Nucleation and precipitation kinetics of M23C6 and M2N in an Fe-Mn-Cr-C-N austenitic matrix and their relationship with the sensitization phenomenon

This work addresses the austenite decomposition in Fe-20Mn-12Cr-0.24C-0.32N steel. Decomposition of austenite in C + N steels leads to the formation of M23C6 and M2N precipitates. Owing to the fact that decomposition is a temperature- and time-dependent phenomenon, thermodynamic and diffusion simulations of nucleation and growth of precipitates were performed based on the CALPHAD method. A well-known consequence of decomposition is sensitization of the areas surrounding the carbides and nitrides, which is normally associated with Cr depletion. In this study, the influence of element redistribution on the Gibbs free energy of the austenitic phase was analyzed. From the electrochemical equivalence of the difference in the Gibbs energy and the difference in the potential, it was found that the sensitized areas exhibit a less noble potential. A mechanism based on a sensitized anode and a matrix cathode is proposed and is correlated with experimental measures of intergranular corrosion.