How to relieve welding stress?
Welding stress and distortion arise from the welding process as internal stresses and changes in the shape and size of the welded components. Uneven temperature distribution during welding, local plastic deformation, and variations in volume are the primary causes of these stresses. Initially, these stresses and distortions are transient, appearing while the temperature differences still exist. Once the temperature normalizes, these stresses and distortions become residual.
Welding stresses can significantly affect the quality and functionality of welded structures. Therefore, reducing and eliminating residual welding stresses is a critical topic in the welding industry. Multiple methods have been developed, including heat treatment, vibration, hammering, shot peening, mechanical stretching, and ultrasonic impact treatment. Often, a combination of these methods is used.

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1.Stress Relief Heat Treatment
This method involves heating the component to a temperature between 550°C and 650°C, but below the material’s phase transformation point or tempering temperature. The component is then held at this temperature for some time before being slowly cooled. Heating the steel reduces its yield strength, turning the elastic strain into plastic strain, thereby relaxing the stress. The effectiveness of stress relief heat treatment depends on controlling parameters such as heating temperature, holding time, and temperature uniformity. Studies have shown that this method can eliminate 60-80% of residual stresses.
2.Vibration Stress Relief
This technique places a vibration unit on the workpiece or repair area, using a control system to manage the motor speed. The unit applies periodic loads to the workpiece, forcing it to resonate within its frequency range. The resulting microplastic deformation reduces peak residual stress and redistributes it uniformly. Compared to heat treatment, vibration stress relief is cost-effective, reduces energy consumption by 90%, and shortens processing time from over 10 hours to under 1 hour. It also offers consistent size accuracy. Domestic research indicates that vibration stress relief can reduce stress by 50-70%. However, concerns remain about equipment reliability and the potential for material fatigue.
3.Explosive Method
This method uses a small amount of explosives to generate high temperatures and intense pressure. The shock load from the explosion combines with residual stress, creating plastic deformation and releasing stress. The explosion-induced stress waves reduce residual stress levels throughout the component. Despite its low cost and short processing time, this method has not gained wide acceptance due to the lack of detailed studies and concerns about its accuracy and reliability.
4.Hydraulic Overload Method
This approach applies an external load, slightly higher than operational stress levels, under controlled conditions. The combined stress exceeds the yield strength, causing plastic deformation in localized areas, which helps release some residual stress when the external load is removed. This technique is typically executed through hydrostatic testing, which is often required for welded pressure vessels. Hydrostatic testing creates expansion similar to mechanical stretching, helping to alleviate residual stresses.
5.Hammering Method
This method is effective for long welds and overlay welds. As the weld cools, light hammering can be applied while the metal is still hot, causing the weld to expand laterally and compensating for shrinkage. This reduces residual tensile stress in the area. However, hammering should be done at high temperatures, avoiding the material’s blue brittleness range. While hammering is commonly used during welding for stress relief, it has not been standardized due to a lack of quantitative indicators and guidelines.
These methods, while varying in technique and application, provide effective ways to manage and reduce welding stress, ensuring the integrity and longevity of welded structures.