Machining heat induced phase transformation on the surface hardening layer of high strength ferrous-based biomedical stainless steel

The present study investigates the drilling characteristics of a high-strength Fe8Al30Mn1C1Si3Cr (mass%) biomedical alloy. After machining, a surface-hardening layer with a Vickers hardness number (H _V) equal to 600 was observed. In addition, a γ→(γ +κ) phase transition was observed in the matrix and at the stress-induced twin boundaries of the surface-hardening layer. κphase carbides ((Fe,Mn) ₃AlC _x) having an L'1 ₂ structure with the lattice parameter a = 0.375nm were precipitated. Furthermore, the heat transfer coefficient of the present alloy was 0.083 cal/(cm ² s °C), which was lower than that of AISI 304 stainless steel, which was 0.098 cal/(cm ² s °C). The instantaneous cutting temperature of the present alloy was approximately 650°C during the machining process. It is believed that the formation of κ-phase carbides not only decreases the machinability of the present alloy, but also reduces the life of the cutting tool. These features could be useful in further understanding the relationship between the machinability and the microstructure of FeAlMnC- based alloys, and thus provide information that would be allow these alloys to be used in biomedical and industrial applications.