The function of heat treatment is to improve the mechanical properties of materials, eliminate residual stress and improve the machinability of metals. According to 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 machinability, eliminate internal stress and prepare good metallographic structure for 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 hot worked blanks. Carbon steel and alloy steel with carbon content greater than 0.5% are often annealed in order to reduce their hardness and easy to cut; For carbon steel and alloy steel with carbon content less than 0.5%, normalizing treatment is adopted in order to avoid tool sticking during cutting. Annealing and normalizing can refine the grain and uniform structure, so as to prepare for future heat treatment. Annealing and normalizing are often arranged after blank manufacturing 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 accuracy, an aging treatment can be arranged before finishing. However, for parts with high accuracy requirements (such as the box of coordinate boring machine, etc.), two or more aging treatment processes shall be arranged. Aging treatment is generally not required for simple parts.

 

In addition to castings, for some precision parts with poor rigidity (such as precision lead screw), in order to eliminate the internal stress generated in machining and stabilize the machining accuracy of parts, multiple aging treatment is often arranged between rough machining and semi-precision machining. For the processing of some shaft parts, aging treatment shall also be arranged after the straightening process.

 

(3) Conditioning

 

Quenching and tempering is the high-temperature tempering treatment after quenching. It can obtain uniform and fine tempered sorbite structure and prepare for reducing deformation during surface quenching and nitriding in the future. Therefore, quenching and tempering can also be used as preliminary heat treatment.

 

Due to the good comprehensive mechanical properties of the quenched and tempered parts, some parts with low requirements for hardness and wear resistance can also be used as 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 small deformation, oxidation and decarburization. Moreover, surface quenching also has the advantages of high external strength, good wear resistance, 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) - rough machining - quenching and tempering - semi finishing - surface quenching - finishing.

 

(2) Carburizing quenching

 

Carburizing and quenching is applicable to low carbon steel and low alloy steel. Firstly, increase the carbon content of the surface layer of the parts, and obtain high hardness after quenching, while the core still maintains a certain strength and high toughness and plasticity. Carburizing can be divided into integral carburizing and local carburizing. During partial carburization, anti-seepage measures (copper plating or anti-seepage material plating) shall be taken for the non carburized part. As the carburizing quenching deformation is large and the carburizing depth is generally between 0.5 ~ 2mm, the carburizing process is generally arranged between semi finishing and finishing.

 

The process route is generally as follows: blanking forging normalizing roughing and semi finishing carburizing quenching finishing. When the non carburized part of local carburized parts adopts the process scheme of cutting off the excess carburized layer after increasing the allowance, the process of cutting off the excess carburized layer shall be arranged after carburization and before quenching.

 

(3) Nitriding treatment

 

Nitriding is a treatment method to make nitrogen atoms penetrate into the metal surface to obtain a layer of nitrogen-containing compounds. Nitriding layer can improve the surface hardness, wear resistance, fatigue strength and corrosion resistance of parts. As the nitriding treatment 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 late as possible. In order to reduce the deformation during nitriding, high-temperature tempering to eliminate stress is generally required after cutting.