In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in the machinery industry, the microstructure of steel is complex and can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment.

In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance.

Heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but by changing the microstructure of the workpiece, or changing the chemical composition of the workpiece surface, to give or improve the performance of the workpiece. It is characterized by improving the intrinsic quality of the workpiece, which is generally not visible to the naked eye.

The role of heat treatment is to improve the mechanical properties of the material, eliminate residual stress and improve the machinability of the metal. According to the different purposes of heat treatment, the heat treatment process can be divided into two categories: preparatory 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 of more than 0.5% are often annealed to reduce their hardness and easy to cut. Carbon steel and alloy steel with a carbon content of less than 0.5% are normalized to avoid sticking to the knife when cutting with low hardness. Annealing and normalizing can still refine the grain, uniform tissue, for future heat treatment preparation. Annealing and normalizing are often arranged after the manufacture of the blank and before roughing.

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, parts with high precision requirements (such as the box of the coordinate boring machine, etc.) should be arranged twice or several times. Simple parts are generally not subject to aging treatment.

In addition to castings, for some poor rigidity of precision parts (such as precision screw), in order to eliminate the internal stress generated in the processing and stabilize the machining accuracy of the parts, it is often arranged for multiple aging treatments between rough machining and semi-finishing. Some shaft parts processing, in the straightening process also want to arrange aging treatment.

3) Quenching and tempering

Quenching is the high temperature tempering treatment after quenching, it can obtain uniform and fine tempering sothosome tissue, for the future surface quenching and nitriding treatment to reduce deformation preparation, so the quenching can also be used as a preparatory heat treatment.

Due to the comprehensive mechanical properties of the parts after quenching and tempering, some parts with low hardness and wear resistance can also be used as the final heat treatment process.

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

There are surface quenching and overall quenching. Among them, surface quenching is widely used because of its small deformation, oxidation and decarburization, and surface quenching also has the advantages of high external strength, good wear resistance, and good internal toughness and strong impact resistance. In order to improve the mechanical properties of surface quenching parts, heat treatment such as quenching and tempering or normalizing is often required as a preparatory heat treatment. The general process route is: blanking-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 steel and low alloy steel. First, the carbon content of the surface layer of the part is increased. After quenching, the surface layer obtains high hardness, while the core still maintains certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and local carburizing. Anti-seepage measures (copper plating or anti-seepage material plating) shall be taken for the non-carburized part during local carburizing. Due to the large deformation of carburizing and quenching, and the depth of carburizing is generally between 0.5 and 2mm, the carburizing process is generally arranged between semi-finishing and finishing.

The process route is generally: material-forging-positive fire-rough, semi-finishing-carburizing 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 margin, the process of removing the excess carburized layer should be arranged after carburizing and before quenching.

3) Nitriding treatment

Nitriding is a treatment in which nitrogen atoms 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 surface of the part. Because the nitriding treatment temperature is low, the deformation is small, and the nitriding layer is thin (generally not more than 0.6~0.7mm), the nitriding process should be arranged as far as possible. In order to reduce the deformation during nitriding, it is generally required to eliminate stress after cutting.