Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and chemical composition of the whole workpiece, but endows or improves the performance of the workpiece by changing the internal microstructure or chemical composition of the workpiece surface. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye.

　　In order to make the metal parts have the mechanical, physical and chemical properties required, heat treatment process is often necessary in addition to the reasonable selection of materials and various forming processes. Steel is a widely used material in mechanical 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, the mechanical, physical and chemical properties of aluminum, copper, magnesium, titanium and their alloys can also be changed by heat treatment to obtain different service properties. Annealing quenching tempering

　　1、 Types of annealing

　　1. Complete annealing and isothermal annealing

　　It is mainly used for casting, forging and hot rolling sections of various carbon and alloy steels with hypoeutectoid composition, and sometimes for welding structures. It is usually used as the heat treatment of some non heavy work pieces or as the pre heat treatment of some workpieces.

　　2. Spheroidizing annealing

　　Tools and tools used in the manufacture of eutectoid steel, such as carbon steel. The main purpose is to reduce the hardness, improve the machinability and prepare for quenching later.

　　3. Stress relief annealing

　　Stress relief annealing, also known as low temperature annealing (or high temperature tempering), is mainly used to eliminate the residual stress of castings, forgings, weldments, hot rolled pieces, cold drawn parts, etc. If these stresses are not eliminated, the steel parts will be deformed or cracked after a certain period of time or in the subsequent cutting process.

　　2、 During quenching, the common cooling media are brine, water and oil. Salt water quenched workpiece is easy to have high hardness and smooth surface, and is not easy to produce hardened soft spots, but it is easy to cause serious deformation and even cracking of the workpiece. Oil as quenching medium is only suitable for quenching some alloy steel or small size carbon steel workpieces with high stability of undercooled austenite.

　　3、 Purpose of steel tempering

　　1. Reduce brittleness, eliminate or reduce internal stress. There are great internal stress and brittleness in steel parts after quenching. If the steel parts are not tempered in time, the steel parts will deform or even crack.

　　2. Obtain the mechanical properties required by the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the requirements of different properties of various workpieces, the hardness can be adjusted by proper tempering to reduce the brittleness and the required toughness and plasticity.

　　3. Stabilize the workpiece size

　　4. For some alloy steels which are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing) to make carbides in steel gather properly and reduce hardness, so as to facilitate cutting.