Defective or effective? - Tramp elements in steels

Tramp elements such as copper (Cu) or tin (Sn) cause a severe hot shortness. Many efforts have been done to remove such elements from molten steel but found hopeless. The only possible way to avoid the problem was thought to dilute the contaminated steels by virgin iron sources but it is costly. Hot shortness is due to the fact that the preferential oxidation of iron on the surface causes the enrichment of Cu and then formed liquid Cu penetrates into the austernite grain boundaries to tear the grains. Based on this mechanism there are two solutions. The first solution is to avoid the oxidation and mechanical stress at the temperature around 1150°C. The second is to enhance the heterogeneous nucleation of liquid Cu at MnS precipitates inside the austenite grains. Once this problem is solved, Cu or Sn may be utilized as effective micro-alloy elements. In this paper we compare the behaviour of the heterogeneous nucleation of ultra-fine Cu precipitates using Fe-10mass% Cu alloys with or without MnS. The Cu precipitates are classified into three types. "A" type is liquid Cu at the grain boundaries and "B" type is the globular Cu particle nucleated at MnS or inclusions. "C" type is ε-Cu of the fee structure and the size is 20-100nm which may be effective for physical properties. Finally, the effect of Sn on the precipitation will be mentioned.