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 reduced or 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.
a) Pre-setting of fillet joint to prevent angular distortion;
b) Pre-setting of butt joint to prevent angular distortion;
c) 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.
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.
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 clamps are transferred into the joint when the clamps are removed, the level of residual stress across the joint can be quite high.
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.
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 minimum 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.
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 which can be purchased here.