Distortion Control - 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
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
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:
a) 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
b) tack weld one end and then use a back stepping technique for tacking the rest of the joint (Fig 1b)
c) 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.
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
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.
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.
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.
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
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.
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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