THE CALCULATION OF EQUILIBRIUM PHASE-COMPOSITION OF AUSTENITE, M23C6 AND M7C3 CARBIDES IN FE-CR-NI-C SYSTEM AT 1000-DEGREES-C

Equilibrium states of the multicomponent Fe-Cr-Ni-C system containing up to 25 % Cr and 35 % Ni at 1000-degrees-C are calculated using thermodynamic data verified experimentally in a more simple systems and appropriate Hillert-Staffansson model of regular solid solution described by two sublattices. The boundaries of two- and three-phase regions as well as the chemical composition and the amount of occurring phases (austenite, M23C6 and M7C3 carbides) were determined. The results obtained were verified experimentally from the carburizing tests and the phase analysis of high alloy Cr steels of similar composition. Observed differences do not exceed usual discrepancies arising from the confrontation of various literature sources, that mostly, however, deal with the Fe-Cr-C system. From the data given in the present work quick and easy determination of supposed phase composition of CrNi steels in equilibrium state at 1000-degrees-C not only in the dependence on varying carbon content, but also in the carburization state after operation in the carburizing atmosphere is possible. The calculation can be eventually extended to other exploitation temperatures and to a more complex chemical composition.