Thermodynamic consideration is an effective procedure for calculation of inclusion composition prediction. In general, Henrian standard state is used, and the activity coefficients of the constituents are represented by Wagner's formalism in thermodynamic calculations. This formalism is commonly used in low alloy steel, and this is not recommended for high alloy steel. High alloy steel is detailed thermodynamically using the Raoultian standard state, and the activity coefficients of the constituents are converted by Miki and Hino's formalism in this study. The equilibrium relation between Al2O3, MgO, spinel (MgO·Al2O3) and dissolved Al, Mg and O in Fe–40mass%Ni–5mass%Cr alloy (Spacer Frame for fluorescent display) were discussed. Plant data of Fe–40mass%Ni–5mass%Cr alloy were compared thermodynamically with the calculated results. It was found that Wagner's formalism can't be useful for inclusion composition prediction of Fe–40mass%Ni–5mass%Cr alloy, Miki and Hino's formalism should be utilized for high alloy steel.
