Sulfides and oxides in Fe−Mn alloys: Part I. Phase relations in Fe−Mn−S−O system

Abstract

This is a critical review of available equilibrium data between phases involving Fe, Mn, S, and O. Using the Morey-Williamson theorem, and that modified by Darken, the sulfur and oxygen potential diagrams are constructed for the Fe−S−O system involving nine univariant and three invariant equilibria. The solubility of sulfur in wustite-saturated iron is evaluated; the sulfur content of iron in equilibrium with wustite and liquid oxysulfide reaches a maximum of 143 ppm at 1200°C; this is about one half of that corresponding to the solidus of the Fe−S system. An estimate is made of the phase equilibria in the Fe−Mn−S−O quaternary system involving gamma iron and Mn(Fe)O phases. There is a eutectic invariant at ∼900°C, and the liquid miscibility gap invariant is estimated to be at ∼1225°C. From the expected phase relations and equilibria, it is deduced that if sufficient oxygen and sulfur,i.e. Mn(Fe)O and Mn(Fe)S phases, are present in Fe−Mn alloys, there may be a liquid oxysulfide phase present at temperatures above 900°C, depending on the concentration of manganese in solution. The higher the manganese content in solution, the higher is the temperature above which a liquid phase is present,e.g. for 10 ppm Mn, 900°C and for ∼90 pct Mn, ∼1225°C. A mechanism is suggested for the precipitation of sulfides and oxysulfides near the surface of steel during heating in an oxidizing atmosphere. In the Appendix by Darken and Gurry, results are given of the melting temperatures of mixtures of wustite and pyrrhotite in equilibrium with iron, from an investigation carried out about thirty years ago.