Thermodynamics on the Modification of Inclusions by Ca and Mg Treatment in GCr18Mo Bearing Steel

The effect of Mg and Ca treatment on the modification of inclusions in GCr18Mo bearing steel at 1873K was studied by thermodynamic calculations. The stability phase diagrams among Mg, Ca, Al, and O were investigated to understand the fundamentals about the formation of inclusions. The calculated results showed that: (1) Both the alumina and spinel can be modified to MgO by Mg treatment in the case of the dissolved [Mg] = 5.9ppm ~ 40ppm, corresponding the dissolved [Al] ≤ 0.056%, which can control the [O] ≤ 3ppm. (2) The MgO·Al2O3 spinel can be modified to 12CaO·7Al2O3 liquid inclusion by Ca treatment in the case of the optimal [Al] = 0.07% ~ 0.31% and the dissolved [Mg] ≤ 5.2ppm, the [O] can be controlled at about 3ppm. (3) The alumina can be modified to liquid calcium aluminate inclusions by Ca treatment in the case of the dissolved [Al] = 0.03% ~ 0.29% and the dissolved [Ca] = 13ppm ~ 80ppm, correspondingly the equilibrium [O] is about 3ppm. These results of thermodynamic calculation will provide a detailed thermodynamic reference for GCr18Mo bearing steel smelting.