What is welding stress?
During the welding process, the increase in local temperature causes thermal expansion in the weld area, but this expansion is restricted by the surrounding cooler areas and cannot stretch freely. Subsequently, during the cooling process, these restricted areas shrink and deform, resulting in residual stress in and around the weld. This residual stress is formed due to the mechanical incompatibility between the weld area and the area not affected by heat. This phenomenon is usually explained by "inherent strain" to help analyze and predict stress and deformation after welding.
Effects of stress
First, welding stress often causes deformation of the structure.
Due to the high temperature in the welding area, the thick plate is heated unevenly, and deformations such as bending and twisting are easy to occur during cooling. This deformation not only affects the dimensional accuracy and appearance of the product, but also increases the difficulty of subsequent assembly, because irregular shapes often require additional correction processes.
Secondly, the presence of welding stress may lead to a reduction in material strength.
In the weld and the area near the weld, welding stress will affect the internal structure of the material, weakening its bearing capacity. This problem is particularly evident in structures that are subjected to dynamic loads or in harsh environments, because after the material strength decreases, cracks are more likely to expand, and even cause overall failure of the structure. This not only affects the durability of the structure, but also brings greater safety hazards.
In addition, welding stress may also cause cracks in the weld joint area.
Due to high stress concentration during thick plate welding, hot cracks and cold cracks are prone to occur near the weld. Hot cracks usually occur during or immediately after welding, mainly due to rapid cooling of the metal in the welding area, while cold cracks may occur some time after welding is completed. These cracks will significantly weaken the integrity and reliability of the structure, increase the risk of failure, and may even cause sudden fractures in key areas.
How to eliminate welding stress of welded parts
Use hammering
It is to control residual stress. In this case, welding residual stress is generated when the weld cools and shrinks. Therefore, using a small hammer to hit the weld and the surrounding surface during welding can expand the metal and reduce the generated stress.
Use vibration
It is to eliminate the generated stress. After the load of the component reaches a certain value and after several cycles of loading, the stress in the structure can be reduced. And this method requires relatively simple costs, short time, and relatively low costs.
Heat the stress reduction zone
It is to eliminate the generated stress. That is, during the welding process, by hindering the heating of the expansion position of the welding zone, it can be expanded and expanded to achieve the purpose of eliminating welding stress. However, the right heating position must be selected.
Reduce the local stiffness of the component
It is to control welding stress. During welding, the stiffness of the welding position can be appropriately reduced through professional process measures to eliminate the generated stress.
Preheating method
It is to achieve the purpose of reducing stress. The temperature difference on the component is relatively large, resulting in greater stress, so preheating can be performed to control the temperature difference, which means reducing the stress of welding.
High temperature tempering
It is to reduce or eliminate stress. The residual stress of the component can be the same as the yield value of the material at most, and the yield value of the metal will decrease with the increase of temperature, that is, increasing the temperature can reduce the stress.
