Atomistic Monte Carlo simulations on the influence of sulphur during high-temperature decarburization of molten iron–carbon alloys Original Research Article

We report a Monte Carlo simulation study of the molten Fe–C–S system with the aim of developing a theoretical understanding of the influence of sulphur during decarburization reactions in Fe–C alloys. Focussing specifically on the role played by free surfaces, computer simulations were based on the hexagonal atomistic model of Fe–C–S system using isotropic atomic interaction parameters; free surfaces were characterized by a missing layer of atoms. Three geometrical configurations, namely a liquid bath, a prismatic block and a spherical droplet, were investigated. Simulations were carried out as a function of melt carbon and sulphur concentration, temperatures and surface/volume ratios of the simulation cell. Sulphur atoms were found to preferentially concentrate in the top few layers, with the second layer showing the highest amounts of sulphur; very little sulphur was observed in the bulk liquid. This trend was observed in all three simulation configurations over a wide carbon/sulphur concentration range and temperatures. Significant levels of iron were observed in the top surface layer. The influence of free surfaces on atomic concentration profiles was found to be a strong function of the surface/volume ratio. The surface segregation of S was more pronounced for small exposed surfaces and was much smaller for liquids with large exposed surfaces. The presence of surface-active sulphur resulted in a major re-distribution of carbon. Carbon tended to concentrate deeper in the bulk, with the surface region being severely depleted of carbon. In addition to several new findings and a better understanding of liquid surfaces, these simulations have helped overcome major limitations of Sain and Belton’s model. Key experimental results on decarburization have been explained within the framework of our simulations. These simulation results have significant implications for surface decarburization reactions and carbon-boil phenomena in smelting technologies.