【Weekly Topic】【Static Equipment Discussion Forum...Question 1】 December 8-16, 2018...Discussion on welding deformation – HaiChuan Coin rewards
Thread Content
This post was last edited by kareale88 on 2018-12-9 at 13:06: bg2.png【Weekly Topic】【Static Equipment Discussion Forum...Question 1】 December 8–16, 2018...Discussions on welding deformation. Do not repeat others’ posts, do not post content unrelated to the topic, and do not reply with meaningless messages. Reward rules: HaiChuan coins ranging from 1 to 10 points are awarded based on quality. Multiple people can participate in scoring (wealth, charm, HaiChuan coins). The management attaches great importance to this, and there will be generous rewards – so please participate actively. The main hazards of welding deformation are as follows: 1. It affects the accuracy of the structure’s shape and dimensions as well as its aesthetic appearance. 2. It reduces the quality of assembly of the entire structure. 3. Correcting such deformation reduces productivity and increases manufacturing costs. 4. It lowers the structural load-bearing capacity. Welding deformation refers to the changes in the shape and dimensions of the workpiece that occur during welding due to the uneven temperature distribution acting on it. Welding deformation is inevitable, but by taking appropriate measures in terms of design and welding processes, it can be prevented or reduced, thereby avoiding or minimizing the amount of work required to correct the deformation after welding. Welding deformation includes longitudinal and transverse contraction, angular deformation, bending deformation, torsional deformation, wave deformation, etc. In other words, welding deformation has a significant impact on the installation accuracy of structures, and excessive deformation will markedly reduce the structural load-bearing capacity. In welding, the metal in the high-temperature zone adjacent to the weld is constrained by the surrounding metal, resulting in uneven compressive plastic deformation. When cooling after welding, this compressed and deformed metal is also constrained by the surrounding cold metal and cannot contract freely, which leads to tensile stress and deformation. Harm of deformation: Unacceptable dimensions after welding, impact on aesthetics, and reduced load-bearing capacity. Harm of stress: Reduced ability of the structure to bear static loads, decreased structural stiffness, reduced dimensional stability of components, and increased tendency to stress corrosion cracking. Please discuss the following topics by referring to cases encountered in production (examples, illustrations): 1. The reasons for welding deformation; 2. How to correct and repair such deformation when it occurs; 3. Measures to reduce or prevent welding deformation. Case format: Cause analysis------------------------------------------------- Repair solution-------------------------------------------------- Preventive measures--------------------------------------------------- Illustrations---------------------------------------------------------- Try to keep the presentation neat and clear, and pleasing to the eye.
I once made a screw for a boiler slag extractor, and it deformed while being welded.
Discussion on the deformation caused by single-sided cladding of tube sheets: Conditions and causes: In many heat exchangers, due to the difference in the media in the tube side and shell side, single-sided cladding of tube sheets occurs. Since tube sheet cladding is a welding process that covers a large area over an extended period of time, even tube sheets with high rigidity can experience deformation. Fix solution: None available, seeking advice. Based on past engineering experience, this situation is taken into account during the process measures phase, with methods such as using fixtures for reinforcement or carrying out intermittent operations to reduce deformation. In the case of double tube sheets, the deformation is counteracted by fixing both tube sheets and performing surfacing welding simultaneously.
bg4.png: In submerged arc welding, failure to control welding deformation. Causes and consequences: During welding, it is not sufficient to pay attention to controlling deformation by considering factors such as the welding sequence, personnel arrangement, groove design, selection of welding parameters, and operating methods; as a result, significant deformation occurs after submerged arc welding, making correction difficult and increasing costs. This is especially true for thick plates and large workpieces, where correction is particularly challenging. Mechanical correction can lead to cracks or layered tearing, while flame correction is costly, and improper use can cause the workpiece to overheat. For workpieces with high precision requirements, if no effective measures are taken to control deformation, the installation dimensions will not meet the required standards, which can even lead to rework or waste of the products. Control measures: Adopt a reasonable welding sequence and select appropriate submerged arc welding specifications and operating methods; in addition, use counter-deformation and rigid fixation measures.
This post was last edited by xkrsyh on 2018-12-14 at 11:22. Causes of welding deformation: The main causes of welding deformation are temperature and residual stress; the fundamental reason is uneven heating of the welded parts, while other factors include the contraction of the weld metal, changes in the microstructure, and differences in the stiffness of the welded parts. The position of the welds in the welded structure, the sequence of welding operations, the welding method, the welding current, and the welding direction also have a certain influence on welding stresses and deformations. Methods for repairing welding deformation (common methods for eliminating residual stress): 1. Heat treatment, which is divided into overall heat treatment and local heat treatment. It takes advantage of the decrease in the yield limit of materials at high temperatures, allowing plastic flow to occur in areas with high internal stress and thereby reducing that stress. 2. The mechanical stretching method involves applying certain tensile stresses to the component in various ways, thereby inducing tensile plastic deformation in the weld and its surrounding areas to reduce welding residual stresses. 3. The temperature difference stretching method involves using a locally heated area to create a region of plastic deformation due to temperature difference stretching, thereby reducing welding residual stresses. 4. Hammering the weld involves gently tapping the weld and its surrounding area with a small hammer after welding, in order to release the stress between the metal grains. 5. The vibration method utilizes the cyclic stresses generated by resonance in the structure caused by a vibrator to reduce welding residual stresses. Measures to reduce or prevent welding deformation: 1. Conduct proper welding structure design: Arrange the weld locations reasonably to avoid excessive concentration; try to use as few welds as possible and keep their lengths short, while also minimizing the cross-sectional size of the welds. 2. Adopt reasonable assembly process measures: use the method of reserving shrinkage allowance, the counter-deformation method, or the rigid fixation method, and select an appropriate assembly sequence. 3. Adopt reasonable welding process measures: Select the appropriate welding method to minimize the input of welding energy, and choose the welding sequence and direction wisely. https://gss0.baidu.com/9vo3dSag_xI4khGko9WTAnF6hhy/zhidao/pic/item/5243fbf2b2119313b93e96ca65380cd791238d4e.jpg
It is believed that many manufacturers have encountered deformation during the butt welding of the heads and cylinders of thin-walled stainless steel containers. As the welding process progresses, the amount of misalignment increases; the greatest misalignment is found at the areas that are welded last. In such cases, the only option left is to reshape the parts by hammering, which results in an unsightly appearance. Sometimes heating is required, which affects the material’s corrosion resistance. In fact, for this situation, the following needs to be done well: 1. The spot weld joints at the mating points should be as long as possible ； 2. Welding process heat input control point ； 3. Symmetric welding.
Case 1: In 2017, an LNG storage tank was constructed with a diameter of 42 meters; it had 120 anchor straps, 240 anchor strap gussets, and the anchor straps were relatively dense. The thickness of the bottom ring wall plate is 16 mm, while that of the anchor strap gasket is 18 mm. The designed weld leg height for the anchor strap gasket is 12 mm. Upon completion of welding, multiple indentations appeared on the bottom ring wall plates, with the largest one being 80 mm deep. It severely affects the appearance and also poses potential safety hazards. Finally, steel profile supports were welded to the inner wall of the outer tank, and jacks were used to push them back; the results were average. Cause analysis: Due to the tight deadline, more welders were employed. Excessive welding stress causes the weld to contract, leading to deformation of the wall panel. Solution: Use 4 welders, arranged symmetrically. Use low-current welding; after completing the root pass, change position and weld again, leaving at least one anchor strap between them, and repeat this process. (Another tank in 2017 was welded using this method, and the shape obtained after welding was excellent.) Case 2: Welding deformation of arch beams. The arch beam steel for large storage tanks is basically formed by cutting steel plates on-site and welding them together. In a project from 2014, after the H-shaped steel was welded, its two flange plates were not in the same vertical plane; in other words, the angle between the web and the flange plates was not 90°. Cause analysis: Interrupted welding was not used, resulting in high welding line energy. And the symmetrical side is welded only after the unilateral welding is completed. Solution: Both the base layer and the top layer are welded using symmetric intermittent welding, with a welding length of around 500 mm.