Not all post-weld heat treatments are beneficial

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Welding residual stress is caused by the uneven temperature distribution in the welded parts resulting from welding, as well as the thermal expansion and contraction of the weld metal; therefore, residual stress is inevitably generated during welding processes. The most common method for eliminating residual stress is high-temperature tempering, which involves placing the welded component in a heat treatment furnace and heating it to a certain temperature for a specified period of time. By taking advantage of the decrease in the yield strength of the material at high temperatures, plastic flow occurs in areas with high internal stress; as a result, elastic deformation decreases while plastic deformation increases, thereby reducing the stress levels. 01 Selection of heat treatment methods: The effect of post-weld heat treatment on the tensile strength and creep limit of metals is related to the temperature and holding time of the heat treatment. The effect of post-weld heat treatment on the impact toughness of weld metal varies depending on the steel type. Post-weld heat treatment generally involves either single high-temperature tempering or normalizing followed by high-temperature tempering. For gas welding joints, normalizing followed by high-temperature tempering heat treatment is applied. This is because the grains in the welds and heat-affected zones of gas welding are coarse, and it is necessary to refine these grains; therefore, normalizing treatment is employed. However, single normalizing cannot eliminate residual stresses; therefore, high-temperature tempering is required to remove them. Single medium-temperature tempering is only applicable to the assembly and welding of large ordinary low-carbon steel containers assembled on-site, with the aim of partially eliminating residual stresses and dehydrogenating the material. In the vast majority of cases, a single high-temperature tempering is chosen. The heating and cooling during heat treatment should not be too rapid, in order to ensure uniformity across the inner and outer walls. 02 Heat treatment methods used for pressure vessels There are two types of heat treatment methods used for pressure vessels: one type is for improving mechanical properties ； One category is post-weld heat treatment (PWHT). Broadly speaking, post-weld heat treatment refers to the heat treatment applied to the welded area or welded components after welding is completed. The specific methods include stress-relief annealing, full annealing, solution treatment, normalizing, normalizing followed by tempering, tempering, low-temperature stress relief, and precipitation heat treatment. In a narrow sense, post-weld heat treatment refers only to stress-relief annealing, that is, the process of uniformly and thoroughly heating the welded area and related parts below the temperature at which metal phase transformation occurs, in order to improve the properties of the welded area and eliminate harmful effects such as residual welding stresses, followed by uniform cooling. In many cases, the post-weld heat treatment discussed is essentially post-weld stress-relief heat treatment. 03 Purpose of post-weld heat treatment: 1. To relieve welding residual stresses. 2. Stabilize the shape and size of the structure to reduce distortion. 3. Improve the properties of the base material and welded joints, including: a. Enhance 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 resistance to stress corrosion. 5. Further release harmful gases from the weld metal, especially hydrogen, to prevent the occurrence of delayed cracks. 04 Determination of the necessity for PWHT: It is necessary to specify clearly in the design whether post-weld heat treatment is required for pressure vessels, and the current pressure vessel design codes have relevant requirements in this regard. In welded pressure vessels, there are significant residual stresses in the welded area, and these residual stresses have adverse effects. It only manifests under certain conditions. When residual stress combines with hydrogen in the weld, it causes the heat-affected zone to harden, leading to the formation of cold cracks and delayed cracks. When the residual static stress in the welds, or the dynamic stress under operating loads, combines with the corrosive effect of the medium, it can lead to crack-like corrosion, namely stress corrosion. Welding residual stresses and the hardening of the base material caused by welding are important factors that lead to stress corrosion cracks. The research results show that the main effect of deformation and residual stress on metal materials is to cause the metal to shift from uniform corrosion to localized corrosion, that is, to intergranular or transgranular corrosion. Of course, both corrosion cracking and intergranular corrosion of metals occur in media that possess certain characteristics specific to that metal. In the presence of residual stress, the nature of corrosion damage can change depending on the composition, concentration, and temperature of the aggressive medium, as well as differences in the composition, microstructure, surface condition, and stress state of the base material and the welded area. Whether a welded pressure vessel requires post-weld heat treatment should be determined by taking into comprehensive consideration factors such as the vessel’s purpose, size (especially the wall thickness), the properties of the materials used, and the operating conditions. Post-weld heat treatment should be considered in any of the following situations: 1. In cases of severe operating conditions, such as thick-walled containers that operate at low temperatures and are at risk of brittle fracture, or containers that are subjected to large loads or alternating loads. 2. Welded pressure vessels with a thickness exceeding a certain limit. There are specific regulations and standards for items such as boilers and petrochemical pressure vessels. 3. Pressure vessels with high dimensional stability. 4. Containers made of steel materials with a high tendency to harden. 5. Pressure vessels at risk of stress corrosion cracking. 6. Other pressure vessels that are specified by specialized regulations, standards, and drawings. In steel welded pressure vessels, residual stresses reaching the yield point are formed in the area near the welds. The generation of this stress is related to the structural transformation involving austenite. Many researchers have pointed out that, in order to eliminate residual stresses after welding, tempering at 650 degrees can have a positive effect on steel welded pressure vessels. It is also believed that without appropriate heat treatment after welding, corrosion-resistant welded joints can never be obtained. Stress-relief heat treatment is generally considered to be a process in which the welded workpiece is heated to 500–650 degrees and then cooled slowly. The reduction in stress is caused by creep at high temperatures, which begins to occur in carbon steel at 450 degrees and in molybdenum-containing steel at 550 degrees. The higher the temperature, the easier it is to eliminate stress. However, once the original tempering temperature of the steel is exceeded, its strength decreases. Therefore, in stress-relieving heat treatment, it is essential to control both temperature and time accurately; neither can be omitted. However, in the internal stresses of welded joints, tensile stresses and compressive stresses always coexist, with stresses and elastic deformation appearing simultaneously. As the temperature of steel rises, its yield strength decreases; the existing elastic deformation then turns into plastic deformation, resulting in stress relaxation. The higher the heating temperature, the more thoroughly the internal stress is eliminated. However, excessively high temperatures will cause severe oxidation of the steel surface. Furthermore, regarding the PWHT temperature for quenched and tempered steel, it should not exceed the original tempering temperature of the steel; generally, it is about 30 degrees lower than that temperature. Otherwise, the quenched and tempered effects will be lost, resulting in a decrease in strength and fracture toughness. This point should be given special attention by those working in heat treatment. The higher the temperature used for post-weld heat treatment to eliminate internal stresses, the greater the degree of softening of the steel. Generally, heating the steel to its recrystallization temperature is sufficient to eliminate internal stresses, 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 in eliminating residual stresses. 04 Consideration of the overall effects of PWHT: Post-weld heat treatment is not always advantageous. Under normal circumstances, post-weld heat treatment helps to relieve residual stresses, and it is only carried out when strict requirements exist regarding stress corrosion. However, impact toughness tests on the test specimens showed that post-weld heat treatment is detrimental to improving the toughness of the deposited metal and the weld heat-affected zone; sometimes intergranular cracking may even occur within the range of grain coarsening in the weld heat-affected zone. Furthermore, PWHT relies on the reduction in material strength at high temperatures to relieve stress; therefore, during PWHT, the structure may lose its rigidity. For structures that undergo overall or partial PWHT, it is necessary to consider the support capacity of the welded joint at high temperatures prior to 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 aspects that improve it and those that reduce it; a reasonable judgment should be made by taking both aspects into comprehensive consideration.