Precipitation Mechanism of Copper Sulfides in Solid Iron

The solubilities of copper and sulfur in solid iron have been investigated at temperatures of 1073, 1173, 1273 and 1573 K. The samples were prepared from electrolytic steel plate, Cu2S powder, FeS powder and iron powder, and then subjected to analyses by an electron micro probe analyser (EPMA). The equilibrium concentrations of copper and sulfur in solid iron are much higher than the apparent solubilities of copper sulfide reported for Fe-3mass%Si steel. The possible reason is that the copper sulfide precipitates form in the concentrated region of copper and sulfur induced by micro segregation during solidification. In addition, the existence of copper sulfide has been verified in samples having the composition of Fe-0.1mass%Cu-0.01mass%S-0.1mass%C, without and with annealing for the homogenization after melting and solidification process. The annealed sample did not cause any copper sulfide precipitation in SEM images, while as-cast sample yielded the one. On the basis of Scheil's equation, super-saturated region has been calculated to be 0.1vol%, which ratio is a little larger than the actual copper sulfide precipitates shown in the SEM images. Thus, micro-segregation will contribute to copper sulfide precipitation in solid iron even when the copper and sulfur concentrations are low.