1. Causes of welding stress and deformation
During the welding process of non-standard welding parts, the root cause of welding stress and deformation is the uneven local heating. When the welding heat source acts on the weldment, the weld area heats up and expands rapidly, while the temperature of the surrounding parent material is relatively low, which limits the free expansion of the weld, thereby generating compressive stress. As the welding process proceeds, the weld cools and shrinks, and is constrained by the surrounding parent material, thereby generating tensile stress. This uneven stress distribution causes deformation of the weldment, such as longitudinal shrinkage deformation, transverse shrinkage deformation, angular deformation, bending deformation, and twisting deformation. For example, for non-standard welding parts with long welds, the longitudinal shrinkage force may shorten the weldment along the weld direction; when welding thin plates, angular deformation is prone to occur, which changes the flatness of the weldment.
2. Welding process optimization measures
Optimizing the welding process is an important means to control welding stress and deformation. First, reasonably select the welding method, such as using energy-concentrated welding methods, such as laser welding and electron beam welding, which can reduce the heat-affected zone and reduce welding stress and deformation. Secondly, adjust the welding parameters, including welding current, voltage, welding speed, etc. Appropriately reducing the welding current and increasing the welding speed can reduce heat input, thereby reducing the degree of deformation. Furthermore, the use of a suitable welding sequence is also extremely critical. For non-standard welding parts with symmetrical structures, symmetrical welding can offset the welding stresses; for complex structures, welds with large shrinkage should be welded first, and welds with small shrinkage should be welded later, so that the welds can shrink freely and reduce residual stress and deformation. For example, when welding large non-standard frame structures, welding gradually from the middle to both ends can effectively control the overall deformation.
3. Pre-deformation and rigid fixation method
The pre-deformation method is to pre-deform the weldment in the opposite direction of the welding deformation before welding to offset the deformation during the welding process. For example, before welding the butt weld of thin plates, the weldment is pre-processed into a certain anti-deformation angle, and it can be restored to a relatively flat state after welding. The rigid fixation method is to fix the weldment by using clamps, positioning welding and other means to increase the rigidity of the weldment and limit its deformation. However, this method may cause welding stress to increase. In some cases, it is necessary to remove the fixation in time after welding to allow the weldment to have a certain amount of free shrinkage space and release some stress, otherwise cracks and other defects may occur. For example, when welding small non-standard thin plates, using a strong clamp to fix it on the workbench can effectively control the angular deformation during welding.
4. Post-weld treatment measures
Post-weld treatment is also indispensable for eliminating welding stress and correcting deformation. Common post-weld heat treatment methods include annealing and tempering. By heating the weldment to a certain temperature and keeping it warm, and then slowly cooling it, the welding stress can be relaxed. For example, for some non-standard welding parts made of high-strength steel, stress relief annealing after welding can significantly improve the dimensional stability and fatigue resistance of the weldment. For welded parts that have already deformed, mechanical correction methods can be used, such as using presses, roller beds and other equipment for correction; flame correction methods can also be used to use flames to heat specific parts of the weldment to cause plastic deformation, thereby offsetting welding deformation, but during flame correction, attention should be paid to controlling the heating temperature and range to avoid adverse effects on the material properties of the weldment.
