Phase equilibrium in liquid iron-carbon alloys doped with yttrium and chromium

In the framework of thermodynamic modeling, the interaction between the metallic melt components and oxygen is considered, as applied to the analysis of deoxidation and alloying processes in the low-carbon low-alloy steels containing chromium and yttrium. In this paper a plotting method solubility surfaces of components in a liquid metal of the Fe-Y-Cr-O-C thermodynamic system was used. Within the scope of implementing that method, the Y2O3-Cr2O3 system has been studied in detail, the enthalpy and entropy values of Y2O3•Cr2O3 have been determined, and the coordinates of eutectic points in the Y2O3-Cr2O3 system have been adjusted. Based on the adjusted coordinates of the liquidus curves for oxide binary FeO-Cr2O3, FeO-Y2O3, and Y2O3-Cr2O3 systems, the energy parameters of Qijkl in the FeO-Cr2O3-Y2O3 oxide melt component activity coefficient-composition equations for the sub-regular ionic solutions model have been determined and the coordinates of the liquidus surface of the FeO-Cr2O3-Y2O3 system have been calculated. Since carbon is not soluble in an oxide system conjugated with a field of liquid metal, it has been found that oxides Y2O3 and Cr2O3, Y2O3•Cr2O3, the oxide melt (FeO, Cr2O3, Y2O3) and the gas phase of {CO, CO2} form as the condensed non-metallic phases after chrome alloying and yttrium modification of the metal. Additional thermodynamic calculations suggested the possibility of the oxide melt containing up to 2 % of bivalent chrome (Cr2+). The system of phase equilibria developed during the study allowed to establish the formation constants for oxides, oxide compounds, oxide melt components, and gas phases. Activity coefficients for the metal melts components have also been calculated using Wagner’s method. The set of thermodynamic parameters established made it possible to plot the solubility surface of liquid metal components and link the liquid metal phase compositions to the conjugated oxide phase composition.