Effects of MnS on the heterogeneous nucleation of Cu precipitates in Fe-10 and-5mass%Cu alloys

Copper in steel has been thought harmful because of the hot shortness at grain boundaries, but Cu precipitates finely distributed inside of each grain increase the strength. If the problem of hot shortness is solved, scrap recycling will become much easier and even Cu can be used as an important alloying element. It is therefore most important to investigate the behavior of Cu precipitation in Fe-Cu alloys at high temperatures. In this study, in-situ observation of the Cu precipitation on cooling in Fe-10mass%Cu and Fe-5mass%Cu alloys was made using a confocal scanning laser microscope. The precipitates were analyzed by SEM-EDX after in-situ observation experiments. As a result the effect of MnS inclusion on the heterogeneous nucleation of Cu precipitate was made clear. In Fe-10 mass%Cu alloy containing MnS inclusions, the starting temperature of Cu precipitation became higher and its distribution became more uniform than the same alloy without MnS inclusions. In case of Fe-5mass%Cu alloy containing MnS inclusions, some of Cu precipitates appeared in gamma phase before the gamma/alpha phase transformation. In case of the same alloy without MnS inclusions, many fine Cu precipitates appeared in alpha phase after the gamma/alpha phase transformation. The complex precipitate of MnS and Cu was also found indicating that Cu precipitated at MnS inclusion. The reason for these facts was explained by the heterogeneous nucleation theory. The undercooling for the Cu precipitation was estimated to be smaller in the alloy with MnS inclusions than in another case. Based on these results, if the nucleation sites such as MnS inclusions are finely distributed beforehand, the heterogeneous nucleation of Cu precipitates will be accelerated and its distribution will be made very fine.