Deformation of welding parts is a common problem caused by uneven heat input and material shrinkage during welding. To correct deformation, it is necessary to select appropriate methods according to the deformation type, material properties and structural form to restore the dimensional accuracy and performance of the component. The core of correction is to redistribute the stress inside the material through external force or heat energy, offset the internal stress caused by deformation, and return the component to the designed shape.
Mechanical correction is a method of using external force to force the deformed part to produce reverse plastic deformation. It is suitable for materials with good plasticity and welding parts with simple shapes. During operation, presses, straighteners and other equipment are usually used to apply pressure to the deformed part, or the shrinkage area is extended by hammering. For example, for beam-type welding parts with bending deformation, it can be placed on the support point, and concentrated pressure can be applied to the convex part to make the material bend in the opposite direction; for wave deformation after thin plate welding, a wooden hammer or rubber hammer can be used to evenly knock around the convex part to extend the compressed plastic deformation area, thereby flattening the plate surface. The key to mechanical correction is to control the size and point of action of the external force to avoid new cracks caused by stress concentration.
Flame straightening is to use flame to heat the welding parts locally, and produce reverse deformation by shrinkage during cooling to offset the original deformation. This method is suitable for low carbon steel and some low alloy steels. During operation, the heating position needs to be determined according to the deformation direction. Generally, point, line or triangle heating is performed on the shrinkage part or the side of the bulge of the component. For example, when correcting angular deformation, linear heating can be performed on the back of the weld. After heating, the cooling and shrinkage of the material will drive the weld to deform in the opposite direction; when correcting bending deformation, triangular heating is performed on the bending bulge. After cooling, the contraction of this area will cause the component to bend in the opposite direction. Temperature control of flame straightening is crucial. It is usually heated to 600-800℃. At this time, the material is in a thermoplastic state and can produce effective shrinkage stress after cooling. Too high temperature will lead to coarse grains, and too low temperature will not be effective.
The overall annealing treatment in heating straightening is suitable for welding parts with complex deformation or high mechanical properties requirements. By heating the entire component to above the recrystallization temperature, keeping it warm and slowly cooling it, the welding stress can be eliminated and the deformation can be corrected. This method can improve the organization and performance of the material at the same time, but it consumes a lot of energy and is suitable for important structural parts. For some welding parts with severe local deformation that cannot be corrected by external heating, electric heating or induction heating can be used to accurately heat specific areas, combined with external force to assist correction, which can not only ensure the correction effect, but also reduce the impact on the overall structure.
The process measures combined with prevention and correction can reduce the difficulty of deformation correction from the source. Before welding, the reverse deformation method can be used. According to experience or simulation calculation, the workpiece is bent or tilted in the opposite direction of deformation in advance. After welding, the reverse deformation amount and welding deformation are offset; during welding, the rigid fixation method is used to fix the workpiece with a clamp or support to limit the free deformation during welding, but attention should be paid to the distribution of the fixing force to avoid new deformation caused by the release of residual stress after the fixation is released. Reasonable arrangement of the welding sequence can also effectively reduce deformation, such as symmetrical welding, segmented de-welding and other methods, which can make the welding heat input evenly distributed and reduce stress concentration.
Different materials and structural forms of welded parts require different correction strategies. For high-hardness or brittle materials, mechanical correction is prone to cracking, so flame correction or low-temperature annealing should be used first, and both the heating and cooling rates should be controlled slowly; for thin-walled pipes or precision welding parts, the deformation is small but the precision requirement is high. Local heating combined with manual fine-tuning can be used to avoid secondary damage caused by excessive external force; for large and complex structures, multiple correction methods may be required in combination, first correcting the main deformation by mechanical methods, then fine-tuning with flame correction, and finally stabilizing the size through overall stress relief.
The corrected welding parts need to be re-inspected for quality to confirm that the deformation meets the design requirements and to check whether new defects are caused by the correction. Whether it is the hammer marks of mechanical correction or the local color changes of flame correction, it is necessary to evaluate whether they affect the mechanical properties and appearance quality of the components. For key load-bearing components, non-destructive testing should also be carried out after correction to ensure that there are no microcracks inside the material caused by the correction process. The essence of welding parts deformation correction is to readjust the stress state of the material. Only by combining the selection of correction methods, the control of process parameters and the actual working conditions of the components can the reliability of the overall performance be guaranteed while restoring the shape.
