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21 Completing the joint with a small number of large weld deposits results


in more longitudinal and transverse shrinkage than a weld completed over a larger number of small passes. In a multi-pass weld, previously deposited weld metal provides restraint, so the angular distortion per pass decreases as the weld is built up. Large deposits also increase the risk of elastic buckling, particularly in thin section plate.


Use of balanced welding Balanced welding is an effective means of controlling angular distortion in a multi-pass butt weld by arranging the welding sequence to ensure that angular distortion is continually being corrected and not allowed to accumulate during welding. Comparative amounts of angular distortion from balanced welding and welding one side of the joint first are shown schematically in Fig. 4 below. The balanced welding technique can also be applied to fillet joints.


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Fig. 4: Balanced welding to reduce the amount of angular distortion


If welding alternately on either side of the joint is not possible, or if one


side has to be completed first, an asymmetrical joint preparation may be used with more weld metal deposited on the second side. The greater contraction resulting from depositing the weld metal on the second side will help to counteract the distortion on the first side.


Best practice The following design principles can control distortion:


Eliminate welding by forming the plate and using rolled or extruded sections Minimise the amount of weld metal Do not over-weld Use intermittent welding in preference to a continuous weld pass Place welds about the neutral axis Balance the welding about the middle of the joint by using a double-V joint in preference to a single-V joint.


Adopting best practice principles can have surprising cost benefits. For example, for a design fillet leg length of 6mm, depositing an 8mm leg length will result in the deposition of 57 per cent additional weld metal. Besides the extra cost of depositing weld metal and the increased risk of distortion, it is costly to remove this extra weld metal later. However, designing for distortion control may incur additional fabrication costs. For example, the use of a double-V joint preparation is an excellent way to reduce weld volume and control distortion, but extra costs may be incurred in production through manipulation of the workpiece for the welder to access the reverse side. ■


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