Hardening involves heating to temperatures above the critical points and consists of several phases:
It is important that the tempering temperature, that is the temperature to which the steel must be brought in the heating phase, must be greater than 50 ° at maximum AC3, i.e. when the austenite / ferrite transformation is complete and which varies according to carbon content and steel alloying agents.
The cooling is then carried out very quickly, so as to obtain the formation of Martensite.
Rapid cooling is ensured by the hardening means, i.e. liquids that abruptly cool the steel.
The steel at 800 ° C (this is an example; the temperatures of the critical points vary as the percentages of the alloy elements vary) is immersed in a liquid: water, oil, polymer.