In order to control contraction in sheet metal, the welding procedure should be devised so that contraction stresses will be held to a minimum order to keep the desired shape and strength of the welded part. Some of the methods used for controlling contraction are described below. b. The backstep method as shown in view A, figure 6-43, may be used. With the backstep method, each small weld increment has its own shrinkage pattern, which then becomes insignificant to the total pattern of the entire weldment.
In welding long seams, the contraction of the metal deposited at the joint will cause the edges being welded to draw together and possibly overlap. This action should be offset by wedging the edges apart as shown in view B, figure 6-43. The wedge should be moved forward as the weld progresses. The spacing of the wedge depends on the type of metal and its thickness. Spacing for metals more than 1/8 in. (3.2 mm) thick is approximately as follows:
|Metal||In. per ft|
|Steel||1/4 to 3/8|
|Brass and Bronze||3/16|
Sheet metal under 1/16 in. (0.16 cm) thick may be welded by flanging the edges as shown in figure 6-20, and tacking at intervals along the seam before welding. A weld can be produced in this manner without the addition of filler metal.
Buckling and warping can be prevented by the use of quench plates as shown in figure 6-44. The quench plates are heavy pieces of metal clamped parallel to the seam being welded with sufficient space between to permit the welding operation. These quench plates absorb the heat of welding, thereby decreasing the stresses due to expansion and contraction.
Jigs and fixtures may be used to hold members in place for welding. These are usually heavy sections in the vicinity of the seam (fig. 6-45). The heavy sections cool the plate beyond the area of the weld.
In pipe welding, spacing as illustrated in figure 6-43, is not practical. Proper alignment of pipe can be best obtained by tack welding to hold the pieces in place. The pipes should be separated by a gap of 1/8 to 1/4 in. (0.32 to 0.64 cm), depending on the size of the pipe being welded.
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