Welding stress and distortion is the stress generated in welded components due to welding. It includes internal stresses generated during the welding process and changes in shape and size caused by the welding heat process. The non-uniform temperature field during welding, and the resulting local plastic deformation and microstructures with different specific volumes, are the fundamental causes of welding stress and deformation. When the non-uniform temperature field caused by welding has not yet disappeared, this stress and deformation in the weldment is called transient welding stress and deformation; the stress and deformation after the welding temperature field disappears are called residual welding stress and deformation. Under the condition of no external force, welding stress is balanced within the weldment. Welding stress and deformation can affect the function and appearance of the weldment under certain conditions.
Because welding stress has many adverse effects on the quality of welded structures, how to reduce and eliminate residual welding stress has become an important issue in the field of welding.
Currently, various methods for eliminating and reducing residual welding stress have been proposed, including heat treatment, hammering, vibration, shot blasting, mechanical tensile testing, and ultrasonic impact testing, sometimes in combination.
This involves heating the container to 550-650 degrees Celsius, not exceeding the material’s phase transformation point or the steel’s tempering temperature, holding it at that temperature for a period, and then slowly cooling it.
As the steel’s temperature rises, its yield strength decreases, causing the original elastic strain to become plastic strain, thus relaxing the stress.
The quality of stress relief heat treatment hinges on controlling process parameters such as heating temperature, holding time, and temperature uniformity.
The higher the heat treatment temperature and the longer the holding time, the more thorough the stress relief. Studies have shown that stress relief heat treatment can generally eliminate 60% to 80% or more of the stress in a workpiece.
Currently, this technology mainly includes three methods: in-furnace overall heat treatment, out-of-furnace overall heat treatment, and localized heat treatment.
This method involves placing a vibrator on the container or welding repair location, using a control system to control the motor speed, and repeatedly applying periodic loads to the workpiece through the vibrator, mechanically forcing the workpiece to resonate within its resonance range.
When the material’s yield limit condition is met, minute plastic deformation occurs at the peak residual stress points in the workpiece, reducing the peak residual stress and redistributing it, thus releasing stress.
Compared to stress-relieving heat treatment, vibration aging requires less investment, reduces energy consumption by 90%, and shortens the processing time from over 10 hours to less than 1 hour. Furthermore, it produces no oxidation, maintains stable dimensional accuracy, and its stress-relieving effect reaches or approaches that of heat treatment.
Domestic research has shown that vibration aging can eliminate 50%–70% of stress. However, current vibration aging technology still has relatively low equipment reliability and automation levels, and there is a lack of necessary verification regarding whether it can cause other defects in the material, such as fatigue damage.
This method uses the high temperature and immense pressure generated by the explosion of a small amount of explosive, calculated and arranged appropriately, to treat the workpiece. In the weld area close to the explosive, the combined explosive impact load and residual stress exceed the material’s dynamic yield strength, resulting in plastic deformation and the release of the original residual stress.
Meanwhile, after 2-3 reflections, or in other parts of the pressure vessel, the peak value of the stress wave, when superimposed with the residual stress, is less than the dynamic yield value of the material. However, due to the stress-relieving effect of vibration, the residual stress in various parts of the pressure vessel can be reduced to varying degrees.
The explosive method is low-cost, short-term, and requires almost no equipment or site conditions. From a quality perspective, it can effectively eliminate welding residual stress and create a certain compressive stress in the treated area.
Due to a lack of necessary in-depth research, although this technology has certain advantages compared to stress-relieving heat treatment, its application and control accuracy and reliability are not yet fully convincing, and therefore it has not been widely adopted in the entire pressure vessel manufacturing industry.
This refers to applying one or more external loads slightly larger than the vessel’s operating condition under controlled conditions. The stress generated by this load is superimposed with the welding residual stress existing locally in the vessel. When the combined stress reaches the material’s yield limit, plastic deformation occurs in the local area.
As the applied stress increases, the range within which the combined stress reaches the yield limit increases, and the range of plastic deformation should also increase accordingly. However, the stress value does not increase or increases only slightly.
Since the container itself is continuous, during the removal of the external load, both the yield deformation region and the elastic deformation region simultaneously recover in an elastic state. The residual welding stress existing inside the container is thus released and partially eliminated.
This technique is generally performed through hydrostatic testing, which is particularly significant for welded containers that require post-weld hydraulic testing.
Because the test pressure on the container during hydrostatic testing is greater than the container’s working pressure (e.g., for steel pressure vessels, the test pressure is 1.25 times the working pressure), the container material undergoes an expansion equivalent to mechanical stretching during the hydrostatic test, thereby eliminating some of the residual welding stress.
Test results show that, when the container material is selected, the residual stress elimination effect is directly proportional to the hydrostatic test pressure. Therefore, the hydrostatic test pressure can be appropriately increased to facilitate the elimination of residual stress.
Since hydrostatic testing is an essential step in pressure vessel manufacturing, this method requires no additional equipment investment, resulting in a short construction period, low cost, and good economic benefits.
The hammering method is suitable for long welds and weld overlays. When the weld metal cools, stress is generated due to the obstruction of weld shrinkage. While the weld and weld overlay are still red-hot, the weld area is lightly tapped with a hammer. The weld metal undergoes lateral plastic stretching under rapid and uniform hammering, compensating for some of the weld shrinkage. This relaxes the elastic strain of the tensile residual stress in that area, thus partially eliminating the welding residual stress.
Hammering should be performed at a relatively high temperature, but should avoid the blue brittleness range of the material. In multi-layer welding, the first and last weld layers do not need to be hammered, but each subsequent layer must be hammered.
The first layer is not hammered to avoid root cracks. The last weld layer should be welded thinner to eliminate work hardening caused by hammering.
In principle, the hammering method can inhibit stress corrosion cracking to some extent and is widely used in the manufacture of pressure vessels.
However, due to the lack of quantifiable indicators and strict operating procedures in practice, it is greatly affected by human factors. Furthermore, insufficient comparative verification has prevented it from being adopted by current standards and thus cannot be used as a final stress-relieving treatment. Currently, it is mostly used as a stress relaxation method during welding and can also be applied to the welding of austenitic stainless steel that is difficult to heat treat.
Post time: Feb-22-2026