Estimation of True Hardness and Quantitative Evaluation of Auto-Tempering in As-Quenched Martensitic Steels

The hardness of martensitic steels with high M_s temperatures is reduced by auto-tempering after transformation, therefore the true hardness of martensite with carbon in fully solid solution is not known. In this study, we investigated a method to quantitatively evaluate the true hardness of quenched martensite unaffected by auto-tempering and the effect of auto-tempering was quantitatively evaluated by the diffusion area of carbon in bcc iron at temperatures below 400°C. As a result, it was clarified that the effect of auto-tempering is more pronounced in steels with an M₅₀ temperature higher than 300°C and that the softening behavior of martensitic steels can be uniformly evaluated regardless of the carbon content if the activation energy of carbon diffusion is known. Furthermore, it was clarified that the degree of auto-tempering can be quantitatively evaluated by calculating the integral diffusion area S (= ∑Dt) below the M₅₀ temperature during quenching.