Precipitation Behavior of Copper Sulfides in Fe–Si–Cu–S Ferritic Steel

Abstract

Precipitation behavior of copper sulfides in Fe-3.2%Si-0.4%Cu-0.02%S (mass%) ferritic steel was studied by X-ray diffraction (XRD), microstructure examination by transmission electron microscope (TEM) and scanning electron microscope (SEM). From the TEM results, it was found that FeS and cubic Cu_2-xS, precipitated in as-cast specimens. XRD peaks due to the cubic Cu_2-xS almost disappeared at 1173 K. XRD peaks due to the hexagonal Cu_2-xS were identified in specimens annealed above 1373 K. Assuming that the hexagonal Cu_2-xS is metastable in steel, which precipitates during cooling after dissolution of the stable cubic Cu_2-xS, it can be suggested that the cubic Cu_2-xS was dissolved at least 1173 K. Results of thermodynamic calculations are consistent with this suggestion, which indicates that the dissolution temperature of the cubic Cu_2-xS is lower than those of other sulfides such as MnS and FeS in the ferritic steel.