Methods for eliminating welding residual stress

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Residual stress: After welding pressure vessels, residual stress is generated in the welded areas of the structure. It results from internal stresses generated in areas where the heating is uneven during welding, reaching the material’s yield limit and causing plastic deformation in those local regions. Even after the temperature returns to its original uniform state, these internal stresses remain within the structure, which is why they are called residual stresses. The magnitude and distribution of welding residual stress have a direct adverse effect on phenomena such as fatigue failure and stress corrosion cracking in containers. Research has shown that residual stresses inevitably arise once containers are welded. Although the mechanisms behind their formation have been partially understood, the levels of residual stress vary greatly due to differences in the external dimensions of pressure vessels, welding techniques, welding procedures, and the degree of constraint. Moreover, their distribution is highly complex. Therefore, it is necessary to determine appropriate strategies for eliminating (or reducing) welding-induced residual stresses, so as to ensure quality and economic efficiency during the manufacturing of pressure vessels, as well as safe operation and accident prevention during their service life. The overload method involves applying, under controlled conditions, one (or multiple) external loads to the container that are slightly greater than those applied during its normal operation. The stress generated by this load combines with the residual welding stresses present in certain areas of the container. When the resultant stress is below the material’s yield limit, the material remains in an elastic state, and there is a linear relationship between stress and strain. When the resultant stress reaches the material’s yield limit, plastic deformation occurs in those local areas. As the applied stress increases, the range of stress values at which the yield limit is reached expands, and accordingly the range of plastic deformation also increases, although the stress value itself 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 return to their elastic state simultaneously; the welding residual stresses present within the container are partially eliminated, and the amount of residual stress that is eliminated is equal to the stress value generated by the external load. The overall heat treatment method involves heating the entire welded container to a temperature of 500°C to Ac1 at a certain heating rate, and holding it at that temperature for a period of time. This allows the deformed metal to recrystallize, forming new equiaxed grains; various crystal defects are essentially eliminated. The strength of the metal decreases while its toughness increases, thereby allowing the welding residual stresses to be relieved and eliminated. Pressure vessels generally have a large size, making it impossible to place them in a heating furnace for heat treatment like other small devices or mechanical parts. An internal combustion method can be used, in which an insulating layer is applied to the outer wall of the container, and fuel is injected inside to burn and generate high temperatures for heating; alternatively, an electric heating method can be employed to heat the entire container. The principle of local heat treatment is the same as that of full-scale heat treatment. Currently, infrared plate heaters or crawler-type resistance heaters are commonly used to heat the weld area. Since heating is applied locally, its effect on eliminating residual stresses is inferior to that of full-scale heat treatment; it can only reduce the peak value of internal stresses and make the stress distribution more gradual, but it cannot eliminate these stresses completely. However, local heat treatment can improve the mechanical properties of welded joints, and it is usually applied only to relatively simple welded joints. The temperature difference stretching method is a technique that takes into account the distribution of residual stresses in the weld area; it utilizes the thermal effect of temperature differences to create an opposing stress field, thereby eliminating those residual stresses. The key to the elimination effect of this method lies in the selection of the temperature difference Δt, which is related to the material’s yield limit σs, modulus E, and coefficient of thermal expansion β. As long as the heating zone and Δt are chosen appropriately, so as to avoid plastic deformation that would result in a loss of plastic reserve and to not affect the metal’s microstructure, a good stress-relief effect can be achieved; the degree of stress relief can reach 50%–70%. This method has certain practical value for plate and shell structures with relatively regular welds and not too large thicknesses. The hammering method involves applying rapid and uniform hammer blows to the weld metal, which causes lateral plastic deformation and thus compensates for some of the weld contraction. This relieves the elastic strain resulting from tensile residual stresses in that area, allowing those residual welding stresses to be partially eliminated. The explosion method involves detonating strips placed at and around the weld; by utilizing the interaction between the shock waves generated by the instantaneous explosion in the weld area and the residual stresses, appropriate plastic deformation of the metal is induced, thereby reducing the residual stresses. The explosion method not only effectively eliminates welding residual stresses but also generates a certain amount of compressive stress in the treated area, thereby enhancing the resistance of the welded joint to failure related to tensile stresses; heat treatment is powerless in this regard. The explosion method has unique advantages in eliminating residual stresses in weld repair work during the tank inspection of operational pressure vessels.