributes

Zinc-rich primers emit some lead and cadmium

into the fume, and the occurrence of nitrous gases, hydrogen cyanide and

40 phosgene has been reported (72).

Therefore, fume removal by capture at

source is very important, and again the self-shielded welding is the ideal process for this purpose (Figs. 2.27-2.29).

2.9.5.

Welding Zn-galvanized steel

Numerous steel products intended for outdoor use are coated with metallic zinc, either by hot dip galvanizing or electroplating, to prevent rusting in service.

Semi-finished steel products are often galvanized

before

fabrication and welding, and when the galvanized steel is welded, problems arise:(a)

Zn-vapour forms readily in the heat of the arc, for the Zn-vapour pressure is even greater than that of Mg ,(Fig. 2.9).

Porosity occurs

usually in the root passes of fillet welds, but even surface breaking porosity is not unknown. (b)

White

zinc

oxide

particles enter

the welding fume,

producing a

pronounced cloud which obstructs the view and, if inhaled, can cause fume fever (77). The arc welding of galvanized steel, with either flux-covered electrodes or

semi-automatic

straightforward

gas-shielded

job

techniques (77).

so

far,

(MIG-MAG/GMAW) and

it

requires

wires,

has

special

not

been

attention

a and

Yet again, because of the inadequate perception of the

self-shielded welding as a process in its own right, the good performance of some self-shielding wires (Class E71T-GS) on galvanized steel is not appreciated.

Only

searches

through

manufacturers

data

sheets

and

advertising material reveal that Lincoln INNERSHIELD NR-152 , CORTEX SELF-SHIELD IIGS and ALLOY RODS Coreshield 15 are the consumables specially recommended for the welding of galvanized steel (Appendix C gives the names of manufacturers and their brand names for self-shielding wires). The profuse evolution of zinc oxide fume requires vigorous fume removal and again this can be best accomplished using guns with mounted (Fig. 2.27) or built-in (Fig.

2.28) fume extractors, rather than moving the

exhaust hood along the joint during welding.

Thus, like with the welding

of primed steel, the self-shielded welding appears to be the ideal process for the welding of Zn-galvanized steel.

41

References 1.

BONISZEWSKI, T. "Self-shielded arc welding". Paper 36 in "Advances in Joining and Cutting Processes 89", Harrogate, Oct. /Nov. 1989, The Welding Institute, Cambridge, CB1 6AL, England.

2.

TULIANI, S.S., BONISZEWSKI, T. and EATON, N.F • "Carbonate fluxes for submerged-arc welding of mild steel", Welding & Metal Fabrication, 1972, Vol.40, No.7, pp.247-259.

3.

BONISZEWSKI, T. "Manual metal arc welding - old process, new developments. Part II: Understanding MMA electrodes", The Metallurgist and Materials Technologist, 1979, Vol.11, No.11, pp.640-643.

4.

PISARSKI, H.G., JONES, R.L. and HARRISON, P.L. "Influence of welding procedure variables on the fracture toughness of welds made with self-shielded flux-cored wire". 6th Intern. Symp. of Offshore Mechanics and Arctic Engineering (OMAE) , Houston, Texas, March 1987.

5.

RODGERS, K.J. and LOCHHEAD, J.C. "Self-Shielded Flux Cored Arc Welding - The Route to Good Fracture Toughness", Welding Journal, July 1987, Vol.66, No.7, pp.49-59.

6.

BONISZEWSKI, T. "Manual metal arc welding - old process, new developments. Part 1: Introductory considerations", The Metallurgist and Materials Technologist, Oct. 1979, Vol.11, No.10, pp.567-574.

7.

KRIVENKO, L.F. and SLUTSKAYA, T .M. "Effects of alloying elements on the residual nitrogen content of the weld metal after open arc welding", Automatic Welding, 1967, Vol. 20 , No.3, pp.12-14.

8.

KRIVENKO, L. F. et al. "Research into the nitrides in the weld metal when steel is welded by open arc process", Automatic Welding, 1967, Vol.20, No.7, pp.6-12.

9.

KAPLAN, H.I. and HILL, D.C. "Thermodynamics of Air-operating Flux Cored Electrodes and an Analysis of Weld Toughness", Welding Journal Res. Suppl., 1976, Vol.55, No.1, pp.13s-19s.

10.

KILLING, R. "Welding with self-shielded wires - the mechanism of shielding and droplet transfer", Metal Construction, Sept. 1980, Vol.12, No.9, pp.433-436.

11.

POKHODNYA, I.K., ORLOV, L.N., SHEVCHENKO, Y.A. and SHLEPAKOV, V.N. "The influence of alloying on mechanical properties of flux-cored wire welds", Automatic Welding, 1985, No.7, pp.8-11.

12.

SIEWERT, T .A. and ZIEGENFUSS, H.G. "Welding in the Soviet Union - A Closer View", Welding Journal, Nov. 1987, Vol. 66 , No.ll, pp.27-34.

13.

POKHODNYA, I.K., SHLEPAKOV, V.N., ORLOV, L.N. and GAVRILYUK, Yu.A. "Technology and equipment for position butt welding of pipes". Paper 53 in "Welding and Performance of Pipelines", London, 1986, The Welding Institute, Cambridg·e, CB1 6AL, England.

14.

ORLOV, L.N. and GAVRILYUK, Yu.A. "The effect of structure on mechanical properties of one-layer flux-cored welds". Joint Soviet-Swiss Seminar "Metallurgy of Weld Metal", 1988, Proceedings in English by Oerlikon-Welding Ltd., CH-8050 ZUrich, Switzerland.

15 •

MALLETT, M. W. and RIEPPEL, P .J • "Arc Atmospheres and Under bead Cracking", Welding Journal Res. Suppl., Nov. 1946, Vol. 25, pp. 748s759s.

42 16.

MATI'HEWS, "Pipeline Conference Cambridge,

G. T., FREEMAN, R.M. , MIDDLETON, T. and WIDGERY, D.J. welding in the '80s". Paper 8 in "Second International on Pipewelding", London, Nov. 1979, The Welding Institute, CB1 6AL, England.

17.

SMITH, D.C. and JOHANNES, K.P. "Development of a Notch-Tough SelfShielded Flux-Cored Electrode", Welding Journal, March 1968, Vo1.47 , No.3, pp.207-214.

18.

SMITH, D.C. "Flux-Cored Electrodes - Their Composition and Use", Welding Journal, July 1970, Vol.49, No.7, pp.535-547.

19.

KOTECKI, D.J. and MOLL, R.A. "A Toughness Study of Steel Weld Metal From Self-Shielded Flux-Cored Electrodes" - Part 1, Welding Journal Res. Suppl., April 1970, Vol.49, No.4, pp.157s-165s.

20.

MILLINGTON, D. "Self-shielded arc welding", The Welding Institute Research Bulletin, 1973, Vol.14, No.2, pp.31-35.

21.

DAVEY, T.G. "Self-shielded welding of ferritic steels - a literature review", The Welding Institute Research Bulletin, Apr. 1978, Vol. 19 , No.4, pp.113-120.

22.

KILLING, R. and OTTO, J. "Metalibogenschweissen mit Fulldrahtelectrode", Report No.2854, 1979, ISBN 3-531-02854-5, Schweisstechnische Lehr - und Versuchsanstalt, Duisburg, Germany.

23.

ABSON, D.J. and EVANS, G.M. "A Study of the Manganese-oxygen System in Low Hydrogen MMA All-Weld-Metal Deposits". International Institute of Welding Doc. II-A-770-89; also in "Recent Trends in Welding Science and Technology TWR '89", Gatlinburg, Tennessee, 1989, publ. ASM International 1990.

24.

SUR IAN , E.S., TROTTI, J .L. and BONISZEWSKI, T. "Effect of Oxygen Content on Charpy-V Toughness in 3% Ni Steel SMAW Weld Metal." International Institute of Welding Doc.II-A-795-89, November 1989.

25.

BONISZEWSKI, T. "Manual metal arc-welding - old process, new developments. Part III: New solutions and challenges", The Metallurgist and Materials Technologist, Dec. 1979, Vol.11, No.12, pp.697-705.

26.

BAKER, R. "Process considerations and options available for the production of low residual steel from the oxygen converter", The Metallurgist and Materials Technologist, Dec. 1984, Vol. 16 , No .12, pp.624-627.

27.

LEDER, P .L.J. "The Application of Carbon Dioxide Shielding to the Continuous Flux-Covered Electrode Process", Brit. Welding J., June 1957, Vol. 4 , pp.274- 281.

28.

TERASHlMA, H. and TSUBOI, J. "Submerged arc flux for low oxygen and hydrogen weld metal", Metal Construction, Dec. 1982, Vol. 14 , No.12, pp.648-654.

29.

KAKOVINE, O.S. "Role des Fluorures dans Ie Soudage a l'Arc". national Institute of Welding Doc. II-A-634-84, 1984.

30.

McKEOWN, D. "Hydrogen and its control in weld Construction, Oct. 1985, Vol.17, No.10, pp.655-661.

31.

ALLEN, J . S. and WIDGERY, D. "Core wire developments and the objectives of BS 7084", Welding and Metal Fabrication, June 1990, Vol.58, No.5, pp.274-276.

metal",

InterMetal

43 32.

EMSLEY, J. "The hidden strength of hydrogen", July 1981, Vol.91, No.1264, pp.291-293.

New Scientist,

30

33.

PATON, B.E. and SLUTSKAYA, T .M. "Arc welding with bare electrode wire and no shielding gas", Automatic Welding, 1962, No.6, pp.1-4.

34.

KOBAYASHI, T. "Nonshielded arc welding of steel", J., 1967, Vol.14, No.3, pp.101-106.

35.

PRIOR, H., CLARK, J., STODDARD, D.W., BROWN, M.A.S. and YEO, R.B.G. "Welding with self-shielded flux cored wire Scottish Branch sponsored meeting", Metal Construction, Aug. 1986, Vol.18, No.8, pp.491-494.

36.

KOTECKI, D.J. and MOLL, R.A. "A Toughness Study of Steel Weld Metal from Self-Shielded Flux-Cored Electrodes" - Part II, Welding Journal Res. Suppl., March 1972, Vol.51, No.3, pp.138s-155s.

37.

LANDIS, G.G. and PATTON, D.M. "Method and Means of Bare Electrode Welding." U.S. Patent 2,909,778, October 20th, 1959, The Lincoln Electric Company, Cleveland, Ohio, U.S.A.

38.

BRAIN, A.G. and SMITH, A.A. "The Mechanical Properties of C02 Weld Metal", Brit. Welding J., Dec. 1962, Vol. 9, pp .669-677 •

39.

BONISZEWSKI, T. "Titanium in steel wires for C02 Construction, May 1969, Vol.1, No.5, pp.225-229.

40.

UL'YANOV, V.I., PARFESSA, G.I. and SHEVCHUK, R.N. "Effects of the Aluminium in Electrode Wire on the Strength of CO 2 Weld Metal in St.3 Steel", Automatic Welding, Dec. 1974, Vol.27, No.12, pp.14-18.

41.

HANNERTZ, N.E. and WERLEFORS, T. "The influence of parent material aluminium content on microstructure, inclusion distribution, and mechanical properties of submerged-arc weld metal". Paper 43 in "Weld Pool Chemistry and Metallurgy", London, April 1980, The Welding Institute, Cambridge, CB1 6AL, England.

42.

TERASHIMA, H. and HART, P.H.M. "Effect of Aluminum on C-Mn-Nb Steel Submerged Arc Weld Metal Properties", Welding Journal Res. Suppl., June 1984, Vol.63, No.6, pp.173s-183s.

43.

TERASHIMA, H. and HART, P.H.M. "Effect of flux Ti0 2 and wire Ti content on tolerance to high Al content of submerged-arc welds made with basic fluxes." Paper 27 in "The Effects of Residual, Impurity and Micro-Alloying Elements on Weldability and Weld Properties", London, Nov. 1983, The Welding Institute, Cambridge, CB1 6AL, England.

44.

MATSUMOTO, T., YOSHIDA, T. and MAKITA, M. "Non-S.hielded Arc Welding", Paper 6 in National Meeting, Spring 1969, pp.11-12, Japan Welding Society.

45.

KEELER, T. and BONISZEWSKI, T. "Effect of Travel Speed on Toughness of INNERSHIELD NR-203 Ni -C Weld Metal." Report MDR 034, March 1985, Brown & Root - Wimpey, HIGHLANDS FABRICATORS Ltd., Tain, Scotland.

46.

KENNY, B.G., KERR, H.W., LAZOR, R.B. and GRAVILLE, B. transformation characteristics and C C T diagrams in weld Metal Construction, June 1985, Vol.17, No.6, pp.374R-381R.

47.

YEO, R.B.G. "Specifications for the welding of offshore oil structures", Australian Welding Journal, Fourth Quarter 1988, pp .1526.

British Welding

welding",

Metal

"Ferrite metals",

44 48.

GRONG, 0., KLUKEN, A.O. and BJt>RNBAKK, B. "Effect of nitrogen on weld metal toughness in self-shielded flux-cored arc welding", Joining & Materials, Oct. 1988, Vol.1, No.4, pp.164-169.

49.

KOTECKI, D.J. "Welding Parameter Effects on Open-Arc Stainless Steel Weld Metal Ferrite", Welding Journal Res. Supp1., April 1978, Vo1.57, No.4, pp.109s-117s.

50.

HOULDCROFT, P.T. "Welding process University Press, Cambridge, England.

51.

SHLEPAKOV, V.N., SUPRUN, S.A. and KOTELCHUK, A.S. "Estimating the Characteristics of Flux-cored Wire Welding under the Wind Flow Effect". Paper IV.1 in "Welding Under Extreme Conditions", Helsinki, Sept. 1989, IIW Confer., Pergamon Press, Oxford, 1989, pp.171-179.

52.

MILLINGTON, D. "Gas shielding efficiency Welding Institute Research Bulletin, 1970, -352.

53.

YEO, R. "Cored wires for lower cost welds", Joining & Materials, 1989, Vol.2, No.2, pp.68-72.

54.

VERHAGEN, J ,G., LIEFKENS, A. and TICHELAAR, G• W• "Gas shielding for CO 2 welding", Metal Construction, 1972, Vol.4, No.2, pp.47-50.

55.

" K. "Detrimental effects of air AUTIO, J., KETTUNEN, P. and STROM, currents and their elimination in MIG-welding." Paper 32 in "Weld Pool Chemistry and Metallurgy", London, April 1980, The Welding Institute, Cambridge, CB1 6AL, England.

56.

BADA, T., ASAI, Y., NAGO, K., OHTSUKA, Y., MOROTOMO, I. and NAKAYAMA, H. "On the Development of the Non-Shielded Arc Welding Process for a Flux-Cored Wire." IIW Doc.XII-317-66, 1966, International Institute of Welding.

57.

HENRY,K.W. and LOND, R.E. "Effect of Wind on Radiographic Quality of Weld Metal Deposited with Low-Hydrogen SMAW Electrodes", Welding Journal, 1982, Vol.61, No.4, pp.47-50.

58.

MORIGAKI, 0., MATSUMOTO, T. and TAKEMOTO, Y. "Some Improvements in Self-Shielded Flux Cored Electrodes for Arc Welding", Welding Journal Res. Suppl., 1976, Vol.55, No.8, pp.241s-248s.

59.

MISKOE, W. 1. INNERSHIELD",

60.

ANON. "Self-Shielded FCAW Speeds High-Rise Construction", Journal, 1984, Vol.63, No.4, pp.47-49.

61.

de KONING, A.C. "Developments in materials welding technology for offshore structures", Metal Construction, 1985, Vo1.17, No.11, pp.727-734.

62.

ANON. "Office BUilding Columns Field Spliced with Self-Shielded Welding Wire", Welding Journal, 1984, Vol.65, No.10, pp.53-54.

63.

BJ0RNBAKK, B. and BOEKHOLT, R. "Self-shielded Welding Review, 1987, Vol.6, No.4, pp.272-275.

64.

ANON. "Self-Shielded FCA Welding is a Breeze in the Windy City", Welding Journal, Vol.67, No.3, pp.47-48.

65.

EVANS, S.R. "Latin America - Welding Technology in Contrasts", Welding Journal, 1989, Vol.69, No.1, pp.33-36.

technology".

1977,

Cambridge

in MIG welding." The Vo1.11 , No.12, pp.347-

"Continued development brings wider applications for Metal Construction, 1983, Vo1.15, No.12, pp.738-741.

flux

Welding

cored

a

wire",

Land

of

45 66.

CULLISON, A. and IRVING, B. "Record Number of Exhi bi tors Paces AWS Welding Show" - "A Look at Technology and Economics", Welding Journal, July 1991, Vol.70, No.7, p.33.

67.

SMITH, A.A. "C0 2 shielded consumable electrode arc welding." British Welding Research Association (now The Welding Institute), 1962, Abington, Cambridge, England. JENKINS, N. (Editor) "The Facts About Fume". 2nd Edition, 1986, The Welding Institute, Abington, Cambridge, CB1 6AL, England. IIW Fume Information Sheet No.4, Welding in the World, 1989, Vol.27, No.5/6, pp.138-148, Pergamon Press. FISs were produced by Commission VIII "Health and Safety" of International Institute of Welding. ANON. "Eliminate Fume Problems at the Source", Welding Journal, Sept. 1991, Vol.70, No.9, pp.69-70. YEO, R. "Welding speeds navy turnround", Welding and Metal Fabrication, Oct. 1989, Vol.57, No.8, p.406. " " BOHME., D., HEUSER, H., FISSAN, H. and KORBER, D. "Investigations into welding fume emission when welding over production-applied coatings." IIW Doc.II-C-692-83, Schweisstechnische Lehr und Versuchsanstalt, Duisburg. ANON. "Welding primed plate", TWI CONNECT, October 1991, No.26, p.7. !WI - The World Centre for Materials Joining Technology, Cambridge, CB1 6AL, UK. " BOHME, D. and HEUSER, H. "The Development of the Recommendation - DVS 0501. The Present Situation of the Shop Primers (FB) in the Federal Republic of Germany." IIW Doc. II -C-680-82 , 1982, Schweisstechnische Lehr - und Versuchsanstalt, Duisburg. ANON. "Effect of shop primer painting on the occurrence of pits and blowholes in fillet weld." Report of Subcommittee No.7 of Technical Committee of Welding Electrode Division, Japan Welding Engineering Society, 1978/79. YEO, R.G.B. Lincoln Weld Ro Ltd., Aston, Sheffield, S31 OBS, UK. Private Communication. ANON. "Welding galvanized steel", !WI CONNECT, July 1991, No.23, TWI - World Centre for Materials Joining Technology, Cambridge, CB1 6AL, UK.

68. 69.

70. 71. 72.

73.

74.

75.

76. 77.

46

Table 2.1.

Examples of typical oxygen contents in the all-weld metal deposits of mild and C-Mn steel composition.

Process/consumable type

Oxygen ppm

Flux-covered electrodes E6010/11

cellulosic

400 - 500

E6013

rutile all-positional

E7016

basic without Fe-powder

E7018

basic with 30% Fe-powder, all-positional

E7024

rutile with 50% Fe-powder

835

E7028

basic with 50% Fe-powder

500

750 - 850 250 - 350 350 - 490

C02-gas shielded welding Wire with 1.5% Mn - 0.8% Si

400

Submerged-arc welding Flux BI * 0.6

600 - 1200

1.0

400 -

600

1.5

250 -

500

2.0

200 -

400

3.0

170 -

350

50 -

100

Self-shielded welding Single-tube wire

* BI = Basicity

Index of the welding flux/slag

BI = CaO + MgO + CaF2 + BaO + SrO + Na20 + K20 + Li20 + !(MnO+FeO) Si02 + !(AI203+Ti02+Zr02)

47

Table 2.2.

Comparison of flux-carrying capacities, consumption of flux per unit mass of metal and weld metal nitrogen contents for different welding processes.

Process

Sub-arc

Flux-carrying capacity

Mass of flux consumed per 1 kg of metal

Unlimited

1 - 2 kg

60-110 ppm

Typical nitrogen contents in weld metal

Flux-covered electrode E7018

Approx. 40 % of electrode

0.7 kg

80-150 ppm

Self-shielded tubular wire

Approx. 20 % of electrode

0.25 kg

200-550 ppm

48

Table 2.3.

Volumes of shielding gases liberated from fluxes of cellulosic and basic low-hydrogen electrodes (15,16), from a self-shielding double-tube wire with 17%CaC03 in the outer tube (10), and from some self-shielding single-tube wires (10).

Welding consumable

Shielding-gas volume liberated from flux NTP

Typical nitrogen contents in weld metal

E8010-G cellulosic electrode

5.4 l/min

50-80 ppm

E7018 basic electrode

1.4 l/min

80-150 ppm

Self-shielding double-tube wire

1.2 l/min

220 ppm

Self-shielding single-tube wire

0.1-0. 2 1 / min

380-480 ppm

49

Table 2.4.

Decomposition, melting and boiling/vapourizing temperatures of some flux ingredients used in self-shielding cored wires, and of some reaction products.

SUBSTANCE

PHYSICAL PROPERTIES Decomposition Temperature, °C

Carbonates

520-825 350-900 1450 1310

CaC03 MgC03 BaC03 Li2C03 Fluorides

Melting Point, °C

CaF2 MgF2

1423 1261

BaF2 AIF3 NaF Na 3AlF 6

1355 1291 993 1000

KF

858 845

LiF

Boiling Point, °C ca

2500 2239 2137

sublimates 760 1695 sublimates from 1200-1300 1505 1676

Deoxidants/ denitriders Al Mg Ti Zr

660 649 1800 1857

2056-2467 1090-1107-1110 3287 4377

Nitrides AIN

-

TiN

2930

Al2S3 Notes:

sublimates 2000-2200

sublimates 1500

1. Main source: CRC HANDBOOK of CHEMISTRY and PHYSICS, 69th Ed., 1988-1989, CRC Press, Inc., Boca Raton, Florida. 2. Some carbonates decompose over a temperature range. 3. Somewhat different values are from different sources.

50

Table 2.5.

Relationship between the composition of core fluxes of three experimental self-shielding wires of single-tube construction and nitrogen and aluminium contents in the resulting weld metals (after Killing and Otto, 1979).

Different wire designs

Wire core ingredients

1

2

3

No gas and negligible vapour shield

Low CO2 gas-shield

Mg-vapour shield

40 %

40 %

40 %

AI-powder

10 %

10 %

10 %

Fe-powder

45.5 %

41.5 %

45.5 %

Mg-powder

0.5 %

0.5 %

4.5 %

CaF2

Minerals

4% MgO

8% MgC03

Graphite

Graphite

----

Graphite

WELD METAL: N Al

NOTE.

650 ppm 1.52 %

590 ppm 1. 72 %

500 ppm 2.55 %

The increasing recovery of Al resulting from the increased degree of shielding had no measurable effect on the recoveries of C, Mn and Si which remained approximately at: C = 0.2 % Mn = 0.35 % Si = 0.11 %.

Table 2.6.

Examples of core-flux ingredients in some self-shielding wires marketed in the 1980s for the welding of mild and C-Mn micro-alloyed structural steels. Values in weight %.

WIRE TYPE

WIRE CHARACTERISTICS

AWS A5.20 E70T-4 AWS A5.20 E71T-7

STRONG KILLING AGENTS

OTHERS

64 CaF2

13 Al 8 Mg

4.4 FeMn 2.8 FeSi 0.8 Na

9 BaF2 25 CaF2

13 Al 11 Mg

30 Fe 3 FeSi 4 FeMn

CARBONATES

FLUORIDES

Flat and horizontal positions Crack resistant fillets Desulphurising capacity High deposition rate

7 CaC03

All-positional Desulphurising slag Crack resistant High deposition rate

5 CaC03

VI I-'

AWS A5.20 E71T-8

AWS A5.29 E7ITS-Ni2

AWS A5.29 E71T8-K6 ---_.-

-

-----

-

All-positional Good low-T impact at -30°C Desulphurising slag Crack resistant All-positional AW good impact at -30°C Desulphurising slag Crack resistant All-positional Impact at -40°C and CTOD Desulphurising slag Crack resistant

2 SrC03

5 CaC03 0.5 SrC03

14 LbC03 1 SrC03

45 BaF2 2 CaF2 11 LiF

11 Al 7 Mg 1 FeZr

8 FeMn 2 FeSi 11 Fe203

47.5 BaF2 4.5 CaF2

Al 11 7.5 Mg 1.0 FeZr

8 10 2 3

Fe Ni Mn FeSi

45 BaF2 2 CaF2 2 LiF

14 A1Mg 2 FeZr

3 12 3 2

Ni FeO FeMn FeSi

52

Table 2.7.

Chemical analysis of experimental self-shielded weld metals showing the de-sulphurising effect of killing with Al on top of killing with Si. After Kotecki and Moll (1972).

Bulk Analysis, % Electrode C

Note.

Mn

Si

S

P

Al

S-l

0.083

1.40

0.50

0.022

0.010

-

S-2

0.060

1.51

1.17

0.023

0.010

-

SA-1

0.115

1. 75

0.82

0.014

0.010

0.38

SA-2

0.083

1.80

1.40

0.011

0.008

0.37

Comparing S-l with SA-1 and S-2 with SA-2 shows that with other aspects of the experiments being constant, the addition of Al increased the recovery of C, Mn and Si whilst the recovery of S fell by nearly a half.

53

600~-------------r~----------~~------------~

Key:

500

r--Melting point of iron

Oxides

400

1 / ~'/ " ",'"

Nitrides - - - - -

II

300

I

200

'2 Fe4~..J---:::;?'

_ C\I

~

:is ~

5 C\I

0

'0 Q)

"0

(5

0

~.::e.

0

>.

c: °in

c:

1!? 0

-="E

c: :;,

c. E 0 Cl

lIS

.E

-100

-:::..-...-

I

/"

/"",

/

--

/].,/"

",,,~;r'l."'/ c/iJo'!>", _ ~--", ~ ./

",

--

-

.., - -

'/2 s~!---

_---

_-

----

'2p..\~---- -'l°n~__ - - -

-L---

_-- _--

_--

~

c:

,.

, -" / ""~~::::-I

-200

2FeO _---.-" ZfN ..... ---I ' ------

-.- ..... --..... .,.,,_-- .....

~

_--

--

Q) Q) Q)

.!!!

JO/

",

o

Q)

lIS "t:I

---),

-

Z

/ //

-- .....I -- .....

'IN!OO

-600

en

-900

-1100~

____________

1000

~~

1500

____________-L______________ 2500 2000

~

Temperature, "C

Fig. 2.1.

The standard free energies of formation of oxides and nitrides as a function of temperature. Note the high stability of the oxides (deep sub-zero

positions)

and

the

lower

stability

of

nitrides (nearer zero levels): Mn - the weakest steelmaking deoxidant is more effective energetically in its function than the strongest denitrider Zr. - AIN.

FeO is more stable than aluminium nitride The iron nitride is completely unstable

in molten steel.

54

<

1000

,>

I

1'----...,.ln_c_re_as_i.... ng_ .. k,...,iI,..,lin,..,g:-.._____

.

900

Increasing shielding

.

I

/ /

800

/

700

600

/

/

I

500

400

300

/

200 E6013 MIG

100

o

-~

V/

-0

/

E7018

IE70::' - -

The "Nitrogen Scale": ferri tic

steel

weld

~

Killing dominant

shielding and deoxidation

ielding dominant

/

/

~r---~~

TI~~ - - E6010/11 ill Combined

Sh

Fig. 2.2.

/

/

/

/

I I / /

nitrogen metals

Killing almost exclusive

contents of deposi ted

by

different arc-welding consumables or processes which

rely

on

different

shielding/protection, pool

contaminated

by

or

degrees killing

oxygen

and

of

either

the

molten

nitrogen.

E7018: dashed line - some recent developments.

,

470~

0

450 430

Wire AWS E61T8-K6 SourceA 0 Source B ll. SourceC'

410

+

390 E

370

iii

350 L

0. 0.

Qj

0

Q

E "0

~

330

.E c: Q)

310

E z

290

I I I I

270

230

-

__ _

+

+

I1 I

210

lJ1 lJ1

4+ o , -- - - 0

250~-- - - - - - - -

,

0

0

0

d 0

0 0

I

0 ________

o

0

o

190

+ 170 0.80

0.90

1.00

o 1.10

1.20

1.30

Aluminium in weld metal, wt%

Fig. 2.3. Inter-relationship between the aluminium (deoxidant/denitrider) and nitrogen recoveries in the weld metal deposited from a single brand and size of self-shielding wire.

56

Cherepovets plant "Double-tube" SS-FCAWwire

Strip Width 22-30 mm Thickness 0.18-0.20 mm

Step 1. Filling with flux

Step 2. Rolling

Step 3. Filling with alloys, deoxidants, etc

Step 4. Closure

StepS. Drawing like a conventional FCAWelectrode

Fig. 2.4.

Soviet designed and manufactured self-shielding wire of double-tube construction which resembles the basic flux-covered electrode: the outer space contains CaC0 3 + CaF2 and the inner space contains deoxidants and various metal powders as required for alloying.

57

,

1000

900

/

800

/

/

,/ 700

E

,

600

II

/

/

300

II

D_ . ..;//

100

~

@

OL-______

/

/

/

200

________

Double tube

Fig. 2.5.

/

~

Z

400

/

/

/

__________

Single tube

~

/

Cl

/

~

500

.5 c:: Q)

g

I

/

E

"0

Gl :it

/

____

~

Gi

~

c. c. 1ii

/

/

Solid wire

The positions on the nitrogen scale of the three different types of self-shielded welding consumables: (a)

Double-tube construction,

(b)

Single-tube construction,

(c)

Solid wire.

and

58

700~-----------------------------------------------------'

600

500

\

\

400

300

,

""

200

\

\

\

\

\

\

,,

,B,

" . . . . " " ~' " .........

..................

100

" """',---

................. ........

2345681

Fig. 2.6.

........

2

3

4 5 6

........

8 10

~ C 2

0.1-0.2 Vrnin Single tube

1.3Vrnin Double tube

15-201lrnin

Internally generated

Internally generated

Externally applied

3

4 5 6

8100

Relationship between the arc gas-shield and the weld metal nitrogen content for

tubular or cored (flux or metal)

welding wires: A

self-shielded wire / single tube

B

self-shielded wire / double tube

C

gas-shielded wire.

Data for the self-shielding wires after R. Killing (1980).

59

2

7

5 1-droplet 2 - tubular wire 3 - core and flux 4 - shielding gas flow 5-weldpool 6-arc 7 - gas cushion

Fig. 2.7.

Hindrance of droplet detachment by the gas cushion which makes the droplet hover at the tube wall, for SS-FCAW wires with the highest gas-shield measured. After Robert Killing (1980).

60

--------------

2.0

1.5

.............

............

............

...........

............

...........

.............

j CD

N

.(jj

1.0

iii

c.

e

o

Gas volume at 1500·C. IImin

Fig. 2.8.

Relationship between the volume of the shielding gas generated by SS-FCAW wires and their droplet size factors. NB.

Droplet size = the size factor x wire diameter. The data plotted taken from Killing (1980).

61

1~r-------~------~--------~------~--------~------~

Zn

10-10

10-12

10-14

o

500

1000

1500

2000

2500

3000

Temperature,OC

Fig. 2.9.

Relationship

between

vapour

pressure and

temperature

for some metallic elements involved in composition of self-shielding wires used for welding C-Mn steels. The original curves extracted from:

"BRAZING MANUAL", 1976 American Welding Society

62

700r-----------------------------------------------------------------,7

• 600

E

500

6

5

0

Co Co

C)

0

0 .,....

:m

~

E

"Qj ~

.5 .l!l c::

400

•

0

C)

e

300

'E

"c::

III

c:: CI)

~ )( 0

200

Qj ~

[0]

•

8

10

3

c::

8 III

C)

"J:

>.

2 [H2]

0

12

14

16

18

0

0

•

•

20

22

24

26

CaF2 content in flux, %

Fig. 2.10.

~

e

• 6

.5 C

c::

100

4

•

•

. _ [N]

2

CI)

"

-"-"----=0_ ~ o

S E

"'--\

~0

c:: CI)

4

The effects of CaF2 additions to the flux of basic low-hydrogen electrodes in suppressing the oxygen, contents in the weld metal,

nitrogen and hydrogen

with other factors

being kept

constant. After Kakovine (1984), IIW Doc. II-A-634-84.

63

40r-----------r-----------~----------r_----------r_--------~

o

20

10

40

30

50

CaF2 content in flux, %

Fig. 2.11.

Improvement

in

steel

weld

metal

ductility

with

increasing content of calcium fluoride (CaF 2) in the flux

of

a

continuous

rutile

electrode

used

with

C02-shielding.

After Leder (1957).

64

0.9 I-

0.8

-

1\ \

\ 0.7

\

-

\ \

0.6 I-

rfl.

"i Ii Qi

E

"C

0.5 I-

Gi ~

.5 :l

'E

:l

<{

0.4

\

\

\

\

\

,,

"', ------0

E

'c

\

-

.........

0.3

-

0.2 f-

0.1 --

O~~I--_~I~I--_~I~I--_~I~I--_~I~I--_~I~I--~ o 0.1 0.5 1.0 Gas volume at 1500°C. IImin

Fig. 2.12.

gas-shield self-generated between Relationship volume (at 1500°C) evolution rate and aluminium content

in

the weld

metal

for

three commercial

self-shielding wires. After Killing (1980).

65 1.0r---------------------r------...,

Air (0.89)

--------------Porous weld metal

0.7

0.6

-

-~ z'"

Non-porous weld metal

0.5

a..

0.4

0.3

0.2

0.1

O~---~---~---~---~~~------------1.2 1.0 0.2 0.4 0.6 o 0.8 Aluminium in weld metal, wt%

Fig. 2.13.

Relationship between the aluminium content recovered in the weld metal and necessary for the prevention

of

nitrogen-porosity, and the N2 -content in the atmosphere when welding with SS-FCAW wires in 02-N2 atmospheres and in air. After Kaplan and Hill (1976).

66 700r----------------------------------------------------------,

650

E

Co Co

E

~

600

8 c: CD

Cl

e

~

550

500L-----------~----------~----------~----------L---------~

200

250

300

400

350

Welding current, A

Fig. 2.14.

Nitrogen content in mUltipass self-shielded weld metal as a function of welding current. After Kaplan and Hill (1976).

Current = 350A Electrode extension 50 mm Travel speed = 30 em/min

=

500

E

Co Co

E

400

.Sl c: 0

()

c: CD

Cl

g Z

300

200~--~--~----~--~---L--~----~--~--~--~----~--~

22

24

26

28

30

32

34

Welding voltage, V

Fig. 2.15.

Nitrogen content in multipass self-shielded weld metal as a function of welding voltage. After Kaplan and Hill (1976).

67

1000

Welding current 400 A Travel speed 20 cm/min Wire extension 45 mm

900

800

o 700

E

Q. Q.

600

E CI) E 0 0

c:

CI)

Cl

g Z

500

400

Bead-on-plate welds 200

23

25

29

27

31

Arc voltage. V

Fig. 2.16.

Effect

of

arc

voltage

on

the

nitrogen

content

measured at the apex of bead-on-plate weld metal. After Tadashi MATSUMOTO, Takashi YOSHIDA and Mibuo MAKITA, Japan Welding Society, National Meeting, Spring 1969.

68 Heat input, kJ/mm 1.5

o

Individual

•

Average

1.0

Wire diameter 2 mm Electrode DC negative Wire feed speed 105 in/min Current 240 A Arc voltage 21 V Stick-out 25 mm

450

400

E

a. a.

SCD

E

~

350

CD

~

.5 c::

CD Cl

~ z

300

250

200~--~~--------~----------~----------~----------~--~

10

15

25

20

30

Travel speed, cm/mm

Fig. 2.17.

Nitrogen content of weld metal cap as a function of travel speed

or

heat

input,

in

single-V

butt

welds

made

automatically in 38 mm thick BS.4360 Gr.50D steel plate, in flat position (ASME 1G) using Lincoln INNER SHIELD NR-203Ni-C wire. Keeler and Boniszewski (1985).

69 Heat input. kJ/mm

300

0.7

1.25

2.3

3.2

4.5

290

•

280

270

260 E Q. Q.

250

~ CD E "0

~

240

.5: c:

CD

Cl

e

230

2

220

Time-te-cool. tBOO--500oC • sec

Fig. 2.18.

Nitrogen content in self-shielded weld metal as a function of

the

cooling rate in the bead, expressed as time to cool between 800°C and SOQoC, or as a function of heat input.

Bead-on-plate welds

were made on 12.7 mm thick steel plate using Lincoln INNER SHIELD (1. SMn-Q •3Si) wire.

The pIa te had been buttered with the same

weld metal to minimize the effect of dilution. Kenny et al. (1985).

70

450r--------.--------~------~--------~------~------~

•

- Min.-max. )

A

..u. - Average

Pisarski et ai, 1987

o - Grong et ai, 1988 400

E

0. 0.

350

~

E

"0

Gi

;= .5

C

$

c

/

300

/

/

/

/

/

/

/

/

/

/

/

/

/

P

0 0

c

CD OJ

g Z

250

200

5

10

15

20

25

30

35

Electrode extension, mm

Fig. 2.19.

Effect of electrode wire extension (electrical stick-out) on the nitrogen content in the weld metal, with other welding parameters being approximately constant. Self-shielding wire:

E61T8-K6.

71

Short electrode extension

long electrode extension

Wire feed speed

Fig. 2.20.

Schematic relationship

between wire feed

speed

(WFS) ,

electrode extension (electrical stick-out) and current. Note that the current falls when the electrode extension is lengthened at a constant WFS.

After Ralph Yeo (1988).

72

km/h

15

10

20

500

E a. a.

400

~

E

'C

CD ~

350

.5 c:

Q)

Cl

g Z

A

300

B

250

2001~----~----~~----~----~~----~----~--~

o

2

3

4

5

6

Wind speed, m/sec

Fig. 2.21.

The effect of cross-wind speed on the total nitrogen content of three self-shielded weld metals deposited from three different tubular consumables.

After Houldcroft (1977). Courtesy of Cambridge University Press.

73 400r-------------------------------------------------------------------,

300 E c.. c.. I§

Wire 2

Q)

E "C

Qj

3:

200

.s:

Wire 1

c: Q)

Cl

g Z

100

Wind velocity. m/sec

Fig. 2.22.

Nitrogen content in the deposited weld metal as a function of wind velocity in welding zone for two different tubular self-shielding wires. After Shlepakov, Suprun and Kotelchuk (1989). Courtesy of Pergamon Press. 50~--------T--------r------_,--------~------_r~

\. \

40

• "# C 0 'Wi

30

Cl

c: 0

jjj

\

20

•

•

10

O~--------~------~ 2

__

----~~------+_------~~

Side wind speed. m/sec

Fig. 2.23.

Comparison of the effect of side-wind speed on tensile elongation of: (a) CO 2 -shielded and (b) self-shielded ferritic steel weld metals. (a) Autio, Kettunen and StrOm (1980); (b) Japanese work, IIW Doc. XII-317-66 (1966).

74

150r-~------~----~------~----~------T------'

...,

100

-c:i m

~Ul

.c
Self-shielded weld metal

>-

e' Q) c:

Q)

>

•

e--

.r:.

()

50

10 01L-~

____

~~

____L -____- L____

~

____

~

4

____- J

5

Side wind speed, m/sec

Fig. 2.24.

Comparison of the effect of side wind speed on Charpy V-notch impact toughness of: (a)

CO 2 -shielded

and

(b)

self-shielded ferritic steel weld metals.

(a)

Autio,

Kettunen

and

Str8m

(1980),

data

generated at room temperature; (b)

Japanese work, IIW Doc. XII-317-66 (1966), data generated at O°C.

75

500r---------~----------_r----------~~~----~----_r----~

400

E

00-

300

~

E :2 Q)

:=

.f:

c:

(I)

Cl

g

200

Z

100

O~

________

o

__________ __________ 40 60 20

~

~

~~~~~~~~~

____

~

100

80

Loss of nozzle area, %

Fig. 2.25.

Effect

of

spatter

build-up

in

the

gun

nozzle

on

nitrogen content in C02-shielded weld metal. Welding conditions:

150 A, 20 V. After A.A. Smith (1962).

......

'"

Fig. 2.26.

A welding gun for self-shielded welding, K126 Innershield Gun, featuring no gas nozzle and a well protruding guide tube from which the wire would emerge. Courtesy:

The Lincoln Electric Co.

Rating: 350 A.

...... ......

Fig. 2.27.

A welding gun for self-shielded welding, K309 Innershield Gun, with a fume exhaust nozzle mounted above the wire guide tube. Courtesy:

The Lincoln Electric Co.

Rating: 250 A.

'-l

00

Fig. 2.28.

A welding gun for self-shielded welding, K206 Innershield Gun, with an integral fume exhaust nozzle surrounding the wire guide tube. Courtesy:

The Lincoln Electric Co.

Rating: 350 A.

79

Fig. 2.29.

Self-shielded welding being carried out with K206 Innershield Gun with integral fume extraction nozzle giving almost complete removal of the fume plume. Note that no visible fume is rising past the welder's chest into his breathing zone. Courtesy:

The Lincoln Electric Co.

80

3.

SOKE ASPECTS OF PHYSICAL METALLURGY

3.1.

Consequences of Heavy Killing on Phase Transformations

In

the majority

of

self-shielding wires

of single-tube construction,

aluminium is the preferred killing element (see Appendix B) and it is used in quanti ties to give about 1 % (10 000 ppm) recovery metal, to ensure freedom from porosity (Fig. 2.13).

in

the

weld

Even at aN-content

of 500 ppm (see Figs. 2.2 and 2.5) the AI/N ratio would be about 20 which is well in excess of what is normally required in steel (i.e. Al/N ~2) to capture and bind most of the nitrogen in the metal.

Therefore, the

bulk of the AI-content recovered in the weld metal is "unconsumed" and it will act as an alloying element in iron (1, 2). The state of Al compared to that of Ti in the self-shielded weld metal was determined (1, 2) by comparing the bulk chemical analysis of the whole material with the electron-probe micro-analysis (EPMA) of the metal matrix alone. (a)

The results are plotted in Fig. 3.1 which shows that:-

for AI, the results of the two analyses are close to the 1:1 line, showing the bulk of Al residing in the weld metal matrix;

(b)

for Ti, the metal matrix analysis gave markedly lower values, thus indicating that a portion of Ti is bound in non-metallic inclusions which were found to be titanium carbo-nitrides and oxy-sulphides

(2).

Coincidentally with the amount of Al (about 1 %) needed to kill effectively the nitrogen porosity (Fig. 2.13), it happens that at about 1% Al in iron, the gamma loop is closed in the Fe-AI phase diagram, and the austenite-ferrite transformation is eliminated (Fig. 3.2).

This has been reported

to occur in self-shielded weld metal at 1.2% Al (1), at which AI-level the metal solidifies as delta-ferrite and remains untransformed during cooling down to room temperature.

The large columnar crystals produced

during

undergone any phase

solidification,

having

not

transformations

which cause grain refinement, are prone to easy cleavage and impart brittleness to the material.

At and above 1% Al in the Fe-AI system, this

situation is analogous to that in ferritic stainless steel with Cr-content above 17 %, also brittle.

in which the non-transformable ferritic microstructure is

81 To

restore the

grain-refining delta-gamma-alpha transformations,

it is

necessary to alloy the material with austenite forming elements such as C, Mn and Ni,

either singly or in various combinations.

Depending on

the level of toughness required, the following compositional groups can be distinguished among the various wires currently on the market (see Appendix B):-

Group A AWS Classes: E70T-3, E70T-4, E70T-7,

E71T-11, E71T-GS.

These wires are designed for less-onerous "general purpose" fabrications where there are no demanding toughness requirements.

They are capable

of replacing rutile flux-covered electrodes, E6013 and E7024 , which are fit-for-purpose in numerous non-coded applications, whether for Single-pass or multi-pass welding. Wi th these self-shielding wires,

the most economical way of restoring

the gamma-alpha transformation in the weld metal, is to add carbon in quantities of up to 0.3 % to the

deposit which contains on average:

0.5% Mn, 0.2-0.3% Si and 1-1.6% Al (3, 4).

In ferritic weld metal deposi-

ted by other arc-welding processes, such a carbon content is detrimental to ductility and toughness on account of generating hard bainitic/martensitic microstructure,

and a

high volume fraction of carbides.

However,

this is not the case in the self-shielded weld metal where at 1.2-1.3 % AI, the carbon addition restores the gamma-alpha transformation, thus producing some grain refinement.

As shown in Fig. 3.3, with the increaSing carbon

content in the range between 0.18-0.28% C, the Charpy toughness increases because the ductile-brittle transition temperature is lowered (3).

Thus,

a modicum of toughness, like with rutile electrodes, is assured for many non-critical applications.

Group B AWS Classes: E70T-6, E71T-8, E71T-G and E70T4-K2. For fabrications, for which weld metal with good toughness down to -30°C (27 J) is required and where ordinary E7018 electrodes would normally be used, the C-content must be kept at or below 0.1 % in the self-shielded weld metal.

In such weld metal,

the AI-content is usually kept closer

to 1 % rather than 1.5 %, and there are wires with 0.5-0.9% Al on the

82 market (see Appendix B). must be used,

However with C ~0.1 %, other austenite formers

and here self-shielded weld metals with the Mn-contents

of 1-2.4 % can be found.

Also, for some wires small Ni-additions of 0.5-

1 % are made.

Group C AWS Classes: E71T-8, E71T8-Ni1, E61T8-K6, E91T8-G. For critical and coded structures, such as offshore platforms, and oil and gas transmission pipelines, high toughness levels are required.

For

such and similar applications, there are self-shielding wires, the allweld-metal of which either gives 100-200 J at -30°C, or at least 35 J at -40°C/-50°C.

In some cases, where plate thickness exceeds 40/50

lDDl,

it is necessary to carry out the CTOn testing before a weld procedure is approved for a

given joint,

and a min.

of 0.25 mm CTOn is often

specified at -10°C. Self-shielding wires selected for such applications (4-6) always deposit weld metal with the C-content well below 0.1 %, usually controlled at 0.06-0.08% C. (0.5-0.9 %)

In addition,

a combination of Mn (0.75-1.65 %)

and Ni

is used for effecting full gamma-alpha transformation and

for enhancing the ferrite toughness.

Some wires developed more recently

for high toughness applications deposit weld metal containing additionally about 0.1% Cr (see INNER SHIELD NR-400 in Appendix B).

3.2.

Microstructure and Toughness

Since the late 1970s,

there have been numerous world-wide studies of,

and voluminous literature on, the effect of microstructure on toughness in C-Mn steel weld metal, deposited from flux-covered electrodes and by submerged-arc and gas-shielded processes.

Although such information for

the self-shielded weld metal is still scant (7-14), some comparison with the other weld metals is possible.

Of primary interest here is the as-

deposited microstructure because in the reheated and refined regions of multipass welds,

toughness is enhanced by grain refinement as is usual

for steels in general.

83 The current consensus (15) holds that in the as-deposited microstructure, the very best constituent conducive to high weld metal toughness is the so-called acicular ferrite which fills the columnar grains with a

fine

maze of needle-like crystals (Fig. 3.4a), appearing to have formed from austenite

according

relationships

(16,

to

Kurdjumov-Sachs/Nishiyama-Wasserman

17).

Such an arrangement

orientation

gives mainly high angle

boundaries between the adjacent crystals and, in combination with their fine grain size, confers high toughness by virtue of high resistance to crack propagation from grain to grain. The current consensus (15) also holds that a certain optimum oxygen content is required in C-Mn weld metal to provide a sufficient number and the right type of fine oxide inclusions for the heterogeneous nucleation of acicular

ferrite.

The

mjnjmum

oxygen

content

associated

with

high

toughness and large (over 50 %) volume fractions of acicular ferrite is found at about 250 ppm.

Depending on the Mn-content, such large volume

fractions of acicular ferrite and high toughness are achieved with E7016 electrodes

at

oxygen

contents

of

250-300

ppm

(18),

and

with

E7018

electrodes with oxygen contents of up to 400 ppm (19). However, as shown in Table 2.1, it is now well established (5, 6, 8-10, 12,

20,

21)

that self-shielded weld metal has a remarkably low oxygen

content, usually not much in excess of 100 ppm, and often below that level. This

is

markedly

below the minimum of 250

ppm required

for

profuse

nucleation of acicular ferrite, and it should be compared with the oxygen contents

of

very

high

toughness

weld

metals

deposited

from

other

consumables:(i)

250-350 ppm found in the weld metal obtained from E7016-1 electrodes which have the CTOD-pedigree, and

(ii)

250-300 ppm found in the weld metal deposited under high basicity submerged-arc fluxes.

Clearly,

as

explained earlier, the strong deoxidation/killing with AI,

done to balance the initially high oxidation by air, results in the selfshielded weld

metal

electrode deposits.

being

the cleanest one

In turn,

among

all

the

consumable

this high cleanliness must be reflected

in the paucity of oxide inclusions needed for the nucleation of acicular ferrite.

84

Consequently,

the typical microstructure within the columnar grains in

the as-deposited regions of the self-shielded weld metal looks like that in Fig.3.4b.

Here, in addition to some polygonal ferrite grains, there are

massive colonies of ferrite side-plates aligned parallel to each other like upper-bainitic sheaves.

It is well known that in such sheaves of

plates, the boundaries between individual crystals are of low angle type. A colony of aligned plates represents an effectively enlarged grain size, prone to ready cleavage and resulting in a lowered toughness. self-shielded weld metal, minimal,

and

the

the

volume

fraction

of

few acicular crystals that do

In the

acicular ferrite

form are rather

is

large

because there are few oxide nuclei to trigger their formation at the O-content of 100 ppm. Using sub-size Charpy specimens extracted solely from the as-deposited microstructure

regions,

toughness

transition

curves

were

obtained

(9)

and are shown in Fig. 3.5 for the two types of microstructure:(a)

in the weld metal from basic low-hydrogen E7018 electrode where there is a profusion of acicular ferrite (Fig. 3.4a), and

(b)

in the self-shielded weld metal ("SSFCAW") where polygonal ferrite and large ferrite side-plates predominate.

Clearly, the toughness of as-deposited regions in the self-shielded weld metal is markedly inferior to that in similar regions of the MMA/SMAW E7018 weld metal. However,

in multipass welds there are successive layers of alternating

as-deposited and reheated regions.

Reheating above the Ac 1-Ac 3 temperature range is analogous to normalizing and it leads to grain refinement which is

much

the

same

regardless

of

the

arc-welding

process

used.

The

appearance of the grain-refined weld metal is shown in Fig. 3.4c for the self-shielded weld metal, deposi t.

Incidentally,

shielded

weld

metal

but this would

look similar in an MMA/SMAW

the occurrence of grain-refinement in the selfdemonstrates

the

efficacy

of

the

gamma-alpha

transformation restoration with austenite-forming additions (Mn and Ni) and it shows that there is nothing inherently wrong with steel weld metal containing about 1% Al in solid solution. Figure

3.5

shows also

Charpy

transition

curves

for

the grain-refined

85 microstructural regions alone.

Here, the two weld metals:-

- one from a basic low-hydrogen E7018 electrode, and - the second from a self-shielding wire, give very similar toughness performance, as would be expected from similar microstructures.

Incidentally,

the

self-shielded weld metal tends to

have a somewhat higher upper shelf on the transition curve, and this could be associated with it being cleaner than the E7018 weld metal. It can be deduced from Fig. 3.5 that the overall toughness of multipass self-shielded weld metal can be raised by maximizing grain refinement, as shown in Fig. 3.6, through judicious re-heating (11).

This involves

the deposition of thin beads/layers which are then affected relatively deeply (about 50 %) by the heat of subsequent passes.

As shown in Fig.3.6,

with

sub-zero

about

50

%

grain-refinement,

temperatures are achieved.

Suitable

quite

low

transition

procedures developed along these

lines with the objective of achieving high toughness, including the CTODs adequate for offshore structures, will be discussed later (SECrrON 4).

Here however,

it is interesting to note how some Charpy test results

obtained from a production weld fit in with the data of Fig. 3.6 generated in the laboratory.

Where a Charpy specimen notch covers different amounts

of the as-deposited (blank) and refined (dotted) microstructures as in Fig. 3.7, variation in toughness must be expected, according to Fig. 3.5. In Positions 1 and 3 (Fig. 3.7), the notch samples about 40 % of the refined microstructure, and the toughness is relatively low.

In Position

2, the notch samples more than 50 % of the refined microstructure and

the toughness is quite high, fitting well with the data in Fig. 3.6.

3.3.

Nitrogen and Toughness

Like in other arc-weld metals

(15,

22)

cleanliness (oxygen content) and strength,

with a

given microstructure,

the increasing N-content in

the self-shielded weld metal is detrimental to toughness.

However, it

will be seen from some data that in addition to general similarities, the high AI-content in the self-shielded weld metal brings about some differences in the nitrogen-toughness relationship.

86 Kaplan and Hill (20) used two self-shielding wires to deposit welds at a

constant heat input of 1.7-1.8 kJ/mm, but excluding the nitrogen of

air by means' of 02-Ar atmospheres and in one experiment N2 -Ar atmosphere was used.

Figure 3.8 shows that when the weld metal N-content was lowered,

the Charpy toughness increased consistently and vice versa. To

decrease N-contamination when welding with. nOminally self-shielding

wire,

Darling

of Ar-2%02. the

and Rogerson

(11)

introduced a

supplementary gas-shield

The results in Fig. 3.9 show that there is a tendency for

toughness

to

improve as

the weld metal

N-content

decreases.

In

Fig. 3.9, some results from another work (12) are included and they fit in with the trend. With otherwise reasonably similar welding conditions, Grong et al.

(12)

increased the weld metal N-content by increasing the electrode extension (see Fig. 2.19), and the full Charpy transition curves for the two weld metals with the N-contents of 220 ppm and 420 ppm are shown in Fig. 3.10. Again,

the increased N-content was detrimental to toughness,

with the

100 J level transition temperature (Tt ) rising by about 22°C. With the N-content nearly doubled, the upper shelf also decreased somewhat, but it still remained at quite a high level of 175 J.

This moderate effect

of nitrogen on the upper shelf in the self-shielded weld metal (Fig. 3.10) is in agreement with the small effect of AIN particles on the upper shelf energy in C-Mn micro-alloyed steels (23) in which the AIN particles only raise the Tt • The moderate effect of nitrogen on the Charpy upper shelf of the selfshielded weld metal is very different from that of oxygen in other arcweld metals (15, 24, 25), where for instance the oxygen rise in the range of 200-400 ppm can reduce the upper shelf energy from above 100 J down to 30-75 J as shown in Fig. 3.11 (26a, 26b).

This is because, as explained

in SECTION 2, oxides form early in the molten metal and are thus able to

coagulate

into

large

particles,

some

of

which

become

trapped

on

solidification.

Such particles enable easy metal decohesion during plastic

deformation and

lead

26b, 27).

to void-coalescence fracture at low strain (26a,

Nitrides, on the other hand, form at much lower temperatures,

mainly in the solid metal and thus form a multitude of small, isolated and scattered particles.

87 There is some evidence (15, 18) that nitrogen in solid solution increases the strength and hardness of other arc-weld metals, as it does for steel in which it is not bound in particles.

The AlN particles, however, make

no contribution to strength in steel (23), and this is almost certainly true for the weld metal.

Although Grong et al. (12) have not reported

strength measurements, the hardness data for the two weld metals were:at N = 220 ppm

191 ± 12 HV5, and

at N = 420 ppm :

183 ± 16 HV5,

with the higher N-content weld metal appearing, if anything, slightly softer.

This would suggest very low, if any, nitrogen in solid solution

whilst the total N-content is very high relati ve to other weld metals (Fig. 2.2). The

relatively

high

total

N-content in

the

self-shielded weld metal

deposited from single-tube wires (Fig. 2.5) has been of some concern to designers responsible for material selection and welding process approval. The fear of embrittlement caused by nitrogen is understandable.

However,

it is necessary to appreciate that there is a difference between the two different effects of nitrogen:(i)

that resulting from the nitrogen bound and immobilized in nitride particles which can have a one-off or fixed effect on the Tt (e.g. Fig. 3.10), and

(ii)

that

resulting

from

the

free

(mobile,

"soluble")

nitrogen

in

interstitial solid solution where strain-ageing can occur as a result of nitrogen atoms migrating in time to and interacting with crystal dislocations, after the metal has been subj ected to some plastic strain, often as a result of an accident. It has been generally accepted that in other arc-weld metals, the bulk of their N-content is in the free and mobile state because in those weld metals, there are normally no intentional AI-additions to bind the nitrogen in

the

AIN

particles.

However,

few

data

are

available

because

of

analytical difficulties in separating the free nitrogen from the fixed nitrogen.

For E7018 weld metal, the following proportions of the N-content

have been recently reported to remain free in the as-welded condition: 45 % (28), 50-60 % (29) and over 80 % (30).

88 As

a

general

benchmark,

the

100

ppm N-level

can

be

taken as

being

representative of a number of other arc-weld metals (see Fig. 2.2), and this is

true for high quality weld metals deposited from E7016-1

(18)

and ~7018-1 (19) electrodes, and under high-basicity submerged-arc fluxes (24,

31,

32).

In the absence of intentional AI, Ti and Zr additions,

at least 50 ppm nitrogen is likely to be free to cause strain ageing embrittlement which can be quite severe (30). The data for the free nitrogen in the self-shielded weld metal are also very sparse, but the following values have been reported and are listed chronologically:1982:

5 ppm

(11) ,

1986:

50 ppm

(4),

1987:

25 ppm

(5),

1988:

<: 2 ppm

(12) •

Compared to the total N-content shown by the box in Fig. 2.2 where the very minimum lies at the 200 ppm level, some of the above values are very low indeed.

Even the value of 50 ppm (4) does not make the self-shielded

weld metal appear un favourably in comparison with the other weld metals. Whatever

the uncertainties resulting from the scarcity of data,

it

is

almost certain that in the self-shielded weld metal, the free and hence embrittling N-content not only does not exceed,

but also very likely is

much lower than that present in other arc-weld metal.

This appears to

be corroborated by the absence of any relationship between the N-content and the strength/hardness in the self-shielded weld metal. an increase in

In contrast

the strength/hardness with the increasing N-content has

been reported for the MMAjSMAW deposits (15, 18). As mentioned earlier, the AIN particles make no contribution to strength and most of the N-content in the self-shielded weld metal is clearly fixed in such particles.

Therefore in the self-shielded weld metal,

the N-

content should be viewed in a manner analogous to that in which the oxygen is considered in the other arc-weld metals.

Any changes in the bulk N-

content should be considered in terms of the effects caused by non-metallic or carbide particles on fracture, and not much in terms of strain-ageing in solid solution.

89 The above view is corroborated by comparing the Charpy toughness in weld roots and filling passes,

in multipass butt welds in thick plate.

In

the arc-welds made by other processes, there is through-thickness variation in the weld toughness (33-35), with the root regions showing sometimes marked embrittlement relative to sub-surface passes.

This is ascribed

to the consequences of strain-ageing which occurs following the plastic strain experienced by the root region, as the remainder of the joint is being completed. In contrast, as shown in Figs. 3.12-3.15 (8), in the self-shielded welds the roct run toughness was never measurably worse than in the sub-surface region.

This total immunity from through-thickness toughness variation

was found to be true for a number of different welding conditions.

This

is a remarkable asset of self-shielded welding because the middle of the plate is subject to the highest constraint (triaxiality of stress), and hence it is the root region which is normally most vulnerable to fracture initiation from any pre-existing weld discontinuity. Clearly, the same toughness in the roots and filling passes of the selfshielded weld metal must be connected with the minimal, if any, strainageing because of the insignificant content of free nitrogen available in this material, being accompanied by the high AI-content.

Furthermore,

it may sometimes be found (6-8) that the total N-content is somewhat lower in the root (R) than in the sub-surface (8) regions (Fig. 3.16).

This

is because in the narrow space of the root, the heat rarefies the air and the fume dilutes it, thus decreasing the mass of nitrogen available for metal contamination. However, there are cases where weld root toughness in the self-shielded weld metal may be worse than in the sub-surface regions.

This is a result

of causes other than nitrogen and its strain-ageing capability.

The weld

volume at the root is lower than further up the V-preparation and in the root the degree of dilution from the plate is correspondingly greater (Fig. 3.17) •

Thus,

the weld root can become markedly contaminated by

the various residual, impurity and micro-alloying elements from the plate. The only impurity element which the self-shielded process can well suppress is sulphur (Fig. 3.17) because of the de-sulphurizing capability of the high-fluoride slag and the high AI-level.

Niobium is added to C-Mn steel

90 as a micro-alloying element and when it is diluted into the root (Fig. 3.17) ,

the

Charpy

toughness

falls

with

the

increasing

Nb-content

(Fig. 3.18). And finally,

the very low availability of free and mobile nitrogen in

the self-shielded welds may be corroborated by the paradoxical fracture toughness behaviour at 1.20-1.30% Al

(3).

In the Robertson-type test,

the self-shielded weld metal was found to have a higher resistance to a fast running crack than to fracture initiation in the 'static' test.

CTOD

This behaviour was opposite to that found in the parent (base)

plate and to that expected from other weld metals. semi-killed variety (0. 18C-1.32Mn-o.025Si-0.005Al) N-content

would

dislocations.

be

in

a

free

form

and

The plate was of the in which all of

available

for

its

pinning

With little free nitrogen and dislocation pinning,

the some

plastic deformation ahead of the running crack would moderate its progress.

3.4.

Non-Metallic Inclusions

In contrast with other weld metals, there have been very few studies of non-metallic inclusions and their effects in the self-shielded weld metal. Undoubtedly, this is because of the inadequate perception of the selfshielded welding in the last 15 years. two

recent

papers

Notwithstanding

this

by

Grong scarcity

et

al.

of

The notable exceptions are the

(12)

data,

it

and is

Es-Sonni possible

appreciation of how the self-shielded weld metal fits

et to

al. get

(14). some

in and compares

with what is known about inclusions in other arc-weld metals. Oxygen and sulphur are the main impurities leading to the non-metallic inclusion formation and determining the total inclusion volume fraction in ferritic weld metal (36).

Nowadays unlike till the mid-1970s, oxides

are dominant inclusions in the weld metal because the S-contents are now very low, usually well below 0.010 %.

Also, some sulphur becomes dissolved

within oxide inclusions when they are still molten (37-38).

Therefore,

weld metal oxygen content will be used as the main reference criterion in this comparison. Figure 3.19 shows a relationship between oxygen content and the volume fraction (in %)

of non-metallic inclusions for various weld metals.

A

91 rather large scatter in the older data due to Widgery (39) is almost certainly contributed to by the variation in the S-content of the weld metal.

The newer data due to Kluken and Grong (40) show a reasonably

good relationship, and in general there is some continuity between the two sets of data (39, 40).

The self-shielded weld metal is the lowest

in oxygen content (see Table 2.1) and it does fit in broadly with the overall pattern,

but

the inclusion volume fractions reported (12,

14)

are clearly higher than what would be expected from the oxygen content alone.

This is undoubtedly caused by the elevated N-content in the self-

shielded weld metal (Fig. 2.2). Unlike in other weld metals where predominantly oxide inclusions appear round/spherical (37,

38), Es-Souni et al.

(14) found that in the self-

shielded weld metal, the majority of the inclusions have a faceted/angular morphology with well defined edges. multiphase in character,

consisting of spherical oxides in the centre

surrounded mainly by hexagonal AIN. Ti(C,N)

was

also

thought

formation of AIN (12,

The bodies of the inclusions are

(14)

Some formation of the cubic ZrN and

possible.

However,

it

is

mainly

the

14) on top of the oxides that would account for

the upward shift in the volume fraction in the plot of Fig. 3.19. The spherical oxide centres indicate the oxide-particle formation when the weld metal is still mol ten,

and the faceted/ angular morphology of

the nitrides indicates their formation mainly in the solid state.

This

is compatible with the dependence of the free energies of oxide and nitride formation on temperature as shown in Fig. 2.1, and the discussion in SubSection 2.2. Another characteristic of non-metallic inclusion population is the average inclusion diameter or arithmetic mean of the particle diameters measured. Figure 3.20 shows the scatter band of the relationship between weld metal oxygen content and the average inclusion diameter for other arc-weld metals presented by Pargeter (41).

The two results available (14) for the self-

shielded weld metal fit in with the general trend, despite the contribution to the inclusion size which must come from nitrogen/nitrides.

This trend

compatibility indicates that self-shielded weld metal fits harmoniously with other weld metals, and is not an unusual creation to be viewed with suspicion.

92 Some data are also available (12, 14) for the size distribution of nonmetallic inclusions and an example is shown in Fig. 3.21.

For the self-

shielded weld metal studied so far, the two-dimensional inclusion diameters lie generally in the range of

0.1-1.0~m.

This is in contrast with other

weld metals, in which inclusions with diameters of up to 2.0)'om (40) are found as frequently as those with 1.0)'om diameter in the self-shielded weld metal. The smaller size of inclusions (Figs. 3.20 and 3.21) in the self-shielded weld metal can be accounted for by the combination of:(a)

the very low oxygen content (see Table 2.1), and

(b)

the precipitation of nitrides (primarily AIN) mainly within the solid metal where inclusions cannot coagulate as they do in the mol ten pool.

The low oxygen content, and the small and dispersed nitride particles make the self-shielded weld metal appear to be metallographically cleaner than the other arc-weld metals. The inclusion size is very important for fracture toughness:(i)

With the increasing inclusion size,

the

fibrous/ductile fracture,

occurring by void coalescence, causes rupture at decreasing strain. The dimple size on the fracture surface increases and the Charpy upper shelf energy is lowered (12). (ii) The inclusions with diameters greater than l~m, usually with about 2fom diameter, are ready initiators of cleavage cracks in the weld metal (42-43).

This leads to low CTOD values in fracture toughness.

When self-shielded welding is not controlled properly, and the electrode extension is unjustifiably high

(e.g.

30 mm)

for

the

filling

passes,

excessively high N-level (e.g. 420 ppm) is obtained (see Fig. 2.19) and inclusions with about 2)(-m diameter can form.

Like spherical oxides in

other weld metals (42, 43), angular inclusions of such size in the selfshielded weld metal also serve as sites for the initiation of cleavage/ /brittle cracks (12).

93

3.5. The

A1uminium in Weld Metal and Corrosion aluminium

added

to

the

necessary to bind nitrogen,

self-shielded

weld

metal,

in

quantities

has an extra beneficial, though originally

unintended, effect that of retarding corrosion in sea water (44). effect was first demonstrated for the C02

The

weld metal deposited from a

series of experimental Mn-Si bearing wires alloyed additionally with up to 1.8% AI. The

tests

were carried out

in

synthetic

sea water of

the

following

composition

(g/l):. 26.52 NaCl, 2.45 MgC12, 3.30 MgS0 4 , 1.14 CaC12 , 0.73 KCl, 0.20 NaHC03 and 0.08 NaBr. The solution was kept at 32-35°C and the water flow at the specimen surface was 10 mise

The total testing

time was 1000 h, but a fresh solution was applied every 10 days. Figure 3.22 shows the results: increasing

up

to

0.5

%,

the

with the AI-content in the weld metal maximum corrosion· depth

decreased

0.25 mm for the AI-free material, down to 0.15 mm at 0.5% AI. same

time

the

depths

of

corrosion

in

the

parent/base

steel

from

At the plate

(0.17C-D.55Mn-D.25Si) and in the HAZ remained at the same high level for all welded joints. The above effect of Al was verified and confirmed by The Lincoln Electric Co.

(45) in a series of tests carried out in a solution Simulating sea-

water composition.

Four test butt-welds were made in ASTM A537 steel

plate (C-Mn-Si, pressure vessel quality):-

two with self-shielded wires, Innershield NR-203M and NR-203Nil% (see Appendix B),

- one with E7018 electrode (LH-70), and - one with L61 wire under F.860 submerged-arc flux. The tests were carried out for a whole year at 73° ± 5° F (23 0 ± 2.5°

C). The E7018 weld was severely corroded preferentially with respect to the steel plate.

The submerged-arc weld metal was also markedly corroded

with respect to the plate. weld metal, unaffected.

with

the

There was little corrosion in the self-shielded

Innershield NR-203Ni1% deposit being

practically

94

References 1.

KOTECKI, D.J. and MOLL, R.A. "A Toughness Study of Steel Weld Metal From Self-Shielded Flux-Cored Electrodes" - Part I, Welding Journal Res. Suppl., April 1970, Vol.49, No.4, pp.157s-165s.

2.

KOTECKI, D.J. and MOLL, R.A. "A Toughness Study of Steel Weld Metal from Self-Shielded Flux-Cored Electrodes" - Part II, Welding Journal Res. Suppl., March 1972, Vol.51, No.3, pp.138s-155s.

3.

GUNN, K.W., SQUIRES, LF. and KOHLER, G.E. "Paradoxical fracture toughness behaviour of self-shielded, flux cored arc weld metal". Paper 9 in "Fracture Toughness Testing", 1982, The Welding Institute, Cambridge, CBl 6AL, England.

4.

PRIOR, H., CLARK, J., STODDART, D.W., BROWN, M.A.S. and YEO, R.G.B. "Welding with self-shielded flux cored wire Scottish Branch sponsored meeting", Metal Construction, 1986, VoL 18 , No.8, pp.491494.

5.

PISARSKI, H.G., JONES, R.L. and HARRISON, P.L. "Influence of Welding Procedure Variables on the Fracture Toughness of Welds Made with Self-Shielded Flux-Cored Wire." 6th Intern. Symp. on Offshore Mechanics and Arctic Engineering (OMAE) , Houston, Texas, March 1987.

6.

RODGERS, K.J. and LOCHHEAD, J .C. "Self-Shielded Flux Cored Arc Welding - A Route to Good Fracture Toughness", Welding Journal, July 1987, Vol.66, No.7, pp.49-59.

7.

DORLING , D • V., RODRIGUES, P. E. L • B. and ROGERSON, J. H • "A comparison of the toughness of self-shielded arc and submerged-arc weld metals in C-Mn-Nb steels" : Part 1 - "Effect of consumables and process variables on weld metal toughness". Part 2 - "Through-thickness toughness variation". Welding and Metal Fabrication, 1976, Vo1.44, No.6, pp.419-423; No.7, pp.479-481.

8.

DORLING, D. V. and ROGERSON, J.H. "The effect of heat input on the weld metal properties of self-shielded arc welds in BS .4360 steels", Welding and Metal Fabrication, 1977, Vol.45, No.3, pp.157-161.

9.

DORLING, D.V., RODRIGUES, P.E.L.B. and ROGERSON, J.H. "A comparative study of the effect of welding conditions on the microstructure and toughness of self-shielded arc and manual metal-arc weld metals". Paper 16 in "Trends in Steels and Consumables for Welding", 1978, The Welding Institute, Cambridge, CBl 6AL, England.

10.

KEELER, T. "Innershield welding" : Part 1 "Development and applications." Part 2 - "Properties." Metal Construction, 1981, Vol.13, No.l1, pp.667-673; No.12, pp.750-753.

11.

DORLING, D.V. and ROGERSON, J.H. "The factors which control the toughness of self-shielded flux-cored arc weld deposits in carbonmanganese structural steels. " Paper 26 in "Fracture Toughness Testing", 1982, The Welding Institute, Cambridge, CBl 6AL, England.

12.

GRONG, t>., KLUKEN, A.O. and BJt>RNBAKK, B. "Effect of nitrogen on weld metal toughness in self-shielded flux-cored arc welding", Joining & Materials, Oct. 1988, Vol.l, No.4, pp.164-169.

13.

BJ0RNBAKK, B. and BOEKHOLT, R. "Self-Shielded Flux-Cored Arc Welding for Offshore Fabrications." IIW Doc .XII - 1080-1988, International Institute of Welding.

95 14.

ES-SOUNI, M., BEAVEN, P .A. and EVANS, G.M. "Microstructure and AEM Studies of Self-shielded Flux-Cored Arc Steel Weldments." Doc.IIA-847-91, 1991, International Institute of Welding.

15.

ABSON, D.J. and PARGETER, R.J. "Factors influencing as-deposited strength, microstructure and toughness of manual metal arc welds suitable for C-Mn steel fabrications." International Metals Reviews, 1986, Vol.31, No.4, pp.141-194.

16.

YANG, J.R. and BHADESHIA, H.K.D.H. "Orientation relationships between adjacent plates of acicular ferrite in steel weld deposits", Materials Science and Technology, Jan. 1989, Vol-S, No.1, pp.93-97.

17.

BHADESHIA, H.K.D.H. "Keynote Address: Modelling the Microstructure in the Fusion Zone of Steel Weld Deposits", in "Recent Trends in Welding Science and Technology", TWR ' 89 , Gatlinburg, Tennessee, USA, 14-18 May 1989, ASM International 1990. pp.189-197.

18.

SUR IAN , Estela and BONISZEWSKI, T. "Effect of Manganese and the Type of Current on the Properties and Microstructure of All-WeldMetal Deposited from E7016-1 Electrodes." IIW Doc.II-A-734-88, 1988, International Institute of Welding.

19.

EVANS, G.M. "Effect of Manganese on the Microstructure and Properties of All-Weld-Metal Deposits", Welding Journal Res. Supp1., March 1980, Vol.59, No.3, pp.67s-75s.

20.

KAPLAN, H.I. and HILL,:D.• C. "Thermodynamics of Air-Dperating Flux Cored Electrodes and an Analysis of Weld Toughness", Welding Journal Res. Suppl., 1976, Vol.55, No.1, pp.13s-19s.

21.

MORIGAKI, 0., MATSUMOTO, T. and TAKEMOTO, Y. "Some Improvements in Self-Shielded Flux Cored Electrodes for Arc Welding", Welding Journal Res. Suppl., 1976, Vol.55, No.8, pp.241s-248s.

22.

van NASSAU, L. and van der MEE, V. "Ni trogen in Manual Metal Arc Weld Metal". - A literature review. IIW Doc. 11-1127-89 , 1989, International Institute of Welding.

23.

BEPARI, M.M.A. "Effects of precipitates on strength and toughness of vanadium structural steels",· Materials Science and Technology, April 1990, Vol.6, No.4, pp.338-348.

24.

TULIANI, S. S. , BONISZEWSKI, T. and EATON, N.F • "Notch Toughness of Commercial Submerged-Arc Weld Metal" , Welding and Metal Fabrication, Aug. 1969 , Vol.37, pp.327-339.

25.

BONISZEWSKI, T. "Manual metal arc welding - old process, new developments. Part II: Understanding MMA electrodes" , The Metallurgist and Materials Technologist, Nov. 1979, Vo1.11 , No.11, pp.640-643.

26a. FARRAR, R.A. "The Role of Inclusions in the Ductile Fracture of Weld Metals", Welding and Metal Fabrication, Oct. 1976, Vo1.44 , No.8, pp.578-581. 26b. PAXTON, H.W. "The Metallurgy of Steels for Large Diameter Linepipe." In "Alloys for the Eighties", Ann Arbor, Michigan, June 1980, Climax Molybdenum Co., Greenwich, Connecticut 06830, USA. 27.

PICKERING, F .B. "The Effect of Composition and Microstructure on Ductility and Toughness" • In "Toward Improved Ductility and Toughness", Oct.1971, Climax Molybdenum Development Co. (Japan) Ltd., pp.9-31.

96 28.

den OUDEN, G. and PEEKSTOK, E.R. "Internal friction measurement of carbon-manganese steel weld metal." Irw Doc.II-A-798-90, International Institute of Welding, 1990.

29.

EVANS, G.M. "Effect of Aluminum in Shielded Metal Arc C-Mn Steel Multipass Deposits", Welding Journal Res. Suppl., Jan. 1991, Vol. 70, No.1, pp.32s-39s.

30.

KOCAK, M., ACHAR, D.R.G. and EVANS, G.M. "Strain Age Embrittlement of C-Mn Steel MMA All-Weld MetaL" IIW Doc.II-A-852-92, International Institute of Welding, 1992. Also: KOCAK, M., ACHAR, D.R.G. and EVANS, G.M. "Influence of Shielded Metal Arc Weld Metal Nitrogen Content on Its Fracture Toughness Behaviour. " Paper No. ISOPE-92-C5-39, 2nd International Conference on "Offshore and Polar Engineering" - ISOPE 1992, 14-19 June 1992, San Francisco, USA.

31.

TERASHIMA, H. and TSUBOI, J. "Submerged arc flux for low oxygen and low hydrogen weld metal", Metal Construction, Dec. 1982, Vol. 14, No.12, pp.648-654.

32.

DALLAM, C.B., LIU, S. and OLSON, D.L. "Flux Composition Dependence of Microstructure and Toughness of Submerged Arc HSLA Weldments", Welding Journal Res. Suppl., May 1985, Vol.64, No.5, pp.140s-150s.

33.

ROBINSON, J.L. "Through-thickness toughness variations in multipass arc welds." Paper 40 in "Trends in Steels and Consumables for Welding", London, Nov. 1978, The Welding Institute, Abington, Cambridge, CB1 6AL, England, 1979, pp.151-166.

34.

TAYLOR, D.S. "The effect of manganese on the toughness of "E7016" type weld metal", Welding and Metal Fabrication, Nov. 1982, Vol. 50 , No.9, pp.452-460.

35.

ABSON, D.J. "The Influence of Current Supply Type on the Composition, Microstructure, and Mechanical Properties of C-Mn and C-Mn-Ni Shielded Metal Arc Welds." In "Residual and Unspecified Elements in Steel", STP 1042, ASTM, 1989, pp.169-191.

36.

WIDGERY, D.J. "New ideas in submerged-arc welding". Paper 26 in "Trends in Steels and Consumables for Welding", London, Nov. 1978, The Welding Institute, Cambridge, CB1 6AL, England, 1979, pp. 217-229.

37.

BONISZEWSKI, T., LE DIEU, S.E. and TREMLETT, H.F. "Sulphur behaviour during deposition of mild steel weld metal", British Welding Journal, Sept. 1966, Vol.13, pp.558-577.

38.

BONISZEWSKI, T. and LE DIEU, S.E. "Sulphur behaviour in mild steel weld metal with respect to the control of oxidation during deposition~ British Welding Journal, March 1967, Vol.14, pp.132-144.

39.

WIDGERY, D.J. "Deoxidation Practice for Mild Steel Weld Metal", Welding Journal Res. Suppl., March 1976, Vol.55, No.3, pp.57s-68s.

40.

KLUKEN, A.O. and GRONG, 9). "Mechanisms of Inclusion Formation in AI-Ti-Si-Mn Deoxidized Steel Weld Metals", Metallurgical Transactions A, Aug. 1989, Vol.20A, pp.1335-1349.

41.

PARGETER, R.J. "Investigation of Submerged Arc Weld Metal Inclusions~ The Welding Institute Research Report No.151/1981 presented as Paper at Denver Symposium "Welding Metallurgy of Structural Steels", February 1987, The Metallurgical Soc. of AIME.

97 42.

McROBBIE, D.E. and KNOTT, J.F. "Effects of strain and strain aging on fracture toughness of C-Mn weld metal", Materials Science and Technology, May 1985, Vol.l, No.5, pp.357-365.

43.

TWEED, J.H. and KNOTT, J.F. "The effect of preheat temperature on the microstructure and toughness of a C-Mn weld metal", Metal Construction, March 1987, Vol.19, No.3, pp.153R-158R.

44.

UL'YANOV, V.I. and LOS', E.P. "Effects of the Aluminium Content of the Weld Metal on the Corrosion Resistance in Sea Water of Welded Joints in VSt.3sp Steel", Automatic Welding, 1976, Vol. 29 , No.9, pp.46-48.

45.

KRAMER, A. "Corrosion Behaviour of Welds in Sea Water." April 25, 1977, The Lincoln Electric Co., Cleveland, Ohio. Report available from the Company.

98

1.0 Aluminium 0 Titanium.

0.9

0.8

o o

0.7

:::e c

0.6

j

iii 'iii >iii c: ca

0.5

.~

iii

~

0.4

0.3

0.2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.9

1.0

Bulk analysis, wt%

Fig. 3.1.

Relationship between bulk chemical analysis of the material and metal matrix electron-probe micro-analysis of Al and Ti recovered in self-shielded weld metals.

After Kotecki and Moll (1970, 1972).

99

Liquid

1600 of Fe - -153S -C- -Melting - -point --------------------0

bFe

1200

Solid

yFe

1000

t)

°ai

:s

iii

_.!!O°CCurie~per~~

CD

_________________ _

c.

E CI)

I-

aFe

600

400

200

O~

Fe

_

_L_~_

_~_~_ _~_

0.1

_ L_ _~_

0.5

_ L_ _~_~_ _~_~_~

1.0

Aluminium, wt%

Fig. 3.2.

Iron (Fe) -

aluminium (AI)

binary phase transformation

diagram at the iron-rich end. Drawn on the basis of data on p.260 in ASM Metals Handbook 8th Edition, Vol. 8, 1973, Metals Park, Ohio, USA.

1.3

100

90

I

80-

70

-

I

I

I

I

I

Mn = 0.45-0.61 % Si = 0.11-0.23% AI = 1.20-1.30%

" ., •

-

'" ' ..""-

..........

" "" ",.

60 -

-

••

" "" . " ""-

50-

" \

401-

I 0.18

I

I 0.20

I

I

I

0.22

I 0.24

•

-

-

I

Carbon content in weld metal, wt%

Fig. 3.3.

Relationship between carbon content

in self-

-shielded weld metal and its Charpy fracture appearance transition temperature, showing

that

at about 1% Al the increasing C-content within the

limi ts

shown

improves

the

weld

metal

toughness. Data extracted from the work of Gunn, Squires and Kohler (1982).

101 (a)

Modern 'CTOD-quality' E7018

As-deposited

x 500

(b)

'CTOD-quality' all-positional SS-FCAW weld metal

As-deposited

x 500

(c)

'CTOD-quality' All-positional SS-FCAW weld metal

Grain-refined

x 500 Fig. 3.4.

Comparison of microstructures relevant to the control of toughness in modern 'CTOD-quality' weld metals (at about 1 kJ/mm):(a) (b) (c)

E7018 MMA/SMAW electrode - as-deposited SS-FCAW wire - as-deposited SS-FCAW wire - grain-refined all x 500

102

SSFCAW refined

60

E7018 refined

------50

-

----------

40

E7018 as-deposited

30

10

Test temperature, °C

Fig. 3.5.

Comparison of

sub-size

Charpy-V

impact

toughness

in

as-deposited and refined regions of two different weld metals deposited from: 1)

AWS E7018 flux-covered electrode: O.08C-1.2Mn-O.2Si at 1.3 kJ/mm,

2)

and

SS-FCAW wire: O.13C-1.6Mn-O.SAI at 1 kJ/mm.

After Dorling, Rogerson and Rodrigues (1978).

103

o

Result from Fig. 3.7: Position 2 I

-

30 f-

-

20 f-

-

"+"

0

10 f--

CD

Co

E

.s

Of--

c:

0

:;::;

·in

c:

-10 f--

jg >.

Co

(ij

-20f-

0

-

"

,, +

-

+,+, "",,+,+ + ,,,

-30 f-

-

0

(0

,

,, +',, + " +'

-40 f-50~

-60~

-70 0

-

, +"",, ",+, ,,

.t:: (.)

-)

I

I

I

40 f-

(.)

~ ~ iii

I

I

I

I

I

I

10

20

30

40

50

"1"

60

-

-

J

70

80

90

100

Grain-refined weld metal, vol%

Fig. 3.6.

Improvement

in

the Charpy V-notch toughness of

self-shielded weld metal with increasing grainrefinement. Multipass deposits in 25-32-38 mm thick plates.

After Dorling and Rogerson (1982).

104

Charpy notch positions

1

2

3

l l l Refined_-~::mS[J~ Omm

35mm

Notch position

Fig. 3.7.

Charpy energy (J) at -30·C

1

23, 20,

2

58, 76, 104;

3 Requirement

6, 23, 19; Min. 30 J indiv.,

9;

ave. 17 ave. 79 ave. 16 35J ave.

Refined microstructure

43% 54% 37%

An example of the effect of weld macrostructure sampled by the notch of the specimen on the level of toughness measured. This is a cut-out from a real production weldment in 35 mm thick BS.4360-50E steel plate welded vertically-up (ASME 3G) with Lincoln INNER SHIELD NR-203Ni-C wire.

Tested in the as-welded condition.

2 mm dia.

105

120 100 ~

~ ~

m

80

~ ~

~

~

~

~

60

~

£

>

~

~

40

~

u

20 0 0

E 200 ~

~

~

0

~

~ ~

400

£ ~

~ ~

g

Z

Fig. 3.8.

Relationship between

Charpy

V-notch

toughness

at room temperature (RT) and the nitrogen content in the various weld metals deposited from consumables designed for self-shielded welding.

After Kaplan and Hill (1976).

106

I

I

20f-

I

I

-

+ Dorling and Rogerson (1982)

o Grong, Kluken and Bj0rnbakk (1988) +

10

,

o

+

-

I+

-10

-

I

I I I I

-20

-30

/ /

/

/ -

",/ ,/' + .,."".." +---------+

-50-

-

o

-60f-

o

-

+ +

+

/

+

+

/

-40

-

+

I

50

100

I

150

200

250

I

I

300

350

400

Nitrogen content in weld metal, ppm

Fig. 3.9.

Effect of

the total nitrogen content on the Charpy-V

transition temperature of mUltipass self-shielded weld metals. Note the sharp deterioration of toughness at the N-content of 250 ppm and above.

107

Electrode extension 15mm 30mm

N, ppm 220 420

300r------------.------------~----------_.------------_r~

200 -:>

>.

~---_o'-

~

Q)

/'"

c:: Q)

"0 Q)

.c

is rn

/

.c

«

/

/0

100

•

/

0/ o~

0---" __________

-100

N=220ppm

0 N=420ppm

/'

~

____________

~

__________

~

o

-50

____________

50

~~

100

Temperature, ·C

Fig. 3.10.

Charpy toughness transition curves for two self-shielded weld metals with different nitrogen contents obtained by

welding

with

two

different

electrode

extensions,

but with other conditions being quite close. Wire: AWS E61T8-K6 (INNERSHIELD NR-203Ni-C). After Grong, Kluken and

Bj~rnbakk

(1988).

108 SA-GMA

• Cl ref 3 0 ref 4 A ref 5 ref 6 • ref7 ... ref2

•

o

} SA GMA } MIG-MAG

0

Cl

Cl Cl

Cl Cl

c

100

200

400

500

700

Oxygen concentration (0) ppm

(a)

Flux type • CaO-Ti02-Si02 o CaO-CaF2"Ti02-Si02 o CaO-CaF2-Si02 V MnO-MgO-AI203-Si02 A MnO-CaF2"AI203-Si02

120

100

75

80

~ ::: 50 >.

"")

>. E" CD

~

60

CD

0

t:

W

t: W

A

40

A 0

•

•

20

oL-______L-______ 0.01

• • • ••

•

25

______L __ _ _ _ _ _~_ _ _ _ _ _~_ _ _ _ _ _~_ _ _ _ _ _~------~~~O

0.03

0.04

0.05

0.06

Weld metal oxygen content, %

(b)

Fig. 3.11.

~

0.02

•

Effect of oxygen content on Charpy upper shelf energy of the transition curve where fracture occurs primarily by ductile shear and void coalescence: (a)

Farrar (1976): ref. Nos. in the legend are quoted in Welding and Metal Fabrication, Oct. 1976, Vol.44, No.8, pp.578-581;

(b)

Paxton (1980): submerged-arc weld metals tested at -25°C.

109 180 Subsurface Root

}AW

Subsurface

160

} SR

Root

.,..., 140

/

/ Subsurface

120

'/

V

100

80

/ 60

/ '/ V

I

/

/

I

I

/

I

I

I

I

/~ " , . , , - - - /'

/

I

./

Root

I

/

40

32mm

-90

-60

-30

o

30

________

60

~

______- L________L-______-L______

~

______

~

~

O

20

90

Test temperature. °C

Fig. 3.12.

Charpy notch toughness of self-shielded NR-203M weld metal deposited at 1.0 kJ/mm in symmetrical double-V butt weld in 32 mm thick SOD steel plate.

Toughness measured in the

as-welded and post-weld heat treated (600°C/l~ h) states. Root back-ground to sound metal. After Dorling and Rogerson (1977).

110

Subsurface Root Subsurface

240

Root

200

-:>

>.

AWroot

160

~ CD c: CD

>~

as

~

U

120

80

38mm

40

O~

______

-90

~

______

-60

~

________

~

______- L______

o

-30

30

~

________

60

~

90

Test temperature, ·C

Fig. 3.13.

Charpy metal

notch

toughness

deposited at

of self-shielded

1.0

kJ/mm in

NR-203M weld

symmetrical double-V

butt weld in 38 mm thick 40E steel plate. measured (600°C/I!

in h)

the

as-welded and

conditions.

Root

Toughness

post-weld heat back-ground

to

treated sound

metal.

After Dorling and Rogerson (1977).

111

Subsurface } Root AW Subsurface }

240

SR

Root

200

.,

160

>. ~

Q)

c:

Q)

e-as>-

~

u

120

80

40

-.--

_/

./

-60

/

/

/

/

/

/

/

/

/

/

/

/

/

'/

38mm

o

-30

30

60

90

Test temperature, ·C

Fig. 3.14.

Charpy

notch

toughness

metal deposited

at

2.0

of

self-shielded

NR-203M

kJ/mm in symmetrical double-V

butt weld in 38 mm thick 40E steel plate. measured

in

the

as-welded

(600°C/I! h) conditions.

weld

Toughness

and post-weld heat

treated

Root back-ground to sound metal.

After Dorling and Rogerson (1977).

112

Subsurface } AW Root

280

Subsurface } SR Root 240

--,,---

SR root

/

//

200

/

/

AWroot

/1' -,

:>. ~ CD t:

160

CD

>.

Co

a;

.c ()

120

I

I

I

I/ /

40

/'

/

/

/

I

I

/

/

/

I

/

/

/

/

/

/

/

I

I

/

/

/

/

/

/

/

/

/

/

/----

/

/

/

/

/

/

/

/

/

/

./

SR subsurface

/

38mm

/

-~~0--------~60~--------30~------~0~-------3~0------~6~0------~9~0~----~

Test temperature, °c

Fig. 3.15.

Charpy notch toughness of self-shielded NR-203M weld metal deposited at 1.0-4.0 kJ/mm in symmetrical double-V butt weld in 38 mm thick 40E steel plate Toughness

measured

in

the

using weave technique.

as-welded and

treated (6000°C/l! h) conditions.

post-weld heat

Root back-ground to sound

metal.

After Dorling and Rogerson (1977).

113

R = Root S = Subsurface

400-

-

3001-

1!., R

1!.

E c. c.

E CD E 0

R R 200~

0

c:

CD

Cl

g Z

100 -

..... .

;~;~~:t

O~~~~~~~~~---f~-L-+

c

B

A

~.:' ;.:~ :. ~: . : ~. : ;~.

__-+~-L~__-?~-L~__-E~-L~~ D

F

E

Weldment codes

Fig. 3.16.

Comparison of nitrogen contents in the roots (R) and the subsurface

regions

(S)

in

self-shielded

weld

INNER SHIELD NR-203M) in double-V butt welds. Welds A and B: Dorling et al. (1976); Welds C

F: Dorling and Rogerson (1977).

metal

(Lincoln

114 Carbon content. wt%

o

0.2

0.1

Subsurface

C S Nb

Ni

Cu

Root

C

S Nb

Ni

Cu

Plate

C

S Nb

Ni

Cu

0.01

0.02

0.03

0.05

0.06

Other elements. wt%

Fig. 3.17.

Comparison of some element contents in the plate and in the root and surface regions of the self-shielded weld metal in a double-V butt weld (INNERSHIELD NR-203M). Dorling at al. (1976). Note that Nb and C-contents in the root, in comparison with those in the subsurface region, indicate significant degree of dilution. However, the S-content in the root is not increased by this dilution, despite its high level in the plate.

115

80

-

+

60 ~

r40 r-

\;-' ~ ;:,

/

r-

it!CD

/

20 r-

c.

/

/

/

/

E

/

~

c:

g

f0-

jg >

Or-

/ +

·iii c:

:l c.

lii

r-

.c

..,

U 0

<0

/

/

/

/

/

-20r-

y+

-40

-

-60 0

/

/

+/

/

I

I

I

0.01

0.02

0.03

0.04

Nb content, wt%

Fig. 3.18.

The detrimental effect

of Nb-dilution,

from the plate

into the weld metal, on the toughness of self-shielded weld metal in the as-deposited condition.

After Darling and Rogerson (1982).

116 0.80r----~--""T"""--""T"--.....,....--.....,....--___r--_--~_--.__-_

Widgery (1976) C02/GMAW 0.70

x

)(

0.60

X X

0.50

X

X

eft.

r:: 0

n ~

CD

E ::l "0 >

XX

0.40

c: 0

·iii ::l

"0

E

0.30

l'

+

1

+ +

0'I1 I 10

0.20

Kluken & Grong (1989)

+ ._t------ SAW/GMAW +

0.10 Self-shielded welds • Grongetal(1988) o Es-Souni et al (1991)

O'~

__

~~

__

~~

__

~

____

~

____- L____

~

____~____~____L -__~

o

800

1000

Weld metal oxygen content, ppm

Fig. 3.19.

Relationship between weld metal oxygen content and volume fraction (%) of non-metallic inclusions.

117

0.9

A

0.8

E

c

c

0.7

::l.

iii CD

E ttl ti c

.2 Ul

:l

"0

.E

0.6

+

Q)

O"l

!!! Q)

«>

0.5

0.4

o } o

0.3~_--L

o

Es-Souni et al (1991) Self-shielded weld metals

_ _ _..I..-_ _.L..-_ _..L-_ _--l.._ _--L_ _--L_ _---1._ _ _..l.-_----l

1000

600

1200

1400

1600

1800

2000

Weld metal oxygen content. ppm

Fig. 3.20.

Relationship

between

weld

average inclusion diameter.

metal

oxygen

content

and

118

40 Number of inclusions = 500 Average diameter = 0.36 I-Im

30

~

20

~

-

eft. >. 0

c: CD

::J

0-

~ CD

>

......--

iii Qj a:

~

10

-

o

1111 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

r---1

r---l

0.9

Inclusion diameter, 1-1 m

Fig. 3.21.

Two dimensional size distribution of non-metallic inclusions

o =

85

in

self-shielded

ppm and

N

weld

metal,

with

= 275 ppm, deposited from

E61T8-K6 wire.

After Es-Souni at al. (1991).

119

0.30r------~-----_r_-----~----

~. •

c:-

Steel plate

o

0

----------------~~----------------~------------O-----Oo

E E o

__r__----__r-.....,

0.20

'iii

g 8

o

'5

..c:

C.

CD '0

E

g

0.15

'x

ItS ~

0.10

0.05L-_ _ _ _~L-----~-----~-------~~--------~~-J o 0.1 0.4 0.5 0.3 0.2 Aluminium content in weld metal, %

Fig. 3.22.

Effect of aluminium content in the weld metal on the maximum depth of corrosion in simulated sea water.

After Ul'yanov and Los' (1976).

120

4.

PROCEDURES FOR HIGH FRACTURE TOUGHNESS

4.1.

Preamble

There are critical structures,

built from C-Mn microalloyed steel with

a yield stress of about 350 N/mm 2 high applied stress, e.g.

,

which operate at low temperatures under

platforms in the North Sea.

Dynamic loading

by sea waves can cause fatigue cracking at welded joints and such cracks can initiate brittle fracture leading to catastrophic collapse. brittle fracture,

the various weld

regions,

To avoid

including the weld metal,

must be tough enough to resist fracture initiation.

To this effect, the

designers usually specify a minimum CTOD of 0.25 mm at -10°C in a 50 mm thick weld in the as-welded condition. It has been known for a few decades that welding procedure variables affect the Charpy toughness of various weld metals, and the same is true for the CTOn fracture toughness.

To achieve good results with flux-covered

electrodes, it is necessary to maintain the electrode run-out-length (ROL) within certain limits, and in submerged-arc welding the heat input must be controlled within certain limits.

The toughness of the self-shielded

weld metal is also influenced by some. procedural variables as shown in the preceding Section.

However,

characteristics

achieve

and

controlled in a

to

each good

process results,

has

its own individual

each

process

manner compatible with those characteristics.

must

be

In the

early 1980s, before the appropriate knowledge had been generated,

some

self-shielded welds gave poor toughness results because the process was not used in a suitable manner. Two key

publications

procedures must

be

(1,

2)

developed

which appeared and

controlled

toughness in the self-shielded weld metal. were

carried

out

with

INNER SHIELD

in 1987 show how welding to achieve high fracture

In both the cases, the tests

NR-203Ni-G

wire

(Appendix

B),

the

consumable recommended by the manufacturer for coded fabrications where high toughness is required.

The two studies (1, 2) have shown that, as

with any other arc-welding process, there are essentially two fundamental factors

of physical metallurgy

which

provide

the

basis for

toughness

control in the self-shielded weld metal:(i)

the macro- and microstructure, i.e. the degree of grain refinement

121 achieved

by

reheating

the

as-deposited

metal,

and

keeping

the

effective ferrite grain/colony size small in the remaining un-refined as-deposited regions, and (ii)

the nitrogen content which must be kept to a minimum by appropriate selection of welding parameters (see Sub-Section 2.8).

For the full details of all the

test conditions and results the two

publications (1, 2) should be consulted. of

Here, only the general principles

the welding technique will be given to show how welding procedure

development and

trials should be conducted with any appropriate self-

shielding wire.

The satisfactory results will be compared with the less

satisfactory ones obtained under non-optimum conditions. Development of self-shielding wires is continuing and new products have entered the market since the two publications (I,

2) appeared.

Before

undertaking any new work based on the principles described, the user is advised to consult a

given wire manufacturer about the precise values

of optimum wire feed speed (current), arc voltage and electrode extension appropriate to a given product, for the achievement of maximum toughness.

4.2.

Making the Welds

The principles of the welding technique described below will apply to any wall thickness,

but the validation testing is usually carried out

in 50 mm thick plate, on a single-sided single-V butt weld, the root of which will

remain intact

(Fig.

4.1).

The general conditions for

the

welding procedure are given in Table 4.1. In all the welding positions (ASME IG, 2G, 3G, 4G and 5G), the welding must be carried out by depositing a narrow stringer bead. This means that in the 3G position,

the travel direction must be vertical-down (V-D),

and a horizontally fixed pipe must be welded from the 12 6

o'clock position.

Any

side-weave

along

the

stringer

0'

clock to the

bead must

be

avoided because this will increase the local heat input and decrease the cooling rate.

This will cause the coarsening of the ferrite grains and

plate packets in the as-deposited microstructure (see Fig. 3.4b).

Only

when making a root run (the very first pass), weaving is permissible and

122 in the 3G position the root pass is made vertically-up (V-U). To maximize the reheating and refinement of the as-deposited microstructure (see Fig. 3.6), in addition to being narrow the original beads must also be as shallow as practicable.

This mode of welding, as shown in Fig.4.l,

is referred to as the "thin layer stringer bead technique".

Using this

technique, each subsequent weld bead is placed at the toes of the preceding one. With the thin layer stringer bead technique, were

numerous successful welds

produced in pipes of different wall thickness and in thick wall

tubular structures, especially in the 3G/V-D and 5G/V-:D positions.

The

important feature to note in Fig. 4.1 is that for all the wall thicknesses in the 3G/V-D position, the weld layer thickness is below 3.0 mm.

Some

authors (3) quote 3.5 mm as the general maximum restriction on the weld layer thickness, but some discrimination is necessary between the different positions.

Table 4.2 shows the realistic values for the average layer

thicknesses which yielded good CTOD toughness results for welds produced in different positions. For a given wire and welding position, once the WFS/current, arc voltage and electrode extension have

been fixed,

the travel speed is used to

control the layer thickness as shown in Fig.4.2.

Naturally, with the

other parameters being constant, the increasing travel speed will decrease the heat input (nominal arc energy).

This must be taken into account

in adopting anti-hydrogen cracking measures. As shown in Fig. 4.2, to achieve a weld layer thickness of about 3.5. mm and less, travel speeds of 25 em/min (10 in/min) and higher will be used (2).

Such travel speeds are over 2-times as high as those used in welding

with flux-covered electrodes.

To make sure that in production welding,

the required travel speeds are maintained,

the QC inspection personnel

can be issued with graphs (Fig. 4.3) showing the bead width as a function of

travel

speed

for

the

various

WFSs

inspection plus a simple measurement,

(currents).

Thus,

by

visual

the compliance of welders with a

given Welding Procedure Specification (WPS) can be verified. The thin layer stringer bead

technique has been used

successfully

in

123 production welding (2 , 4).

by some fabricators,

both in the UK and in Norway

Figure 4 .4 shows a macrosec tion of a test piece related to a

real production weld of 75 mm thickness.

4.3.

The eTOD Test Results

In the two studies (1, 2), the sampling and testing were carried out in From 50 mm thick plate, full thickness

conformance with BS.5762:1979.

bend-type specimens were obtained with their longitudinal axes transverse to the welding direction, with the weld being in the middle of the beam. The preferred geometry rectangular cross-section (t x 2t) was used for maximum constraint.

The specimens were notched in the through-thickness

(t) direction on the centre-line of the V-weld. weld

panels

had

been

given

hydrogen

release

Prior to sampling, the treatment

at

150°C

for

48 hours. Prior

to

fatigue

pre-cracking,

compression to obtain a

the

specimens were

fatigue crack with a

subjected to

local

relatively smooth front.

The CTOD testing was carried out at -10°C. Figure 4.5a (2) and Table 4.3 (1) give the CTOD results accompanied by the nitrogen contents for the welds made strictly according to the thin layer stringer bead technique.

In Fig. 4.5a,

the bars marked "C" and

"R" represent the N-contents in the weld caps (C) and roots (R), and the full points on the adjacent lines represent the individual CTOD values measured.

The majority of the results lie above 1.0 mm CTOD.

In general, the procedures based on and controlled according to:(a)

the thin layer stringer bead technique, and

(b)

the

operating

parameters

conducive

to keeping

the N-content down

and close to 200 ppm, gave welds with excellent CTOD fracture toughness, and in most cases well in excess of the customary minimum of 0.25 mm specified for the avoidance of brittle fracture initiation.

However, Table 4.3 shows that one out

of the 12 specimens gave a low CTOD result of 0.084 mm.

Metallographic

examination showed the microstructure to be the same as in other specimens,

124 but at the point of cleavage fracture 308-434 ppm. 'hiccup'

initiation,

the N-content was

It would appear that there had been the proverbial welding

there, leading to a local high N-content pocket which happened

to lie ahead of the crack tip. inclusions with about

2~m

At such high N-content, AIN non-metallic

diameter are likely to form (3), and inclusions

of such size are known to facilitate cleavage fracture initiation. As

shown

in Fig.

4.2,

the deposition of

thin weld

layers

requires

relatively fast travel speeds.

In semi-automatic welding, this requires concentration, dexterity and diligence, and some welders may not be up to those qualities. It has been thought that to accommodate the lessproficient welders, the required thin layer welds may be deposited by slowing down both the travel speed and the WFS simultaneously. The proposition of the "slow" procedure has been put to a series of tests (2) which refuted the idea. Figure 4.5b shows that welding procedures based on the slow WFS/travel speed technique produced inferior CTOD results.

Here, only two CTOD values

exceeded 1.0 mm and two values were below 0.25 mm, with one of them registering a "pop-in" (cross) at 0.13 mm for the 3G/V-D position. The reason for the inadequacy of the "slow" procedure lies in the increased N-content in the weld metal (see bars in Fig. 4.5b).

To maintain a given

thin layer dimension when the travel speed is lowered, the WFS must be lowered too so that a lower mass of metal is delivered at a given point along the bead.

But with the decreased WFS, the welding current falls

and this is conducive to increased nitrogen pick-up as shown in Fig. 2.14. Therefore, no departure from the WFS recommended by a manufacturer for good welding practice is permissible, where good fracture toughness is required.

4.4.

Corroboration of the Fast Travel Speed Thin Layer Stringer Bead Technique

4.4.1.

Fully automatic welding in flat pOSition

The effect of the variation in travel speed and the resulting change in the weld layer thickness was tested (5) for butt welds in 38 mm thick

125 plate

of Grade SOD

NR-203Ni-C

steel

2.0 mm diameter

to BS.4360:1979, wire,

in

the

using Lincoln INNER SHIELD jOint

preparation shown in

Fig. 4.6 with the procedural conditions given in Table 4.4.

With other

welding parameters being constant, as the travel speed increased the layer thickness decreased, becoming 3.34 mm at 30 em/min. The CTOD tests were carried out at -WoC according to BS.5762:1979 on full plate thickness, t x 2t specimens, pre-cracked on the weld centreline in the through thickness direction.

The results are given in Fig. 4.7

showing an outstanding rise in the CTOD values at the travel· speeds of 20 and 30 cm/min.

4.4.2.

Welding in overhead position

Lincoln· INNERSHIELD NR-207 wire can be used vertically down and it is recommended

by

the

manufacturer

for

pipe

welding.

When

welding

a

horizontally fixed pipe of a large diameter and travelling from 12 to 6 o'clock location, between 4-6 o'clock the welding is carried out almost in the overhead (ASME 4G) position.

Gravity pulls the weld pool down

and it is difficult to deposit thin and flat beads (see Table 4.2). Therefore, tests were carried out to find out the effect of bead thickness on Charpy toughness. Test welds were made in single-V preparation in 42 mm thick plate of Grade SOD steel to BS.4360:1979.

The wire feed speed (WFS) was 80 in/min and

the arc voltage was 20 V in four tests in which travel speed was varied to obtain different bead thicknesses.

In one test, the WFS was lowered

to 70 in/min and the travel speed was slowed down as well, to obtain a bead thickness comparable with those obtained at the faster travel speeds. The average Charpy-V test results are plotted in Fig. 4.8 against the bead

thickness as

measured

specimens were located.

at

the

capping

passes,

where

the Charpy

At -40°C, quite high toughness of over 75 J was

obtained for the bead depth of 5.5 mm, and the toughness fell sharply wi th the increasing bead thickness.

The weld made with the slower WFS

of 70 in/min was out of line with the general trend, showing drastically diminished toughness.

Almost certainly, this was a result of the increased

N-content, as could be expected from Fig. 2.14.

126 4.4.3.

Vertical welding of aT-butt

To achieve good toughness in the vertical position, it is necessary to deposit thin layers and this can be done only when welding verticallydown.

To demonstrate the case for a given T-butt joint, two procedural

tests were carried out:- one, referred to as "BAD" and welded vertically-up, and a - second, referred to as "GOOD" and welded vertically-down. The steel plate was 25 mm thick and the joint preparation was double-bevel T-butt as shown in Fig. 4.9a.

Procedural details for the two techniques

are given in Table 4.5 and the records of passes deposited are given in Figs. 4.9b and 4.9c:

the joint welded vertically-up had only 14 passes,

but the one welded vertically-down had 29 passes.

Wide bead weaving and

a slow travel speed (10 cm/min) were possible when welding verticallyup, but narrow and thin beads were obtained welding vertically-down because a fast travel speed (20 cm/min) had to be maintained, to prevent the molten metal running ahead of the arc under the pull of gravity. The Charpy-V toughness results

for

the two T-butt welds are given in

Table 4.6.

The test specimens were extracted from both the cap and root

locations.

The "bad" procedure with the slow travel speed gave inadequate

toughness,

for which the minimum requirement at -30°C was 35 J average

minimum and 30 J individual minimum.

The "good" procedure with the fast

travel speed gave excellent toughness, about 3-times as high as the minimum required,

both in the cap and the root, with the latter location being

if anything the tougher one.

References 1.

PISARSKI, H.G., JONES, R.L. and HARRISON, P.L. "Influence of Welding Procedure Variables on the Fracture Toughness of Welds Made with Self-Shielded Flux-Cored Wire". 6th Internat. Symp. on Offshore Mechanics and Arctic Engineering (OMAE), Houston, Texas, March 1987.

2.

RODGERS, K.J. and LOCHHEAD, J. C. "Self-Shielded Flux Cored Arc Welding - A Route to Good Fracture Toughness", Welding Journal, July 1987, Vol.66, No.7, pp.49-59.

3.

GRONG, 0. KLUKEN, A.D. and BJ9)RNBAKK, B. "Effect of nitrogen on weld metal toughness in self-shielded flux-cored arc welding", Joining & Materials, Oct. 1988, Vol.l, No.4, pp.164-169.

127 4.

5.

BJ0RNBAKK, Band BOEKHOLT, R.

"Self-Shielded Flux-Cored Arc Welding for Offshore Fabrications." IIW Doc.XII-1080-1988, International Institute of Welding. KEELER, T. and BONISZEWSKI, T. "Effect of Travel Speed on Toughness of INNER SHIELD NR-203Ni-C Weld Metal." Report MDR 034, March 1985. Brown & Root - Wimpey HIGHLANDS FABRICATORS Ltd., Nigg, Tain, Ross-shire, Scotland, IV19 1QY.

128

Table 4.1.

General procedural conditions for the deposition of self-shielded welds with high CTOD toughness.

Plate

C-Mn micro-alloyed steel Yield stress about 350 N/mi thickness = 50 mm

Weld preparation Electrode--Wire

EXXT8-:XX,

Wire feed speed (WFS)

About 100 in/min* (about 2.5 m/min) adjusted as required in different positions

Arc voltage

17 - 20 V

Electrode extension

20 mm max., typically 10 - 15 mm

Polarity

Usually DC- (but consult wire data sheet)

Electrode/gun angle

Held in constant position without oscillation

Deposition technique

Stringer bead throughout, strictly without weaving, except in the root (the 1st pass); 3G - welded vertically-down (V-D)

2 mm dia.

Preheat and min. interpass temperature Maximum interpass temperature

*

The WFS in in/min is conceptually more tangible than in m/min or mm/min. Most popular wires are of USA manufacture and the data sheets quote WFS in in/min. Some data sheets printed in Europe give em/min.

129

Table 4.2.

Realistic weld

layer thicknesses achieved with the thin

layer stringer bead technique in different positions, and resulting in high CTOD toughness. Wire:

ASME POSITION

*

INNER SHIELD NR-203Ni-C

2 mm diameter.

AVERAGE LAYER THICKNESS*, mm.

IG

3.2 - 3.3

2G

3.4 - 3.8

3G/V-D

2.4

4G

3.8

Calculated by dividing the plate thickness (50 mm) by the number of layers lying within the plate thickness.

130

Table 4.3.

Weld Position

IG

The CTOn test results at -woe for 50 mm thick welds deposited from INNER SHIELD NR-203Ni-C 2 mm diameter wire, in strict compliance with the thin layer stringer bead technique. After Pisarski, Jones and Harrison (1987).

40 J Toe

-65

CTOn

mm

1.03 0.77 1.51 1.40 0.60

- - -- - -

3G

V-D

-60

Result Type Ou 8u au &u

~u_

0.084

~c

1.85 2.04 1.65 0.26 2.36 1.16

Sm

Nitrogen Content, ppm

202 - 261

- - - - - - 308 - 434

~ du ~c c5m ~u

202 - 261

--

131

Table 4.4.

Procedural and welding conditions used in ASME 1G (flat) position to examine the effect of travel speed on toughness. For joint preparation see Fig. 4.6.

Welding Position Plate thickness Steel Groove Root gap Root land Pre-heat Max. interpass Layer build-up

1G (flat) 38 mm. Gr.50D BS.4360 Single-V 50 o ±5° 4-6 mm 0-2 mm. RT 150°C Passes placed at toes of previous beads

Power source: Lincoln DC 600 Wire feeder Lincoln LN-7 Consumable :INNERSHIELD NR-203 Ni-G Wire dia. 2mm. Electrode DC-negative WFS 105 in/min Current 240 A Arc voltage 21 V Stick-out 25 mm.

·.' ··

Root Pass: Manual at 17-20 cm/min Filling and capping: Automatic Test No. 1 2 3 4

Travel speed cm/min. 12 15 20 30

Heat input kJ/mm 2.5 2.0 1.5 1.0

Layer thickness mm 4.85 4.60 4.03 3.35

132 Table 4.5.

Procedural details for the welding of a T-butt joint in the vertical (ASME 3G) position by two different techniques: (b) vertical-up and (c) vertical-down.

Material: Thickness tested: Joint preparation:

BS.4360, Grade 50E Mod., Z-quality 25 mm Fig. 4.9(a): i-i double-bevel, 45° included angle, root gap 5±1.5 mm, root land 0-2 mm. Arc-air back gouge, grind and MPI.

Root treatment: Pre-heat: Interpass temperature: Filler material: Electrode: Welding position:

100°C minimum. 250°C maximum. Lincoln INNER SHIELD NR-203Ni-G 2 mm dia. wire. DC-negative. ASME 3G

Welding conditions: "BAD" PROCEDURE: Pass No.

WFS. in/min

A

1 2-9

60-65 70

150-170 170-190

vertically-up

[Fig. 4.9(b)]

Travel Speed cm/min

V

18-19 18-20

Heat Input kJ/min

5.0 max. 10.0 max.

3.2-3.9 1.8-2.3

10.0 max.

1.8-2.3

BACK GOUGE, GRIND & MPI 10-14

70

170-190

18-20

"GOOD" PROCEDURE: vertically-down [Fig. 4.9(c)] except for Pass 1 (Root) and capping passes being welded vertically-up. Pass No.

WFS in/min

A

V

l/up 2-15 Cap 16-17/up

60-65 70 70

150-170 150-170 150-190

18-19 18-19 18~19

Travel Speed cm/min

Heat Input kJ/mm

5.0 max. 20.0 max. 10.0 max.

3.2-3.9 0.80-0.86 1.8-2.2

20.0 max. 10.0 max.

0.80-0.86 1.8-2.2

BACK GOUGE, GRIND & MPI 18-27 Cap 28-29/up

70 70

150-170 150-190

18-19 18-19

133

Table 4.6.

Charpy

V-notch

toughness

test

results

in

the

as-welded

condition at -30°C comparing two weld metals (see Fig. 4.9) deposited vertically-up (b) and vertically-down (c).

Toughness results:

Charpy-V tests at -30°C in weld metal, "Bad" Procedure

Sampling

(J)

"Good" Procedure

Position Individual

Ave.

Individual

Ave.

CAP

Centreline ROOT Centreline

38,

20,* 92,

With the

106

71

38

50

Re-testing: 47,

*

69,

25,* 45

fabrication

74,

146,

107

109

145,

107,

100

117

39

specification requirement

of

30 J

individual

minimum and 35 J average min., the "Bad" Procedure failed the toughness criterion. Note that the "Good" Procedure gave as high a toughness in the root region as in the filling passes under the capping runs.

134 (a) Joint preparation

\

I

55-60°

\ I

\

I

t = 25-50mm

Strong backs Root gap = 4-6 mm Root face = 1-2 mm

(b)

Joint fill-up pattern:

3G vertically-down

-------- --- r----r-----,

Pass 1: Root run vertically-up Further passes: stringer beads vertically down Layers: each subsequent stringer at the toes of the preceding one. No of layers 2Smm 3Smm 4Smm SOmm

Fig. 4.1.

Joint

14

18 21

preparation

Layer thickness (LT) 2.8mm 2.Smm 2.Smm 2.4mm

9

(a)

and

filling-up

pattern

(b)

for Single-sided single-V butt welding of 25-50 mm thick SOD steel plate in ASME 3G position, vertically-down,

using Lincoln INNER SHIELD NR-203Ni-C 2.0 mm

dia. wire, for achieving high CroD toughness in the weld metal, in the as-deposited (as-welded) condition.

135

5

4

E E

en

rJ)

m c:

.Jt:

o

~

!

...J

3

2~--1~O------------~15~----------~2~O------------~2~5~----------~3~O--~

Travel speed, em/min

Fig. 4.2.

Effect of the travel speed on the weld layer thickness in butt welds made with INNER SHIELD NR-203Ni-C wire at the WFS of 105 in/min and 21 V.

136

17 16 15 14 13 12 11 10 E E

.c

9

:0 .~

"C I'll Q)

CD

7 6 5

WFS,ipm

•

4

0

•...

3 2

60 70 80 90

I:::.. 100

O~--~----------~~------------L-----------~~----------~--~

15

20

25

30

35

Travel speed, em/min

Fig. 4.3.

Combined effect of travel speed and wire feed speed (WFS) on the width of bead-on-plate welds made with 2 mm diameter INNERSHIELD NR-203Ni-C wire on 25 mm thick plate in ASME 1G (flat) position.

137

Mag. x 1 Fig. 4.4.

Macrosection of a ring stiffener type weldment made in 75 mm thick BS.4360-50E t1od. steel plate using Lincoln INNERSHIELD NR-203 Ni-C

2 mm dia.

wire, in ASME 3G vertical-down position. Weld layer thickness

= 2.5 mm.

138 (a)

400

2.0

300

1.5

c E

E E

0. 0.

c:CD

Cl

~ Z

ci

200

~

()

(b)

400

2.0

c

c c

1.5

: --- - --- I R

E

0. 0.

c: CD

g

c

R

E E

-R--

ci

1.0

I

Z

:!.~ i:.

0.5

:::.::.:::i:: .. ::.::i:.:::

..

O~~~~~L-

Fig. 4.5.

____

~~~~~

______

j jlj l l~

~~~~~

0.25

______

~~~~L--J

The effect of N-content on the eTOD toughness at -10 c e in self-shielded single-V one-sided welds in 50 mm thick plate. Microstructural

condition

approximately

constant

and

the

N-content altered by procedural parameters. e = cap; R = root. (a) Fast procedures;

(b) Slow procedures. After Rodgers and Lochhead (1987).

0

~

()

139

50 ± 5° 0

38mm

Backing strip

Fig. 4.6.

Joint preparation in 38 mm thick steel plate to BS.4360 Grade SOD used plates

with

for

the fully automatic welding of four

INNERSHIELD

NR-203Ni-C

wire,

with

four

different travel speeds and all the other factors being kept constant. For procedural and welding parameters see Table 4.4.

140 Layer thickness, mm

4.03

3.35

2.0

1.5

E E

.

c5 0

~

I

Cii 0

0 t-

o

1.0

• 0.5

• •

10

15

20

25

30

Travel speed, cm/min

Fig. 4.7.

Through-thickness CTOD toughness results at -lOoC and the lower bound for their scatter, obtained for Lincoln INNERSHIELD NR-203Ni-C welds deposited automatically in ASME 1G (flat) position, in 38 mm thick Grade SOD (BS.4360) steel plate. The results are plotted against the weld travel speed and weld-layer thickness.

141 90r-------------------------------------------------------------------------~

80-

o

,, ,

Lincoln Innershield NR-207 4G Weld cap results .

,,

,

",

70f-

WFS~80lnImin~"

60r-

,, ,

o

" ,,

" , "0',,

50-

40f-

,,

,, ,, , o

WFS = 70 in/min-------High[N]

20-

,

"

10 -

0~

__

-~1

5.0

________

~1

5.5

________ ________ ~1

6.0

~1

__________

6.5

~1

7.0

________

~~

7.5

________

~1~

8.0

Weld bead depth at cap, mm

Fig. 4.8.

Average Charpy-V notch toughness results obtained at -40°C from samples located at weld cap centre-line in as-welded Lincoln INNERSHIELD NR-207 single-V butt welds deposited in the overhead position in 42 mm thick C-Mn steel plate BS.436D-SOD. Toughness results are plotted against the weld bead thickness as measured at the cap.

142 (a)

Joint preparation Rootgap5± 1.5mm Root land 0 - 2 mm

45o~5:/ o

Side 1

25mm 25mm

Side 2

(b)

Pass location for vertical-up/"8ad" procedure

Side 1

25mm

Total 14 passes

25mm

Side 2

(c)

Pass location for vertical-down/"Good" procedure

Side 1

25mm

Total 29 passes

25mm

Side 2

Fig. 4.9.

Joint

preparation

(a)

and

the

filling-up

patterns

for

(b) "bad" and (c) "good" procedures for the welding of full-penetration

double-bevel

T-butt joints

in ASME

3G

position with Lincoln INNERSHIELD NR-203Ni-C 2 mrn diameter self-shielded wire.

143

5.

SPECIFICATIONS FOR SELF-SHIELDING WIRES

5.1.

Historical Background

There are no national or international standard specifications specifically intended for

self-shielding wires.

As mentioned in the INTRODUCTION,

such wires are treated jointly with gas-shielded wires, within the general group referred to either as the "flux-cored" or "tubular cored" electrodes. Therefore, unlike with other arc-welding processes,

the introduction of

self-shielded welding to a novice presents a special problem:

how to

combine the fundamental principles of the process with the classification of different types of wire? in

this

Publication,

unavoidably

some

the

however

In a sense, this problem is intractable and

fundamentals limited

have

references

been had

classifications to identify the products discussed.

treated to

first,

be made

to

yet wire

Such a dilemma would

not exist in the treatment of flux-covered electrodes, for which process fundamentals and flux-covering classifications are intimately connected. Apart from some obscure historical curiosities,

flux-cored or tubular

wires did not enter the industrial scene seriously until the 1960s, and this has been reflected in the relatively recent appearance of their standard specifications.

The first in the free economic world was the

American Welding Society

(AWS) Specification A5.20 issued in 1969 and

revised in 1979.

In comparison,

welding with flux-covered electrodes

became significant in the early 1900s.

The first British Standard BS.639

for those electrodes was published in 1935 and the flux-coverings much as they are known today were described and coded in BS.1719 in 1951. The very first AWS Specification A5.1 for covered electrodes was issued in 1940.

Thus, in comparison with the flux-covered electrodes, the self-

shielded welding is today where those electrodes were in the early 1940s. Perhaps 10-20 years from now, the self-shielded welding will be recognised as a distinct process in its own right. In October A81-359

1986,

for

a

"bare

series

of

flux-cored

French specifications from wires"

were

published,

each

A81-350 as

to

"Norme

Experimentale", by AFNOR-Comite de Normalisation de la Soudure, with the invitation to comment upon them till January 1989. cover

tubular

wires

not

only

for

the

Those specifications

self-shielded

welding, but also those for submerged-arc welding.

and

gas-shielded

144 A British

Standard

which

includes

was published as late as 1989. carbon

and

carbon-manganese

self-shielding wire

classifications

This is BS. 7084: 1989 "Specification for

steel

tubular

cored

welding

electrodes".

Although like with AWS AS. 20-79, BS. 7084: 1989 treats together the wires to be used with and without shielding gas, the title of BS. 7084: 1989 is a step in the right direction:

the restrictive reference to "flux-cored"

wire has been omitted to accommodate metal-cored wires. Currently, Sub-Commission XII-D of the International Institute of Welding is working on specifications for "Tubular cored wires for gas shielded and

self shielded metal arc welding ••. ".

In 1991,

the 3rd draft for

carbon-manganese steels and the 1st draft for low alloy steels have been produced. The classification principles of the above mentioned specifications will be presented briefly, but the reader interested in self-shielded welding is

advised

to

study

the

original

documents

in

full.

This

applies

especially to AWS A5.20-79 Specification which is given prominence because of its track record and the use of the AWS Classifications in literature in the last two decades.

5.2.

AWS AS.20 Specification for Carbon Steel Electrodes

The AWS classification scheme is shown in Fig. 5.1. arc-welding processes, electrode.

the letter "E"

is the first

In common with other symbol denoting an

The letter "X" stands for different variable symbols, in this

case digits, which must be inserted to discriminate fully between various product types.

The letter "T" before the hyphen has been obviously derived

from the word "tube", but in the AWS system it refers currently only to a "flux cored" electrode. The first "X" stands for the strength classification of which there are two levels, with the classification being based on the minimum tensile strength (TS):(i)

the digit "6" stands for the TS

= 62

(ii)

the digit "7" stands for the TS

=

ksi minimum, and

72 ksi mimimum.

145 The second "X" is for the usability position which is:- either "0" for the flat (ASME 1G and IF) and horizontal (ASME 2G and 2F) positions, or "1" for the welding in all-positions. In the place of the last "X", i. e. after "T-", it is necessary to insert a number, as in Table 5.1, to designate a wire with a set of specific characteristics.

For self-shielding wires the numbers:

10 and 11 are reserved. pattern

relating

the

3, 4, 6, 7, 8,

Table 5.1 does not appear to convey any specific numbers

to

the

various

wire

characteristics.

Presumably, as the product development and refinement occurred, consecutive numbers have been used to extend the list of classifications.

In general

it is necessary to memorize the numbers ascribed to the different types of self-shielding wires.

The only aid to memory is that the all-positional

usability has arrived at a later stage (T-7, T-8 and T-11) and the Charpy toughness capability is also a later arrival, e.g. T-7 vs. T-8. Examples of full classifications and their meanings are (see Table 5.1):(a)

E7OT-3:

a

self-shielding wire

positions

only,

for

for

use

single-pass

in

flat

and

high-speed

horizontal welding

of

thin sheet. (b)

E71T-8:

a self-shielding wire for use in all-positions, for both single- and multi-pass welding and depositing weld metal with good Charpy toughness (like an ordinary As.1 E7018 electrode).

5.3. AWS AS.29-80 Specification for Low Alloy Steel Electrodes As the self-shielded welding is still relatively little known and used compared to other arc-welding processes, there are very few self-shielding wires available on the market for the deposition of low-alloy steel weld metals (Appendix B).

Therefore at this stage, there is not much point

in considering the full range of the different compositions classified in As.29-80 Specification. As indicated with the asterisks in Table 5.1, performance

classifications

are

specified

for

only two usability and low-alloy

steel

self-

146 shielding wires, i.e. those designated with the digits "4" and "S". classification scheme is shown in Fig. 5.2. in Fig.

5.1 ,

designating

The

In comparison with the scheme

the hyphen is eliminated between the "T" and the digits

the

usability and performance,

and the two classifications

are denoted thus:"T4" -

for flat and horizontal welding on DC+ at high deposition rates, and

"'1'8" - for all-positional welding on DC-, both being suitable for single- and multi-pass welding. Unlike for the carbon steel electrodes, the Charpy toughness requirements are related to different weld metal compositions, rather than usability characteristics. As shown in Fig. 5.2, the hyphen is now used to separate the usability and performance designations, T4 and TS, alloy steel types (see Appendix B).

from the designations of low-

For instance, for the two low-alloy

steel types available on the market, A5.29-S0 specification requires the following compliance for the weld metal composition:-

= O.SO

- "Nil" must contain Ni "K6" must contain Ni

5.4.

= 0.40

BS.7084:1989

- 1.10 %, and - 1.10 %.

Specification

for

Carbon

and

Carbon-

-Manganese Steel Electrodes The British classification scheme is given in Table 5.2. "T" which stands for a tubular electrode wire, five

characters

in

the

characters are either (a) -

designation

digits

of

a

After the letter

there must be at least

given

product

type.

or letters as defined in the four

Those boxes

(d), plus the letter "G" for gas-shielded wire, or the letter "N"

for self-shielding wire. Where appropriate, the letter "H" can be added at the end of the mandatory classification to denote a hydrogen controlled consumable. An example

of a

classification for a

wire

welding may be thus (refer to Table 5.2):-

intended

for

self-shielded

147

T5-0NU where,

"5"

is for the tensile strength of 510-650 N/nun 2 (a),

"-" is for no Charpy toughness requirement (b), "0" is for usability in flat and horizontal/vertical positions (c), "N" is for self-shielding,

"u"

Such a

and

is for high deposition rate in flat and horizontal/vertical positions (d).

classification describes a typical welding consumable intended

primarily for the welding of fillets where the weld throat thickness is small

enough

unimportant.

(usually

below 10 nun)

for

the Charpy

toughness

to

be

The self-shielding wire so classified can replace E7024

flux-covered electrode in many applications. In comparison with the AWS-scheme (Table 5.1), this BS-scheme requires less to be committed to memory.

For the self-shielding wires alone, only

the meanings of the five letters "u" - "y" must be memorised [see box (d)

in Table 5.2].

This need not be difficult using

the

following

associations:- the wide looking "U" - the single-spiked "V" - the multi-spiked "W" However,

for high deposition rate, for single-pass welding, and for multi-pass welding.

it is strange that for

the self-shielding wires the welding

positions are classified twice, using boxes (c) and (d) in Table 5.2. This can lead to confusion as shown by the example taken from technical literature of a wire manufacturer:-

T532NW, where,

- the third digit "2" indicates all-positional welding capability, except vertical-down, the

but

letter "W" indicates usability

in flat

and horizontal/

/vertical positions, thus excluding the vertical-up and overhead positions indicated by the digit "2". Another weakness of BS. 7084: 1989 is that the letters "G" and "Nil standing for the type of process which normally is the first consideration, like the choice of a tool, appear after the digits denoting strength, toughness

148 and welding

positions.

It

would have

been better,

if

those

letters

appeared immediately after the letter "T", thus:"TG"

- for gas-shielded welding, and

"TN"

- for self-shielded welding.

It will be seen below that such an arrangement has been adopted in the French classification of tubular cored wires, and it is considered in the current IIW draft classifications. Unlike AWS A5.20-79 and A5.29-80 Specification, BS.7084:1989 Specification has not been in existence long enough to acquire a documented industrial track record.

Therefore at present, the experience with the application

of self-shielded welding can be considered only in terms of the AWS Classification.

5.5.

NF A81-350 Specification for Unalloyed Steel

for

Bare

Flux-Cored

Wires

The French principles for classifying tubular wires incorporate the three arc-welding processes as shown in Table 5.3:"TSS"

stands for self-shielded welding,

"TGS"

stands for gas-shielded welding,

"TSA"

stands for submerged-arc welding.

and

Compared to the British method (Table 5.2), the French scheme indicates immediately the process application of a given tubular wire.

It is also

noteworthy that the process designation symbols adopted by the French are fully compatible with the English welding terminology, and consequently should be well suited for any future international classifications, e.g. by the IIW. An example of classification for a self-shielding wire could be as follows (refer to Table 5.3):-

TSS51. 2 . 2 . H where,

"SS"

is for self-shielding wire,

149

"51"

is for the tensile strength of 510-650 N/mm2,

"2"

(the first one) is for 35 J at -20°C,

"2"

(the second one) is for all-positions, except vertical-down,

"H"

and

is for diffusible hydrogen content of more than 5 and up to 10 ml/l00g.

The French classification scheme is still in its tentative stage,

and

like

any

with

the

BS.7084:1989

scheme,

there

does

not

appear

to

be

documented track record of its application to self-shielded welding in fabrication.

5.6.

llW Draft Specification for Gas-Shielded and Self-Shielded Tubular Cored Wires

Submitted

in

1991,

the

IIW Doc.XII-D-138-91

is the

3rd

draft of

the

classification requirements proposals for wires to be used for the welding of carbon-manganese steels.

The general outline of the classification

scheme is shown in Table 5.4, but as this is still under discussion, its full details are being omitted. At this stage suffice it to note that the IIW scheme uses the letter "T" for tube, and in general follows the overall patterns of the British and French schemes,

but it is closer to the French one

process type immediately after the "T".

However,

by indicating the

the choice of letter

"H" for self-shielding wires is most unfortunate, because almost everywhere in the welding world,

the letter "H"

hydrogen for welding consumables.

Also,

is used for denoting diffusible this is so in this IIW scheme

(Table 5.4) where the letter "H" accompanied by a digit is used as an optional symbol for the diffusible hydrogen content in the weld metal. According to the IIW scheme,

an example of classification for a self-

shielding wire may appear thus:TH4821B where,

"H"

is for the self-shielding wire,

"48" is for the tensile strength of 480-650 N/mm2 ,

150

"2"

is for 27 J at -20°C,

"1"

is for multi-pass welding in all-positions, except vertical-down,

"B"

5.7.

and

is for basic flux in tube core.

General Comment

In the applications of self-shielding wires described in the literature of

the

1980s,

references

Classifications. scheme, but

have

been

made

exclusi vely

to

the

AWS

The British and French schemes, as well as the IIW draft

may convey more information directly than the AWS designations,

they

still

lack

applications

history.

Furthermore,

the

AWS

Classifications for welding consumables in general are also used in the ASME Codes, and are thus readily recognizable and recognized world-wide, especially in the dynamic economies of the Pacific basin and in Latin American countries.

The same cannot be said about the existing British

and French Specifications. Currently

in Europe,

the CEN are working on a draft European Standard

for the "Classification of Cored Electrodes for Metal Arc Welding With or Without Gas Shield of Carbon Steels, Carbon-Manganese Steels and Micro Alloyed Steels."

Unlike in the USA, former Soviet Union and Japan, self-

shielded

has

welding

not

been appreciated much in Continental Europe,

and it has been rather emaciated in the latest CEN draft (April 1990), allegedly at the insistence of the Germans.

After that European Standard

has been finalized and published, it will be some years before its use generates any industrial experience, especially for self-shielding wires. Therefore, it is considered that at present only the AWS Classifications are

meaningful

in

conveying

the

applications

of

and

experience

with

self-shielded welding. It could be argued that in the AWS scheme, the letter "E" is superfluous and it could be omitted to vacate the space for other symbols. reluctance

to

change

the

AWS

system

Yet, the

radically is understandable

(like

with the English spelling) in view of the existing records and literature in world-wide use.

However,

an early improvement of the AWS scheme by

the obvious flagging of the self-shielding wire with the letter

"s"

would

151 be

helpful

in

improving

the

visibility

of self-shielded welding.

For

instance, the letters "TS" could be used in the systems of Figs. 5.1 and 5.2 to indicate self-shielding wires.

Table 5.1.

Summary of usability, performance and some metallurgical characteristics extracted from the Guide in the AWS AS.20-79 Specification. T-1 *

T-2

T-3

T-4 *

T-5 *

T-6

T-7

T-S *

T-10

T-11

Gas-shield

CO2

CO2

NONE

NONE

CO2

NONE

NONE

NONE

NONE

NONE

Electrode current

DC+

DC+

DC+

DC+

DC+

DC+

DC-

DC-

DC-

DC-

Positions

All

F, H

F, H

F, H

F, H

F, H

All

All

F, H

All

Single

Single

Single

Single

Single

Single

Single

&

&

&

&

&

multi-

multi-

multi-

multi-

multi-

High deposit.

FEATURE

Single Passes

&

multiHigh speed; sheet

High deposit. low penetr.

-

Deep penetration

Desulphurizing

Basic

-

Desulphurizing

Desulphurizing

-29°C 27 J

-29°C 27 J

Not required

-29°C 27 J

Special features

-

Tolerates mill scale & rust

Flux/slag

Rutile

Rutile

-

Charpy

-lSoC 27 J

Not required

Not required

-

-

----

----------

---

Not required

-

Single Single

&

multiHigh speed

High speed

-

-

Not Not required required

---

T-G * and T-GS classifications are for newly developed products, for multi- and single-pass welding respectively, which do not fit into any currently defined classifications, and for which all the other characteristics, operating parameters and requirements are not defined.

* Only

these classifications are used in AWS AS.29-S0 for low alloy steel tubular/cored wires. F

=

flat position;

H

= horizontal

position.

......

V1 N

153

Table 5.2.

Classification scheme in BS.7084:1989 "Specification for carbon and carbon-manganese steel tubular cored welding electrodes" which includes wires for self-shielded welding.

--'1

The letter T for tubular cored _ _ _ _ _ _ _ _ _ electrodes for arc welding

(3 )

(b) (c) ( ) (d)

1

(a) Strength, Table 1 (b) Toughness,Table 2 (c) Welding position,Table 3 Shielding: G N

= gas shielded

= self-shielded

-----------------'

(d) Applications and - - - - - - - - - - - - - - - - - - - - - - ' characteristics,Table 4

(a)

(c)

Table 1. Designation for tensile properties Digit

Table 3. Welding positions

Tensile strength range

Minimum yield stress

Minimum elongation

Digit

Welding position

N/mm'

N/mm'

%

0

Flat and horizontal/vertical positions

4

430-550

330

20

I

All positions

5

510-650

360

18

2

All positions except vertical-down

9

Any position or combination of positions not classified above

(b) Table 2. Designation for toughness

(d) Table 4. Application and characteristics

Digit

Temperature for minimum average impact value of 47 J

Lener

Description

Gas shield

-

°C No requirement

R

For spray transfer in nat and horizontal/venical positions

CO~

p

For spray transfer in all p0sitions

0

0

Argon based ur CO~

2

-20

3

-30

4

-40

5

-50

6

-60

7

-70

Note.

The table numbers in the four boxes are the same as those in the original BS.7084:1989.

B

Basic flux

CO,or Argon based

M

Metal-cored (total non-metallic materials less than I % of electrode weight)

Argon based or CO 2

U

For high deposition rate in flat and horizontal/venical positions

Self-shielded

V

For single run. high travel speed In flat and horizontal/vertical positions

Self-shielded

W

For multi·run nat and horizontal/vertical positions

Self·shlelded

X

High metal powJer/low nux. All p<»itions

Self·shlelded

y

High lluxilo,," metal powJer. All positions

Self·shielded

S

Other types

-

154 Table 5.3.

Classification scheme in the French Specification NF A81-350 "Arc welding.

Bare flux-cored wires

depositing an unalloyed steel weld-metal. Symbolisation. Requirements.

Acceptance."

October 1986.

T ( ) (a). (b) • ( ). ( )

The Letter T for tubular _ _ _ _ _----11 cored electrode wire

I

SS = sel f-shielding wire GS = gas-shielded wire - - - - - - - - - ' SA = submerged-arc wire (a) Strength symbol, Table 1 - - - - - - - 1 (b) Toughness Symbol, Table 2 - - - - - - - - - - - 1 Positions: 1 - all-positions 2 all-positions, except vertical-down 3 flat on a slope and in groove 4 flat in a sloping groove 5 vertical-down Hydrogen (H) ~ 5 5 < H ~ 10 H > 10

I------~

symbol BH symbol H No symbol (a) Table 1

Symbol ...--.

~

Tensile strength N/mm 2

43

430 - 550 47 ~ - 590 S1 510 - 650

-----

I

Yield stress or 0.2% PS N/mm 2

Elongation 5d

320 min. 355 min. 390 min.

24 min. 22 min. 20 min.

%

-------'

(b) Table 2

Symbol

Test temperature

Average

°C X A

o 2 J

Min. Charpy toughness, J/cm 2

+ 20 0

- 20 - 30

35 35 35 35

Individual 26 26 26 26

_l_L__ =~g ___ ,- -_._.__ji~ ~~ ___J

155

Table 5.4.

Classification scheme proposed in the IIW Doc.XII-D-138-91 "Tubular cored wires for gas shielded and self shielded metal arc welding of carbon-manganese steel." Draft 3, Goteborg, October 1991.

T ( ) ( ) ( ) ( ) ( ) The letter T for tubular cored wire

----

D - C02-gas shielding G - Ar-C02/Ar-02 shielding - - - - - - - '

H - self-shielding Tensile strength symbols ----"43" or ""48" Toughness symbols for 27 J "N", "0", "2" or "3" --Application characteristics digit

---------~

Core ingredients symbols _________________________~ "R", "B", "M" or "G"

Optional additions after the hyphen: ( ) a symbol for 47 J toughness ( ) diffusible hydrogen designation H5, HI0 or HIS

156

r---------

r--------

Designates an electrode. Indicates the minimum tensile strength of the deposited weld metal in a test weld made with the electrode and in accordance with specified welding conditions.

Indicates the primary welding position for . - - - - - - which the electrode is designed:

*L

EX

T -X r-

0 - flat and horizontal positions 1 - all positions Indicates usability and performance capabilities.

L......._ _ _

Fig. 5.1.

Indicates a flux cored electrode.

Classification scheme in AWS A5.20-79 "Specification for Carbon Steel Electrodes for Flux Cored Arc Welding" which includes wires for self-shielded welding.

157

r--"---.. . -----

Designates an electrode.

I i I

: Iii,

!

f - .. -----..- - - -

I

:I 11 Ir-----L

Indicates the minimum tensile strength of the deposited weld metal in a test weld made with the electrode and in accordance with specified welding conditions.

-'-

EX XJ X-~

-

Indicates the primary welding position for which the electrode is designed: 0 - flat and horizontal positions 1 - all positions Designates the chemical composition of the deposited weld metal. Specific chemical compositions are not always identified with specific mechanical properties in the specification. A supplier is required by the specification to include the mechanical properties appropriate for a particular electrode in classification of that electrode. Thus, for example, a complete designation is:

E61T8-K6, see Appendix B.

I '-,--_

L Fig. 5.2.

Indicates usability and performance capabilities. Indicates a flux cored electrode.

Classification scheme in AWS AS.29-80 "Specification for Low Alloy Steel Electrodes for Flux Cored Arc Welding" which includes wires for self-shielded welding.

158

6.

SOME ASPECTS OF PRODUCTION WELDING

6.1.

Logistic Convenience of the SSAW

Unlike the

gas-shielded welding,

the self-shielded arc welding

(SSAW)

in common with the MMA/SMAW requires at the work station only:(i)

the welding equipment, and

(ii)

the welding filler wire.

Because of the high resistance of the SSAW to side winds, there is no need for protective screens which are normally required where gas-shielded welding is used in large fabrication shops. can also

be arranged

close

to

disturbing

the arc environment:

and 2.28.

Whereas,

the

arc

In the SSAW, fume extraction (1),

without much danger

see Sub-Section 2.9.3 and Figs.

of

2.27

the AWS Welding Handbook warns (2) that with gas-

shielded flux-cored arc welding

(FCAW) "...

fume extractors may cause

welding problems by disturbing the gas shielding." The

freedom

from

encumbrance

by

the

gas

supply

equipment,

and

the

accessories for the gas pressure and flow control, makes the self-shielded welding equipment sufficiently simple and manoeuvrable to allow access to tight and difficult places, where hitherto welding could be carried out only with flux-covered electrodes. in a

tight

space,

inside a

Figure 6.1 shows a welder working

transportable concrete mixer:

the semi-

automatic self-shielded welding was carried out with E71T-ll and E71T-GS wires, using smoke removal guns.

As can be seen in the photograph,

there is no visible fume plume. Figure 6.2 shows a welder working perched on a girder at a nodal junction of a beam-to-column connection of a large and massive steel structure. Again, the self-shielded welding was the only semi-automatic process that could be realistically applied,

in lieu of the flux-covered electrode,

under the difficult access conditions, where the erection of screening necessary for the protection of the gas-shielded welding from draughts would be extremely difficult and time consuming, if hardly possible. Welding current and deposition rate being equal, the guns used for the SSAW are not as heavy, large and cumbersome as those required for gas-

159 shielded welding, especially with tubular wires. advantage

for

accessibility

in

the

dimensional lattice structures (3).

This is a very positive

fabrication

of

nodes

in

three-

Figure 6.3 (3) shows the situation

in a narrow gap which exists at a junction between a chord and a brace stub.

The nozzle of a gas-shielded welding gun is about 20 mm in diameter

and that of a SSAW gun is only about 15 mm in diameter.

To reach the

weld root, the electrical extension (stick-out) of 40 mm would be required in

gas-shielded

welding,

but

with

sufficient to reach the root.

the

SSAW gun

30

mm stick-out

is

The normal values of stick-outs quoted

(4) for gas-shielded welding with tubular wires are between 10-25 mm, and the recommended limits given in the AWS Welding Handbook (5) are 19-38 mm

a-I! in.). Whereas, with the self-shielding wires such limits

can be 19-95 mm depending on the application.

Thus in gas-shielded welding,

the stick-out of 40 mm in Fig. 6.3a would be outside the recommended range and it would lead to some loss of gas-shield at the arc.

6.2.

Welding Equipment for the SSAW

For the

general

description

of

principles

characterizing the welding

equipment used for semi-automatic and automatic arc-welding with tubular cored wires,

including

the

self-shielding

Handbook should be consulted (i.e. Ref. 2).

varieties,

the AWS Welding

Here, the items of equipment

preferred for the self-shielded welding and available on the market will be illustrated.

Power Sources

It

is

not

necessary

to

procure

a

separate

power

source

dedicated

specifically to self-shielded welding.

There are multi -process DC* arc

welding

used

power

electrodes,

sources

which

can

be

with

flux-covered

(stick)

self-shielding and gas-shielded tubular wires, gas-shielded

solid wires, as well as for TIG/GTA welding, submerged-arc welding and air carbon arc gouging. Figure 6.4 shows such a multi-process source, Lincoln IDEALARC DC-400,

*

Table 5.1 shows that self-shielding wires are used with DC.

160 with the multi-process switch on the bottom panel.

For the self-shielded

welding the constant voltage mode is used and the multi-process switch is turned either to DC+ or DC- depending on the wire used (see Table 5.1). This power source is a solid state controlled, three phase input power transformer/rectifier (SCR) rated at 400 A, 36 V and 100 % duty cycle. It has had a good application track record in the 1980s for semi-automatic self-shielded welding. A competitive power source, Hobart MEGA-FLEX 450 RVS, with a similar multiprocess capability is shown in Fig. 6.5.

In the constant voltage (CV)

mode this power source is rated at 450 A, 38 V and 100 % duty cycle. It is recommended by the manufacturer for use with self-shielding wires. By virtue of their medium-to-high current rating, these two power sources can be used with self-shielding wires of up to 2.4 mm (3/32 in.) diameter. For welding with wires of 2.8 and 3.0 mm diameter, currents of up to

600 A

may be required and there are similar power sources to those shown in Figs. 6.4 and 6.5, but rated up to 600/650 A. In 1991, a very modern power source was put on the market, for relatively light welding fabrications with self-shielding wires of up to 1.2 rom diameter (Fig. 6.6).

This power source is a single-phase input DC arc

welding machine for welding with constant voltage only, i.e. for MIG/GMAW and tubular/cored wires,

both gas-shielded and self-shielding.

It is

rated at 145A/26V at 100 % duty cycle or 250A/26V at 35 % duty cycle. The power source incorporates microcomputer control and has a liquid crystal display on the panel. German, Japanese and Spanish.

It works in five languages:

English, French,

It is pre-programmed with numerous welding

procedures which can be called by pressing an appropriate key.

The wire

feed unit and the wire reel are enclosed within the casing of the machine shown in Fig. 6.6.

Wire Feeders Figure 6.7 shows a heavy duty wire feeder, Lincoln LN-9, which has been very

popular during

the 1980s for

welding with self-shielding wires.

This wire feeder can be coupled with the power source shown in Fig. 6.4. The attractive features of the LN-9 wire feeder are as follows:-

161 (a) The digital display of the wire feed speed and the arc voltage settings are accessible to QC inspection. (b) Once set,

these parameters remain virtually constant throughout the

welding operation. (c) The pre-set procedural conditions can be locked with a special key by the supervisory for

the

arc

or QC

voltage,

the

nitrogen pick-up (see Figs.

personnel.

This is especially important

increase of which leads

to an enhanced

2.15 and 2.16) and the associated loss

of weld metal toughness (see Figs. 3.8 and 3.9). (d) If and when the arc voltage occurs outside the pre-set values,

the

LN-9 stops wire feeding automatically and the welding ceases. A somewhat simpler and Fig. 6.8.

lighter wire feeder,

Hobart 2410,

is shown in

This machine has no voltage display and locking facilities,

but it has been designed to tolerate line voltage variations and to control the wire feed speed precisely.

It could be used for self-shielded welding

where fabrication specifications do not require deep sub-zero Charpy values or the ClOD toughness. In some applications, important.

equipment portability and lightweight design are

Figure 6.9 shows a

portable wire feeder, Lincoln LN-25 and

Fig. 6.10 shows a competitive portable wire feeder, Hobart HEFTY CC/CV. Without wire reels, both those wire feeders weigh about 14 kg (about 30 lb) and hence they are lightweight equipment.

The LN-25 can be coupled to

the DC-400 power source shown in Fig. 6.4 and the HEFTY can be coupled to the MEGA-FLEX power source shown in Fig. 6.5.

Guns Some guns for self-shielded welding have been illustrated in Figs. 2.262.28 in connection with the discussion of fume extraction at the point of origin.

Those guns are rated at 250-350 A.

A gun designed for use

wi th up to 450 A is shown in Fig. 6.11 and it features a heavier hand shield than those furnished for the other guns, to protect the welder's hand from the heat of the arc and the radiation from a massive molten pool.

A competitive gun rated at 500 A is shown in Fig. 6.12.

162

6.3.

Welder Training for the SSAW

This Sub-Section is based on three seminal papers (6-8), especially those due to Yeo (7, 8), and i t reflects the considerable experience of The Lincoln Electric Co.,

the market leaders in self-shielded welding,

as

is readily apparent from Appendix B. In general, the dexterity and skill required for successful semi-automatic welding with self-shielding wires are similar to those in the MMA/SMAW and the semi-automatic MIG/GMA welding.

However, compared to the MMA/SMAW

the SSAW is carried out at significantly higher travel speeds.

Therefore,

young welders can be taught to make satisfactory welds easier and quicker than the older welders. Experience has shown that in general (7), welders skilled in the MMA/SMAW adapt themselves better to the SSAW than those proficient in the MIG/GMAW. In a way this is fortunate, because the self-shielding wires are just right for replacing many flux-covered electrodes (Table 6.1).

In self-

shielded welding, in common with the MMA/SMAW, there is no need to concern oneself with the consequences of losing the auxiliary gas-shield which does not exist. The three parameters which must be considered in the self-shielded welding are:(i)

the wire feed speed (WFS) which determines approximately the general level of the welding current,

(ii)

the arc voltage which is related to the arc length and influences the nitrogen pick-up (see Figs. 2.15 and 2.16), and

(iii) the stick-out (the electrode extension) which provides the means for fine and momentary current adjustment (see Fig. 2.20). The first two parameters are normally taken out from the manufacturers data sheets, and are set in and controlled by the equipment.

The control

of the third parameter - the stick-out is in the hands of the welder who can exercise significant initiative and exploit the flexibility of the SSAW.

However, this is not the case with the semi-automatic gas-shielded

welding where the gas-shield can be lost as the gun is moved away from

163 the workpiece.

Therefore, the welders accustomed to gas-shielded welding

may feel constrained by habit from exploiting the advantages of stickout adjustment to the full. The advantages of stick-out control in the SSAW within a permitted limit for a given wire are these:(a)

At a given wire feed speed (an approximate current setting), a change in

the

stick-out

brings about

some

change

in

the

current

(see

Fig. 2.20). (b)

If a welder notices that too much material at the edge is being melted and "burned" away, the penetration can be reduced by moving the gun away from

the workpiece.

The increasing stick-out increases the

ohmic resistance of the wire, thus decreasing the welding current. (c)

If and where increased penetration is required, this can be achieved by moving the gun towards the workpiece, thus decreasing the stickout and increasing the current.

The consideration of substitution possibilities (Table 6.1) of flux-covered electrodes by self-shielding wires indicates that,

in both the cases,

there are spectra of different classes which cover a broad applications,

range of

ranging from simple non-coded work to strictly coded and

QA-controlled work on highly critical structures.

Consequently like for

the MMA/SMAW, there is a wide spectrum of skill and training applicable to the SSAW. and

This is also apparent from the consideration of the usability

performance characteristics

summarized in Table 5.1.

of

Therefore,

the different wire

classifications

the time of training for a given

application depends on:(i)

the type and thickness of the steel to be welded and the application of the finished product,

(ii)

the type of weld, welding position and the weld metal toughness, if any, to be achieved,

(iii) previous experience, age,

eagerness to learn,

learning ability of an individual, and (iv)

the equality of welding equipment used.

Experience shows (6) that the training times may vary:-

dedication and the

164 from 2-days for

simple,

single-pass welds deposited in the flat

(IG, IF) position, to 3-weeks for

unbacked single-sided butt welds in pipes in 6G

position. However, the very minimum of I-week should be allowed for training which must include:(a)

safety instructions:

(b)

setting up, adjusting and maintaining the equipment, and

(c)

understanding the use of correct welding conditions (the WFS, arc voltage,

stick-out,

electrical and environmental,

gun-angle,

travel

speed and arc manipulation

techniques). When training welders experienced in the MMA/SMAW, the emphasis is on (7):- setting up and operating constant voltage power sources and wire feeders, - understanding the role of wire feed speed, - understanding the consequences of changes in the arc voltage, and - using the role of stick-out and the benefits and consequences of its variation. When training welders experienced in the MIG/GTAW, the emphasis is on (7):- controlling the slag by pulling the gun instead of pushing it, - using the gun angle to control the slag, and - using a longer stick-out than is customary in gas-shielded welding. Experience has shown (8) that successful training can be structured as follows:(i)

start with a classroom session to explain semi-automatic welding,

(ii)

start welding with self-shielding wires by depositing bead-on-plate welds,

in the flat position,

followed by stringer bead placement

and fillet welds (2-days to I-week), (iii) release the welders to production work on IF and 2F fillets on plate, (iv)

after about 6-weeks experience of fillet welding, welders can be

165 introduced

to

butt welding,

including pipework and tubulars,

and

trained to the 6G standard. It has been claimed (3, 8) that after such a form of training, the repair rates in self-shielded welding were about 0.5 %.

6.4.

Productivity Benefits

It is impossible to give one general and definitive figure which would indicate how much more productive the self-shielded welding can be in comparison with the MMA/SMAW.

This is because, as shown in Table 5.1,

there are different self-shielding wires for different applications like there are different flux-covered electrodes which can be replaced by those wires (Table 6.1). The information on the productivity of different processes expressed simply in terms of deposition rates, given in product data sheets or handbooks, may be misleading and the following must be borne in mind when considering the substitution of self-shielding wire for the ubiquitous flux-covered electrode:(a)

The deposi,tion rate values in "kg/h" given in manufacturers' sheets apply to the "arc-hour", i.e. 100 % duty cycle.

data

Almost always,

they are obtained under comfortable laboratory conditions by welding for

a

few

obtained

by

minutes simple

without

getting

proportional

tired.

The

hourly

multiplication of

figure

is

the metal mass

deposited in those few minutes. (b)

In a given job and in a given position, what matters in productivity is the ability to keep the arc burning as long as possible without interruptions.

With a flux-covered stick electrode there are enforced

stops and electrode changes; with a continuous wire the welder can carryon much longer and can stop when necessary. When

considering whether

. welding,

the

impact

of

to the

opt

for

following

the

gas-shielded or self-shielded

factors

on the

productivity and

profitability (costs) must be assessed:(i)

The supply of the shielding gas to the work station, and the possible supply of water where high currents and heavy guns are to be used.

166

NB.

At a given current the SSAW guns run cooler than those for

gas-shielded welding, probably because there is no hot gas bouncing off the workpiece. (ii)

Any

need to screen the work from draughts.

In large shops,

air

currents can disturb the gas-shield. (iii) The

guns used for gas-shielded welding with tubular cored wires

are heavy and cumbersome, and welders get tired more quickly and need breaks more often than with the lighter self-shielded welding guns. (iv)

As they are more complex, the gas-shielded welding guns may be more prone

to abuse and breakdown,

and

require more maintenance

than

the simpler self-shielded welding guns. In butt welding, shielding

wires

narrower V-groove preparations are possible with selfthan

either

with tubular/cored

flux-covered electrodes (Fig. 6.13).

gas-shielded

Consequently,

wires,

or

less weld metal will

be needed to fill the groove, and less time may be spent welding it up, other factors being equal.

Because of the total thickness of the covered

electrode, or the diameter of the gas-shielding nozzle, the alpha angle of the V-groove must be wide, e. g. 60°, to allow access to the root of the weld preparation.

In self-shielded welding, the combination of the

small contact tube and the extended stick-out permits the narrowing of the beta angle to for instance 45°.

For examples of j oint preparations

for self-shielded and gas-shielded welding with tubular electrodes (FCAW), see the AWS Welding Handbook (5). In the fabrication of large and heavy items, where access is reasonably good, the following benchmark values for the average deposition rate can be used for the all-positional basic low-hydrogen electrode E7018:- 1.5 kg/arc-hour

(100 % duty cycle),

- 0.5 kg/work-hour

(30 % duty cycle).

With reference to these figures, the following claims published for selfshielded welding can be considered:(a)

In the fabrication of a E70T-4 wire,

drag line machine,

using a high-deposition

deposition rates as high as 50 lb (about 23 kg)

per

167 8

hour

per

day

man

have

been

reported

(9);

this

converts

to

2.8 kg/work-hour, i.e. over 5-times faster than E7018 electrode. (b)

On a high rise building project (10), also 23 kg of weld metal per day per welding operator have been claimed for E70T-4 (Innershield NS-3M) wire.

On the same proj ect, using an all-positional E71 T-8

wire vertically-up, a certain j oint was completed in 22 h, whilst an

identical

joint

welded

with

the

flux-covered

electrode

took

40 h to achieve 75 % completion. (c)

For some offshore structural welding in Norway (3, 11), it' has been claimed that:the overall time saving in comparison with a flux-covered electrode was in the range of 30-40 %, and on some specific joints where very narrow preparations could be used (e. g.

see Fig. 6.13) , the cost saving was up to 80 % in

comparison with the MMA/SMAW because the welder could maintain the arc for long periods without forced interruptions. (d)

A British

offshore

structures

fabricator

has

reported

(12)

the

following comparison between the flux-covered electrode E7018-G and E61T8-K6 self-shielding wire, for use in the ASME 3G and 4G positions only:Deposition rate, kg/arc-hour

Consumable E7018-G

1.2 - 1.8

E61T8-K6

1.5 - 2.0

Here on average, the self-shielded welding appeared only 1.2 times (i.e. 20 % more) as productive as the flux-covered electrode,

but

this was so in the two difficult positions, vertical and overhead. Perhaps,

if

the

flat

(lG)

position

were

included,

the

British

experience (12) could have been closer to the Norwegian one (3, 11). (e)

For vertical-up (3G) butt welding on the revolving frame of a dragline assembly with E71 T-8 self-shielding wire,

deposition rates similar

to those in (d) above have been quoted (13):1.4-1.8 kg/arc-hour At first glance,

(3-4 lb/arc-hour).

these figures may suggest a performance similar

to that of E7018 electrode, but it should be borne in mind that the

168 duty cycle (or the operating factor) the most,

for the MMA/SMAW is 30 % at

but it can be up to 60 % with the semi-automatic self-

shielded welding. Recently (1990), Fern and Yeo (14) considered welding costs in general structural

fabrication

effective deposition

where

rates

fillet

in

welding

fillets

for

is used different

extensively. welding

The

processes

are compared in Table 6.2 (14) using values relative to the flux-covered electrode being "1.0" when used vertical (3G) and overhead (4G).

in

the

two most difficult

positions:

The figures show a remarkable potential

improvement in productivity that can be achieved using continuous electrode (wire) processes, even semi-automatically let alone when fully mechanized. The semi -automatic self-shielded welding is about 4-times faster in the lG and 2G positions,

but it is 8-times faster in the difficult 3G and

4G positions, than the MMA/SMAW. open sites,

Furthermore, in structural welding on

the self-shielded welding does not require protection from

wind, unlike the gas-shielded welding whether with solid or tubular/cored wires. And last but not least, in site welding where only flux-covered electrodes and self-shielding wires can be used, the SSAW can increase producti vi ty (7) because being resistant to the effects of wind it deposits inherently sound weld metal.

On a welded steel framed building in London (7), the

repair rate for the MMA/SMAW was 2 %, but it was zero for the self-shielded welds.

6.5.

Hydrogen Control

6.5.1.

General considerations

All arc-welding deposits contain initially some diffusible hydrogen which is mobile at and around room temperature, and which in combination with weld residual stresses can lead to cracking in the HAZ and the weld metal, at temperatures below 300°C.

To avoid this hydrogen cracking, welding

consumables must be cared for to minimize moisture content in their fluxes and to keep the surface of corrosion products.

bare wire

free

from organic material and

This care consists of proper storage, drying/baking

immediately before use and limiting the exposure time of consumables to

169 the ambient environment during production welding. Nowadays,

there are some brands of basic flux-covered electrodes which

may be used directly from their packs, without prior baking, and which have a low moisture pick-up rate on exposure.

However, any mineral flux

whether on a rod or in the form of loose powder, and which is bonded with sodium and potassium silicates (water glasses) will absorb some moisture from air in due time.

Therefore, a flux-covered electrode, however high

its moisture pick-up resistance,

is always vulnerable to dampness,

and

especially so when in contact with the human palm. Self-shielding wires, their

flux

within

in common

the metal

Furthermore, unlike the flux

with gas-shielded

sheath which

is

tubular wires,

impermeable

have

to moisture.

bonded onto the core wire of an MMA/SMAW

electrode, the flux contained within the tube does not require the use of silicate binders.

In the extrusion of flux-covered

electrodes the

flux is in the form of water-based paste and it contains plasticizers which are hygroscopic, whereas completely dry powder free from plasticizers can be poured into the metal trough during the tubular wire manufacture. Therefore, the self-shielding wires are scarcely vulnerable to moisture pick-up when in use. no

limit on the

In comparison with flux-covered electrodes, usually

exposure

production personnel.

time

Neither is

of self-shielding wire by

is

required

for

wire

reels issued

to

there any requirement for the baking

the fabricator

prior to use.

This greatly

simplifies the housekeeping and control of welding consumables when selfshielded welding is adopted in lieu of flux-covered electrodes.

Naturally,

there are attendant cost savings.

6.5.2.

Diffusible hydrogen levels

There are basic flux-covered

electrodes (EXX16 and EXX18) which comply

with the Hydrogen Scale D to BS.5135, thus giving hydrogen contents of less than and up to and including 5 ml/lOOg. electrodes give consistently 1.5 - 3 ml H2/100g.

Some modern high quality There are also tubular

wires for gas-shielded welding which can comply with Hydrogen Scale D. However at the time of writing,

no self-shielding wire has been known

to consistently comply with the Scale D, but only with the Scale C which

170 permits up to 10 ml/lOOg.

The diffusible hydrogen contents reported (8)

for high quality self-shielding wires used in North Sea structures lie in the range of 3 - 7 ml/100g. Thus,

the question arises:

why is it possible to obtain consistently

less than 5 ml Hz /100g from the gas-shielded tubular wires, but not from the self-shielding wires?

After all,

during

the wire manufacture all

the same remedies and precautions can be applied to any tubular cored wire whatever the nature of its ultimate use. The answer would appear to lie in the effect of the ambient absolute humidity on the arc environment during welding (15). a flux-covered electrode with a

When welding with

given indigenous moisture content,

the

climatic humidity of the air surrounding the arc can contribute 1-2 ml Hz /100g into the weld metal.

Some ingress of air into the arc occurs

despite the evolution of shielding gas by the coating flux (see Table 2.3).

In gas-shielded welding,

the air with its moisture is largely

excluded from the arc environment by the shroud of the shielding gas. However,

as demonstrated by the Ni trogen Scale (Fig. 2.2) ,

there is a

massive ingress of air into the arc environment in self-shielded welding. Therefore at present, the compliance with the Hydrogen Scale C would appear to be the best that can be expected from self-shielding wires.

6.5.3.

History of hydrogen cracking

Despite this

disadvantage when compared with the gas-shielded welding,

at the time of writing the author has been unaware of any case of hydrogen cracking

in

self-shielded

welding,

whereas

gas-shielded

tubular wires has a history of hydrogen cracking.

welding

with

True, the volume of

self-shielded welding is still low, but the special nature of the selfshielded weld metal may have some effect on the behaviour of the diffusible hydrogen, to moderate its noxious effect. Most self-shielded weld metals contain about 1% Al (Appendix B).

At the

time of writing, no data have been located on the effect of such AI-content on the diffusion rate (diffusivity) of hydrogen in ferritic steel.

However,

it is conjectured that at about 1% Al in iron, the hydrogen diffusivity is likely to be retarded, thus minimizing the deli very of hydrogen atoms

171 (ions) to the potential sites of crack nucleation.

This conjecture is

based on the fact that 1% Si addition to "pure" /ferritic iron slows down hydrogen diffusivity at room temperature by about an order of magnitude (a factor of nearly 10), as shown by Geller and Sun in 1950 (16), thus:Material

Activation energIz cal/mol

DiffusivitI at 25°C z cm 2 /s

Fe-1.06% Si

1.6 x 10-5 1.2 x 10-6

4450,

Fe-1.85% Si

3.5 x 10-7

5250.

Pure «Fe

2900,

Silicon and aluminium are close neighbours in the periodic system of elements.

They occupy the same Period III and lie in Groups IIIb and

IVb, and differ only by one electron in their outer shell which determines chemical properties of elements:Element

Atomic No.

Al

13

26.97

Si

14

28.06.

Atomic weight

Both Al and Si close the gamma loop in iron and they are both ferrite formers.

Therefore,

in

view

of

the

similarity

of

their

electron

configurations, in general they exert similar effects as alloying elements in iron. It is thus not unlikely that like Si, Al might slow down the hygrogen diffusion rate in iron.

At present no fundamental data on this subject

have been located in literature, but some data available for self-shielded weld metal might support the case.

Keeler (17) has reported the following

weld metal hydrogen contents for Innershield NR-203Ni-C (E61TB-K6) wire:HIdrogen content z ml/100g

Wire

Diffusible

Condition As received

Residual

Total

3.2

2.9

6.1

Exposed 1 day at 18°C, 50%RH

4.0

3.2

7.2

Exposed 7 days at 18°C, 50%RH

4.B

2.9

7.7

The diffusible hydrogen determined according to BS .639: 1976 is obtained after 3 days I

(72 h) evolution time at 25±5°C and the residual hydrogen

is extracted at 650°C.

In the above weld metal, the residual hydrogen

172 content was

nearly as high as the diffusible hydrogen content and

comprised 37-47 % of the total.

it

This is quite unlike what is known about

hydrogen in other weld metals in which the residual hydrogen seldom exceeds 1 ml/100g, even at 10-15 ml/l00g total level. The corollary of the supposed slow hydrogen diffusivity in, and evolution from the AI-bearing self-shielded weld metal, extra susceptibility fracture

to the

surfaces of

manifestation

of

so-called

tensile

the

"fish-eyes"

specimens.

slow

strain

is its reported

The

rate

develop

when

maximum

load

on

the

hydrogen

specific

embrittlement

tensile

stress-strain

17)

which appear on the

fish-eyes are a

condi tions of massive plastic deformation in a

(8,

curve

test, is

under

and

they

approached.

Because hydrogen is slow to come out from the self-shielded weld metal, even after the delay of sampling and machining, to cause fish-eyes may remain. either

by

hydrogen

release

Fish-eyes can be

treatment

at

150°C

a

quantity sufficient

minimized for

or

24-48

removed

hrs,

or

by

prolonged holding of specimens at room temperature (28-40 days) before testing. The occurrence bears

no

of

fish-eyes

relationship

to

on fracture

the

problem

surfaces of tensile specimens

of

hydrogen

cracking

under

the

(constant) residual stress which does not cause gross plastic strain of several percent.

It

is probably because hydrogen movement within

the

self-shielded welds is retarded that such welds have had no known history of hydrogen cracking.

After weeks and months, although more slowly than

for other weld metals, hydrogen escapes from the self-shielded welds in the normal way.

6.6.

Root Pass Welding

When assembling large lattice structures,

such as jackets for offshore

platforms, quite large gaps and mismatches can occur where tubular braces are connected to leg nodes ( 17) •

As Keeler ( 17 ) pointed out, although

the leg node itself is positioned within 4 mm tolerance,

the abutting

brace can meet it with up to 10 mm mismatch in some places and with a root gap of 12 mm or more. (outside)

only,

as there is

The single-V weld must be made from one side no access for

back-gouging and

re-welding

173 the root from the second side.

When large tubulars are so joined, access-

holes can be cut to allow welders to back-gouge and re-weld the roots of the leg node-to-brace butt welds, but even then the access-holes have to be welded up with cover plates.

Again, this can be done from one side

only. The welding of wide and open roots in single-sided closure welds presents a number of difficulties, one of which is the extra contamination of the molten pool by air.

Because the self-shielding wires are designed to

cope with the heavy contamination from air, by killing such contamination with AI, they are eminently suitable for the welding of wide and open roots (3, 17).

The Classifications which have been found effective are

E71T-8, E7lT8-Ni1 and E61T8-K6 (see Appendix B). Additionally,

the judicious

use

of

the

variable electrical stick-out

(electrode extension) enables the welder to place the metal at the root with a greater precision with a self-shielding wire than with a fluxcovered electrode (Fig. 6.13).

This is likely to be conducive to achieving

a smoother blending of the weld root bead at the junction with the parenti /base material

than is

possible

with

a

flux-covered

electrode.

improvement in the weld root profile is beneficial to

Any

increasing the

resistance of a welded joint to fatigue cracking. A special study

(18)

of single-sided closure welds

in large

structures was carried out at The Welding Institute in the UK.

tubular Among

other things, that study showed that in general the frequency and magnitude of root flaws were significantly lower where E61T8-K6 self-shielding wire was used compared to E70l6 flux-covered electrode.

This was thought to

be attributable to:(a)

fewer interruptions

to welding with the continuous wire compared

to stick electrode, (b)

improved manipulation at the edges with narrow wire, especially where misalignment existed, and

(c)

lowered risk of porosity in self-shielded weld metal.

Consequently as shown in Fig. 1.3, the fatigue lives of welds made with the self-shielding wire were longer compared with those related to the use of the E70l6 flux-covered electrode.

174

6.7.

Limitations

Where the root pass has been welded with a self-shielding wire, it is best to use

the self-shielded

welding

throughout,

from root to

cap.

This is because toughness problems have been encountered with mixed or "hybrid" welding procedures (19) where the root was made with a selfshielding wire and the remainder of the joint was filled by some other arc-welding process. For instance in submerged-arc welding, it is common to use a flux-covered electrode or gas-shielded welding for the joint with the SAW process.

the root pass,

before completing

Naturally, some of the metal from the

root pass becomes diluted into the filling passes. Tests have shown

(19)

that

the submerged-arc weld metal can tolerate

dilution and its toughness is not degraded where it is deposited on top of the root made with E7016 flux-covered electrode.

But this is not so

where the root had been welded with a self-shielding wire. Figure 6.14 shows the j oint preparation and some details of a "hybrid" weld procedure in which the root pass (Pass No.1) was made:either with E7016 electrode, or

E61T8-K6

with

self-shielding

(Lincoln

wire

INNER SHIELD

NR-203Ni-C). Following a

given root

pass,

the V-preparation of each test weld was

filled with Oerlikon SD3 wire/OP121TI flux combination, and subsequently a two-pass back-weld (Pass Nos. 22-23 in Fig. 6.14) was made:either with SD3 wire/OP121TI flux combination, or with Tibor 22 wire/OP121TT flux combination. In total, four test welds were produced thus:Test Weld

Root Pass

Filling Passes

Back-Weld

(i)

E7016

SD3/0P121TT

SD3/0P121TT

(ii)

E61T8-K6

SD3/0P121TT

SD3/0P121TT

(iii)

E7016

SD3/0P121TT

Tibor 22/0P121IT

(iv)

E61T8-K6

SD3/0P121TT

Tibor 22/0P121TT

175 Charpy and CTOD toughness tests were carried out and the results are shown in Tables 6.3 and 6.4.

In Test Welds (i) and (ii), regardless of the

consumable used for the root pass, the cap centre-line Charpy toughness was unaffected

(Table 6.3)

because even with the self-shielded deposit

in the root, the effects of dilution could not extend that far.

However,

in Test Weld (ii) with the E61T8-K6 weld bead in the root, the root centreline Charpy toughness was only about

!

of that obtained for Test Weld

(i) containing E7016 weld bead in the root. For each type of back-weld, the full plate thickness CTOD toughness (see Tables 6.3 and 6.4) was lower with E61T8-K6 than with E7016 in the root. However, to

the Tibor 22/0P121TT back weld [Test Weld (iv)] was less prone

toughness

deterioration

(Table 6.4)

than

the SD3/0P121TT back weld

[Test Weld (ii) in Table 6.3], and it gave CTOD values above 0.25 mm. Although caution is required,

this shows that sometimes it is possible

for other arc-welding processes to find some welding consumables which may be reasonably tolerant to dilution from the self-shielded weld metal. For instance, local repairs in some self-shielded welds were carried out with suitably

chosen flux-covered

using

electrodes

E7018-G

normally very tough.

electrodes.

depositing

1%

Ni

Gougings were filled

steel

weld

metal

which

in is

Although its toughness was nearly halved by dilution,

the minimum specification requirements were exceeded by a wide margin. The

toughness

variation

in

of the

the

MMA/SMAW

AI-content,

and

and

SAW deposits

this

is

bound

is to

sensitive occur where

deposits are made on top of the self-shielded weld metal. addition,

to

the

those

However in

the possible detrimental effect of the high nitrogen content

being also diluted from the self-shielded weld metal (Fig. 2.2) into the other weld metals must not be ignored. The self-shielded weld metal can be deposited on top of any other weld metal without metallurgical mild

or

C-Mn

steel.

problems,

However,

like it can be used to weld any

it must

be always

remembered

that

the

toughness of other weld metals can be degraded by dilution from the selfshielded weld metal, ascertain

and

the degree and

shielding wires must not

therefore appropriate

tests must be

acceptability

degradation.

of any

done

to

The self-

be used indiscriminately either for root-pass

176 welding or for tack welding where other processes are to follow, if and where high toughness is required. Where tack welding with flux-covered electrodes is to be carried out before self-shielded welding, basic low-hydrogen electrodes should be used. Tack welds made with rutile flux-covered electrodes tend to upset the deslagging behaviour of some self-shielded deposits.

References 1•

PRIOR, H., CLARK, J., STODDART, D•W., BROWN, M. A. S • and YEO, R•B•G• "Welding with self-shielded flux cored wire - Scottish Branch sponsored meeting", Metal Construction, August 1986, Vol.18, No.8, pp.491-494.

2.

AWS WELDING HANDBOOK, 8th Edition, Vo1.2: "Welding Processes", 1991, p.165, American Welding Society, Miami, Florida 33135, USA.

3.

BJ~RNBAKK,

4.

HOULDCROFT, P. and JOHN, R. "Welding and cutting", Woodhead-Faulkner, New York, London, 1988, p.113.

5.

As Ref.2, p.176.

6•

PRIOR, H., CLARK, J., STODDART, D•W., BROWN, M. A•S • and YEO , R•B • G• "Welding with self-shielded flux cored wire - Scottish Branch sponsored meeting", Metal Construction, Aug. 1986, Vo1.18, No.8, pp.491-494.

7.

YEO, R.B.G. "Fast track starts from Fenchurch Street station", Metal Construction, Dec. 1986, Vol.18, No.12, pp.741-745.

8.

YEO, Ralph B.G. "Specifications for the Welding of offshore oil structures", Australian Welding Journal, 1988, 4th Quarter, pp.15-26.

9.

GILBANK, J .S. "Analytical Approach Cuts Welding Costs on Dragline Project", Canadian Welder and Fabricator, 1982, Vo1.73, No.10, pp.15-26.

10.

TORCHIO, P. "Mechanised welding speeds high rise construction", Metal Construction, June 1984, Vol.16, No.6, pp.344-345.

11.

BJ~RNBAKK, B. and BOEKHOLT, R. "Self-shielded Welding Review, 1987, Vol.6, No.4, pp.272-275.

12.

RODGERS, K.J. and LOCHHEAD, J.C. "The Use of Gas-Shielded FCAW for Offshore Fabrication", Welding Journal, Feb. 1989, Vo1.68, No.2, pp.26-32.

B. and BOEKHOLT, R. "Self-Shielded Flux-Cored Arc Welding for Offshore Fabrications." IIW Doc. XII-1080-88, 1988, International Institute of Welding.

flux cored wire",

177 13.

ANON. "Consumable Guide Saves Time in Dragline Assembly", Welding Journal, Nov. 1990, Vol.69, No.11, pp.51-52.

14.

FERN, D.T. and YEO, R.B.G. "Designing cost effective weldments." Paper 3 in "Welded Structures '90", London, November 1990, The Welding Institute, Abington, Cambridge, CB1 6AL.

15.

DICKEHUT, G. and HOTZ, U. "Effect of Climatic Conditions on Diffusible Hydrogen Content in Weld Metal", Welding Journal Res. Suppl., Jan. 1991, Vol.70, No.1, pp.1s-6s.

16.

GELLER, W. and SUN, T .H. "Effect of alloying additions on the diffusion of hydrogen in iron and contribution to the iron-hydrogen system", Archiv EisenhUttenwesen, 1950, Vol.21, No.11-12, p.423. Also: SMIALOWSKI, M. "Hydrogen in Steel." Pergamon Press, Oxford, 1962, p.lO!.

17 •

KEELER, T. "Innershield welding", Metal Construction, 1981, Vol. 13 : "Part 1: Development and applications", No .11 , pp. 667-673; "Part 2: Properties", No.12, pp.750-752.

18.

JONES, R.L., ANDREWS, R.M. and FOR SHAW , M.E. "Single-Sided Welding of Closure Joints in Large Tubular Fabrications." - Final Summary Report Prepared by The Welding Institute for the Department of Energy (UK), Report No. OTH 90 335, London: HMSO, 1991.

19.

KEELER, T. and GARLAND, J.G. "How SA welding adapted to the offshore challenge - part two", Welding and Metal Fabrication, May 1983, No.5, pp.193-199.

178

Table 6.1.

Possibilities for the replacement of flux-covered stick electrodes by self-shielding wires.

MMA/SMAW AWS Classifications

SSAW/SS-FCAW AWS Classifications

E6013 sheet welding

E70T-3 E71T-ll E71T-GS

Toughness requirements

No Charpy requirement

E70T-6 for 2F fillets E71T-8

Charpy 27 J

E70l4

E71T-7

No Charpy requir.

E7024

E70T-4 E70T-7 E70T-IO

No Charpy requirement

E7024-1

E70T-6

Charpy 27 J

E6013 shipbuilding

at -20 o /-30°C

at -20 o /-30°C E7028

E70T-6

E7016 E7018

E71T-8

Charpy 27 J at -30°C

E7016-1 E7018-1 E7018-G(1Ni)

Charpy 27 J at -20 o /-30°C

E61T8-K6 E71T8-K6

Charpy ~lOO J at -40°C and CTOD at -10°C

179

Table 6.2.

Relative effective deposition rates for different welding processes used for fillets in the fabrication of structural steelwork. MMA/SMAW in 3G/4G positions = 1.0 After Fern and Yeo (1990).

Welding process

MMA/SMAW MIG/GMAW Solid wire GS-FCAW Gas-shielded tubular wire SS-FCAW Self-shielded tubular wire SAW Sub-arc

Positions

Welding method

IF

2F

3F

4F

Manual

12

12

1.0

1.0

Semi-automatic

24

24

NA

Mechanized

48

48

NA

NA NA

Semi-automatic

48

48

10

10

Mechanized

96

96

20

20

Semi-automatic

44

44

8

8

Mechanized

88

88

16

16

Semi-automatic Mechanized

80 160

80 160

NA NA

NA NA

"NA" indicates that those processes are not applicable in the given positions.

180

Table 6.3.

Comparison of submerged-arc weld toughness in 50 mm thick BS.4360 Grade SOD steel plate (see Fig. 6.14 for procedure) when the root passes are made with self-shielded flux-cored wire and £7016 electrode. SD3 back-weld.

Test Weld

Root Consumable

(i) (ii)

£7016 £61T8-K6

Charpy-V average at -25°C CAP centre-line

ROOT centre-line

120

100 J 35 J

120

CTOD at -10°C on 2B x B samples (i)

E7016

(ii)

E61T8-K6

*

Table 6.4.

Test Weld (iii) (iv)

0.92, 0.49, 0.71 mm Test 1: Test 2:

Unacceptable values:

0.12, 0.17, 0.13 mm 0.03, 0.04, 0.05 mm

*

all below 0.25 mm

Comparison of submerged-arc weld toughness in 50 mm thick BS.4360 Grade SOD steel plate (see Fig. 6.14 for procedure) when the root passes are made with self-shielded flux-cored wire and £7016 electrode. TIBOR 22 back-weld.

Root weld

E7016 £61T8-K6

*

Acceptable values:

CTOD at -10°C in 2B x B samples

0.94, 0.58, 0.39 mm 0.49, 0.36, 0.31 mm all above 0.25 mm.

*

181

Fig. 6.1.

Excellent accessibility of self-shielded welding demonstrated by a welder's ability to work inside a transportable concrete mixer. AWS E7lT-ll and E71T-GS wires were used with a welding gun fitted with a fume extraction nozzle and no fume plume can be seen. Courtesy:

The Lincoln Electric Co.

182

Fig. 6.2.

A welder perched on a girder at a beam-to-column connection of a large structure is using self-shielded welding under conditions hardly suitable for the erection of draught screens necessary for gas-shielded welding. Courtesy:

The Lincoln Electric Co.

183

Self-shielded

Gas-shielded

(a)

(b)

Electrode extension 40 mm

Electrode extension 30 mm

Joint preparation angle: 22"

Fig. 6.3.

Root gap: 5 mm

Comparison of access to the root in a narrow gap of 22° using: (a)

Gas-shielded welding gun,

(b)

Self-shielded welding gun. After

Bj~rnbakk

and

and Boekholt, IIW Doc. XII-1080-88.

Original Lincoln Electric Co. information.

184

Fig. 6.4.

A mUlti-process DC arc welding power source, Lincoln IDEALARC DC-400 popular in self-shielded welding applications during the 1980s. This is a three-phase input power transformer/rectifier incorporating solid state silicon control rectifiers (SCR) and rated at 400 A, 36 V and 100 % duty cycle. Courtesy:

The Lincoln Electric Co.

185

Fig. 6.5.

A multi-process DC arc welding power source, Hobart MEGA-FLEX 450 RVS, suitable for eight different processes, including self-shielded welding.

This is a three-phase input power

transformer/rectfier (SCR) rated for constant voltage welding at 450 A, 38 V and 100 % duty cycle. Courtesy:

Hobart Brothers Co.

186

Fig. 6.6.

A constant voltage DC arc welding power source incorporating wire feeder, Lincoln IDEALARC SP-250, suitable for self-shielded and gas-shielded welding with solid and cored tubular wires. This is a single-phase input power source intended for light fabrication and rated at 250 A, 36 V at 35 % duty cycle. It features a microprocessor, liquid crystal display and is programmed with numerous procedures in five languages: English, French, German, Japanese and Spanish. Courtesy:

The Lincoln Electric Co.

187

Fig. 6.7.

Heavy duty wire feeder, Lincoln LN-9, used with self-shielding wires.

Note the digital display of the arc voltage which can be

pre-set and locked, with the consequence that any departure from it would stop welding. Weight: Courtesy:

37 kg (80 lb.). The Lincoln Electric Co.

188

Fig. 6.8.

Hobart 2410 wire feeder suitable for welding with self-shielding wires where weld metal toughness requirements are not very critical. Weight: 24 kg (52 lb.)

Courtesy:

Hobart Brothers Co.

189

Fig. 6.9.

Lincoln LN-25 portable wire feeder designed for use with self-shielding wires of up to 2.0 mm (5/64 in.) diameter only. Weight:

Courtesy:

about 14 kg (30 lb.) without wire reel.

The Lincoln Electric Co.

190

Fig. 6.10.

Hobart HEFTY CC/CV portable wire feeder designed for use with self-shielding wires of up to 2.0 mm (5/64 in.) diameter only. Weight: about 14 kg (30 lb.) without wire reel.

Courtesy:

Hobart Brothers Co.

I-'

\0 I-'

Fig. 6.11.

A heavy duty gun for self-shielded welding, Lincoln K-115-45 Innershield gun, rated at 450 A. Courtesy:

Lincoln Electric Co.

192

Fig. 6.12.

A heavy duty gun for self-shielded welding, Hobart Model GS-2 rated at 500 A. Courtesy:

Hobart Brothers Co.

193

(a)

(b)

(e)

Fig. 6.13.

Comparison of V-groove preparation angles for butt welding with: (a)

flux-covered electrode,

(b)

gas-shielded process

(c)

self-shielded process.

Courtesy:

and

The Lincoln Electric Co.

194

CONSUMABLES FLUX

WIRE SD3FiII Back-weld: TIBOR22

OP121TT

PLATE TYPE

BS 4360 Grade 500

orSD3 THICKNESS 50 mm

Preheat

100"C

InIerpaSS

25O"C

JOINT PREPARATION

Post weld heiIl-treatment Pass No.

1

Root:

2

Electrode size(mm)

Cunwt, _A

Volts

Speed, em/min

375

29

44

3

4.0

450

30

44

4.0

550

32

44

9-21

4.0

550

34

4.0

600

34

4.0

575

32

ROOT GAP o-3mm

-

RUN SEQUENCE

lI8~20l21) ~6 17}

DC/AC

55

4.15KJ1mm

44

Tibor22

or

SD3

Fig. 6.14.

~~

E61 T&-K6 or E7016 4.0

4-8

22-23

Remarks

"t"'t~~:-:J IW'

-.L

~~6-8mm

-t

Welding procedure used for testing the effect of dilution from different root passes on the toughness of submerged-arc welds: Test Weld

Root Pass

Filling Passes

Back-Weld

(i) (ii) (iii) (iv)

E7016 E61T8-K6 E7016 E61T8-K6

SD3/0P121TI' SD3/0P121TI' SD3/0P121TI' SD3/0P121TI'

SD3/0P121TI' SD3/0P121TI' Tibor 22/0P121TT Tibor 22/0P121TI'

After Keeler and Garland (1983).

195

7.

SPECTRUM OF PROVEN APPLICATIONS

7.1.

Market Share of the SSAW

It is not possible at present to establish with any reasonable accuracy the market share for the self-shielded arc welding (SSAW) filler metal among all the filler metals used for different arc-welding processes. In

the

published

statistics

(1)

which show relative

consumption of filler metals used for

changes

different processes,

in the

the self-

shielding wires are buried within the FCAW together with gas-shielded wires. Judging from the combination of professional contacts and published papers, it appears that the self-shielded welding has been used mostly in the USA,

followed by the former Soviet Union and Japan.

Although the USA

is claimed (2) to be ahead of any other country in the use of selfshielding wires, the figure of 16 % quoted (2) for the weld metal deposited in the mid-1980s embraces all the cored/tubular wires, gas-shielded ones as well.

The comparable figures for both the UK individually (2) and

Western Europe as a whole (1) are 5 % each. In Scotland, where offshore structures are fabricated for the North Sea, self-shielded welding is used probably relatively more than anywhere else in the world.

One offshore structures fabricator (3) gives the following

breakdown for the

use of different processes in the bulk of general

fabrication:(a)

50/50 division between the submerged-arc welding (SAW) and the handheld welding comprising flux-covered electrodes and self-shielding wires, and

(b)

the usage of self-shielding wires varying from project to project and "lying in the range of 15-30 % of the total hand-held welding.

The relatively low application of self-shielded welding in Europe compared to

the USA

and the

former

USSR,

appears to have had

the

following

underlying reasons:(i)

There appears to be an element of the "not invented here" (NIH) syndrome.

The self-shielding wires have been developed, marketed

196 and hence patented mainly by The Lincoln Electric Co. in the USA, and only recently other consumables manufacturers have been emulating the Lincoln efforts. (ii)

Some attempts by European manufacturers to produce self'-shielding wires under the then Soviet licence have not proven very successful.

(iii) Because of its unique know-how and marketing position in the selfshielded welding, The Lincoln Electric Co. has not, until recently, given in its data sheets the full product information, including the weld metal composition, been accustomed.

to which the European engineers have

Consequently in Europe, including the UK,

there

have been cases, some personally known to the writer, of the selfshielding wires being viewed wi th suspicion and mistrust,

to the

point of being barred from applications. It can be said that in the UK, and especially in Scotland, the first significant use of self-shielding wires in the late 1970s, immediately for the fabrication of offshore platforms started somewhat on the wrong footing,

compared to their use in the USA.

Being the cradle of self-

shielded welding, the American industry adopted this process in a 'natural' way, starting with the less demanding applications, where self-shielding wires could replace first and foremost E6013 and E7024 rutile flux-covered electrodes (see Table 6.1) which are easy to use.

Until its application

to the welding of offshore structures in Scotland (4), the self-shielded welding was almost unknown and hardly heard of in the UK.

Being applied

straight at the most demanding end of the spectrum, where high sub-zero Charpy and the CTOD toughness were required, was like being dropped in at the proverbial deep end. which although now overcome, self-shielded welding

Not surprisingly, there have been problems, have left an impression that somehow the

is extra difficult to perform and control.

The

real need for extreme care and strict control in the self-shielded welding may apply only to one or two wires in Appendix B, i.e. those wires which are meant to compete with E7016-1 and E7018-G(1%Ni) CTOD pedigree (Table 6.1).

However,

electrodes of the

the high profile of the offshore

industry in the UK and Europe has stamped this unwarranted mark on the self-shielded welding as a whole and clouded the perception of many an engineer.

197 As the perusal of Appendix B and Table 6.1 can show, there is a variety of self-shielding wires which can be used with ease and which can replace profitably

the

applications,

MMA/SMAW

in

numerous

applications.

In

many

such

not only the onerous toughness requirements relevant to

offshore structures are inapplicable, but also there is no need to even carry out the simple Charpy test.

7.2.

Sheet Ketal Work

For welded joints in steel sheet of 1-3 mm thickness, Charpy toughness is not

required and the metallurgical integrity

accepted

on the

containers

and

basis of storage

the

bend

vessels

test.

designed

of welded joints is

This for

applies

domestic,

to

numerous

farming

and

industrial use, as well as to car bodies. In sheet metal work, the self-shielding wires can replace some rutile flux-covered

electrodes of E6013 Classification recommended

for

sheet

metal work and they can compete with CO 2 - welding, especially on farms and in auto-body repair work where welding is often done in almost outdoor conditions. There are now suitably thin self-shielding wires with diameters of: 0.76

DDIl

(0.030")

and

0.9

DDIl

(0.035")

which can operate at currents below 50 A and which are applicable to thin sheet even below 1.0 mm thickness, i.e. 0.8 mm thick. normally on DC- (DCEN) • E71T-GS

classification,

Such wires operate

Wires for single-pass welding conform to AWS and

those

for

multi-pass

welding

conform

to

E71T-11 classification. There are also larger diameter E70T-3 wires used for fully mechanized welding of car bodies at very high speeds of about 5 m/min, and operating on DC+ (DCEP).

Some brands of E70T-3 wires, killed with Ti (see Appendix

B), are suitable for application with robots.

198

7.3.

Earth Moving Equipment

Figure 7.1

shows where different classes of self-shielding wires were

applied in the fabrication of a

digger/bulldozer,

producti ve performance of the welds in service.

to achieve the most

For the characteristics

of the wires used, 'see Table 5.1:(1) E70T-4

(IF) position for heavy 12 mm (!")

wire was used in flat

fillets and lap welds on blades, pivots and liftarms; (2) E70T-7

wire was used for general structural welds around gussets, brackets and reinforcing members where good penetration was required,

yet

it could

be used

with ease at

high travel

speeds; (3) E71T-GS

wire was used for making single-pass fillet and lap welds on

light

gauge sheet metal

tanks were constructed,

from which fuel and hydraulic

and where

leak-proof operation in

service is a must; (4) E71T-8

wire was used for multi-pass welding of digger gear which is subj ect to impact and shock in service, where the weld metal must have high Charpy toughness;

(5) E71T-11

wire,

because

of

its

speed

characteristics,

all-positional and

yet high welding

was

welding of wheel

used

for

the

guards, trailers and trailer tongues. At another end of the size spectrum, in the construction of a huge drag line for open pit coal mining with a boom of 400 ft (about 122 m), E71T-8 wire was used extensively on the massive structure because the wire had an all-positional capability and delivered tough weld metal required by design

(5). had

Because of the size of the machine, a considerable amount of welding to

be

done

outdoors,

where

the

self-shielded

welding

performed

especially well.

7.4.

High Rise Buildings, Plant and Bridges

There have been a number of reports describing the use of self-shielding wires in the erection of structural steelwork for high rise building in

199 various

American

(7) , Cleveland,

cities:

Atlanta,

Georgia

Ohio (8) and Chicago (9).

(6),

Jacksonville,

Florida

The salient feature of all

those applications has been the remarkable resistance of the SSAW to the effects of wind,

especially in Chicago where strong winds blow across

Lake Michigan. Figure 7.2 shows two welders perched high up on a structure, working at a vertical truss and carrying out balanced welding on each flange from two sides.

Figure 7.3 shows a welder working even higher up, welding

a beam on the 52nd floor of Georgia Pacific building (7). conceivable that under the

It is hardly

conditions shown in these two photographs,

the gas-shielded semi-automatic welding, whether with solid or tubular wires,

could

have

been applied.

cylinders,

hoses

productive

alternative

electrodes.

and

restrictive to

the

The

SSAW

shielding

rather

slow

If the SSAW did not exist,

be used perforce.

the

free

from

cumbersome

curtains

offered

welding

with

MMAjSMAW

the

gas only

flux-covered

would

have

to

However, because of the application of the self-shielded

welding, significant economic benefits accrued on all projects in question

(6-9). Depending on the welding pOSitions and weld metal toughness requirements, the

following

effective

when

wire used

Classifications on

the

above

(see

Table

mentioned

5.1)

were

building

found

sites:

very

E70T-4,

E70T-7 and E70T-8. In the construction of Abjar Hotel in Dubai, self-shielded welding was used on steel trusses in positions where submerged-arc and consumable guide processes could not be used (10).

The all-positional E71 T-8 wire

of 2 mm diameter was used in the vertical and overhead positions. The

se1f~shie1ded

welding was used in the rebuilding of the British Steel

Corporation's Redcar blast furnace (11). tubular

wires

could

be

used,

but

for

For shop welding, gas-shielded the

assembly

welding

on site,

E71T-8 wire was specified as the SSAW was the only semi-automatic process which could deposit sound weld metal faster than the MMAjSMAW in windy conditions in all-positions.

Working

in 3-shifts

(24 h),

were employed and on average 80 % duty cycle was achieved.

130 welders

Stop and start

positions were dressed to ensure complete soundness, and as a result there

200 were no cut-outs, i.e. the zero defect condition having been achieved. The self-shielded welding has been used on some bridges. in Scotland was

The Forth Bridge

successfully and expeditiously repaired due,

in large

measure, to the application of the SSAW resistant to inclement weather. Site welding of pre-fabricated sections for the 1800 m bridge at St.Omer/ /Pas de Calais, for the TGV link from Paris to the Eurotunnel, was carried out with E71T-8 and E71T8-K6 self-shielding wires.

Butt welds were made

in ASME 1G and 4G positions and fillet welds in all-positions.

Smooth

and neat structural geometry was achieved while welding on site, as this was required for good protection from corrosion.

On the St.Omer bridge,

the DC-400 rectifiers shown in Fig. 6.4 were used as power sources, and the wire feeders were the LN-9 (Fig. 6.7) and the portable/lightweight LN-25 (Fig. 6.9).

7.5.

Shipbuilding and Dockyard Work

The self-shielded welding is beginning to make inroads into shipyards which traditionally have relied on Lloyds Grade 3 E6013 rutile electrode and E7018 low-hydrogen electrodes for all-positional welding. In Portsmouth Dockyard, the Royal Navy ship HMS Gloucester and other Type 42

destroyers

sections

have

along

the

been

strengthened

deck (12,

13).

by

welding

longitudinal channel

This was done using E71T-8

self-

shielding wire which gave the required weld metal toughness (see Table 5.1). In

the

fighting

severely for

shipbuilder

and took

competitive

industry of merchant shipbuilding,

winning some contracts for steps

bulk-carriers,

to equip the yard with more

machines with the self-shielded welding capability.

than

a

whilst British

one hundred

The SSAW being almost

as flexible as the MMA/SMAW, but much more productive, is being seen as indispensable for surviving in the face of competition from low labour cost countries. In Chile, South America (14), a SOD-ton fishing boat was fabricated almost entirely using the self-shielding wire of E71T-ll Classification.

This

gave excellent all-positional operability, including vertical-down.

The

total fabricating

time was reduced by 30 % with the 40 % increase in

201 productivity and greatly improved profitability of the shipyard.

7.6.

Pipelines

Traditionally, overland pipelines have been welded with cellulosic fluxcovered electrodes, AWS E6010 and E6011, because these electrodes give off the highest volume of gas-shield among all the classes of covered electrodes.

As can be seen from the Nitrogen .Scale in Fig. 2.2,

the

cellulosic electrode weld metal has the lowest nitrogen content among the MMA/SMAW deposits as a result of the highest shielding.

Girth butt

welds in pipelines are made by single-sided welding, with no access to the root from the inside of the pipe, and the high gas volume of the cellulosic electrode arc enables the deposition of sound weld metal in the root pass. Hydrogen is

the main component of the cellulosic electrode gas-shield

and the weld metal hydrogen contents lie in the range of 50-100 m1/100g. Consequently,

preheating

circumvent this handicap,

is necessary

to avoid hydrogen cracking.

To

there have been attempts to use gas-shielded

welding with solid wires, but the need for protection from wind in the open countryside and other problems of the process do not make the gasshielded welding a

serious competitor to

the cellulosic electrodes in

pipeline welding. The self-shielded welding lies at the other extreme of the Nitrogen Scale (Fig. 2.2) from the cellulosic electrodes and being very resistant from the effects of wind, overland pipelines. Self-shi~lding

it should be ideally suited for

the welding of

There are already recorded cases of such applications.

wire was used for the welding of 12.5 mm wall thickness

pipe made from steel Grade X70 (15).

Not only the welding procedure was

compatible with the use of a low hydrogen process, but the welds were resistant

to

porosity.

Also,

a

self-shielding

wire

of

E71T-K6

Classification has been developed for the welding of Arctic-grade pipe (15). In Argentina (14), a natural gas pipeline, longer than the Alaskan one by 10 %, was welded with self-shielding wire of E71T8-K6 Classification, after the root pass was made with E6010 electrode.

Beyond the root pass,

202 the productivity

was increased by 44 %,

and

30 % fewer welders and

machines were required to meet the production schedule, compared to the

MMA/SMAW. In India (16), a 31 mile (50 km) pipeline was welded with self-shielding wire

of

E71T8-K6

Classification

after

E7010-G cellulosic coated electrode.

the

root

pass

was

made

with

The welding was done on a lay barge

where the high resistance of the self-shielding wire to the effects of wind was of great advantage.

The X-ray examination revealed only 0.42%

defect rate relative to the weld seam length.

7.7.

Offshore Structures

The use of self-shielded welding in the' fabrication of offshore platforms for the North Sea, both in Britain and Norway, has already been referred to earlier.

Some offshore installations in the North Sea have been in

service

over

for

15

years

and

naturally

various

troubles

have

been

experienced with materials and welded joints as a result of environmental attack and fatigue loading by sea waves.

However, till the time of writing,

there has not been a single case reported to implicate self-shielded welds in any problems. On the 7th July 1988, a disastrous explosion destroyed the Occidental Oil Piper Alpha platform which operated in the North Sea since 1976. The scene of destruction is well known from the TV newsreels.

However,

it is not so well known that many critical nodes in the jacket structure, supporting the platform, Scottish fabricator.

were welded with a

self-shielding wire by a

The extensive damage to the platform and its heavy

listing could be seen clearly, but the jacket did not collapse into the sea.

There was no evidence that any of the structural nodes had been

destroyed by the tremendous force of the explosion. The self-shielding wire used most extensively by both British and Norwegian fabricators

has been E61 T8-K6,

but

E71 T8-K6 has entered the industry.

recently

a

new wire classified

as

Some Australian offshore structures

have thinner walls and operate at higher temperatures than those in the North Sea.

Those Australian structures were welded with E71T-8 wire which

203 does not contain Ni-additions for enhanced toughness.

7.8.

Assembly and Erection on Site

This has also been mentioned earlier and it is carried out in a number of applications described above.

However,

its importance with respect

to the potential application of self-shielded. welding warrants a special mention. When large structural items are brought on site to be fitted together, the joint root gaps can be not only large, but also they can vary in time within a day as the work progresses.

In outdoor conditions, from morning

to mid-day and then to the evening, a given root gap can vary by a few mi1limetres as a result of the action of the sun rays, regardless of the dimensions specified on the drawings.

With the increasing gap, the molten

pool needed to bridge it increases and increased nitrogen contamination in weld metal can occur.

Some self-shielding wires have a proven track

record (4, 15) of having an excellent capability for bridging wide and variable root gaps, and depositing sound weld metal under such difficult conditions.

References 1.

OLSSON, R., PERSSON, K-A. and MacKAY, L. "Gas selection for increas e d productivity", Welding & Metal Fabrication, Nov. 1991, Vo1.59 , No.9, pp.502-506.

2.

PRIOR, H., CLARK, J., STODDART, D.W., BROWN, M.A.S. and YEO, R.B.G. "Welding with self-shielded flux cored wire Scottish Branch sponsored meeting", Metal Construction, Aug. 1986, Vo1.18, No.8, pp.491-494.

3.

RODGERS, K.J. and LOCHHEAD, J.C. "What Process Can Be Used to Weld 65 ksi Yield Steels?" Welding Journal, Dec. 1991, Vo1. 70, No.12, pp.29-34.

4.

KEELER, T. "Innershie1d welding", Metal Construction, 1981, Vol. 11 : "Part 1: Development and applications", No.ll, pp.667-673; "Part 2: Properties", No.12, pp.750-752.

5.

GILBANK, J. S. "Analytical Approach Cuts Welding Costs on Dragline Project", Canadian Welder and Fabricator, 1982, Vo1.73, No.lO, pp.1O-13.

204 6.

ANON. "Self-Shielded FCAW Speeds High-Rise Construction", Welding Journal, 1984, Vol.63, No.4, pp.47-49.

7.

TORCHIO, P. "Mechanised welding speeds high rise construction", Metal Construction, June 1984, Vol.16, No.6, pp.344-345.

8.

ANON. "Office Building Columns Field Spliced wi th Self-Shielded Welding Wire", Welding Journal, 1986, Vo1.65 , No.lO, pp.53-54.

9.

ANON. "Self-Shielded FCA Welding is a Breeze in the Windy City", Welding Journal, 1988, Vol.67, No.3, pp.47-48.

10.

D'SILVA, B.A. "Jumbo-Section Trusses Need Heavyweight Welding", Welding Journal, April 1991, Vol.70, No.4, pp.75-77.

11.

MATHERS, G.

"A fabricator's view of synergic MIG welding", Joining

& Materials, June 1989, Vol.2, No.6, p.273.

12.

YEO, R. "Welding speeds navy turnround", Welding & Metal Fabrication, Oct. 1989, Vol.57, No.8, p.406.

13.

ANON. "What's new in welding consumables?" , Fabrication, April 1991, Vol.59, No.3, pp.138-142.

14 .

EVANS, S.R. "Latin America - Welding Technology in a Land Contrasts", Welding Journal, Jan. 1989, Vol.68, No.1, pp.33-36.

15.

MISKOE, W.I. "Continued development brings wider applications for Innershield", Metal Construction, Dec. 1983, Vo1.15 , No.12, pp.738-741.

16.

CHEC, L. "Offshore Pipe Welding Benefits from Flux Cored Electrodes", Welding Journal, Aug. 1989, Vol.68, No.9, pp.43-45.

Welding

&

Metal of

N

o

111

Fig. 7.1.

Application of self-shielding wires in the fabrication of the "Ditch Witch" digger/bulldozer built by the Charles Machine Works of Perry, Oklahoma, USA:1 2 3 4 5

-

E70T-4 E70T-7 E71T-GS E71T-8 E71T-ll

Courtesy:

for for for for for

heavy 12mm (!") downhand fillet and lap welds on blades and liftarms, general fast welding with good penetration around gassets and brackets, single-pass fillet and lap welds on sheet metal: fuel and hydraulic tanks, welds which require high Charpy impact toughness, and all-positional mUlti-purpose welding on wheel guards and trailers.

The Lincoln Electric Co.

206

Fig. 7.2.

Self-shielded welding on a truss of a building with two welders working simultaneously on opposite flanges.

Note the suspended work

platform on the left hand side. Source: Metal Construction, June 1984.

207

Fig. 7.3.

Self-shielded welding of a beam on the 52nd floor of a high rise building for the headquarters of Georgia-Pacific in Atlanta. Source: Metal Construction, June 1984.

208

8. 1.

CONCLUSIONS On the fundamental grounds of process metallurgy, the self-shielded arc-welding (SSAW) should be regarded as a distinct welding process which is not just a mere variant of the so-called flux-cored arcwelding (FCAW) or tubular/cored wire welding. wires

together

with

gas-shielded

wires

Treating self-shielding in

literature,

on

the

superficial grounds of their similar appearance and use with the same power sources and wire feeders, obscures the deep differences in their nature.

This is detrimental to the perception of the SSAW by users,

the understanding of the self-shielded weld' metal properties and the beneficial exploitation of the process in appropriate applications. 2.

The distinctive characteristics of the self-shielded welding are that the process relies very little on the protection of molten metal from air,

but

instead

strong

deoxidation and

denitriding,

mainly with

aluminium, are used to counteract the inevitable initial contamination by oxygen and nitrogen. 3.

Because the higher the volume of deoxidation products, the more readily they are removed from the molten metal to the slag, the oxygen content in the self-shielded weld· metal is very low, only about 100 ppm. Thus regarding oxide inclusions, the self-shielded weld metal is the cleanest

among all

the arc-weld metals

depOSited from consumable

electrodes. 4.

Most of the nitrogen picked-up by the molten metal from air is trapped on solidification,

but then it is fixed by strong denitriders in

nitride particles.

Therefore, the bulk analytical or total N-content

is high in the self-shielded weld metal, whereas

in many

60-150 ppm range.

other arc-weld metals

normally above 200 ppm,

the N-contents lie in the

However, the free or mobile nitrogen, capable of

causing strain-ageing embri ttlement, appears to be much lower ill the self-shielded weld metal than in other arc-weld metals which do not contain strong denitriders by design. 5.

The minimal shielding in the SSAW confers a very important operational advantage:

the process is very resistant to the effects of side wind,

209 as there is hardly any gas"";shield to be lost. up to 5 m/ s constant,

With wind speeds of

the nitrogen level in the weld metal remains nearly

and

the

unaffected by wind

tensile elongation and Charpy spee~s

of up to 6 m/ s.

toughness remain

Consequently, the SSAW

is an ideal process for use outdoors and in the assembly and erection of large structures on site. 6.

In the self-shielded welding,

vigorous fume capture and extraction

at source is very easy and effective because there is no auxiliary gas-shield to be disturbed. 7•

Wi th the. vast maj ori ty of single-tube self-shielding wires on the market at present, about 1% Al is recovered in the weld metal to prevent nitrogen porosity.

This AI-content can eliminate the gamma-

-to-alpha transformation in steel, and the transformation is restored by judicious additions of C, Mn and Ni.

B.

In the self-shielded weld metal intended for less-onerous applications, such as sheet metal work, for which rutile flux-covered electrodes, AWS E60l3, are fit-for-purpose, carbon contents of up to 0.3 % are used

in

conjunction

transformation.

with

0.5%

Mn

to

produce

grain

refining

Such weld metal has adequate ductility and a modicum

of toughness compatible with the application requirements. 9.

For sub-zero toughness in the as-welded condition,

low C-contents

are used (C <0.1%) and the transformation is restored with about 1.5% Mn, or with 0.7% Mn + Ni> 0.5%.

The Ni-bearing weld metals

are capable of good Charpy toughness down to -50°C and can deliver consistently the CTOn values above 0.25 mm at -10°C in 50 mm thick plate, when appropriate procedures are used.

Such weld metals are

deposited from E6lTB-K6 and E7lT8-K6 self-shielding wires which can replace E70l6-l and E70lB-G(1%Ni) flux-covered electrodes. 10.

The welding procedures designed for achieving high sub-zero toughness for critical applications must be based on the "thin layer stringer bead" technique and a strict control of welding parameters to ensure that:(i) at least 50% grain-refinement is achieved by reheating the earlier

210 passes with subsequent passes; this requires the deposition of thin weld metal beads/layers; (ii)

the heat input is kept close to 1 kJ/mm which ensures high cooling rate, leading to small ferrite grain/plate packet size in the as-deposited microstructure;

(iii) the wire feed speed, arc voltage and electrode extension are controlled in such a manner that the total N-content is maintained at a level generally not exceeding 250 ppm.

11.

Unlike other arc-weld metals, the self-shielded weld metal does not appear to suffer much from root region embrittlement in thick plate butt welds.

This appears to be associated with the low level of

free nitrogen which is the agent causing strain-ageing embrittlement in the root when subsequent passes are being deposited. 12.

In comparison with gas-shielded welding,

the self-shielded welding

has a number of advantages:(a) only welding machines and filler metal are required, and there is no

encumberance

with and

cost

of

gas cylinders and

gas

regulating and metering equipment, (b) the self-shielding guns are slim and light, enabling good access to tight spaces and narrow gaps, (c) there is no need for protection from air currents and wind, and (d) vigorous fume capture and extraction at source,

on the guns,

does not impair the arc environment. 13.

Currently, there are self-shielding wires for almost any conceivable application with C-Mn steels where flux-covered electrodes have been used:-

14.

from sheet metal work to large structures and pipelines.

The combination of (i) the capacity to kill any nitrogen pick-up, (ii)

good

access

instantaneous

to

current

deep-V

roots and

variation

through

(iii)

the

stick-out

capability changes,

for

makes

the SSAW an ideal process for root-pass welding where there are large, variable and misaligned gaps.

Also for the same reasons, the SSAW

is better than the E7016 electrode and the best process for singlesided butt welding.

211

15.

Being a continuous wire process, unlike the flux-covered electrode, the SSAW does not suffer from enforced stops and starts.

This and

other features of the self-shielded welding make it effective in improving fabrication rate, productivity and profitability. 16.

There are already numerous documented cases of application of the self-shielded

welding with

the attendant

technical

and

economic

advantages.

Acknowledgements

A number of individuals have been generous with their help to make this work possible. The long term encouragement by Mr. Charles R. Hughes and Dr. Ralph B.G. Yeo of The Lincoln Group is highly appreciated, and grateful thanks are due to Dr. Richard L. Jones of TWI for his thorough and constructive comments on the preliminary draft. The author is grateful to Mr. P.T. Houldcroft, Mr. John A. Street and Dr. Damian J. Kotecki for their general encouragement, and to Mr. Roger A. Daemen of Hobart Brothers and to Mr. Richard J. Pargeter, Dr. Nia Francis-Scrutton and Dr. Richard F. Smith of TWI for their provision of photographic material.

208

8. 1.

CONCLUSIONS On the fundamental grounds of process metallurgy, the self-shielded arc-welding (SSAW) should be regarded as a distinct welding process which is not just a mere variant of the so-called flux-cored arcwelding (FCAW) or tubular/cored wire welding. wires

together

with

gas-shielded

wires

Treating self-shielding in

literature,

on

the

superficial grounds of their similar appearance and use with the same power sources and wire feeders, obscures the deep differences in their nature.

This is detrimental to the perception of the SSAW by users,

the understanding of the self-shielded weld' metal properties and the beneficial exploitation of the process in appropriate applications. 2.

The distinctive characteristics of the self-shielded welding are that the process relies very little on the protection of molten metal from air,

but

instead

strong

deoxidation and

denitriding,

mainly with

aluminium, are used to counteract the inevitable initial contamination by oxygen and nitrogen. 3.

Because the higher the volume of deoxidation products, the more readily they are removed from the molten metal to the slag, the oxygen content in the self-shielded weld· metal is very low, only about 100 ppm. Thus regarding oxide inclusions, the self-shielded weld metal is the cleanest

among all

the arc-weld metals

depOSited from consumable

electrodes. 4.

Most of the nitrogen picked-up by the molten metal from air is trapped on solidification,

but then it is fixed by strong denitriders in

nitride particles.

Therefore, the bulk analytical or total N-content

is high in the self-shielded weld metal, whereas

in many

60-150 ppm range.

other arc-weld metals

normally above 200 ppm,

the N-contents lie in the

However, the free or mobile nitrogen, capable of

causing strain-ageing embri ttlement, appears to be much lower ill the self-shielded weld metal than in other arc-weld metals which do not contain strong denitriders by design. 5.

The minimal shielding in the SSAW confers a very important operational advantage:

the process is very resistant to the effects of side wind,

209 as there is hardly any gas"";shield to be lost. up to 5 m/ s constant,

With wind speeds of

the nitrogen level in the weld metal remains nearly

and

the

unaffected by wind

tensile elongation and Charpy spee~s

of up to 6 m/ s.

toughness remain

Consequently, the SSAW

is an ideal process for use outdoors and in the assembly and erection of large structures on site. 6.

In the self-shielded welding,

vigorous fume capture and extraction

at source is very easy and effective because there is no auxiliary gas-shield to be disturbed. 7•

Wi th the. vast maj ori ty of single-tube self-shielding wires on the market at present, about 1% Al is recovered in the weld metal to prevent nitrogen porosity.

This AI-content can eliminate the gamma-

-to-alpha transformation in steel, and the transformation is restored by judicious additions of C, Mn and Ni.

B.

In the self-shielded weld metal intended for less-onerous applications, such as sheet metal work, for which rutile flux-covered electrodes, AWS E60l3, are fit-for-purpose, carbon contents of up to 0.3 % are used

in

conjunction

transformation.

with

0.5%

Mn

to

produce

grain

refining

Such weld metal has adequate ductility and a modicum

of toughness compatible with the application requirements. 9.

For sub-zero toughness in the as-welded condition,

low C-contents

are used (C <0.1%) and the transformation is restored with about 1.5% Mn, or with 0.7% Mn + Ni> 0.5%.

The Ni-bearing weld metals

are capable of good Charpy toughness down to -50°C and can deliver consistently the CTOn values above 0.25 mm at -10°C in 50 mm thick plate, when appropriate procedures are used.

Such weld metals are

deposited from E6lTB-K6 and E7lT8-K6 self-shielding wires which can replace E70l6-l and E70lB-G(1%Ni) flux-covered electrodes. 10.

The welding procedures designed for achieving high sub-zero toughness for critical applications must be based on the "thin layer stringer bead" technique and a strict control of welding parameters to ensure that:(i) at least 50% grain-refinement is achieved by reheating the earlier

210 passes with subsequent passes; this requires the deposition of thin weld metal beads/layers; (ii)

the heat input is kept close to 1 kJ/mm which ensures high cooling rate, leading to small ferrite grain/plate packet size in the as-deposited microstructure;

(iii) the wire feed speed, arc voltage and electrode extension are controlled in such a manner that the total N-content is maintained at a level generally not exceeding 250 ppm.

11.

Unlike other arc-weld metals, the self-shielded weld metal does not appear to suffer much from root region embrittlement in thick plate butt welds.

This appears to be associated with the low level of

free nitrogen which is the agent causing strain-ageing embrittlement in the root when subsequent passes are being deposited. 12.

In comparison with gas-shielded welding,

the self-shielded welding

has a number of advantages:(a) only welding machines and filler metal are required, and there is no

encumberance

with and

cost

of

gas cylinders and

gas

regulating and metering equipment, (b) the self-shielding guns are slim and light, enabling good access to tight spaces and narrow gaps, (c) there is no need for protection from air currents and wind, and (d) vigorous fume capture and extraction at source,

on the guns,

does not impair the arc environment. 13.

Currently, there are self-shielding wires for almost any conceivable application with C-Mn steels where flux-covered electrodes have been used:-

14.

from sheet metal work to large structures and pipelines.

The combination of (i) the capacity to kill any nitrogen pick-up, (ii)

good

access

instantaneous

to

current

deep-V

roots and

variation

through

(iii)

the

stick-out

capability changes,

for

makes

the SSAW an ideal process for root-pass welding where there are large, variable and misaligned gaps.

Also for the same reasons, the SSAW

is better than the E7016 electrode and the best process for singlesided butt welding.

211

15.

Being a continuous wire process, unlike the flux-covered electrode, the SSAW does not suffer from enforced stops and starts.

This and

other features of the self-shielded welding make it effective in improving fabrication rate, productivity and profitability. 16.

There are already numerous documented cases of application of the self-shielded

welding with

the attendant

technical

and

economic

advantages.

Acknowledgements

A number of individuals have been generous with their help to make this work possible. The long term encouragement by Mr. Charles R. Hughes and Dr. Ralph B.G. Yeo of The Lincoln Group is highly appreciated, and grateful thanks are due to Dr. Richard L. Jones of TWI for his thorough and constructive comments on the preliminary draft. The author is grateful to Mr. P.T. Houldcroft, Mr. John A. Street and Dr. Damian J. Kotecki for their general encouragement, and to Mr. Roger A. Daemen of Hobart Brothers and to Mr. Richard J. Pargeter, Dr. Nia Francis-Scrutton and Dr. Richard F. Smith of TWI for their provision of photographic material.

212

APPEHDIX·A Collation of some data on the types and contents of slag, gas and vapour forming ingredients and killing agents used in self-shielding tubular/cored wires and published between 1970-1980. Values in mass (weight) percentages. Code

FLUORIDES

CARBONATES

A

40 CaF2

10 CaC03 7.5 MgC03

B1

11 LiF 12.5 BaF2

B2

45 CaF2

B3 C1

13.1 CaF2 9.3 BaF.2 11.9(LiF)2CaF2 33 CaF2 5 REM fluorides

3 Li2C03

OXIDES 7.5 AI203 7.5 MgO

11

Al

2 25

15

Ca 4Mg3AI3

MgO Al203

7.5 MgO 3.75 Li2Si03 15 10

MgO Al203

15 CaC03

6

MgO

2 4

K20 Si02

C2

50 CaF2

10 CaC03 10 MgC03

C3

30 CaF2 5 AIF3

25 MgC03

Dl

48.5 CaF2

D2

STRONG KILLING AGENTS

7 Al 7 Mg 10.6 Al3Ca2 13.75 Mg 55Ca 45 6 Mg 6 CaSi 8 6 6 10

FeZr FeTi FeAI AIMg

4 CaC03

1 K20 1 Na20 2 Si02 26.5 MgO

10.8 MgAI

46.4 CaF2

4 CaC03

22 6 MgO

10.7 CaAI

E

6-48 CaF2

1.8-3 CaC03 9-15 MgC03

F

67 CaF2

G

58 CaF2

I

63.5 CaF 2

J

22 CaF2

K

15.3 CaF2

L

22 CaF2

4 CaC03

0.6-4 FeTi 1.2...,4.5 Al 11. 7 MgO

15.83 Al

22 "others"

8 Al 8 Mg 15.4 Al

12.6 MgO ~

1 MeC03

3.6 Si02 20.6 Ti02 4.5 MgO

1.6 Al

6 CaC03

4.2 14.7 6.9 0;6 1.2

1.4 Al

3.5 CaC03

Si02 Ti02 S;i02 A1203 Ti02

213

APPENDIX A (continued) Code

FLUORIDES

M

14.8 CaF2

N

45.7 CaF2

0.9 Si02

15.2 Al2Mg3 0.85 Al

0

47.6 CaF2

1.0 Si02

17.1 Al2Mg3 0.5 Al

P

37.2 BaF2 11. 7 LiF

R

I

39 CaF2 0.8 BaF2 1.5 LiF

CARBONATES 17 CaC03 4.5 Na2C03

3.3 CaC03

4.4 CaC03

OXIDES

STRONG KILLING AGENTS

5 Si02 20 Ti02

2.4 2.1 1.4 1.8

Si02 Zr02 SrO K20

0.6 Si02 12 MgO

9.1 Al 7.7 Mg

8.8 Al 4.75 Mg

Sources: AI.

Code letters A - G: DAVEY, T.G. "Self-shielded welding of ferritic steels - a literature review", The Welding Institute Research Bulletin, April 1978, Vol.19, No.4, pp.113-120.

A2.

Code letters I - L: SMITH,D.C. "Flux-Cored Electrodes - Their Composition and Use", Welding Journal, July 1970, Vol.49, No.7, pp.535-547.

A3.

Code letters M - R: KILLING, R. "Welding with self-shielded wires - the mechanism of shielding and droplet transfer", Metal Construction, Sept. 1980, Vol.12, No.9, pp.433-436.

214

APPENDIX B Typical chemical compositions of some all-weld-metals deposited from commercial self-shielded wires for different applications. BRAND NAME

AWS CLASS

C

INNERSHIELD

NR-1 NR-5 NR-131 NR-151 NR-152

NR-202 NR-211 NR-211MP NS-3M NR-311

NR-232 NR-301 NR-302 NR-305

Element contents, mass (weight) % P Al Ti Mn Si S

-

The Single-pass welding E70T-3 0.22 1.25 E70T-3 O.lS 1.00 E7OT-10 0.26 0.60 E71T-GS O.lS 1.00 E71T-GS 0.2S 0.9 Non-coded fabrications E71T-7 0.20 0.70 E71T-ll 0.26 0.50 E71T-GS 0.22 0.60 E70T-4 0.23 0.40 E70T-7 0.26 0.45

Lincoln Electric Co. only - sheet metal 0.50 0.020 0.012 0.35 0.020 0.010 0.20 0.010 0.010 0.22 0.025 0.010 0.22 0.015 0.010

.

products mainly 0.01 O.lS O.OS 0.40 1.40 0.35 1.30 -

no Charpy test requirements 0.20 0.004 O.OOS 1.20 0.16 0.005 0.005 1.60 O.lS 0.003 0.007 1.50 0.26 0.004 0.006 1.40 O.OS 0.005 0.007 1.55 -

Fabrications with Charpy toughness at -30°C : 27 J min. E71T-S 0.17 0.65 0.27 0.004 0.006 0.55 E70T4-K2 0.06 1.12 0.17 0.005 O.OOS 1.16 E70T-6 0.11 1.75 0.14 0.009 0.006 0.S9 E70T-6 0.09 1 0.20 0.006 0.007 O.SO -

Coded fabrications NR-203M E71T-S 0.07 1.50 NR-203Ni1% E71TS-Nil O.OS 1.12 NR-203Ni-C E61TS-K6 0.06 0.72 NR-207 E71TS-K6 0.07 0.S5 NR-20S-H E91TS-G 0.06 1.65 NR-400 E71TS-K6 0.07 O.SO

with extra toughness 0.20 0.002 0.006 0.2S 0.003 O.OOS 0.03 0.002 0.004 0.22 0.003 0.005 0.06 0.003 0.005 0.16 0.002 0.004

0.S4 0.S5 0.S5 1.00 1.0 0.65

Ni

Cr

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

0.95 0.52 0.S5 0.9 0.70

0.04

-

1.15

0.03

0.13

215

APPENDIX B (continued) BRAND

AWS

NAME

CLASS

C

FABSHIELD 23

4

21B

2125

Element contents, mass (weight) % Mn P Al Si S Ti

Ni

Cr

-

-

-

-

Hobart Brothers Co. products

Single-pass welding only; good for galvanized steel E71T-GS 0.2 0.55 0.4 0.005 0.01 1.5 Non-coded fabrications : no Charpy test requirements E70T-4 0.25 0.35 0.15 0.002 0.002 1.15 E71T-ll 0.30 0.50 0.35 0.006 0.010 1.15

Fabrications with Charpy toughness at -30°C : 27 J min. E71T-G 0.06 2.40 0.09 0.009 0.008 1.10 0.48 -

-

216

APPENDIX C Welding consumables manufacturers and their generic brand names for the self-shielding tubular wires.

Manufacturer

Generic Brand Name

The Lincoln Electric Co.

INNERSHIELD

Hobart Brothers Co.

FABSHIELD

Alloy Rods Corp.

CORESHIELD

Corex, Inc., Ohio

SELF-SHIELD

Tri-Mark, Inc.

No special name

Murex Welding Products Ltd.

MURAIR

ESAB Group

Teledyne McKay

No special name for self-shielding wires; TUBROD for all the tubular/ cored wires

TUBE-ALLOY (for surfacing)

Welding Alloys Ltd.

OPEN ARC (for surfacing)

217

INDEX accessibility, 158, 166, 181, 183, 193, 210 acicular ferrite, 83, 84 aircraft, 2 all-positional welding, 30 Alloy Rods, 40 aluminium, iii, iv, 4, 15, 16, 17, 18, 25, 26, 28, 29, 30, 31, 55, 80, 81, 88, 98, 99, 171 content in weld metal, 30, 34, 39, 50, 64, 65, 84, 85, 89, 119, 170, 175 and corrosion, 93 sulphides, 29 arc voltage, 30, 31, 66, 67, 121, 122, 125, 162 austenite, 83 austenite-ferrite transformation, 80 automatic welding, 2, 124-125, 139, 140, 159 bainite formation, 28, 81 barium, 37 carbonate, 25 bridges, 2, 5, 200 brittle fracture, 120, 123 burn through, 32 butt welds, iii, 2, 68, 89, 93, 109, 110, 111, 112, 113, 114, 121, 124, 135, 164, 166 cadmium, 39 calcium, 15, 16 carbonate, 20, 22, 23, 24, 27, 48, 56 fluoride, 26, 27, 56, 62, 63 carbon, 28, 81, 82, 100, 114, 209 carbonates, 49 Charpy toughness, 2, 10, 34, 74, 81, 84, 85, 86, 89, 90, 100, 102, 103, 105, 106, 107, 108, 109, 110, 111, 112, 120, 125, 126, 133, 141, 145, 175, 197, 209 classifications, 11 of wires, 81-82, 173 cleavage, 80, 84, 92, 124 columnar crystals, 80 consumables, iii arc welding, iii, 39 compositional groups, 81-82 flux-covered electrodes, iv, 2, 3, 16, 19, 20, 22, 23, 24, 35, 40, 46, 47, 56, 82, 102, 120, 122, 143, 147, 158, 170, 173, 175, 178 basic low-hydrogen, 22, 35, 48, 62, 84, 85, 176 cellulosic, 22, 48, 201 rutile, 81, 176, 197 gas-shielded, 11, 16, 24, 34, 40, 46, 58, 143, 170 positions on the Nitrogen Scale, 19-21 self-shielding, iv, 3, 4, 5, 7, 11, 13, 14, 16, 20, 25, 26-28, 29, 30, 33, 35, 36, 51, 60, 61, 64, 65, 81-82, 85, 86, 102, 121, 143-157, 160, 173, 178, 180, 197, 198, 212-213 manufacturers, 216 solid, 1, 4, 20, 28, 33, 36, 57

218 tubular/cored, 1, 4, 11, 20, 22, 23, 24, 28, 33, 36, 47, 49, 58, 72, 143, 159, 170 bare flux-cored wires for unalloyed steel, NF specification for, 148-149, 154 carbon and carbon-manganese steel tubular cored electrodes, BS specification for, 146-148, 153 carbon steel electrodes for flux-cored arc welding, AWS specification for, 144-145, 152, 156 double tube construction, 20, 23, 48, 56, 57, 58 gas-shielded and self-shielded tubular cored wires, Irw Draft Specification for, 149-150, 155 low-alloy steel electrodes for flux-cored arc welding, AWS specification for, 145-146, 157 single tube construction, 20, 23, 24, 26, 28, 46, 48, 50, 57, 58, 80, 87 copper, 114 cored-wire welding, iii, 3, Coreshield, 40 corrosion, 93, 119 Cortex Self-Shield, 40 crack growth/propagation, 2, 83, 90, 92 critical applications, 82, 120, 163 CTOD testing/toughness, 82, 83, 85, 90, 92, 101, 120, 122, 123-124, 129, 130, 134, 138, 140, 175 current, 30, 31, 32, 66, 71, 121, 122, 124 delta-ferrite, 80 delta-gamma-alpha transformation, 81 denitridation, 16, 17, 19, 26, 28, 29, 30, 33, 49, 55 deoxidation, 16, 17, 18, 19, 25, 26, 29, 33, 49, 55, 83, 208 dephosphorisation, 27 deposition rates, 179 desulphurisation, 27, 29, 52, 89 diffusible hydrogen levels, 169-170 dilution, 89, 174-176, 194 draughts, 7, 33, 35, 158, 166, 182 ductile-brittle transition, 81 ductility, 15, 26, 27, 34, 63, 81 earthmoving equipment, 198, 205 electrode extension, 31, 32, 70, 71, 86, 92, 107, 121, 122, 159, 162 run-out-length, 120 electron beam welding, 15 -probe micro-analysis, 80, 98 embrittlement, lli, 34, 80, 87, 88, 89 fatigue, 2, 3, 13, 14, 173 FCAW, see flux-cored arc welding ferrite, 32, 82, 84, 121, 171 fillet welds, 38, 40, 147, 168 fish-eyes, 172 fit-up, 31, 172, 203 flaws, 3, 15, fluorides, 26, 49, 89, fluorine, 27

219 flux, iii, 1, 15, 21, 22, 23, 24, 26, 27, 28, 29, 30, 35, 37, 46, 47, 83, 88, 93, 108, 169 flux-cored arc welding, iii, 3, 4, 13, 14, 158 see also consumables, tubular/cored fracture, 88, 89, 92 toughness, 120-126 fume extraction, 37-38, 40, 79, 158, 161, 209 fever, 40 galvanized steel, 40 gamma-alpha transformation, iv, 81, 82, 84, 209 gamma loop, 80, 171 gas shield, iii, 3, 7, 15, 28, 29, 30, 32, 57, 64 and nitrogen content in weld metal, 26, 86 generation in self-shielding wires, 24-26 gas shielded welding, 1, 5, 6, 10, 12, 24, 33, 34, 35, 36, 37, 82, 158, 170, see also consumables, gas-shielde4 GMAW, see metal inert gas grain refinement, 80, 81, 82, 84, 85, 103, 120, 209 GTAW, see tungsten inert gas guns, 161, 166, 183, 191, 192 high-rise buildings, 36, 167, 198-200, 206, 207 Hobart Brothers Co, 36 hybrid welding procedures, 174-176 hydrogen, 26, 27, 146, 201 content in weld metal, 27, 62 control, 168-172 cracking, 122, 168, 170-172 cyanide, 39 inclusions, 15, 17, 18, 29, 80, 83, 90-92, 116, 117, 118, 124, 208 indoor welding, 6 Innershield, 28, 39, 40, 68, 69, 82, 93, 120, 125, 171 iron, 16, 17, 25, 80, 99 killing, iii, 8, 16, 17, 20, 26, 28-30, 32-40, 80-82, 83 lap joints, 38 lead, 39 Lincoln Electric Co, The, 28, 93, 162, 196 lithium, 28 carbonate, 25 fluoride, 27 macrostructure, 104, 120 magnesium, 15, 16, 25, 30 carbonate, 24, 25, 27 manganese, 15, 17, 80, 81, 82, 83, 84, 93 manual metal arc, iv, 1, 3, 6, 10, 84, 88, 108, 158, 162 see also consumables, flux-covered martensite formation, 81 metal inert gas welding, 15, 16, 19, 29, 108, 162 see also consumables, gas-shielded matrix, 80, 98 transfer, iii, 15

220 microstructure, 82-85, 101, 120 MIG, see metal inert gas mill scale, 38 misalignment, 3 MMA, see manual metal arc mUltipass welding, 30, 66, 81, 82, 84, 85, 89, 103, 106, 197 nickel, 81, 82, 84, 114 niobium, 89, 114, 115 nitrides, 49, 53, 86,92 nitrogen, 15, 27, 39 affinity for, 16 and toughness, 85-90 as the contamination gauging medium, 17-18 bound, 87, 93 content in weld metal, iii, 17, 19, 20, 21, 22, 23, 25, 27, 28, 29, 30,31,32,33,34,35,37,48,50,62,66,67,68,69,70,72, 73,75,80,91,92,105,106,107,108,113,121,123,124,125, 138, 175, 208, 210 free, 87, 88, 89, 90, 208 induced porosity, 30, 80 pick-up, 30-32, 33, 124, 161, 208 Scale, iii, 17, 18, 23, 24, 28, 30, 35, 37, 54, 57, 201 non-critical applications, 81, 163 normalisation, 84 offshore structures,S, 36, 82, 85, 120, 167, 170, 172, 195-196, 202-203 outdoor welding, iii, 6, 35 overhead welding, 125, 141 oxides, 53 oxygen, 15, 27, 39, 90 affinity for, 16 content in weld metal, iii, 17, 18, 27, 29, 46, 62, 83, 84, 88, 90, 91, 116, 117, 208 penetration, 31 phase transformation, 80-82 phosgene, 40 pipe welding, 125 pipelines, 82, 201-202 plastic deformation, 86, 89, 90 porosity, 3, 15, 16, 30, 33, 34, 35, 37, 38, 39, 40, 65, 80, 173, 209 power generation, 5 sources, 1, 31, 159-160, 184, 185, 186 pressure vessels, 2 primed steel, 38-40 process selection, 5 production welding, 158-194 productivity, iv, 1, 2, 4, 165-168, 211 profitability, iv quality assurance, iv rare earth metals, 15 Robertson-type test, 90 robots, 197 root-pass welding, iii, 172-173, 210

221 sea water, 93, 119 self-shielded arc welding, a distinct process, iii, 3, 4, 5, 40, 143, 208 and accessibility, 158-159, 181, 183, 193, 210 and wind, iii, 6, 7, 32-36, 38, 72, 73, 74, 158, 168, 199, 208 cleanliness, iii, 83, 92 consumables, see consumables, self-shielded and tubular/cored equipment for, 1, 12, 37, 38, 76, 77, 78, 79, 158-161, 184, 185, 186, 187, 188, 189, 190, 191, 192, 210 limitations, 174-176 market share, 195-197 principles, 5 productivity, iv, 1, 2, 4, 165-168, 211 profitability, iv proven applications, 195-207 weld metal, 29, 34, 35, 52, 74, 80-93, 98, 100, 103, 105, 106, 107, 109, 110, 111, 112, 113, 114, 115, 118, 120, 171, 214-215 semi-automatic welding, 2, 36, 124, 158 sheet metal, 28, 30, 197 shielding gas, iii, 1, 16, 17, 20, 48, 144, 165 vs killing, 18 shipbuilding, 6, 38, 200-201 silicon, 15, 16, 25, 81, 93, 171 single pass welding, 81, 164 single-sided welding, iii, 2, 14, 121, 134, 164, 173 slag, 26, 27, 89 SMAW, see manual metal arc spatter, 24, 26 on guns, 36-37, 75 specifications, 4, 5, 143, 144-145 AWS A5.1, 143 AWS A5.20-79, 4, 152, 156 AWS A5.29-80, 145-146, 157 BS.639, 143 BS .1719, 143 BS.7084: 1989 , 144, 146-148, 153 IIW Doc. XII-D-138-91, 149-150, 155 NF A81-350 - A81-359, 143, 148-149, 154 of wires, 81-82, 143-157 SSAW, see self-shielded arc welding steels austenitic stainless, 32 carbon-manganese, iii, 4, 5, 19, .34, 35, 46, 51, 61, 82, 86, 89, 120, 144, 149, 175, 210 -silicon, 93 chromium-nickel austenitic stainless, 5 ferritic, 29, 32, 54, 73 stainless, 80 low-alloy, iii, 5, 144 mild, iii, 5, 19, 46, 57, 175 primed, 38-40 zinc-galvanized, 40 strain-ageing, 87, 88, 89, 208 strength, 87, 88, 144 stick-out, 31, 32, 70, 71, 86, 92, 107, 121, 122, 159, 162 storage tanks, 2

222 stress, 120 submerged-arc, 1, 2, 6, 10, 13, 15, 19, 21-22, 23, 29, 46, 47, 82, 83, 93, 108, 120, 143, 174, 180, 194 sulphur, 89, 90, 91, 114 surfacing alloys, 5 thin layer stringer bead technique, 122-125, 129, 130, 209 TIG, see tungsten inert gas titanium, iii, 4, 15, 16, 18, 25, 28, 29, 30, 80, 88, 98 carbo-nitrides, 80 oxy-sulphides, 80 toughness, iv, 2, 15, 28, 29, 34, 74, 81, 82, 100, 104, 109, 110, 111, 120-126, 161, 174 and travel speed, 131 microstructure and, 82-85, 101 nitrogen and, 85-90 training, 162-165 travel speed, 31, 68, 122, 124, 125, 126, 135, 136 tungsten inert gas, 15, 18, 19 turbulence, 37 vertical welding, 126 void-coalescence fracture, 86, 92, 108 weaving, 112, 121, 126 weld metal hydrogen content, see hydrogen content in weld metal nitrogen content, see nitrogen content in weld metal oxygen content, see oxygen content in weld metal self-shielded, see self-shielded, weld metal welder training, 162-165 welding parameters, 30 procedures, 2, 10, 82, 120, 132, 133, 142, 160, 194, 209 hybrid, 174-176 Welding Institute, The, 173 wind, iii, 6, 7, 32-36, 38, 72, 73, 74, 158, 168, 199, 208 wire feed speed, 31, 32, 71, 121, 122, 124, 125, 136, 162 feeders, 160-161, 187, 188, 189, 190 wires, see consumables zinc content in primers, 39 galvanized steel, 40 zirconium, iii, 4, 15, 16, 17, 18, 25, 28, 88

This book puts self-shielded arc welding on the map of professional knowledge as a process which ought to be recognised in its own right, and which should not be treated as a variant of flux-cored arc or cored-wire welding. The metallurgical principles of how self-shielding wires work are explained along with their remarkable resistance to the effects of wind. The author suggests which self-shielding wires are appropriate to replace various types of flux-covered stick electrodes and reveals the advantages which can ensue. The physical metallurgy of the weld metal produced is discussed with the help of available data. Principles of toughness control are explained and appropriate procedural guidance is given. 'Self-shielded arc welding' is addressed to a broad readership from the graduate engineer seeking an explanation of how it relates to other arc welding processes to welding engineers and managers seeking to use the information to improve productivity and profitability in their companies. Dr. Tad Boniszewski came to Britain in 1957, having obtained an MSc at the Academy of Mining and Metallurgy in Cracow, Poland. After studying for a PhD at Cambridge University, his first job was with the then British Welding Research Association (now TWI) where he became a Group Leader in the Metallurgy Department. In 1966 he joined the CEGB as a Deputy Head of Welding R&D where his prime responsibility was to study how welding consumables work and how their nature determines weld metal properties. In 1971 he became Technical & Commercial Director at Metrode Products Ltd. Four years later he moved on to ESAB where he created a Technology Department with the aim of modernising and broadening the consumables product range. In 1981 he left ESAB to become an independent consultant in welding metallurgy.

Abington Publishing Abington Cambridge CB21 6AH England

ISBN 978-1- 85573-063-2

ABINGTON PUBLISHING Woodhead Publishing Limited in association with The Welding Institute