Quenching is followed by tempering when the steel is heated below the austenitization temperature and kept at the appropriate temperature for a certain time. The essential purpose of tempering is to reduce the hardness of steel and increase the toughness.
Annealing of steel is called heat-treated when it is heated at the appropriate speed of the material and then held at a suitable temperature in the austenitic range. With proper cooling after holding at temperature, we want to achieve state of microstructure as balanced as possible. The purpose of annealing is to improve dimensional stability, machinability, working, achieve mechanical properties and reduce hardness.
Quenching is a type of heat treatment where the steel is heated in the austenite area. Then follows accordingly to steel grade rapid cooling in various hardening agents. The most used agents are water, oil, salt bath and air. This increases the hardness, making the steel more brittle and less tough. Temperature differences occur between the surface and the inside of material, which increase with the higher cooling rate. These differences can lead to the fault of the material or even to the cracks, so it is important to provide a certain steel with the appropriate hardening agent.
After casting, the melt solidifies in the moulds and thus passes from the liquid to the solid state and the microstructure is called casted. Such a microstructure is mostly unsuitable for the end use of a particular product, so we have to make heat treatment, which means that steel is again in the furnace and treated at high temperatures.
Holding time, temperature and cooling method or cooling rate depends on the quality of the steel and the required mechanical properties. In general, we know three basic methods of heat treatment, other types of heat treatment are used for special purposes. Basic treatments as follows in chronological order:
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