Thickness and plate size determines the area of the heated zone. Number and placement of heating zones are largely a question of experience. For new jobs, tests will often be needed to quantify the level of shrinkage. Spot, line, or wedge-shaped heating techniques can all be used in thermal correction of distortion. Spot heating Spot heating, is used to remove buckling, for example when a relatively thin sheet has been welded to a stiff frame.
Distortion is corrected by spot heating on the convex side.
If the buckling is regular, the spots can be arranged symmetrically, starting at the centre of the buckle and working outwards. Line heating Heating in straight lines is often used to correct angular distortion, for example, in fillet welds. The component is heated along the line of the welded joint but on the opposite side to the weld so the induced stresses will pull the flange flat. To correct distortion in larger complex fabrications it may be necessary to heat whole areas in addition to employing line heating. The pattern aims at shrinking one part of the fabrication to pull the material back into shape.