Distortion - Prevention by fabrication techniques
Distortion caused by welding a plate at the centre of a thin plate before
welding into a bridge girder section. Courtesy John Allen
Assembly techniques
In general, the welder has little influence on the choice of welding
procedure but assembly techniques can often be crucial in minimising distortion.
The principal assembly techniques are:
- tack welding
- back-to-back assembly
- stiffening
Tack welding
Tack welds are ideal for setting and maintaining the joint
gap but can also be used to resist transverse shrinkage. To be effective,
thought should be given to the number of tack welds, their length and the
distance between them. With too few, there is the risk of the joint
progressively closing up as welding proceeds. In a long seam, using MMA or MIG,
the joint edges may even overlap. It should be noted that when using the
submerged arc process, the joint might open up if not adequately tacked.
The tack welding sequence is important to maintain a uniform root gap along
the length of the joint. Three alternative tack welding sequences are shown in
Fig 1:
- tack weld straight through to the end of the joint (Fig 1a). It is
necessary to clamp the plates or to use wedges to maintain the joint gap
during tacking
- tack weld one end and then use a back stepping technique for tacking the
rest of the joint (Fig 1b)
- tack weld the centre and complete the tack welding by back stepping (Fig
1c).
Fig. 1 Alternative procedures used for tack welding to prevent transverse
shrinkage
a) tack weld straight through to end of joint
b) tack weld one
end, then use back-step technique for tacking the rest of the joint
c) tack
weld the centre, then complete the tack welding by the back-step technique
Directional tacking is a useful technique for controlling the joint gap, for
example closing a joint gap which is (or has become) too wide.
When tack welding, it is important that tacks which are to be fused into the
main weld are produced to an approved procedure using appropriately qualified
welders. The procedure may require preheat and an approved consumable as
specified for the main weld. Removal of the tacks also needs careful control to
avoid causing defects in the component surface.
Back-to-back assembly
By tack welding or clamping two identical
components back-to-back, welding of both components can be balanced around the
neutral axis of the combined assembly (Fig 2a). It is recommended that the
assembly is stress relieved before separating the components. If stress
relieving is not done, it may be necessary to insert wedges between the
components (Fig 2b) so when the wedges are removed, the parts will move back to
the correct shape or alignment.
Fig. 2 Back-to-back assembly to control distortion when welding two
identical components
a) assemblies tacked together before welding
b) use
of wedges for components that distort on separation after welding
Stiffening
Fig. 3 Longitudinal stiffeners prevent bowing in butt welded thin plate
joints
Longitudinal shrinkage in butt welded seams often results in bowing,
especially when fabricating thin plate structures. Longitudinal stiffeners in
the form of flats or angles, welded along each side of the seam (Fig 3) are
effective in preventing longitudinal bowing. Stiffener location is important:
they must be placed at a sufficient distance from the joint so they do not
interfere with welding, unless located on the reverse side of a joint welded
from one side.
Welding procedure
A suitable welding procedure is usually determined by
productivity and quality requirements rather than the need to control
distortion. Nevertheless, the welding process, technique and sequence do
influence the distortion level.
Welding process
General rules for selecting a welding process to prevent
angular distortion are:
- deposit the weld metal as quickly as possible
- use the least number of runs to fill the joint
Unfortunately, selecting a suitable welding process based on these rules may
increase longitudinal shrinkage resulting in bowing and buckling.
In manual welding, MIG, a high deposition rate process, is preferred to MMA.
Weld metal should be deposited using the largest diameter electrode (MMA), or
the highest current level (MIG), without causing lack-of-fusion imperfections.
As heating is much slower and more diffuse, gas welding normally produces more
angular distortion than the arc processes.
Mechanised techniques combining high deposition rates and high welding speeds
have the greatest potential for preventing distortion. As the distortion is more
consistent, simple techniques such as presetting are more effective in
controlling angular distortion.
Welding technique
General rules for preventing distortion are:
- keep the weld (fillet) to the minimum specified size
- use balanced welding about the neutral axis
- keep the time between runs to a minimum
Fig. 4 Angular distortion of the joint as determined by the number of runs
in the fillet weld
In the absence of restraint, angular distortion in both fillet and butt
joints will be a function of the joint geometry, weld size and the number of
runs for a given cross section. Angular distortion (measured in degrees) as a
function of the number of runs for a 10mm leg length fillet weld is shown in Fig
4.
If possible, balanced welding around the neutral axis should be done, for
example on double sided fillet joints, by two people welding simultaneously. In
butt joints, the run order may be crucial in that balanced welding can be used
to correct angular distortion as it develops. 
Fig. 5 Use of welding direction to control distortion
a) Back-step
welding
b) Skip welding
Welding sequence
The sequence, or direction, of welding is important and
should be towards the free end of the joint. For long welds, the whole of the
weld is not completed in one direction. Short runs, for example using the
back-step or skip welding technique, are very effective in distortion control
(Fig 5).
- Back-step welding involves depositing short adjacent weld lengths in the
opposite direction to the general progression (Fig.5a).
- Skip welding is laying short weld lengths in a predetermined, evenly
spaced, sequence along the seam (Fig 5b). Weld lengths and the spaces between
them are generally equal to the natural run-out length of one electrode. The
direction of deposit for each electrode is the same, but it is not necessary
for the welding direction to be opposite to the direction of general
progression.
Best practice
The following fabrication techniques are used to control
distortion:
- using tack welds to set up and maintain the joint gap
- identical components welded back to back so welding can be balanced about
the neutral axis
- attachment of longitudinal stiffeners to prevent longitudinal bowing in
butt welds of thin plate structures
- where there is choice of welding procedure, process and technique should
aim to deposit the weld metal as quickly as possible; MIG in preference to MMA
or gas welding and mechanised rather than manual welding
- in long runs, the whole weld should not be completed in one direction;
back-step or skip welding techniques should be used.
Bill Lucas prepared this article in collaboration with Geert Verhaeghe, Rick
Leggatt and Gene Mathers.
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REPRODUCED - COURTESY TWI-UK