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The purpose of heat treatment is to improve the mechanical properties of materials used in steel pipes and precision steel pipes, eliminate residual stresses, and enhance the machinability of the steel pipe metal. ▲ Heat treatment oxygen-free annealing furnace for precision steel pipes and seamless steel pipes ▲ Depending on the different purposes of heat treatment, the heat treatment processes can be divided into two main categories: preliminary heat treatment and final heat treatment. 1. Preliminary heat treatment: The purpose of preliminary heat treatment is to improve workability, eliminate internal stresses, and prepare a favorable microstructure for the final heat treatment. Its heat treatment processes include annealing, normalizing, aging, quenching and tempering, etc. (1) Annealing and normalizing: Annealing and normalizing are used for blanks that have undergone hot processing. For carbon steels and alloy steels with a carbon content of over 0.5%, annealing is often employed to reduce their hardness and improve machinability ； For carbon steels and alloy steels with a carbon content of less than 0.5%, normalizing is employed to prevent the tool from sticking during cutting due to their excessively low hardness. Annealing and normalizing can still refine the grain structure and homogenize the microstructure, preparing it for subsequent heat treatment. Annealing and normalizing are usually carried out after the blank is manufactured and before rough machining. (2) Aging treatment: Aging treatment is mainly used to eliminate the internal stresses generated during blank manufacturing and mechanical processing. To avoid excessive handling, for parts with ordinary precision, one aging treatment is sufficient before finishing. However, for parts with high precision requirements (such as the housing of coordinate boring machines), two or more aging treatment processes should be carried out. Simple parts generally do not require aging treatment. Apart from castings, for some precision parts with poor rigidity (such as precision screws), in order to eliminate the internal stresses generated during processing and maintain the accuracy of part fabrication, multiple aging treatments are often carried out between rough machining and semi-finishing. For the machining of some shaft components, aging treatment is also required after the straightening process. (3) Quenching and tempering: Quenching and tempering involves performing high-temperature tempering after quenching, which results in a uniform and fine tempered sorbite structure. This prepares the material for reduced deformation during subsequent surface quenching and nitriding treatments; therefore, quenching and tempering can also be considered a preliminary heat treatment. Due to the good comprehensive mechanical properties of the parts after quenching and tempering, it can also be used as the final heat treatment process for those parts where high hardness and wear resistance are not required. 2. Final heat treatment: The purpose of final heat treatment is to improve mechanical properties such as hardness, wear resistance, and strength. (1) Quenching: Quenching includes surface quenching and full quenching. Among them, surface quenching is widely used due to its minimal deformation, oxidation, and decarburization. It also offers the advantages of high external strength and good wear resistance, while maintaining good toughness inside and strong impact resistance. To improve the mechanical properties of surface-quenched parts, heat treatments such as quenching and tempering or normalizing are often required as pre-treatments. Its general manufacturing process is: material cutting -- forging -- normalizing (annealing) -- rough machining -- quenching and tempering -- semi-finishing -- surface quenching -- finishing. (2) Carburizing and quenching: Carburizing and quenching is suitable for low-carbon steels and low-alloy steels. It first increases the carbon content in the surface layer of the part; after quenching, the surface layer achieves high hardness, while the core retains a certain degree of strength as well as high toughness and plasticity. Carburizing is divided into full carburizing and local carburizing. During local carburizing, anti-carburization measures must be taken for the areas that are not to be carburized (copper plating or plating with anti-carburization materials). Due to the significant deformation caused by carburizing and quenching, and since the carburized depth is generally between 0.5 and 2 mm, the carburizing process is typically performed between semi-finishing and finishing operations. The general process route is: material cutting – forging – normalizing – rough and semi-finishing – carburizing and quenching – finishing. When, in the process scheme of removing the excess carburized layer by using a larger allowance for the uncarburized parts of locally carburized components, the step of removing the excess carburized layer is to be carried out after carburizing but before quenching. (3) Nitriding treatment: Nitriding is a treatment method in which nitrogen atoms are introduced into the metal surface to form a layer of nitrogen-containing compounds. The nitrided layer can improve the surface hardness, wear resistance, fatigue strength, and corrosion resistance of parts. Due to the low nitriding temperature, minimal deformation, and thin nitrided layer (usually not exceeding 0.6–0.7 mm), the nitriding process should be scheduled as late as possible. To reduce deformation during nitriding, high-temperature tempering to relieve stress is generally required after machining.