Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. To minimize distortion, many different techniques are implemented and one such technique (majorly) for thin sheet welding is back-step welding.
In the back-step technique, the general progression of welding may be, say, from left to right, but each bead segment is deposited from right to left as in figure. As each bead segment is placed, the heated edges expand, which temporarily separates the plates at root gap. But as the heat moves out across the plate to the corner, expansion along outer edges brings the plates back together. This separation is most pronounced as the first bead is laid. With successive beads, the plates expand less and less because of the restraint of prior welds. Back stepping may not be effective in all applications, and it cannot be used economically in automatic welding.
Reference: Lincoln literature
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