During the welding process of welded structural parts, due to uneven heating and cooling, deformation and stress are produced in the welding parts. At the same time, when the molten pool metal changes from liquid to solid during cooling, or from high temperature to low temperature, the weld shrinkage leads to stress and deformation. Welding deformation brings difficulties to assembly, increases manufacturing cost and reduces connection performance and strength, The most serious is to reduce the load-carrying capacity of the welded structure, which will cause accidents, casualties and property losses. According to the experience and lessons of our factory for many years, we will start from improving the quality of welded structural parts. We will talk about our own opinions from the selection of welding materials and the design of welded structure.
1. Reasonable selection of welding materials
The selection of materials is closely related to the structural design. When designing the welded structure, we should first select the materials of the structural parts according to the stress, working conditions and design requirements of the welded structural parts. Materials should be selected according to the following points.
1.4 Try to choose the same material. Welding structural parts are formed by welding several parts or components together. Considering the characteristics of welding, the material section of each part shall be the same as much as possible. In this way, it is easy to purchase materials, select welding methods, formulate welding processes and select welding rods. Of course, for some reasons, other materials must be selected.
1.5 Materials with good welding performance shall be selected as far as possible. When selecting materials for welding structural parts, the strength of materials and requirements for working conditions of welding structural parts shall be considered, such as corrosion resistance, impact resistance, alternating load, etc. When a variety of materials can meet the use requirements at the same time, some of these materials have good welding performance, while some have poor welding performance, some are suitable for this welding method, some are suitable for another welding method. Therefore, when choosing materials, we should choose materials with common welding methods and good welding performance.
1.6 The materials with low price shall be selected as far as possible. When selecting the materials for welding structural parts, besides meeting the basic design requirements, the economy shall also be considered. The materials with low price and rich resources shall be selected for welding structural parts, so as to meet the basic principles of reducing cost and improving product competitiveness.
2. Reasonable design of weldment structure
The design of welded structure is the key of weldment. Whether the structural design is reasonable or not is related to the strength, life of welded structure and whether it can obtain qualified and high-quality welded structure. The structural design of weldment is related to many aspects. Based on our factory experience, we will talk about the following aspects.
2.1 Reduce the number of welds as much as possible. Generally, welded structural parts are assembled and welded by multiple parts. When designing welded structural parts, reduce the number of parts and welds as much as possible. Only in this way can the welding workload and deformation of the welded parts be reduced, and the welding stress be reduced, so as to improve the strength of the welded parts.
2.2 The weld shall be arranged at the place with less stress as far as possible. When the welded structural parts bear the load, the internal stress of the material will inevitably occur. Because of the different shape and stress characteristics of the parts, the stress of the parts may be different in different sections and different parts. If we arrange the welding seam in the place where the stress is less, it will reduce the influence of the welding defect and stress concentration on the damage of the parts, and improve the strength and reliability of the welded structural parts. As shown in the section design of cantilever beam in Figure 2, it is better to change the weld on the left and right sides than on the upper and lower sides.
2.3 Select the appropriate joint type. The performance and quality of welded joint of welded structure are directly related to the performance, safety and reliability of welded structure. Over the years, welding workers have carried out extensive experimental research on welded joints, which has played a great role in improving the performance and reliability of welded structures and expanding the application scope of welded structures. The welding seams of fusion welding mainly include butt weld and fillet weld. The welding joints mainly composed of these two kinds of welds are butt joint, fillet joint, T-shaped (cross) joint, lap joint and plug welding joint. The welding structure should adopt the welding joint type with simple joint, small stress concentration and no damage to structure continuity.
2.4 Minimize the section size of the weld. The welding deformation is closely related to the quantity of deposited metal, so the section size of the weld should be minimized.
If the conditions permit, double U-groove and double V-groove are used to replace V-groove, so that the deposited metal is reduced, and the weld is symmetrical in the thickness direction and has the same shrinkage, which can reduce the welding deformation. The welding deformation caused by fillet weld is large, so the weld leg size of fillet weld should be minimized. When the steel plate is thick, the amount of deposited metal in the groove weld is smaller than that in the fillet weld. When the thickness of the plate is different, the groove shall be cut on the thin plate. As shown in Fig. 3, it is obvious that the weld size in Fig. 3 (c) is much smaller than that in Fig. 3 (a) and (b). The prevention of welding deformation and reduction of welding deformation can be solved from two aspects of design and process. First, measures should be taken from design, in design, the possible welding deformation in manufacturing should be fully estimated, and a reasonable design scheme should be selected to prevent and reduce welding deformation. Because it is much more convenient and effective to solve the problem from the design than to adopt the process measures, it often needs more manpower and energy to rectify the welding deformation after welding, prolongs the production cycle and increases the cost of the product, so we should solve the problem based on the design and process. Production practice shows that welding deformation can be prevented or controlled within the allowable range if a reasonable design scheme and process route are adopted.
2.5 In the design, try to choose symmetrical component section weld position. In this way, the deformation caused by welding can offset each other. As long as the process is correct, the welding deformation is easy to control. According to the structural characteristics of the weldment, the anti deformation method, reasonable assembly and welding sequence method, rigid fixation method, heat balance method, heat dissipation method, etc. are adopted flexibly, and the welding method and welding process parameters are selected reasonably. Only when welding deformation is controlled or reduced, qualified and high-quality welding structural parts can be obtained.
3. Concluding remarks
In order to improve the quality of welded structural parts, the main task is to prevent and eliminate welding stress and deformation. There are many ways to reduce and correct welding deformation, with the deepening of practice, more and better ways will be created, as long as we are good at summing up experience, this is not difficult. But in the practical work, we should pay attention to the correct choice of the material, structure design, welding method and welding process parameters of the weldment according to the structure, shape and use of the weldment, and correctly handle the relationship between the welding stress and deformation. We must give a comprehensive consideration according to the actual situation of the product, so that the product can better meet the technical requirements of the design.
