In order to make metal workpiece have required mechanical properties, physical properties and chemical properties, in addition to reasonable selection of materials and various forming processes, heat treatment processes are often indispensable. Steel is the most widely used material in machinery industry. Microstructure of steel is complex and can be controlled by heat treatment. Therefore, heat treatment of steel is 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 properties.
Heat treatment generally does not change shape of workpiece and overall chemical composition, but by changing internal microstructure of workpiece, or changing chemical composition of surface of workpiece, to give or improve performance of workpiece. Its characteristic is to improve internal quality of workpiece, which is generally not visible to naked eye.

 

Purpose of preliminary heat treatment is to improve processing performance, eliminate internal stress and prepare a good metallographic structure for final heat treatment. Heat treatment process includes annealing, normalizing, aging, quenching and tempering.

Annealing and normalizing are used for hot processed 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; carbon steel and alloy steel with carbon content less than 0.5% are normalized in order to avoid sticking to tool when hardness is too low. Annealing and normalizing can still refine grains and uniform structure, preparing for subsequent heat treatment. Annealing and normalizing are often arranged after blank is manufactured and before rough machining.

Aging treatment is mainly used to eliminate 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 higher precision requirements (such as 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 internal stress generated during machining and stabilize machining accuracy of parts, multiple aging treatments are often arranged between rough machining and semi-finish machining. Some shaft parts are processed with aging treatment after straightening process.

Quenching is high temperature tempering treatment after quenching. It can obtain a uniform and fine tempered sorbite structure to prepare for reduction of deformation during subsequent surface quenching and nitriding treatment. Therefore, quenching and tempering can also be used as a preliminary heat treatment.
Due to better comprehensive mechanical properties of parts after quenching and tempering, some parts that do not require high hardness and wear resistance can also be used as final heat treatment process.

 

Purpose of final heat treatment is to improve mechanical properties such as hardness, wear resistance and strength.

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

Carburizing and quenching is suitable for low-carbon steel and low-alloy steel. First, increase carbon content of surface layer of part. After quenching, surface layer will obtain high hardness, while core part still maintains a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and partial carburizing. In the case of partial carburizing, anti-seepage measures (copper plating or anti-seepage material plating) should be taken for non-carburized part. Because carburizing quenching deformation is large, carburizing depth is generally between 0.5~2mm, carburizing process is generally arranged between semi-finishing and finishing.
Process route is generally: blanking-forging-normalizing-rough and semi-finishing-carburizing and quenching-finishing.
When non-carburized part of partial carburized parts adopts process plan of removing excess carburized layer after increasing margin, process of removing excess carburized layer should be arranged after carburizing and before quenching.

Nitriding is a treatment method that allows nitrogen atoms to penetrate into metal surface to obtain a layer of nitrogen-containing compounds. Nitriding layer can improve surface hardness, wear resistance, fatigue strength and corrosion resistance of part. Since nitriding treatment temperature is low, deformation is small, and nitriding layer is thin (generally no more than 0.6~0.7mm), nitriding process should be arranged as far as possible. In order to reduce deformation during nitriding, high temperature tempering for stress relief is generally required after cutting.