How to reduce welding residual stress?

Thread Content

Welding stress is an important factor that causes various welding cracks in welded joints, while the residual stress remaining in the welded parts after welding affects the service life of the structure. Therefore, to improve the welding quality of the entire component, it is necessary to take measures to control and reduce welding residual stresses. To reduce welding residual stresses, the following process measures should be taken: 1. Adopt a reasonable welding sequence and direction. (1) For welds on the welding plane, it is necessary to ensure that longitudinal and transverse welds (especially the transverse ones) can contract freely. For butt welds, the welding direction should point toward the free end. (2) Weld the joints with greater contraction first. If there are both butt joints and fillet joints in the structure, the butt joints with greater contraction should be welded first, as shown in Figure 7-27. (3) Weld the transverse short welds first. (4) Welds that are subject to high stress during operation should be welded first, so as to ensure a proper distribution of internal stresses. (5) When welding cross-butted joints, a welding sequence must be employed to ensure that defects do not occur at the intersection points. When welding T-joints and cross joints, the weld laid first at the intersection point should be cleaned thoroughly, and welding should be carried out in the correct sequence. This allows for more free lateral contraction of the T-joints and cross joints, helping to prevent cracks from forming at the junctions of the welds. 2. Reduce the local stiffness of the welded structure. As the structural stiffness increases, welding stresses also increase. Therefore, reducing the local stiffness of the welded components of the structure helps to decrease stress. When welding closed joints or joints with high rigidity, the counter-deformation method can be employed to reduce the local rigidity of the structure, or relief grooves can be created near the joint to decrease the local rigidity at the welded area. 3. Heating the “stress-reduction zone”: During welding, by heating those areas that prevent the welding zone from expanding and contracting freely, so that they expand and contract simultaneously with the welding zone, welding stress can be reduced. This method is known as the heating of the “stress-reduction zone” method, or the synchronous contraction method. The area that is heated is called the “stress-relief zone”. 4. Use the “cold welding” method to reduce welding residual stresses. The principle of “cold welding” is to ensure as uniform a temperature distribution as possible in the welded structure; it is necessary to keep the local temperature at the weld as low as possible, while also ensuring that the volume of this area within the overall welded structure is as small as possible. This approach of minimizing temperature differences within the structure can effectively reduce welding residual stresses and lower the tendency for thermal stress cracks. During cold welding, a weld rod with a smaller diameter and a lower welding current are used, and only a short section of weld is created at a time. For example, when patch welding cast iron, only 10–40 mm is welded for each section. When welding components with high rigidity, only one or half a weld rod should be used at a time; after each weld, it is necessary to wait until the area has cooled down enough to no longer be hot to proceed with the next weld. 5. Hammering the weld: During the cooling process of each weld, a small hammer with a round head is used to strike the weld, causing the weld metal to undergo plastic tensile deformation and spread outward, thereby counteracting the contraction of the weld and reducing internal stresses. The hammering should be done evenly and moderately, to avoid excessive hammering that results in too deep marks. 6. Preheating before welding: The purpose of preheating before welding is to reduce the temperature gradient in the welding area and the structure, thereby decreasing the degree of constraint and lowering the welding internal stresses. Preheating of the welded parts can be done overall, or locally in the welding area. Methods of preheating include overall heating inside the furnace, local far-infrared heating, local power-frequency heating, flame heating, etc.