Why should mechanical parts be heat treated?

In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to the rational selection of materials and various forming processes, heat treatment processes are 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. Therefore, the heat treatment of steel is the main content of metal heat treatment.

In addition, aluminum, copper, magnesium, titanium, and the like can also be modified by heat treatment to obtain different mechanical properties, physical properties, and chemical properties.

Heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but rather imparts or improves the performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition of the surface of the workpiece. It is characterized by improved 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: preliminary heat treatment and final heat treatment.

1.Preparatory heat treatment

The purpose of the preliminary heat treatment is to improve the processability, eliminate the internal stress, and prepare a good metallographic structure for the final heat treatment. The heat treatment process includes annealing, normalizing, aging, quenching and tempering.

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 easy to cut to reduce their hardness, and are often annealed; carbon steel and alloy steel with a carbon content of less than 0.5%, in order to avoid the hardness is too low, the cutting knife is used when cutting. It is treated with normalizing. Annealing and normalizing can refine grains and uniform structure for later heat treatment. Annealing and normalizing are often arranged after the blank is manufactured and before the roughing.

2) aging treatment

The aging treatment is mainly used to eliminate the internal stress generated in the blank manufacturing and machining.

In order to avoid excessive transportation workload, for general precision parts, arrange an aging treatment before finishing. However, parts with higher precision requirements (such as the box of the coordinate boring machine) should be arranged twice or several times. Simple parts are generally not subject to aging.

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 the parts, multiple aging treatments are often arranged between roughing and semi-finishing. Some shaft parts are processed and aging treatment is also required after the straightening process.

3) tempering

The quenching and tempering is a high-temperature tempering treatment after quenching, which can obtain a uniform and fine tempered sorbite structure, and prepare for the subsequent surface quenching and nitriding treatment to reduce deformation, so 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 good, parts with low hardness and wear resistance are also required 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

Quenching has surface quenching and overall quenching. Among them, surface quenching is widely used because of deformation, oxidation and decarburization, and surface quenching also has the advantages of high external strength, good wear resistance, and good internal toughness and impact resistance. In order to improve the mechanical properties of surface-hardened parts, heat treatment such as quenching or normalizing is often required as a preliminary heat treatment. The general process route is: blanking-forging--normalizing (annealing)--roughing--tempering--semi-finishing--surface hardening--finishing.

2) Carburizing and quenching

Carburizing and quenching is suitable for low carbon steel and low alloy steel. Firstly, the carbon content of the surface layer of the part is improved. After quenching, the surface layer obtains high hardness, while the core still maintains a certain strength and high toughness and plasticity. Carburizing is a total carburizing and local carburizing. For partial carburization, anti-seepage measures (copper plating or plating impervious materials) shall be adopted for the non-carburizing 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-coarse, semi-finishing-carburizing quenching-finishing.

When the non-carburized part of the partially carburized part is increased in excess, the process of cutting off the excess carburized layer shall be arranged after carburizing and before quenching.

3) Nitriding treatment

Nitriding is a treatment in which a nitrogen atom is infiltrated into a metal surface to obtain a nitrogen-containing compound. 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 after cutting. Perform high temperature tempering to eliminate stress.

Reprinted from the network