4.6 Water-fired bending plate and induction heating forming
Water-fired curved plates are generally heated by flames, usually with natural cooling (air cooling), front tracking water cooling and back tracking water cooling. The author has used three-dimensional thermoelastic finite element method to analyze the water-fired curved plate. The flame heat source is input as a surface heat flow, and the heat flux density is assumed to be Gaussian. The water-cooled zone and the air-cooled zone are treated differently in the finite element analysis. The example calculations yield better results. However, the three-dimensional thermoelastic-plastic finite element method requires a large-capacity computer and a long technical time for large and complex components, which is uneconomical. Therefore, the use of the inherent strain method instead of the great advantage. In addition, plate and shell units are widely used in finite element analysis. In order to facilitate precise control of the forming process, induction heating has been considered a very suitable forming method. Some foreign companies, such as Ishikawajima Heavy Industries of Japan, have developed a computer-controlled automatic induction heating bending machine based on the inherent strain prediction and control induction heating forming, which has been successfully applied in actual production. .
4.7 Residual stress of ceramic/metal joints and their relaxation
The residual stress state caused by diffusion welding of hot pressed silicon nitride (Si3N4) and K-500 alloy was analyzed by thermoelastic finite element method. The three-dimensional residual stress of the square test piece and the two-dimensional axisymmetric residual stress of the round bar specimen were compared, and the effects of different transition layers were discussed. The results show that the axial maximum tensile stress exists in the outer surface of the ceramic near the joint interface, and its position is just the same as the cracking point. It can be used as an index to evaluate the influence of residual stress and select the optimal transition layer. The residual stress level of the square rod specimen is higher than that of the round rod specimen, and the maximum tensile stress is about 1.5 times that of the latter (see Figure 15). In order to obtain a ceramic metal diffusion welded joint with both reduced residual stress and high interfacial bonding ability, a composite transition layer composed of soft metal, low expansion metal and active metal is recommended. Tests show that the Si3N4/K-500 diffusion welded joints have a flexural strength of more than 216 MPa at room temperature and high temperature (800 °C) when combined with the best optimal composite transition layer.
Figure 15 Residual stress of ceramic/metal connections
4.8 Feasibility analysis of post-weld heat treatment of Baosteel's 300-ton converter
Baosteel's 300-ton converter has a diameter of D=8500mm and a wall thickness of t=80mm. The upper and lower sections are connected by two large ring welds during final assembly. Since the furnace body is too large, it has been considered to use a post-weld stress relief treatment and to determine the proper width of the heating belt. A viscoelastic-plastic axisymmetric finite element calculation model was used for the analysis. As shown in Figures 16 and 17, the calculations show that the circumferential axial stress of the inner surface is lower only when the heating width is large (B = 2400 mm). Since the diameter and wall thickness of the converter body are large, the heating width required for the partial post-weld heat treatment is large. Obviously, this is problematic in terms of economy and achievability and is therefore unsuitable. According to the working conditions of the furnace body and the characteristics of the material, if the performance of the welded joint meets the requirements, a stress relieving treatment process without post-weld heat treatment may be considered. At the same time, it is recommended to use hammer welds, post-weld welds and other process measures that are beneficial to reduce and alleviate residual stress and post-weld dehydrogenation treatment to reduce the tendency of weld cracks. Through the feasibility analysis, the adverse consequences of the partial post-weld heat treatment can be avoided, and the manpower, material resources and capital are greatly saved.
Figure 16 The circumferential stress distribution of the inner surface Figure 17 The axial stress distribution of the inner surface
4.9 Application in high technology
Ceramic metal diffusion welded joints with reduced residual stress and high interfacial bonding ability have been obtained by numerical simulation of welding mechanics [9]. Numerical simulation also plays an important role in the study of new technology for non-indented stainless steel energy storage spot welding. Our ongoing tasks include numerical simulation of welding mechanics in spacecraft and numerical simulation of micro-connections in medical devices.
5. Conclusion
With the rapid development of computer technology, the numerical simulation technology of welding mechanics has also been greatly developed and raised to a new stage. The existing numerical research results have enabled us to further understand the nature and laws of complex welding mechanics phenomena, bringing new ideas and new methods to solve and control these problems, and have a lot in engineering. Successful application examples. However, it should be noted that these studies are preliminary and far from meeting the needs of scientific research and practical engineering. It is believed that with the deepening of people's understanding of welding processes and phenomena and the high development of computer technology, welding virtual engineering including prediction theory and control technology of welding mechanics will also become more and more developed and have a broad Application prospects.
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