Residual stress inevitably builds up in welded parts during the manufacturing process. This stress originates from uneven heating and cooling during welding, which creates a self-equilibrium internal force field within the material. If residual stress is not effectively controlled, it can cause deformation and crack propagation in welded parts, potentially reducing the structure's load-bearing capacity and fatigue life. Therefore, reducing residual stress in welded parts through process optimization is a key step in improving welding quality and product reliability.
Optimizing the welding sequence and direction is a fundamental measure for reducing residual stress. During welding, welds with large shrinkage, such as longitudinal butt welds, should be prioritized, followed by short transverse welds. This allows the welds to shrink freely during cooling and reduces restraint stress. For intersecting welds, a reasonable welding sequence is essential to avoid cracking at the weld intersections due to stress concentration. For example, when welding T-shaped welds, the non-intersecting areas should be welded first, followed by the intersections. The weld metal at the intersection should be removed to ensure free transverse shrinkage, thereby reducing residual stress.
Reducing the local rigidity of the welded structure is an effective means of controlling residual stress. The greater the structural rigidity, the higher the restraint stress generated during welding. Therefore, when welding closed welds or rigid structures, a reverse deformation method can be used. This pre-set reverse deformation offsets shrinkage during welding. Furthermore, creating a relaxation groove near the weld can reduce rigidity constraints in the weld area, allowing weld stress to be released. These measures can significantly reduce residual stress levels in welded parts.
Preheating and post-heat treatment are key processes for regulating the welding temperature field and reducing residual stress. Preheating before welding reduces the temperature difference between the weld zone and the overall structure, allowing the weld to cool more evenly, thereby reducing thermal stress. The preheating temperature should be appropriately selected based on the material properties and structural rigidity to avoid stress concentration caused by excessive temperature differences. Post-weld heat treatment relaxes residual stresses by heating the welded parts to an appropriate temperature, holding the temperature for a period of time, and then slowly cooling them. For most structural steels, post-weld heat treatment typically involves annealing, which effectively eliminates weld residual stresses and improves structural stability.
Optimizing welding process parameters is crucial for reducing residual stress. By adjusting welding current, voltage, and welding speed, the heat input can be controlled, minimizing the extent and deformation of the heat-affected zone. Lower welding heat input can reduce thermal stress and the tendency of the weld metal to overheat, thereby suppressing the generation of residual stress. Furthermore, when using multi-layer, multi-pass welding, the welding sequence and direction of each pass must be carefully planned to avoid stress accumulation and ensure uniform weld stress distribution.
Mechanical methods and vibration aging techniques are effective supplementary methods for eliminating residual stress after welding. Peening welds, through gentle percussion with a small round hammer, induce plastic tensile deformation in the weld metal during the weld cooling process, offsetting contraction stresses and reducing internal stresses. Vibration aging, by applying alternating stresses, induces subtle plastic deformation in the welded parts, thereby relaxing residual stresses. Both methods are simple to operate and low-cost, making them suitable for both on-site construction and mass production.
The application of new welding technologies offers new approaches to reducing residual stress. High-energy beam welding methods such as laser welding and electron beam welding offer concentrated heat input and a small heat-affected zone, significantly reducing weld residual stress. Furthermore, friction stir welding achieves material joining through mechanical stirring and frictional heat generation, avoiding the thermal stress issues associated with fusion welding. Residual stress levels are significantly lower than with traditional arc welding. The widespread application of these technologies has opened up new avenues for improving the quality of welded parts.
