Issues related to post-weld heat treatment

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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 stages 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 Do austenitic stainless steel pressure vessels require post-weld heat treatment? Post-weld heat treatment takes advantage of the decrease in the yield strength of metal materials at high temperatures, enabling plastic flow to occur in areas with high stress levels; this helps to eliminate residual welding stresses. It also improves the plasticity and toughness of the welded joint and the heat-affected zone, as well as 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 metal materials with a face-centered cubic structure have more slip planes than those with a body-centered cubic structure, they exhibit good toughness and strain strengthening 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 research 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 current standards, there are no explicit requirements regarding whether post-weld heat treatment is necessary for pressure vessels made of austenitic stainless steel. Clause 8.2.4 of GB150.4 \"Pressure Vessels\" stipulates: \"When post-weld heat treatment is required for austenitic stainless steels and austenite-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 specified 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 increasingly 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 parameter that is usually given considerable attention regarding composite sheets is their bond strength, while the issue of heat treatment for such sheets is often given little consideration, or it is believed that this matter 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, achieving 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 from the Q235 series or Q345R, testing their mechanical properties after explosive processing reveals the aforementioned strain strengthening phenomenon. Therefore, the current relevant technical standards specify the heat treatment for austenitic stainless steel sheets after explosive processing. NB/T 47002.1-2009 \"Explosively Welded Composite Plates for Pressure Vessels\" – Part 1: Stainless Steel-Steel Composite Plates stipulates that: \"The composite steel plates shall be supplied after heat treatment, leveling, and trimming (or cutting); the heat treatment condition of the composite plates shall comply with the requirements for the corresponding base material specified in GB150 or JB4732.\" As required by the buyer and 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 constraints 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. Typical alternatives to overall heat treatment include: local heat treatment, the hammering method for eliminating welding residual stresses, the explosion method for eliminating welding residual stresses and the vibration method, as well as the hot water bath method. Local heat treatment: GB150.4 \"Pressure Vessels\" 8.2.6.5 stipulates that: for welding joints of categories B, C, D, and E, connections between spherical heads and cylinders, as well as areas where defects have been repaired, local heat treatment methods may be employed. ”This regulation means that local heat treatment is not permitted for Class A welds on the cylinder; in other words, local heat treatment is not allowed for the entire equipment, one of the reasons being that the residual welding stresses cannot be eliminated symmetrically. Eliminating welding residual stresses 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, it has not been adopted by current standards due to the lack of quantitative indicators and strict operating procedures in practical application, as well as insufficient verification work using comparative methods. Eliminating welding residual stress by explosion: 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 excellent condition, whereas the weld joints in the tanks treated with the explosion method showed many cracks. Thus, the once-popular method of using explosions to eliminate residual welding stresses faded away quietly. 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 accepted by all parties in the pressure vessel industry for eliminating welding residual stresses.