Microstructure Evolution on the Surface of Fe-20Mn-6Al-0.6C-0.15Si Austenitic Low-Density Steel during Heat Treatment

Microstructure evolution on the surface of Fe-20Mn-6Al-0.6C-0.15Si austenitic low-density steel was studied by comparing with the microstructure at the core. In the present study, the austenite grain growth was in situ observed using laser scanning confocal microscope (LSCM). The microstructure of specimens on surface and at core was analyzed after LSCM experiments. The distribution of Mn and Al along axial direction was analyzed. The results show that the volatilization of Mn on the specimen surface during isothermal holding at high temperatures varying from 900 to 1200 degrees C results in a low stability of the austenite on the specimen surface, leading to the transformation of less stable austenite to ferrite during subsequent cooling process. The ferrite fraction on the specimen surface increases with isothermal temperature, indicating more Mn volatilization at higher temperature. In addition, because Mn volatilizes during isothermal holding at high temperatures, the austenite grain growth on the surface is different from that at the core.