Influence of sulfur on the solubility of graphite in Fe–C–S melts: optimization of interaction parameters

Abstract We report a Monte Carlo simulation study of a molten Fe–C–S system at 1600°C with the aim of developing an atomic model with optimum interaction parameters. This model should account for key features of the system, e.g., homogenous nature of Fe–FeS solution, separation of Fe–C–S liquid in two immiscible layers and a decrease in solubility of graphite in iron melts with the addition of sulfur. The atoms in the ternary Fe–C–S system were arranged on a graphitic hexagonal lattice and pair-wise interactions between them were assumed to be short ranged. The simulations were carried out using a combination of canonical and grand canonical ensembles for a range of interaction strengths, carbon and sulfur concentrations. The strength of ionic Fe–S interaction was one of the significant parameters and could lead to electronic distortions around the Fe atom. Local modifications to various interactions due to these distortions were represented by three parameters: e (Fe–Fe), δ (Fe–S) and δ (Fe–C) and their effect on the Fe–C–S system was systematically analyzed. Optimum modulation parameters for this system, which simultaneously simulate key features of a molten Fe–C–S system are: e (Fe−Fe)=1.0, δ (Fe−S)=1.0, δ (Fe−C)=1.0 and 1.5. Even though a slight increase in Fe–C repulsion gives optimum results, our high temperature simulation results clearly show that electronic distortions around Fe due to strong Fe–S bond do not play a significant role in the molten Fe–C–S system.