THERMODYNAMIC ANALYSIS OF ISOTHERMAL TRANSFORMATIONS OF HYPO-EUTECTOID FE-C AUSTENITES

The thermodynamics of decomposition reactions of undercooled austenite in Fe-C alloys were studied using a regular solution sublattice model for austenite and ferrite and taking cementite as a stoichiometric co mpound. From a Gibbs free energy point of view intermediate transforma tions, not leading to stable phases, are possible: carbide formation a bove the A1 temperature, and carbon enrichment in austenite below the A1 temperature. The differences in chemical potential of Fe and of C b etween both sides of the transformation front were calculated as a fun ction of the degree of transformation and related to the kinetics of t ransformation interface progress and carbon rearrangement, respectivel y. Experimental evidence for the occurrence of carbide formation above the A1 temperature was obtained. A thermodynamic calculation of the o nset of the pearlite formation in hypoeutectoid Fe-C alloys is present ed.