Phase Equilibria of the MnO–SiO ₂ –Al ₂ O ₃ –MnS System

Abstract

Phase equilibria of the MnO–SiO 2 –MnS system in the temperature range of 1185°–1400°C and the MnS solubility in the MnO–SiO 2 –Al 2 O 3 –MnS system at 1200°C under low oxygen partial pressure have been experimentally investigated using equilibration/quenching technique followed by electron probe X‐ray microanalysis and X‐ray diffraction analysis. Based on the experimental results, polythermal projection of the MnO–SiO 2 –MnS system was estimated. The system has two ternary eutectic points; namely, one at 1173°± 3°C where phases of MnS, MnO, “A” (67.0 mass% MnO–23.6 mass% SiO 2 –9.4 mass% MnS), and liquid coexist, and the other at 1187°± 3°C where phases of MnS, Mn 2 SiO 4 , MnSiO 3 , and liquid coexist. All invariant reactions observed above were discussed. The ternary compound “B” containing 74.4 mass% MnO–22.0 mass% SiO 2 –3.6 mass% MnS was observed at 1300°C. In the MnO–SiO 2 –Al 2 O 3 –MnS system, the MnS solubility is strongly affected by Al 2 O 3 content and also the MnO/SiO 2 ratio. The solubility increased as MnO/SiO 2 ratio increased at a constant Al 2 O 3 content and decreased as Al 2 O 3 content increased at a constant MnO/SiO 2 ratio. In addition, depending on the Al 2 O 3 content and MnO/SiO 2 ratio, the liquid phase is possibly saturated with not only MnS but also other solid oxide phases. In view of inclusion utilization for free‐cutting steel, it might have an advantage to decrease the Al 2 O 3 content and increase the MnO/SiO 2 ratio.