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As a traditional and effective method for improving and restoring metallic properties, heat treatment has always been a relatively weak aspect in processes such as the design and manufacturing of pressure vessels. Pressure vessels involve four types of heat treatment: post-weld heat treatment (stress-relief heat treatment) ； Heat treatment to improve material properties ； Heat treatment to restore material properties ； Post-weld dehydrogenation treatment. Here, the issues related to post-weld heat treatment, which is widely used in pressure vessel design, are discussed in detail. 1 Purpose of post-weld heat treatment (stress-relief heat treatment): 1. To relieve the stresses generated during welding. 2. Stabilize the shape and size of the structure to reduce distortion. 3. Improve the properties of the base material and the welding area, including a. enhancing 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. 2. Is post-weld heat treatment required for pressure vessels made of austenitic stainless steel? Post-weld heat treatment takes advantage of the decrease in the yield limit of metal materials at high temperatures, causing plastic flow in areas with high stress levels. This helps to eliminate residual welding stresses. At the same time, it improves the plasticity and toughness of the weld joint and the heat-affected zone, thereby enhancing resistance to stress corrosion. This stress-relief method is widely used in carbon steel and low-alloy steel pressure vessels with a body-centered cubic crystal structure. The crystal structure of austenitic stainless steel is face-centered cubic; since metallic materials with a face-centered cubic structure possess more slip planes than those with a body-centered cubic structure, they exhibit good toughness and strain-hardening properties. Furthermore, in the design of pressure vessels, stainless steel is often chosen for the purposes of preventing corrosion and meeting specific temperature requirements. Additionally, since stainless steel is more expensive than carbon steel and low-alloy steel, its wall thicknesses are not very thick. Therefore, from the perspective of safety in normal operation, there is no need to require post-weld heat treatment for pressure vessels made of austenitic stainless steel. As for corrosion that occurs due to use, as well as material instability resulting from abnormal operating conditions such as fatigue and impact loads, these are difficult to take into account in conventional design. If such situations exist, it is necessary for the relevant technical personnel (from units involved in design, use, research, etc.) to conduct thorough studies and comparative experiments in order to develop a practical heat treatment plan that ensures the overall performance of the pressure vessel is not affected. Otherwise, if the requirements and possibilities of heat treatment for austenitic stainless steel pressure vessels are not fully considered, and heat treatment requirements for austenitic stainless steel are simply derived by analogy with carbon steel and low-alloy steel, it often does not work. Under the current standards, there are no explicit requirements regarding whether post-weld heat treatment is necessary for pressure vessels made of austenitic stainless steel. Article 8.2.4 of GB150.4 \"Pressure Vessels\" stipulates: \"When post-weld heat treatment is required for austenitic stainless steels and austenitic-ferritic stainless steels, it shall be carried out in accordance with the provisions in the design documents.\" Clause 8.2.5 of GB150.4 \"Pressure Vessels\" stipulates that: \"Unless otherwise specified in the design documents, the welded joints of austenitic stainless steels and austenitic-ferritic stainless steels may not require heat treatment.\" Clause 3.2.11 of TSG21-2016 \"Code for Pressure Vessels\" stipulates that: \"Welded pressure vessels made of austenitic stainless steel and non-ferrous metals generally do not require post-weld heat treatment; if heat treatment is necessary due to special requirements, it shall be indicated in the design drawings.\" ” 3 Heat treatment of containers made of explosive stainless steel composite plates. Due to their excellent combination of corrosion resistance and mechanical strength, along with favorable cost-performance ratios, explosive stainless steel composite plates are being used more and more widely in the pressure vessel industry. However, the heat treatment issues related to this material also deserve attention from pressure vessel designers. For pressure vessel designers, the technical indicator they generally pay close attention to regarding clad plates is their bonding rate; meanwhile, they seldom consider the heat treatment of clad plates, or believe that this issue should be addressed by relevant technical standards and manufacturers. The process of explosive machining metal composite sheets is essentially a process of applying energy to the metal surface. Under the action of high-speed pulses, the composite material impacts the substrate at an inclined angle; in the state of metal jet flow, a serrated composite interface is formed between the layered metal and the base metal, enabling atomic-level bonding. The base metal that has undergone explosive processing has, in fact, been subjected to a strain-hardening treatment process. As a result, the tensile strength σb increases, the plasticity index decreases, and the yield strength value σs is not significant. Whether it is steel of the Q235 series or Q345R, after explosive processing and subsequent testing of their mechanical properties, the aforementioned strain hardening phenomenon is observed. Therefore, the current relevant technical standards specify the heat treatment for austenitic stainless steel sheets after explosive processing. NB/T 47002.1-2009 \"Explosion-welded clad plates for pressure vessels\" – Part 1: Stainless steel-steel clad plates stipulates that: \"The clad plates shall be supplied after heat treatment, leveling, and trimming (or cutting); the heat-treated state of the clad plates shall comply with the requirements for the base material specified in GB150 or JB4732.\" At the request of the buyer and as specified in the contract, the surface of the overlay material can be treated by methods such as sandblasting, polishing, or pickling. 4 Can alternative methods be used to replace the overall heat treatment of equipment? Due to limitations imposed by the manufacturing facilities and considerations related to economic benefits, many people have explored alternative methods to replace the overall heat treatment of pressure vessels. Although these efforts are useful and valuable, they cannot yet replace the traditional overall heat treatment process for pressure vessels. Among the currently valid standards and regulations, the requirements for overall heat treatment have not been relaxed. Among the various alternatives to overall heat treatment, typical methods include: local heat treatment, hammer peening to relieve welding residual stresses, explosive treatment to relieve welding residual stresses, vibration methods, hot water bath treatment, etc. Local heat treatment: GB150.4 \"Pressure Vessels\" 8.2.6.5 stipulates that: for welding joints of categories B, C, D, and E, joints where spherical heads are connected to cylinders, as well as areas that have been repaired due to defects, local heat treatment methods may be employed. ”This regulation means that local heat treatment is not allowed for Class A welds on the cylinder; in other words, local heat treatment is not permitted for the entire equipment, one of the reasons being that the residual welding stresses cannot be eliminated symmetrically. Eliminating welding residual stress by hammering: This involves applying manual hammering to create a layer of compressive stress on the surface of the welded joint, thereby partially counteracting the adverse effects of residual tensile stress. In principle, this method does have a certain inhibitory effect on preventing stress corrosion cracking. However, due to the lack of quantitative indicators and relatively strict operating procedures during practical implementation, as well as insufficient validation work for comparative use, it has not been adopted by current standards. Eliminating welding residual stress by explosion method: This involves using **specially designed tape-like materials that are attached to the surface of the welded joint on the inner wall of the equipment; the mechanism is the same as that used in the hammering method to eliminate welding residual stress. It is said that this method can compensate for some of the shortcomings of the hammering method in eliminating welding residual stresses. However, certain organizations conducted comparative tests on two liquefied petroleum gas storage tanks under identical conditions, using either overall heat treatment or the explosion method to eliminate welding residual stresses. After one year, inspections of the tanks revealed that the weld joints in those treated with overall heat treatment were still in perfect condition, whereas the weld joints in the tanks treated with the explosion method showed many cracks. Thus, the once-popular method of using explosion treatment to eliminate residual welding stresses has also faded away silently. There are also other methods for eliminating residual welding stresses, but they have not been adopted by the pressure vessel industry for various reasons. In summary, although post-weld overall heat treatment of pressure vessels (including staged heat treatment in a furnace) has the disadvantages of high energy consumption and long processing times, and faces various challenges in practical application due to factors such as the structure of the pressure vessels, it remains the only method acceptable to all parties in the pressure vessel industry for eliminating welding residual stresses.