Isothermal hardening

Isothermal hardening is the operation of heating steel to an austenitic state, rapid cooling to the temperature range of relative stability of supercooled austenite and holding until the complete or partial transformation of austenite into bainite.

The main purpose of isothermal hardening is to obtain a microstructure of Bainite, which is characterized by high and medium hardness values, sufficient viscosity, has high strength and resistance to impact loads.

Isothermal hardening allows to significantly eliminate the large difference in the cooling rate of the surface and core of products, which is the main cause of the formation of thermal stresses. In addition, during isothermal hardening, the diffusionless martensitic transformation is replaced by a diffusion intermediate transformation, which occurs gradually and simultaneously throughout the entire cross-section. This reduces the formation of structural stresses. When cooling after isothermal holding, due to the completed transformation of austenite, structural changes no longer occur. The absence of significant thermal and structural stresses sharply reduces defects associated with the formation of cracks, warping and changes in the dimensions of parts.

It is not possible to obtain the Bainite structure using other hardening methods.