The function of heat treatment is to improve the mechanical properties of steel pipes and precision steel pipes, eliminate residual stress and improve the machining performance of steel pipes.

According to the different purposes of heat treatment, the heat treatment process can be divided into two categories: preliminary heat treatment and final heat treatment.

1. Preparatory heat treatment
The purpose of preparatory heat treatment is to improve processability, eliminate internal stress and prepare a good metallographic structure for final heat treatment. The heat treatment process includes annealing, normalizing, aging, quenching and tempering, etc.
(1) Annealing and normalizing
Annealing and normalizing are used for hot worked blanks. Carbon steel and alloy steel with a carbon content greater than 0.5% are often annealed in order to reduce their hardness and easy to cut; carbon steel and alloy steel with carbon content less than 0.5%, in order to avoid sticking to the knife when their hardness is too low, And the use of normalizing treatment. Annealing and normalizing can still refine grains and uniform structure to prepare for subsequent heat treatment. Annealing and normalizing are usually scheduled after blank fabrication and before rough machining.
(2) Aging treatment
Aging treatment is mainly used to eliminate the internal stress generated in blank manufacturing and machining.
In order to avoid excessive transportation workload, for parts with general precision, an aging treatment can be arranged before finishing. However, for parts with high precision requirements (such as the box of a coordinate boring machine, etc.), two or several aging treatment procedures should be arranged. Simple parts are generally not subject to 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 processing and stabilize the machining accuracy of the parts, multiple aging treatments are often arranged between roughing and semi-finishing. For some shaft parts, aging treatment should also be arranged after the straightening process.
(3) Quenching and tempering
Quenching and tempering are to perform high-temperature tempering after quenching, which can obtain a uniform and fine tempered sorbate structure and prepare for the reduction of deformation during subsequent surface quenching and nitriding treatment. Therefore, quenching and tempering can also be used as preliminary heat treatments.
Due to the good comprehensive mechanical properties of the parts after quenching and tempering, some parts that do not require high hardness and wear resistance can also be used in the final heat-treatment process.

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 integral quenching. Among them, surface quenching is widely used because of less deformation, oxidation and decarburization, and surface quenching also has the advantages of high external strength and good wear resistance, while maintaining good internal toughness and strong impact resistance. In order to improve the mechanical properties of surface hardened parts, heat treatment such as quenching and tempering or normalizing is often required as preliminary heat treatment. The general process route is blanking — forging — normalizing (annealing) — roughing — quenching and tempering — semi-finishing — surface quenching — finishing.
(2) Carburizing and quenching
Carburizing and quenching are suitable for low carbon steel and low alloy steel. First, the carbon content of the surface layer of the part is increased. After quenching, the surface layer can obtain high hardness, while the core still maintains a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and local carburizing. When local carburizing, anti-seepage measures (copper plating or anti-seepage material plating) should be taken for the non-carburized part. Due to the large deformation of carburizing and quenching, 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: blanking – forging – normalizing – rough, semi-finishing – carburizing and quenching – finishing.
When the non-carburized part of the local carburized part adopts the process plan of removing the excess carburized layer after increasing the allowance, the process of removing the excess carburized layer should be arranged after carburizing and before quenching.
(3) Nitriding treatment
Nitriding is a method of infiltrating nitrogen atoms 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. Because the nitriding temperature is low, the deformation is small, and the nitriding layer is thin (generally no more than 0.6~0.7mm), the nitriding process should be arranged as far back as possible. In order to reduce the deformation during nitriding, it is generally necessary to High-temperature tempering for stress relief.

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