Metal heat treatment is a process of heating a metal workpiece to a suitable temperature in a certain medium, keeping it at this temperature for a certain time, and then cooling it at different speeds to obtain the required mechanical, physical and chemical properties of the metal workpiece. Steel, aluminum, copper, magnesium, titanium and their alloys can also be heat treated.
Heat treatment of metal materials is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the working performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition on the surface of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece, which is generally invisible to the naked eye.
The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes there are only two processes of heating and cooling. These processes are interrelated and uninterrupted.
Metal heat treatment processes can be roughly divided into three categories: integral heat treatment, surface heat treatment and chemical heat treatment. According to the difference of heating medium, heating temperature and cooling mode, each type can be divided into several different heat treatment processes. The same metal can obtain different structures through different heat treatment processes, thus having different properties.
Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to change its overall mechanical properties. The integral heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.
Annealing is to heat the workpiece to a suitable temperature, adopt different holding time according to the material and size of the workpiece, and then slowly cool it, with the purpose of making the internal structure of the metal reach or approach the equilibrium state, obtaining good technological performance and service performance, or preparing the structure for further quenching.
Normalization is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the obtained microstructure is finer, which is often used to improve the cutting performance of materials and sometimes as the final heat treatment of some parts with low requirements.
Quenching is to heat and insulate the workpiece, and then quickly cool it in quenching media such as water, oil or other inorganic salts, organic water solution, etc. After quenching, steel becomes hard, but at the same time it becomes brittle.
In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature but lower than 650℃ for a long time, and then cooled. This process is called tempering.
Annealing, normalizing, quenching and tempering are the "four fires" in the whole heat treatment, among which quenching and tempering are closely related, and they are often used together and are indispensable. The "Four Fires" evolved heat treatment processes with different heating temperatures and cooling modes.
In order to obtain certain strength and toughness, the process of combining quenching with high temperature tempering is called quenching and tempering.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. Generally speaking, it can guarantee and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the microstructure and stress state of the blank, so as to facilitate various cold and hot processing. For example, white cast iron can be annealed for a long time to obtain malleable cast iron and improve its plasticity; Using the correct heat treatment process, the service life of gears can be doubled or even dozens of times longer than that of gears without heat treatment; In addition, cheap carbon steel has some properties of expensive alloy steel by infiltrating some alloying elements, which can replace some heat-resistant steels and stainless steels; Almost all tools and molds need heat treatment before use.