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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KP Bhatt

Residual stresses are those that exist in a part independent of external force or restraint. Neglect of these residual tensile stresses created during welding processes can lead to stress corrosion cracking, distortion, fatigue cracking, premature failures in components, and instances of over design. When applied and residual stresses are accumulated, the net tensile stress seen by the components are much greater than the actual load applied. This is why welded components usually fail at the weld.

The residual tensile stress from welding is created because the weld consumable is applied in its molten state. The weld is applied in its hottest, most expanded state. It then bonds to the base material, which is much cooler. The weld cools rapidly and attempts to shrink from the cooling. Since it has already bonded to the cooler, stronger base material it is unable to shrink. The net result is a weld that is essentially being “stretched” by the base material. The heat affected zone is usually most affected by the residual stress and hence where failure will usually occur. Inconsistency in the weld bonding material, chemistry, weld geometry, porosity, etc. act as a stress risers for residual and applied tensile stress to initiate fatigue failure.

Reference: Evaluation of Welding Residual Stress Levels through Shot Peening and Heat Treating

Keep reading, happy welding

Thank you,

KP Bhatt