Table Of ContentEdited by W Lucas, DSc, PhD, CEng, FIM, Weld1
Process pipe
and tube welding
A guide to welding process options,
techniques, equipment, NDT and
codes of practice
A B I N G T O N
PUBLISHING
Woodbead Publishing Ltd in association with The Welding Institute
Cambridge England
Published by Abington Publishing.
Woodhead Publishing Ltd. Abington Hall. Abington.
Cambridge CB I 6AH. England
First published 1991. Abington Publishing
0 Woodhead Publishing Ltd
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.
British Library Cataloguing in Publication Data
Process pipe and tube welding
1. Piping. Metal tubes welding.
1. Lucas. William
67 I .52
ISBN 1 85573 012 X
Designed by Geoff Green (text) and Chris Feely Cjacket). typeset by BookEns
Limited, Baldock Herts and printed by Crampton & Sons Ltd. Sawston.
Cambridge
Introduction
The welding of tubes is an essential requirement in the fabrication of
components for chemical plant, power generation and aeroengine
industries. The original idea for this book arose from a seminar,
organised by The Welding Institute in 1984, which attracted over 100
engineers concerned with design, fabrication, production and quality
assurance. The speakers were specially invited from major UK com-
panies as acknowledged experts in the fabrication of tubular products.
This book contains contributions from those speakers together with
additional chapters to provide unique, comprehensive and up-to-date
coverage of all aspects of tube welding from initial design considerations
through production to final inspection.
In the first three chapters the authors have outlined the process and
equipment options available for both manual and mechanised welding.
This type of information is essential for design and production planning
when engineers are often faced with difficult choices, for example,
between competing processes (MMG MIG, TIG, plasma) and indeed
process variants such as pulsing. The information contained here will
help engineers make the right choice for particular applications, ensuring
that the most cost effective welding techniques are employed.
Five chapters are devoted to the application of tube welding in the
aeroengine, ship building, power generation, petrochemical and chemical
plant industries. Here numerous applications are described with detailed
information on the choice of process, materials, techniques and equip-
ment. The welding parameters and production data provided by the
authors will be invaluable to engineers seeking to apply tube welding
techniques in a practical situation. In addition, the experiences of the
authors will be a valuable source of information and will help engineers
to avoid making what could be costly mistakes or to overcome problems
when they occur in production.
For completeness, specialised information is also provided on the pro-
duction management aspects of tube welding. For example, in site welding
V
vi Introduction
the need to integrate all activities from material procurement through
fabrication to final inspection is shown to be an essential requirement.
More generally, when high quality applications are considered, the
pipework must be fabricated to meet construction codes and here guid-
ance is given on interpreting the requirements of the major codes for
pressurised pipework. In the final chapter, NDT techniques are described
for inspecting tubes to ensure that weld quality meets the exacting
requirements of steam generation plant.
The information contained in the book represents without doubt the
most comprehensive study of tube welding in industry. The data pre-
sented here will become a source of reference for designers, metallurgists.
welding engineers and production engineers who are concerned with
pipework fabrication. The authors are not only acknowledged experts in
this technology but have also built up a reputation for providing sound
practical advice in their particular industrial sectors. For these reasons
this book, containing technical information and practical experience, is
essential for those engineers wishing to manufacture tube components in
the most productive manner.
W Lucas
Chapter 1
K R SPILLER
Process options and manual
techniques for welding pipework
fabrications
Of the methods available for joining pipe, a principal technique is manual
welding using the manual metal arc (MMA). tungsten inert gas (TIG),
and metal inert gas (MIG, MAG or CO,) processes.
Joining pipework with any of these processes is regarded as a
specialised operation, with the most important requirement relating to
the quality and profile of the penetration bead. The successful achieve-
ment of this critical application is one of the most exacting tasks the
welder encounters, particularly when root runs are made on unbacked,
unrotated butt joints. Where high quality root runs are required, TIG
welding, with or without the use of fusible inserts, is usually preferred.
Where the quality becomes less stringent, MMA with basic, rutile and
cellulosic electrodes, or the MIG process find acceptance.
Whichever process is used, the production of controlled penetration
beads on an unrotated pipe is possible, provided the correct application
of specific techniques is carried out. This chapter discusses, in turn, the
relative merits of the manual techniques of each process.
Physical and geometrical aspects of pipewelding
Welding a pipe butt joint, especially when its axis is in the fixed position,
i.e. vertical, horizontal, or 45" inclined (classified as 2G, 5G, and 6G
respectively, Fig. l.l), is one of the most difficult challenges to the welder,
particularly when making the root run in the overhead quadrant, which
will henceforth be referred to as the 'critical area'. It will be appreciated
that on larger pipe diameters, for example, 300mm OD, a greater degree of
tolerance will be present than when pipes of smaller diameters, i.e.
100mm. are welded.
1
2 K R Spiller
(a) (b) (C)
1. I Classification of non-rotatable pipewelding positions: a) 26:
pipe vertical; b) 56: pipe horizontal; c) 66: pipe 45' inclined.
The techniques used to make the all-important root run in pipe butt
joints are given in Table 1.1.
The limitations imposed on the maximum permissible protrusion of
the penetration bead into the bore of the pipe are given in Table 1.2.
While these acceptance limits are laid down, any concavity should merge
smoothly into the adjacent surfaces and should be of a continuous and
uniform profile around the internal circumference of the welded joint.
The mismatch of the internal bore will also affect the penetration bead
formation and profile. Many codes give allowable tolerances on the
amount of bore mismatch which can be permitted, but this should never
be taken as a licence to tolerate the limits and every effort should be exer-
cised to keep the bore mismatch to a minimum.
Manual metal arc (MMA) welding
The MMA process may be used on a wide range of materials and pipe
sizes and is one of the most commonly practised for welding steel pipe.
Although it is capable of being used to produce high integrity joints, con-
sistent control of the penetration bead profile is difficult to achieve and
can vary from being concave to a typical 3mm convex protrusion. The
design of the joint preparation and the fit-up obtained make a significant
contribution towards the achievement of controlled penetration beads. A
representative sample of joint designs as given in various piping codes is
reproduced in Fig. 1.2. Two techniques are available for making the root
run: vertical-up and vertical-down welding. The choice of method is not
affected by the diameter of the pipe, but depends primarily upon the wall
thickness and type of material being welded.
Process options and manual techniques 3
Table 1.1 Techniques for making root runs on unbacked pipe butt joints
Welding process Manual welding technique
MMA 1 Vertical-up: rutile electrodes
2 Vertical-up: basic electrodes
3 Vertical-down: cellulosic electrodes
4 Vertical-down: basic electrodes
TIC 1 Vertical-up: filler rod additions
L Vertical-up: fusible inserts
3 Vertical-up; fusing the root. No fusible inserts or filler rod
MIG or CO, 1 Vertical-down: solid wire dip transfer
2 Vertical-up: solid wire dip transfer
Table 1.2 Limits of bore protrusion and root concavity
Nominal bore of pipe Maximum penetration in bore, mm
-
Less than 12mm 0.8
l2mm up to but not including 50mm 1.5
50mm up to but not including lOOmm 2.5
lOOmm and larger 3.0
Root concavity: 1 The bore surface of the joint is of smooth contour
2 The depth of root concavity is no greater than 10% of the pipe wall
thickness or 1.2mm. whichever is the smaller
3 The thickness of the weld is not less than the pipe wall thickness
When a vertical-up procedure is used it is customary to start welding at
the 6 o’clock position and weld upwards until the 12 o’clock position is
reached. This method is preferred for thick-walled pipe where the walls
act as a ‘heat sink by dissipating the heat more rapidly from the weld
area. The rapid cooling rates can cause metallurgical changes within the
heat affected zone (HAZ) which could prove detrimental. To overcome
this the cooling rate must be reduced, and this is accomplished by
decreasing the rate of welding and depositing a heavier weld bead.
When vertical-down welding is used, the weld is started at the top side
of the pipe (12 o’clock position) and is then continued downwards to the 6
o’clock position. Vertical-down welding is primarily used to weld relatively
thin-walled steel pipe since it allows fast travel speeds. On heavier walled
pipe it is possible to use the vertical-down procedure with larger diameter
electrodes and current values, but in doing this the welder will experience
considerable difficulty in maintaining control over the weld pool. The
weld pool increases in size which results in the pool over-running the arc
4 K R Spiller
r - 7
30-40"
1
i t
1,5+0.5mm
1.5-4.0 mm
(a)
3&37.5"
iI / \
1.5"
-0.5 mm
(C)
1.2 Typical joint preparatlons for MMA welding of process
pipework. Pipe walls: a) Up to and including 20mm; b) Over
20mm; c) Up to and including 12.7mm, suitable for vertical-down
welding.
Process options and manual techniques 5
and so flooding the joint. Moreover, because of gravitational forces, the
increased weld pool and associated slag coverage cushions the arc and
can cause defects such as pinholes. lack of root fusion and cold lapping.
Formation of root penetration around the pipe
When making the root run on a fixed 5G position pipe using the conven-
tional vertical-up welding, the welder has to maintain correct electrode
angles and execute small changes in welding technique to produce the
desired penetration bead.
In the critical area, the welder will rely on the position of the electrode
tip and combined arc force to give the weld pool some support. As the
weld is progressed to the vertical quadrant, the weld pool tends to sag
downwards under the arc and tries to flow into the bore of the pipe at a
much faster rate than when welding in the critical area, and the penetration
bead becomes markedly convex. In the inclined and flat quadrant, the
weld pool becomes even more fluid and the penetration bead can become
excessively convex and, in more severe cases, burnthrough can occur.
Irrespective of which type of electrode is used, achieving a controlled pene-
tration bead will be highly dependent upon the formation of a keyhole.
The keyhole or pear-shaped enlargement is formed by the root faces
burning away, which causes a localised enlargement of the root gap just
ahead of the arc. Ideally penetration is obtained when the keyhole is
between one and one and one-third times the electrode diameter (core)
being used. If the keyhole becomes enlarged, excess penetration,
burnthrough or internal undercut will develop. Conversely, if the keyhole
is too small, lack of penetration may occur. The keyhole size is controlled
by changing the electrode angle and the manipulation of the electrode tip
and arc.
When vertical-down welding, relatively flat to slightly convex pene-
tration bead profiles are more readily achieved around the pipe bore than
with vertical-up welding. With vertical-down welding, the weld pool,
because it readily flows downwards, is aided by gravity and so requires
high welding speeds to prevent it from over-running the arc.
When a keyhole is correctly formed, the fluid weld metal flows in
behind the hole to form the weld bead and automatically produces the
required penetration bead profile.
€fleet of electrode type on welding technique
Verzical-up welding with basic electrodes
On pipe joints which require the root run to be of high quality it is cus-
tomary to use basic cover electrodes with diameters of either 2.5 or
6 K R Spiller
3.25mm. The 2.5mm diameter electrode is preferred as it results in the for-
mation of a smaller weld pool which can be more easily controlled.
Once the arc has been established. the tip of the electrode is directed
into the root gap and pointed at the weld pool. Extreme care is exercised
at this stage to ensure that the arc does not 'blow' into the root gap. A
short arc length is maintained throughout welding. since porosity will
occur if the arc is too long. The welder can, if so desired. execute a V-
shaped weave of the electrode tip. Fig. 1.3. the weave being made in a con-
tinuous smooth movement. In doing this the arc is brought out of the
weld pool and up along the bevel face with a quick movement. The return
movement is slowed sufficiently to allow just enough time for the weld
pool to lose its fluidity, with the arc being returned to the weld pool and
held for a short pause. This combined movement is then repeated up and
along the other bevel face.
The practice of 'whipping' the tip of the electrode as used with rutile
covered electrodes must be avoided to eliminate the occurrence of porosity
and loss of control over the formation of the penetration bead profile.
Another common practice - that of 'hopping' the electrode tip from one
bevel to the other at the root of the joint - is also detrimental.
When stopping a weld bead which will be continued, care must be
taken to ensure that the arc is not 'broken off abruptly. otherwise a
shrinkage cavity forms in the weld crater. Unless these are dressed and
tapered by grinding it is unlikely that they will be completely remelted out
when a restart is made. The recommended practice to avoid forming a
crater is to 'tail out' the arc to the sidewall of the joint. then break the arc.
The restarting technique requires a preheat to allow fusion and penetration
to take place at the leading edge and underside of the crater. This is
achieved by initiating the arc on the bevel face adjacent to the crater and
1.3 Pattern of V weave when making root run using basic elec-
trodes.
