The hardening and quenching heat treatment does not change the chemical composition of the starting material and is the combination of two phases: quenching and subsequent tempering.
The cooling speed must be all the higher the more the CCT (Continuous Cooling Transformation) curves are shifted to the left; therefore the critical quenching speeds are maximum for steels with basic hardenability and minimum for steels defined as self-hardening that allow a martensitic transformation even with cooling in calm air. The structure to be obtained is a tetragonal martensite.
The oil quenching treatment involves a complete austenitization at a temperature above Ac3, generally higher than 20° ÷ 50°C, followed by a rapid cooling in oil such as to exceed the critical quenching speed, so that the transformation takes place between Ms and Mf.
The martenistic salt quenching and tempering are a thermal treatment applied as interrupted quenching of steels typically in a bath of molten salt at a temperature higher than the martensite initiation temperature. The objective is to delay cooling for a certain period of time to render the temperature uniform throughout the piece. This reduces to a minimum the risk of deformation, fracture formation and residual stresses.
The martensitic quenching in polymer-in-water emulsion can be superficial or deep and the cooling rate is regulated according to the concentration of the polymers in water. The disadvantages of polymers concern the risk of fracture formation and increase in residual stresses due to the severity of the quenching.
The tempering is a complementary subsequent treatment to the hardening and is carried out in FVF furnaces. The tempering temperature depends on the final characteristics to be achieved. For that purpose, we distinguish:
- Stress relieving tempering: to be performed at temperatures between 150 and 250°C. It aims at the transformation of tetragonal martensite (hard and brittle) into tempered martensite.
- Softening tempering: it is carried out at moderately high temperatures (550 ÷ 650°C). It aims at the transformation of tetragonal martensite into sorbite (through the coalescence of finely dispersed cementite) with a reduction of hardness and obtaining an increased toughness.