Detailed Explanation of Heat Treatment for Pressure Vessels

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Heat treatment: Heat treatment of pressure vessels is a task that requires both expertise and practical skills. Based on the purpose of heat treatment, it can be divided into two categories: post-welding heat treatment, and heat treatment aimed at restoring or improving properties (mechanical properties, corrosion resistance, workability). Based on the object of heat treatment, it can be classified into three types: heat treatment of raw materials, heat treatment of components, and heat treatment of finished products. Post-weld heat treatment ① Post-weld heat treatment is a type of heat treatment that takes advantage of the decrease in a metal’s yield strength at high temperatures, thereby enabling plastic flow in areas with high internal stresses; this process serves to eliminate or reduce welding residual stresses and is classified as stress-relief annealing. For pressure vessels made of carbon steel and low-alloy steel, the product is slowly heated to 500–650°C, held at that temperature for a certain period of time, and then cooled evenly along with the furnace. The main functions of post-weld heat treatment are: to eliminate or reduce welding residual stresses and cold work hardening, and to improve the crack resistance of the joint ； Improve the plasticity and toughness of weld joints, and enhance stress corrosion resistance ； Stabilize the shape of the welded components to avoid or minimize deformation during post-weld machining and use ； It promotes the outward diffusion of hydrogen in the weld. For pressure vessels with high safety requirements, post-weld heat treatment can improve their safety. Sometimes, post-weld heat treatment can be combined with dehydrogenation treatment, as well as heat treatment to restore and improve properties. ②Deoxygenation treatment: A heat treatment in which the welded part is heated to a high temperature immediately after welding, thereby increasing the diffusion coefficient of hydrogen in steel and accelerating the diffusion and escape of hydrogen atoms in a supersaturated state from the weld metal, in order to reduce the likelihood of delayed cracking in the container. It is usually heated to 200–350°C, and the holding time should generally be no less than 0.5 hours. For containers that require hydrogen removal treatment, if post-weld stress relief heat treatment is carried out immediately after welding, the post-weld hydrogen removal treatment can be omitted; however, the holding time must be kept within 16–24 hours. Not all metal materials develop delayed cracking when welded. The occurrence of delayed cracking is related to the strength grade and chemical composition of the material; this phenomenon can occur only in low-alloy steels with higher strength grades. Pressure vessels that generally require dehydrogenation treatment need post-weld heat treatment, whereas equipment that requires post-weld heat treatment does not necessarily need dehydrogenation treatment. Heat treatment for restoring or improving performance ① Heat treatment for restoring performance after cold forming and medium-temperature forming: When components are subjected to cold forming or medium-temperature forming and experience large amounts of deformation, work hardening occurs, which reduces the plasticity and toughness of the steel, while also generating significant internal stresses. To restore the properties of steel and eliminate or reduce residual processing stresses, heat treatment for property restoration should be applied to cold-formed and medium-temperature formed compression members when necessary. For compression members made of carbon steel and low-alloy steel, this heat treatment is equivalent to stress-relief annealing or recrystallization annealing. If necessary, the recovery performance heat treatment can be combined with the post-weld heat treatment. ②Heat treatment for restoring properties after hot working: Hot working can alter the condition of the steel material; therefore, necessary heat treatment can be applied to the compressed components after hot working, in accordance with the desired condition of the steel for use as specified in the design requirements. For steel used in the hot-rolled state, compressed components after hot working generally do not require re-heat treatment ； Steel used in the normalized state requires re-normalization after hot working. However, if the heating temperature during hot working is similar to the normalizing temperature of the steel, and test pieces processed using this furnace-based hot working method prove to be satisfactory, re-normalization may not be necessary ； For steel that is in the normalized and tempered condition, it needs to be normalized and tempered again after hot working. If the heating temperature during hot working is similar to the normalization temperature of the steel, and the test pieces processed using this furnace-based hot working method pass the qualification tests after tempering, then such pressure components can be subjected only to tempering after hot working ； Steel used in the quenched and tempered condition should be subjected to quenching and tempering treatment again after hot working of the compressed components ； The weld structure resulting from electroslag welding should be normalized in order to restore its mechanical properties and eliminate stresses. The biggest difference between normalizing and annealing is that normalizing involves a faster cooling rate. ③Solution treatment is a heat treatment process in which austenitic stainless steel is heated to around 1010–1120°C; after maintaining this temperature for an appropriate period of time to allow carbon compounds to dissolve as much as possible into the austenite matrix, it is then rapidly cooled to room temperature. This rapid cooling prevents the carbon compounds from precipitating out, leaving them in a supersaturated state and dissolved within the matrix, thereby resulting in a single-phase austenite structure. The normal condition of delivery for austenitic stainless steel is the solution-treated state. In pressure vessels, solution treatment can serve the following purpose: for non-ultra-low carbon austenitic stainless steels, solution treatment is an important means of preventing intergranular corrosion ； For heat-formed compression members, solution treatment can be used to restore their original properties ； When cold forming or operating conditions alter the austenite structure, the original properties can be restored through solution treatment depending on the actual situation. For example, cold-formed austenitic stainless steel pressure components should undergo post-forming heat treatment when the design temperature falls within the sensitization range and the processing deformation rate exceeds a certain limit. When austenitic stainless steel pressure components are used in cryogenic conditions, their low-temperature toughness can be restored by performing solution treatment after cold forming. ④Stabilization treatment is a heat treatment process in which austenitic stainless steel containing stabilization elements (Ti or Nb) is heated to high temperatures of 850–930°C. After maintaining this temperature for an appropriate period, the stabilization elements added to the steel precipitate sufficiently from the matrix, forming carbides such as TiC and NbC at the grain boundaries, thereby enabling these stabilization elements to exert their full effect. Stabilization treatment is only applicable in cases where austenitic stainless steels containing stabilizing elements (Ti or Nb) are used in an intergranular corrosion environment. ⑤Quenching and high-temperature tempering (tempering) can endow low-alloy steel with good comprehensive mechanical properties. Alloy steels commonly used for high-pressure fasteners, such as 40MnB and 35CrMoA, must undergo quenching and tempering treatment to achieve the required mechanical properties. ⑥Intermediate annealing is an inter-process annealing carried out to eliminate the workpiece’s deformation strengthening effect, improve its plasticity, and facilitate subsequent processing steps. For example, in an environment free from stress corrosion, when the heat exchange tubes and tube sheets are connected by strength expansion bonding, local annealing of the tube ends can be used to reduce the hardness of the heat exchange tubes, thereby meeting the requirements of the expansion bonding process, which stipulate that the hardness of the heat exchange tube material should be lower than that of the tube sheet material. The common feature of normalizing (followed by tempering), quenching (followed by hardening), and solution treatment (followed by stabilization treatment) is rapid cooling after being held at a high temperature for an appropriate period of time; this is difficult to achieve in pressure vessel products that have enclosed spaces. Other heat treatment processes with a more gradual cooling rate can be applied to pressure vessel products. It is necessary to clearly define the objects of heat treatment for the two types of heat treatment processes in pressure vessel manufacturing (raw materials, components, and finished products), in order to avoid mixing up the heat treatment of raw materials or components with the heat treatment of the pressure vessel as a whole. Some of the heat treatment challenges in the manufacturing process of pressure vessels can be addressed by changing material selection, updating structural designs, adjusting processing techniques, and replacing tooling and equipment.