The variation of the hardness of high-strength steels depending on the microstructures specific of tempering

Abstract

The properties of steel depend on its chemical composition and structure. The modification of the structure in order to change the properties in a certain way is not only the main purpose of heat treatment, but also the only purpose, because the heat treatment regime influences the variation of properties through the variation of the steel structure. It is only by correlating the variation in structure with hardness that we can answer how heat treatment influences hardness, which also depends on the stress state. The tempering results are mainly influenced by the heating temperature, holding time and less sensitively by the tempering cooling rate (air, oil) as shown in the research carried out and presented in this paper. Tempering reduces hardness, internal stresses, the amount of residual austenite and increases elongation, necking and resilience at the expense of strength. The characteristics of tempering structures are clearly superior to equilibrium (annealing) structures in that the degree of dispersion of the structure is greater and the shape of the carbides and constituents is fine globular.