Self-Shielded Arc Welding

228 Pages·1992·11.56 MB·English

by  T. Boniszewski (Auth.)

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
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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 arc
welding 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 gas
shield 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 flux
covered 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 arc
welding 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
SELF -SIDELDED ARC WELDING
1. INTRODUCTION
1.1. General Considerations
Self-shielded arc welding (SSAW) is a fusion welding process employing
a continuous (wire) consumable electrode 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). To work with the SSAW, it is
necessary to supply only two~ as opposed to three, 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. Therefore,
despite the current marketing trend 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 self
If
shielding consumable if access permits the use of semiautomatic
welding ?"
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 to root flaws, mainly lack of fusion and/or
penetration and porosity. However, 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. In 1970, D.C. Smith (10) referred to the
self-shielding electrodes as "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 (1990) developments in welding torch design
are considered, 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 because the term FCAW has been
narrowed by new developments. Today, there are metal-cored (flux-fr.e~)
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 involv ement 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. However, even here there are limitations because in large
shops and especially close to large doors, screening is often
necessary for gas-shielded welding. 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), can cause severe quality
problems. Under such conditions, 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. In limited
cases where cost is not a problem, localized weather protection
can be applied to large structures, but this always requires extra
time, resources and manpower. This has been done (25, 26) when
building Phillips Petroleum Co. r s Maureen Field platform for the
North Sea. 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.