Frequently Asked Questions
In a butt joint, the heat input of the welding process and the subsequent solidification and cooling can result in transverse shrinkage, and closing of the root gap. This effect can be reduced through the use of tack welds. 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 (SMA) or MIG (GMA), 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. 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
c) tack weld the centre and complete the tack welding by back stepping
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.
The technical knowledge pages contain a number of items on distortion in welded assemblies. These can be located by using the search engine - try typing 'Distortion' into the free text search box or selecting Distortion from the list of keywords.