Thermodynamics of the solid phases in the system Fe−Mn−C

Abstract

Phase relationships in the Fe−Mn−C system in the temperature range 600 to 1100°C have been studied using metallographic and X-ray methods and the electron microprobe. Isothermal sections of the phase diagram of the system are reported based on the present results and those of earlier investigators. The fcc λ-phase (austenite) containing carbon is stable at all values ofy Mn=x Mn/(x Mn+x Fe) in the range 890 to 1100°C and in a more restricted composition range at lower temperatures. Its composition under conditions of equilibrium with the carbides (Fe, Mn)3C, (Fe, Mn)23C6, e, and liquid are shown for several temperatures. The free energy of formation of the cementite phase, (Fe, Mn)3C, at 1000°C, from γ-Fe, γ-Mn (undercooled) and graphite is ΔG 1273=−35,790y Mn−2760y Fe+3RT (y Mn lny Mn+y Fe lny Fe). The data show that the alloyed cementite is essentially and ideal mixture of Fe3C and Mn3C,i.e., the metal atoms are distributed at random on the metal sites in the lattice.