Distortion - Prevention by pre-setting, pre-bending or use of
restraint
Assembly arrangement for main side plate fabrication of the Stalwart
carrier. (Courtesy of Roland Andrews)
General guidelines are provided as 'best practice' for limiting distortion by
adopting suitable assembly techniques.
In the 'Job knowledge for welders, Distortion - prevention by
design', it was shown that distortion could often be prevented at the
design stage, for example, by placing the welds about the neutral axis, reducing
the amount of welding and depositing the weld metal using a balanced welding
technique. In designs where this is not possible, distortion may be prevented by
one of the following methods:
- pre-setting of parts
- pre-bending of parts
- use of restraint
The technique chosen will be influenced by the size and complexity of the
component or assembly, the cost of any restraining equipment and the need to
limit residual stresses.
Fig. 1 Pre-setting of parts to produce correct alignment after welding
- Pre-setting of fillet joint to prevent angular distortion
- Pre-setting of butt joint to prevent angular distortion
- Tapered gap to prevent closure
Pre-setting of parts
The parts are pre-set and left free to move during
welding (see Fig 1). In practice, the parts are pre-set by a
pre-determined amount so that distortion occurring during welding is used to
achieve overall alignment and dimensional control.
The main advantages compared with the use of restraint are that there is no
expensive equipment needed and there will be lower residual stress in the
structure.
Unfortunately, as it is difficult to predict the amount of pre-setting needed
to accommodate shrinkage, a number of trial welds will be required. For example,
when MMA or MIG welding butt joints, the joint gap will normally close ahead of
welding; when submerged arc welding; the joint may open up during welding. When
carrying out trial welds, it is also essential that the test structure is
reasonably representative of the full size structure in order to generate the
level of distortion likely to occur in practice. For these reasons, pre-setting
is a technique more suitable for simple components or assemblies.
Fig. 2 Pre-bending, using strongbacks and wedges, to accommodate angular
distortion in thin plates
Pre-bending of parts
Pre-bending, or pre-springing the parts before
welding is a technique used to pre-stress the assembly to counteract shrinkage
during welding. As shown in Fig 2, pre-bending by means of strongbacks
and wedges can be used to pre-set a seam before welding to compensate for
angular distortion. Releasing the wedges after welding will allow the parts to
move back into alignment.
The main photograph shows the diagonal bracings and centre jack used to
pre-bend the fixture, not the component. This counteracts the distortion
introduced though out-of-balance welding.
Use of restraint
Because of the difficulty in applying pre-setting and
pre-bending, restraint is the more widely practised technique. The basic
principle is that the parts are placed in position and held under restraint to
minimise any movement during welding. When removing the component from the
restraining equipment, a relatively small amount of movement will occur due to
locked-in stresses. This can be cured by either applying a small amount of
pre-set or stress relieving before removing the restraint.
When welding assemblies, all the component parts should be held in the
correct position until completion of welding and a suitably balanced fabrication
sequence used to minimise distortion.
Welding with restraint will generate additional residual stresses in the weld
which may cause cracking. When welding susceptible materials, a suitable welding
sequence and the use of preheating will reduce this risk.
Restraint is relatively simple to apply using clamps, jigs and fixtures to
hold the parts during welding.
Welding jigs and fixtures
Jigs and fixtures are used to locate the parts
and to ensure that dimensional accuracy is maintained whilst welding. They can
be of a relatively simple construction, as shown in Fig 3a, but the
welding engineer will need to ensure that the finished fabrication can be
removed easily after welding.
Flexible clamps
A flexible clamp (Fig 3b) can be effective not
only in applying restraint but also in setting up and maintaining the joint gap
(it can also be used to close a gap that is too wide).
A disadvantage is that as the restraining forces in the clamp will be
transferred into the joint when the clamps are removed, the level of residual
stress across the joint can be quite high.
Fig. 3 Restraint techniques to prevent distortion
a) Welding jig |
b) Flexible clamps |
c) Strongbacks with wedges |
d) Fully welded strongbacks |
Strongbacks (and wedges)
Strongbacks are a popular means of applying
restraint especially for site work. Wedged strongbacks, Fig.3c, will
prevent angular distortion in plate and help to prevent peaking in welding
cylindrical shells. As these types of strongback will allow transverse
shrinkage, the risk of cracking will be greatly reduced compared with fully
welded strongbacks.
Fully welded strongbacks (welded on both sides of the joint) Fig 3d,
will minimise both angular distortion and transverse shrinkage. As significant
stresses can be generated across the weld which will increase any tendency for
cracking, care should be taken in the use of this type of strongback.
Best practice
Adopting the following assembly techniques will help to
control distortion:
- Pre-set parts so that welding distortion will achieve overall alignment
and dimensional control with the minimum of residual stress
- Pre-bend joint edges to counteract distortion and achieve alignment and
dimensional control with minimum residual stress.
- Apply restraint during welding by using jigs and fixtures, flexible
clamps, strongbacks and tack welding but consider the risk of cracking which
can be quite significant, especially for fully welded strongbacks.
- Use an approved procedure for welding and removal of welds for restraint
techniques which may need preheat to avoid forming imperfections in the
component surface.
Further information
TWI have recently published some useful formulae for
calculating the likely level of distortion in practical situations.
General guidance on practical measures to control distortion is available
from TWI in the form of a multi-media (CD ROM) package.
The information in this article was prepared by Bill Lucas in collaboration
with Geert Verhaeghe, Rick Leggatt and Gene Mathers.
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REPRODUCED - COURTESY TWI-UK