Phase Change of Fe-Cr-Mn Alloy Surface by Selective Vaporization of Mn

Phase change on the surface of the Fe-Cr-Mn alloy through the vaporization of Mn, which has a high vapor pressure, was studied in order to develop a new surface modification method utilizing the vapor pressure differences among the alloy elements. Fe-(10-30 mass%)Cr-(10-30 mass%)Mn alloys were heated at 1373 K, and the phases of the samples were examined before and after the heat treatment. The microstructures of the cross section of the samples were observed, and the concentration depth profiles of the alloy elements were measured after the heat treatment.The surface concentrations of the heat treated samples depended strongly on the atmospheres during heat treatment. The Mn concentration on the sample surfaces heat treated in a closed tube were 7-15% and those heat treated under a constant pressure using a vacuum pump were 1-2%. The gamma phase alloys, containing more than 20%Mn, changed to the alpha phase and carbide due to such shift in the surface concentration. The sigma phase was also formed on the surface of the Fe-20%Mn-20%Cr alloy. Surface layers with different microstructures from the base alloy were observed. These surface layers were 10-50 μm thick and corresponded to the newly formed phases. On the other hand, no phase change was observed for the alpha phase alloys, containing less than 15%Mn.It seems possible that the sigma phase layer is formed singularly on the surface of the Fe-Cr-Mn alloy through controlling the alloy compositions and the heat treatment conditions for Mn vaporization.