1、 During the manufacturing process of pressure vessels, the following issues will arise: cold work hardening caused by excessive cold rolling, cold straightening, and other cold processing. Changes in the microstructure and properties of the weld zone caused by welding. The generation and development of residual stresses and stress corrosion cracking caused by welding. When welding pressure vessels, a sharp temperature gradient with a temperature difference greater than 100 degrees between adjacent areas of the base metal can cause uneven plastic strain in ferritic steel or equivalent materials. During the subsequent cooling process, a residual stress field with peak stress reaching the yield point will be generated. In addition, the uneven plastic strain in the manufacturing of pressure vessels leads to residual strain in the elastic plastic material, which can be caused by mechanical (mainly cold rolling, cold straightening, etc.), thermal (mainly generated during the welding process), or a combination of both, and is also due to thermal mechanical reasons. Therefore, residual elastic strain fields will be left in the final product of pressure vessel processing, and corresponding elastic residual stresses will be borne. The presence of residual stress will affect the performance of pressure vessels. In order to eliminate peak strain in the welding area and achieve uniform distribution of internal strain, various methods can be adopted, such as mechanical vibration method, post weld heating method, etc. However, due to the fact that many potential problems in pressure vessels mainly come from metallurgical damage in the weld zone, using mechanical methods to reduce internal strain is no longer sufficient to prevent many problems that may occur during future operation. In addition, the phenomenon of hydrogen embrittlement in metals has attracted considerable attention. After hydrogen enters the steel, its mechanical properties will significantly deteriorate. The strength and plasticity are significantly reduced, and the hydrogen dissolved in the metal lattice causes brittle failure of the steel during slow deformation. Hydrogen in metal materials can be absorbed during the production process of metal materials, such as hydrogen absorbed by liquid metal during welding and retained in the weld seam, or hydrogen absorbed by materials during service in a hydrogen environment. A more effective method to eliminate hydrogen absorbed in the weld seam is through post weld heat treatment, which can achieve relaxation and relaxation of welding residual stresses, improve the hardening and embrittlement of the welding heat affected zone due to welding, enhance the ductility and fracture toughness of the weld metal, and also allow harmful gases such as hydrogen to diffuse and escape from the welding zone and its vicinity. There are two types of heat treatment methods used for pressure vessels: one is heat treatment to improve mechanical properties, and the other is post weld heat treatment (PWHT). Generally speaking, post weld heat treatment is a heat treatment performed on the welding area or welded component after the workpiece has been welded. The starting content includes stress relief annealing, complete annealing, solidification, normalizing, normalizing and tempering, tempering, low-temperature stress relief, precipitation heat treatment, etc. Narrowly speaking, post weld heat treatment only refers to stress relief annealing, which is the process of uniformly and sufficiently heating and then uniformly cooling the welding area and related parts below the temperature point of metal phase transition 2, in order to improve the performance of the welding area and eliminate harmful effects such as welding residual stress. In many cases, the discussed post weld heat treatment is essentially post weld stress relief heat treatment.
2、 The purpose of post weld heat treatment (PWHT) is to: 1. relax the stress involved in welding, 2. stabilize the shape and size of the structure, and reduce distortion. 3. Improve the performance of the base metal and welding area, including a Improve the plasticity of weld metal. b. Reduce the hardness of the heat affected zone. c. Improve fracture toughness. d. Improve fatigue strength. e. Restore or improve the reduced yield strength during cold forming. 4. Improve the ability to resist stress corrosion. 5. Further release harmful gases, especially hydrogen, from the weld metal to prevent delayed cracking.
3、 The necessity of PWHT judgment: The necessity of post weld heat treatment for pressure vessels should be clearly defined in the design, and the current pressure vessel design specifications have requirements for this. Welded pressure vessels have significant residual stresses in the welding area, and the adverse effects of residual stresses only manifest under certain conditions. When residual stress combines with hydrogen in the weld, it will promote the hardening of the heat affected zone, leading to the formation of cold cracks and delayed cracks. When the static stress remaining in the weld seam or the dynamic load stress during load operation is combined with the corrosive effect of the medium, it may cause crack like corrosion, also known as stress corrosion. Welding residual stress and the hardening of the base material caused by welding are important factors in the occurrence of stress corrosion cracking. The research results indicate that the main influence of deformation and residual stress on metal materials is to transform the metal from uniform corrosion to localized corrosion, that is, to intergranular or transgranular corrosion. Of course, corrosion cracking and intergranular corrosion of metals occur in media that have certain characteristics for that metal. In the presence of residual stress, the nature of corrosion damage may vary depending on the composition, concentration, and temperature of the corrosive medium, as well as differences in the composition, microstructure, surface state, and stress state of the base metal and welding zone. Whether welded pressure vessels need to undergo post weld heat treatment should be determined based on a comprehensive consideration of the vessel's purpose, size (especially the thickness of the wall panels), material properties, and working conditions. If one of the following situations occurs, post weld heat treatment should be considered: 1. Thick walled containers with high risk of brittle fracture when working at low temperatures, containers that withstand large loads and alternating loads. 2. Welded pressure vessels with a thickness exceeding a certain limit. Including boilers, petrochemical pressure vessels, and other specialized regulations and standards. 3. For pressure vessels with high dimensional stability. 4. Containers made of steel with a high tendency to harden. 5. Pressure vessels with the risk of stress corrosion cracking. 6. Other pressure vessels that are specified by specialized regulations, specifications, and drawings. In steel welded pressure vessels, residual stresses that reach the yield point are formed in the area near the weld seam. The generation of this stress is related to the transformation of the microstructure mixed with austenite. Many researchers have pointed out that tempering at 650 degrees can have a good effect on steel welded pressure vessels in order to eliminate residual stresses after welding. At the same time, it is believed that without proper heat treatment after welding, corrosion-resistant welded joints cannot be obtained. It is generally believed that stress relief heat treatment belongs to the process of heating the welded workpiece to 500-650 degrees and then slowly cooling it down. The decrease in stress is caused by creep at high temperatures, starting from 450 degrees Celsius in carbon steel; In steel containing molybdenum, it appears from 550 degrees. The higher the temperature, the easier it is to eliminate stress. But once the original tempering temperature of the steel is exceeded, the strength of the steel will decrease. So the heat treatment to eliminate stress must master the two elements of temperature and time, both of which are indispensable. However, in the internal stress of welded components, there are always tensile stress and compressive stress, and stress and elastic deformation coexist. When the temperature of steel increases, the yield strength decreases, and the original elastic deformation becomes plastic deformation, resulting in stress relaxation. The higher the heating temperature, the more complete the elimination of internal stress. But when the temperature is too high, it will cause severe oxidation on the surface of the steel. In addition, for the PWHT temperature of quenched and tempered steel, it should be based on the principle of not exceeding the original tempering temperature of the steel, generally about 30 degrees lower than the original tempering temperature of the steel. Otherwise, the material will lose its quenching and tempering effect, and its strength and fracture toughness will decrease. This should be given special attention by heat treatment workers. The higher the post weld heat treatment temperature for eliminating internal stress, the greater the softening degree of the steel. Usually, when heated to the recrystallization temperature of the steel, internal stress can be eliminated, and the recrystallization temperature is closely related to the melting temperature. Generally, the recrystallization temperature K=0.4X the melting temperature (K). The closer the heat treatment temperature is to the recrystallization temperature, the more effective it is to eliminate residual stress.
4、 Considering the comprehensive effect of PWHT, post weld heat treatment is not advantageous. In general, post weld heat treatment is beneficial for relieving residual stress and is only carried out when there are strict requirements for stress corrosion. However, the impact toughness test of the specimens showed that post weld heat treatment is not conducive to improving the toughness of the deposited metal and the welding heat affected zone, and sometimes intergranular cracking may occur within the grain coarsening range of the welding heat affected zone. Furthermore, PWHT relies on the reduction of material strength at high temperatures to eliminate stress. Therefore, during PWHT, the structure may lose rigidity. For structures that adopt integral 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 are both aspects that can improve performance and aspects that can reduce performance. A reasonable judgment should be made based on a comprehensive consideration of both aspects