Introduction to Heat Treatment of Steels for Pressure Vessels

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The heat treatment of steel varies depending on the processing techniques and the different requirements for its structure and properties. Normalizing heat treatment can be used to obtain a refined and uniform microstructure as well as better overall mechanical properties. To ensure that high-strength quenched and tempered steel plates achieve the required mechanical properties, quenching followed by high-temperature tempering can be performed. To improve the corrosion resistance of stainless steel sheets, solution heat treatment can be carried out. The specific heat treatment process for steel is as follows. 1. Annealing: The steel component is heated to a temperature higher or lower than the Ac1 point and held at that temperature for a certain period of time (for steel used in pressure vessels, annealing generally involves heating to a temperature below Ac1). After that, it is cooled slowly (usually by cooling it in the furnace); this heat treatment process is known as annealing. Based on the characteristics of the process, annealing can be divided into full annealing, isothermal annealing, spheroidizing annealing, low-temperature annealing, and diffusion annealing. Low-temperature annealing is carried out below the Ac1 temperature, primarily to eliminate residual stresses in the steel, stabilize its dimensions, remove cold work hardening, and reduce hardness. Low-temperature annealing, also known as stress-relief annealing, is the most commonly used heat treatment method in pressure vessel manufacturing. The main purpose of annealing: (1) to reduce the hardness of steel ； (2) Improve or eliminate the compositional or microstructural heterogeneities that arise in the billet during casting, forging, and welding ； (3) Eliminate internal stress and stabilize dimensions ； (4) Refine grains and improve the morphology and distribution of carbides in the steel (complete annealing). 2. Normalizing: Normalizing is a heat treatment process in which steel is heated to a temperature 30°C–50°C above Ac3, or even higher, held at that temperature for a certain period of time, and then cooled in air. The main purposes of normalizing are: (1) to refine the grain structure, improve the toughness of the steel, and stabilize its mechanical properties; (2) to obtain the desired microstructural pattern. 3. Quenching: Quenching involves heating the steel component to a temperature above Ac3, holding it at that temperature for a certain period of time to allow austenitization, and then rapidly cooling it to room temperature. A heat treatment process used to obtain hardened microstructures, such as martensitic structures. The main purposes of quenching are: (1) to increase the strength and hardness of steel; for example, forged parts made of 15CrMo and 12Cr2Mol steels exhibit higher strength and hardness after quenching compared to those after normalizing; (2) to improve the mechanical and chemical properties of certain special steels. For steels used in pressure vessels, the quenched steel components are usually subjected to high-temperature tempering immediately afterward to achieve the desired optimal properties. 4. Tempering is a heat treatment process in which the hardened steel part is heated to a temperature below Ac, held at that temperature for a certain period of time, and then cooled to room temperature at a specific rate. The main purposes of tempering are: (1) to eliminate the thermal stresses and structural stresses generated after quenching or normalizing, reduce brittleness, and prevent deformation and cracking; (2) to transform the hardened structures resulting from quenching or normalizing (such as martensite, bainite, etc.) into more stable structures, thereby ensuring the desired good overall mechanical properties; (3) by tempering at different temperatures, different structures and properties can be obtained. Therefore, by selecting different tempering temperatures, the mechanical properties of the steel parts after quenching or normalizing can be adjusted. 5. Quenching and tempering: In production, the combined heat treatment process of quenching followed by high-temperature tempering is generally referred to as quenching and tempering treatment. This treatment results in a tempered sorbite structure; steel plates such as 07MnMoVR, 07MnNiVDR, and 07MnNiMoDR exhibit good comprehensive mechanical properties. 6. Solution treatment is a process in which austenitic stainless steel is heated to 1000°C–1100°C, held at that temperature for a certain period of time, and then rapidly cooled to obtain a single-phase austenitic structure. Austenitic stainless steel treated in this way possesses certain strength, high toughness, and excellent corrosion resistance. 7. Stabilization: Austenitic stainless steels containing stabilizing elements such as Ti and Nb are heated to 850°C–950°C, held at that temperature for a certain period of time, and then cooled by water, oil, or air. This process causes the stabilizing elements Ti and Nb, along with the carbon present in the steel, to form TiC and NbC, thereby improving the steel’s resistance to intergranular corrosion and other properties.