Morphology of Precipitated Copper Sulfide during Solidification in Eutectoid Steel Containing Copper

Precipitation of copper sulfide in eutectoid steels (Fe-0.8 mass%C-0.3 mass%Cu-0.03 mass%S) was explored and investigated. The present copper concentration in the steel was set to be 0.3 mass%, which is generally considered to cause hot shortness. Steels prepared were heat-treated at a high temperature of 1873 K to be completely melted, followed by subsequent cooling under different conditions to vary cooling rate. Precipitates formed in the steel samples were analyzed using scanning electron microscope (SEM) and transmission electron microscope (TEM) for metallographical observation. The results indicated that copper sulfide precipitation occurs during the cooling process of solidification. The precipitated copper sulfides were mainly Cu2S with face-centered cubic structures. The results of analysis by using field emission electron probe micro analyzer (FE-EPMA), all the samples after experiments exhibited dendritic structures, with copper and sulfur concentrated mainly in the inter-dendritic regions. Analysis using the Clyne-Kurz model revealed that the concentrations of copper and sulfur in the liquid phase exhibited differences in cooling rates where the solid fraction exceeded 0.95. When the solid phase fraction reached 0.99 or higher, the solubility product of copper and sulfur exceeded the solubility curve of Cu2S in eutectoid steel, suggesting that copper sulfide precipitated during solidification. This micro-segregation due to the enrichment of copper and sulfur, along with FeS formed during solidification, was thought to result in the precipitation of copper sulfides with a core of FeS.