1. During the manufacturing process of pressure vessels, the following problems will be brought about: cold work hardening caused by excessive cold rolling, cold straightening and other cold working. Changes in the organization and properties of the weld zone caused by welding. Residual stress generated by welding and the generation and development of stress corrosion cracks caused by it. When welding pressure vessels, when a sharp temperature gradient with a temperature difference of more than 100 degrees is generated in the adjacent areas of the parent material, uneven plastic strain is caused in ferritic steel or other equivalent materials, and in the subsequent cooling process, a residual stress field with a peak stress reaching the yield point will be generated. In addition, the uneven plastic strain in the manufacture of pressure vessels leads to residual strain in elastic-plastic materials, and the residual strain can be mechanical (mainly cold rolling, cold straightening and other cold working) thermal (mainly generated by welding process), or both, that is, thermomechanical reasons. Therefore, a residual elastic strain field will be left in the final product after the pressure vessel is processed, and it will be subjected to corresponding elastic residual stress. The existence of residual stress will affect the performance of the pressure vessel. In order to eliminate the peak strain in the welding area and achieve the goal of uniform distribution of internal strain, various methods can be adopted, such as mechanical vibration method, post-weld heating method, etc. However, since many potential problems in pressure vessels mainly come from metallurgical damage in the weld area, the use of mechanical methods to reduce internal strain is no longer sufficient to prevent many problems that may occur in the future operation process. In addition, the hydrogen embrittlement phenomenon of metals has attracted more attention. After hydrogen enters steel, the mechanical properties will deteriorate significantly. The strength and plasticity are significantly reduced, and the hydrogen dissolved in the metal lattice causes brittle failure of steel during slow deformation. The hydrogen in the metal material can be absorbed during the production process of the metal material, such as the hydrogen absorbed by the liquid metal during welding and retained in the weld, or it can be the hydrogen absorbed by the material in the hydrogen environment. For the hydrogen absorbed in the weld, the more effective elimination method is to perform post-weld heat treatment, which can not only relax and alleviate the welding residual stress, improve the weld heat affected zone that is hardened and embrittled due to welding, and improve the ductility and fracture toughness of the weld metal, but also diffuse and escape harmful gases such as hydrogen in and near the welding area. There are two types of heat treatment methods used in pressure vessels: one is heat treatment to improve mechanical properties, and the other is post-weld heat treatment (PWHT). In a broad sense, post-weld heat treatment is the heat treatment of the welding area or welded components after the workpiece is welded. The contents include stress relief annealing, complete annealing, solution, normalizing, normalizing and tempering, tempering, low-temperature stress relief, precipitation heat treatment, etc. In a narrow sense, post-weld heat treatment only refers to stress relief annealing, that is, in order to improve the performance of the welding area and eliminate harmful effects such as welding residual stress, the welding area and related parts are uniformly and fully heated below the metal phase transformation temperature point, and then uniformly cooled. In many cases, the post-weld heat treatment discussed is essentially post-weld stress relief heat treatment.
2. Purpose of post-weld heat treatment (PWHT for short): 1. Relax welding stress 2. Stabilize the shape and size of the structure and reduce distortion. 3. Improve the performance of the parent material and welding area, including a. Improve the plasticity of the weld metal. b. Reduce the hardness of the heat-affected zone. c. Improve fracture toughness. d. Improve fatigue strength. e. Restore or increase the yield strength reduced during cold forming. 4. Improve the ability to resist stress corrosion. 5. Further release harmful gases in the weld metal, especially hydrogen, to prevent the occurrence of delayed cracks.
3. Judgment of the necessity of PWHT: Whether the pressure vessel needs post-weld heat treatment should be clearly specified in the design, and the current pressure vessel design specifications have requirements for this. For welded pressure vessels, there is a large residual stress in the welding area, and the adverse effects of residual stress are only manifested under certain conditions. When the residual stress combines with the hydrogen in the weld, it will promote the hardening of the heat-affected zone, leading to the generation of cold cracks and delayed cracks. When the static stress remaining in the weld or the dynamic load stress in the load operation is combined with the corrosive effect of the medium, it may cause crack-like corrosion, which is the so-called stress corrosion. Welding residual stress and the hardening of the parent material caused by welding are important factors in the generation of stress corrosion cracks. The research results show that the main effect of deformation and residual stress on metal materials is to transform the metal from uniform corrosion to local corrosion, that is, to intergranular or transgranular corrosion. Of course, metal corrosion cracking and intergranular corrosion both occur in media with certain characteristics for this metal. In the presence of residual stress, the nature of corrosion damage may change depending on the composition, concentration and temperature of the corrosive medium, as well as the composition, structure, surface state, stress state, etc. of the parent material and the weld zone. Whether the welded pressure vessel needs post-weld heat treatment should be determined based on the purpose, size (especially the wall thickness), performance of the materials used, and working conditions of the vessel. Post-weld heat treatment should be considered in any of the following situations: 1. Harsh conditions of use, such as thick-walled vessels with a risk of brittle fracture at low temperatures, and vessels that bear large loads and alternating loads. 2. Welded pressure vessels with a thickness exceeding a certain limit. Including boilers, petrochemical pressure vessels, etc., which have special regulations and specifications. 3. Pressure vessels with high dimensional stability. 4. Containers made of steel with a high tendency to harden. 5. Pressure vessels with a risk of stress corrosion cracking. 6. Other pressure vessels with special regulations, specifications and drawings. In steel welded pressure vessels, residual stress reaching the yield point is formed in the area near the weld. The generation of this stress is related to the transformation of the structure mixed with austenite. Many researchers have pointed out that in order to eliminate the residual stress after welding, tempering at 650 degrees can have a good effect on steel welded pressure vessels. At the same time, it is believed that if proper heat treatment is not performed after welding, corrosion-resistant welded joints can never be obtained. It is generally believed that stress relief heat treatment is a process in which the welded workpiece is heated to 500-650 degrees and then slowly cooled. The reduction of stress is caused by creep at high temperature, which starts from 450 degrees in carbon steel and 550 degrees in molybdenum-containing steel. The higher the temperature, the easier it is to eliminate stress. However, once the original tempering temperature of the steel is exceeded, the strength of the steel will be reduced. Therefore, the heat treatment for stress relief must master the two elements of temperature and time, and neither is indispensable. However, in the stress of the weldment, tensile stress and compressive stress are always accompanied, and stress and elastic deformation exist at the same time. When the temperature of steel rises, the yield strength decreases, and the original elastic deformation will become plastic deformation, thus causing stress relaxation. The higher the heating temperature, the more fully the internal stress is eliminated. However, when the temperature is too high, the steel surface will be severely oxidized. In addition, for the PWHT temperature of quenched and tempered steel, the principle should not exceed the original tempering temperature of the steel, which is generally about 30 degrees lower than the original tempering temperature of the steel, otherwise the material will lose the quenching and tempering effect, and the strength and fracture toughness will be reduced. This point should be given special attention to heat treatment workers. The higher the post-weld heat treatment temperature for eliminating internal stress, the greater the softening degree of the steel. Usually, the internal stress can be eliminated by heating to the recrystallization temperature of the steel. The recrystallization temperature is closely related to the melting temperature. Generally, the recrystallization temperature K=0.4X melting temperature (K). The closer the heat treatment temperature is to the recrystallization temperature, the more effective it is in eliminating residual stress.
4. Consideration of the comprehensive effect of PWHT Post-weld heat treatment is not absolutely beneficial. Generally, post-weld heat treatment is conducive to relieving residual stress and is only carried out when there are strict requirements for stress corrosion. However, the impact toughness test of the specimens shows that post-weld heat treatment is not conducive to improving the toughness of the deposited metal and the weld heat affected zone, and sometimes intergranular cracking may occur within the grain coarsening range of the weld heat affected zone. In addition, PWHT relies on the reduction of material strength at high temperatures to achieve stress relief. Therefore, during PWHT, the structure may lose rigidity. For structures that adopt overall or partial PWHT, the support capacity of the weldment at high temperatures must be considered before heat treatment. Therefore, when considering whether to perform post-weld heat treatment, the advantages and disadvantages of heat treatment should be comprehensively compared. From the perspective of structural performance, there is a side that improves performance and a side that reduces performance. A reasonable judgment should be made based on the basic work of comprehensively considering both aspects.
