Correlation between mechanical stability and hardness of austenite in martensite/austenite mixtures

Abstract

The hardness of austenite in Fe−1.5%Mn−(0.44–1.46)%C steels with microstructures consisting of austenite and martensite was estimated by an extrapolation procedure. The procedure involved heat treatments to obtain different fractions of athermal martensite. For each carbon concentration, the variation of hardness with martensite fraction was non-linear. Linear and inverse rules of mixtures could not account for the observed hardness variations, which was justified by the metastability of austenite. The hardness variations could be described by exponential fits, which were subsequently extrapolated to zero percent martensite to obtain the hardness of austenite. The variation of extrapolated hardness with carbon concentration in the range of 0.44–1.46% indicated maximum hardness at intermediate carbon concentrations. This behavior was interpreted in the context of the influence of carbon on the mechanical stability of austenite and on the hardness of the product martensite. On this basis, the lowest extrapolated austenite hardness was obtained in the steel with the highest carbon concentration due to the suppression of the deformation-induced martensite formation.