Hardening processes are used to give the elements special mechanical properties to ensure that they are suitable for the final application.
Tempering is a low temperature (from 150ºC to 650ºC) heat treatment used to remove the stresses and brittleness caused by the hardening process and ensures that the desired mechanical properties of the object are finally obtained.
Steel hardening methods
Normal hardening – It involves rapid cooling in a cooling bath, usually water or oil bath, below the martensitic transformation start temperature, up to the ambient temperature or preferably to the end temperature of the martensitic transformation. The cooling rate should be chosen so that hardening deformations do not occur. Cooling in water is more intense than in oil.
Gradual hardening – It involves rapid cooling in a quenching bath, usually from molten nitrate, to a temperature just above the martensitic transformation temperature and to maintain at this temperature, so that the temperature in the whole section of the object will be equalized. In the second phase, already in a water or oil bath, further cooling takes place in order to obtain a martensitic transformation. The advantage of this method is to avoid hardening stresses. However, it requires a lot of practice in determining the time for an intermediate bath.
Isothermal hardening – It is hardening, in which martensitic transformation does not occur. The heated object is kept in a bath of molten nitrate or lead at a temperature above the beginning of the martensitic transformation. The name of the method comes from the fact that the bath maintains a constant temperature. In this type of hardening, martensite is not formed, but austenite breaks down into other phases, e.g. bainite, giving the steel properties similar to tempering after quenching. The advantage of the method is the lack of hardening stresses, but it is a long-lasting process, sometimes extending up to several hours.
Surface hardening – A method in which the whole object is not heated (through hardening), but only the surface of the object. Therefore, only the surface layer undergoes hardening. Used wherever it is required to harden only fragments of the surface of the object. There are several surface hardening methods.