Effect of Carbon and Nitrogen on Md30 in Metastable Austenitic Stainless Steel

Md-30 is defined as the temperature at which 50 vol.% of alpha'-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 Md-30 was estimated to be identical to that of nitrogen as shown by Nohara's equation. However, we found in this study that Md-30 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 Md-30 and carbon and nitrogen content is not linear. The effect of carbon and nitrogen content on Md-30 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 Md(30)( )of a Fe-Cr-Ni alloy system. As a result, modified Md-30 equation is suggested as below: Md-30(K) = 800 - 333 root C-eq - 10.3Si - 12.5Mn - 10.5Cr - 24.0Ni - 5.6Mo Carbon equivalent, C-eq is a function of carbon and nitrogen concentrations and temperature. C-eq = C = aN a = 0.931 - 0.000281 exp (0.02197) 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.