Why do steel pipes need heat treatment

Why do steel pipes need heat treatment The role of heat treatment is to improve the mechanical properties of steel pipes and precision steel pipes, eliminate residual stress, and improve the cutting performance of steel pipe metal. According to the different purposes of heat treatment, heat treatment processes can be divided into two categories: preliminary heat treatment and final heat treatment. 1. Preliminary heat treatment: The purpose of preliminary heat treatment is to improve processing performance, eliminate internal stress, and prepare a good metallographic structure for final heat treatment. Its heat treatment processes include annealing, normalizing, aging, and tempering. (1) Annealing and normalizing: Annealing and normalizing are used for blanks that have undergone heat treatment. Carbon steel and alloy steel with a carbon content greater than 0.5% are often annealed to reduce their hardness for easy cutting; carbon steel and alloy steel with a carbon content less than 0.5% are normalized to avoid sticking to the knife when cutting due to their low hardness. Annealing and normalizing can also refine the grains and make the structure uniform, preparing for subsequent heat treatment. Annealing and normalizing are often arranged after blank manufacturing and before rough processing. (2) Aging treatment: Aging treatment is mainly used to eliminate the internal stress generated during blank manufacturing and machining. In order to avoid excessive transportation workload, for parts with general precision, one aging treatment is sufficient before fine machining. However, for parts with higher precision requirements (such as the housing of a coordinate boring machine, etc.), two or more aging treatment processes should be arranged. Simple parts generally do not require aging treatment. In addition to castings, for some precision parts with poor rigidity (such as precision lead screws), in order to eliminate the internal stress generated during machining and stabilize the machining accuracy of parts, multiple aging treatments are often arranged between rough machining and semi-finishing. For some shaft parts, aging treatment should also be arranged after the straightening process. (3) Quenching and tempering: Quenching and tempering is a high-temperature tempering treatment after quenching. It can obtain a uniform and fine-tempered troostite structure, preparing for reducing deformation during subsequent surface quenching and nitriding treatment. Therefore, quenching and tempering can also be used as a preliminary heat treatment. Since the comprehensive mechanical properties of the parts after quenching and tempering are better, it can also be used as the final heat treatment process for some parts with low requirements for hardness and wear resistance. 2. Final heat treatment The purpose of the final heat treatment is to improve mechanical properties such as hardness, wear resistance, and strength. (1) Quenching: Quenching includes surface quenching and overall quenching. Surface quenching is widely used because of its small deformation, oxidation, and decarburization. Surface quenching also has the advantages of high external strength and good wear resistance, while maintaining good toughness and strong impact resistance inside. In order to improve the mechanical properties of surface quenched parts, heat treatments such as tempering or normalizing are often required as preliminary heat treatments. The general process route is: cutting-forging-normalizing (annealing)-rough machining-tempering-semi-finishing-surface quenching-finishing. (2) Carburizing quenching: Carburizing quenching is suitable for low-carbon steel and low-alloy steel. It first increases the carbon content of the surface layer of the parts, and after quenching, the surface layer obtains high hardness, while the core still maintains a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and local carburizing. During local carburizing, anti-seepage measures (copper plating or anti-seepage material plating) should be taken for the non-carburized parts. Since carburizing and quenching cause large deformation and the carburizing depth is generally between 0.5 and 2 mm, the carburizing process is generally arranged between semi-finishing and finishing. The process route is generally: cutting-forging-normalizing-rough and semi-finishing-carburizing and quenching-finishing. When the non-carburized part of the local carburized part adopts the process plan of increasing the allowance and cutting off the excess carburized layer, the process of cutting off the excess carburized layer should be arranged after carburizing and before quenching. (3) Nitriding treatment: Nitriding is a treatment method that allows nitrogen atoms to penetrate into the metal surface to obtain a layer of nitrogen-containing compounds. The nitriding layer can improve the hardness, wear resistance, fatigue strength, and corrosion resistance of the part surface. Since the nitriding treatment temperature is low, the deformation is small, and the nitriding layer is thin (generally not more than 0.6-0.7 mm), the nitriding process should be arranged as late as possible. In order to reduce the deformation during nitriding, high-temperature tempering is generally required to eliminate stress after cutting. Post time: Feb-06-2025