Thermodynamic Calculations on the Stability of Cu2S in Low Carbon Steels

Thermodynamic stability of Cu2S sulfide in low carbon steels has been investigated using a CALPHAD type thermodynamic calculation method. Thermodynamic properties of the Cu–S binary and Fe–Cu–S ternary systems were critically assessed. By combining the newly assessed thermodynamic parameters to an existing thermodynamic database for steels, a thermodynamic description for low carbon steels involving sulfur and Cu could be obtained and be used to calculate phase equilibria and thermodynamic stability of precipitating phases such as AlN, MnS, and Cu2S. It was predicted that the Cu2S sulfide often observed in low carbon steels is actually a thermodynamically unstable phase and can precipitate when thermodynamic equilibrium state is not reached during steel making processes. Probable reasons and conditions for the formation of this unstable phase are discussed