Effect of carbon and nitrogen on Md30 in metastable austenitic stainless steel

Abstract

Md30 is defined as the temperature at which 50 vol.% of α-martensite is formed at a true tensile strain of 0.3 in metastable austenitic steels. It has been generally believed that the effect of carbon content on Md30 was estimated to be identical to that of nitrogen as shown by Nohara's equation. However, we found in this study that Md30 in carbon-added steel is lower than that in nitrogen-added steel, which indicates that the effect of carbon content on the mechanical stability of austenite is more significant than that of nitrogen. In addition, the relationship between Md30 and carbon and nitrogen content is not linear. The effect of carbon and nitrogen content on Md30 is higher at lower carbon and nitrogen content region (<0.1%). As this effect was not considered in the previous study, the austenite-stabilizing effects of both the elements were underestimated. Therefore, in this study, new equations are proposed to accurately estimate Md30 of a Fe-Cr-Ni alloy system. As a result, modified Md30 equation is suggested as below: Md30(K) = 800 - 333√Ceq - 10.3Si - 12.5Mn - 10.5Cr - 24.0Ni - 5.6Mo Carbon equivalent, CCeq is a function of carbon and nitrogen concentrations and temperature. CCeq = C + aN a = 0.931 - 0.000281exp (0.0219T) Above equations show that the difference in austenite-stabilizing effects of carbon and nitrogen increases with rising temperature, owing to the difference in stacking fault energy between carbon-added and nitrogen-added steels.