Precipitation behavior of alloy nitrides during plasma nitriding in Fe-0.6mass%C-1mass%M (M=Al, Cr) martensitic steels

Change in surface hardness and precipitation of alloy nitrides in Fe-0.6C binary and Fe-0.6C-1M(M=A1, Cr) (mass%) ternary martensitic steels during plasma nitriding was investigated by means of hardness test, scanning electron microscopy and transmission electron microscopy. In the Fe-0.6C binary alloy, surface hardness is hardly increased by nitriding at 823K. On the other hand, in Fe-0.6C-1A1 and Fe-0.6C-lCr alloys the surface hardness increases largely and reaches HV650 and HV550, respectively. In the Fe-0.6C-1A1 alloy, fine NaCl type A1N platelets of which length is about l00nm and thickness is a few nm, precipitate mainly on dislocation lying on {001} planes of the matrix inside martensite laths holding Baker-Nutting orientation relationship with respect to ferrite matrix. In the Fe-0.6C-lCr alloy, NaCl type CrN platelets about 20nm in length and several atomic layers in thickness precipitate much more uniformly than in the Fe-0.6C-1A1 alloy also on {001} planes of the matrix. Those precipitate phases in Fe-0.6C-1M ternary martensite are similar to those in Fe-M ferritic binary alloys reported previously. A1N dispersion in martensite is finer due to higher density of dislocation as nucleation sites, resulting higher hardness density, than in ferrite. On the other hand, hardness of martensite is lower than ferrite presumably due to coarser dispersion of CrN resulted from in-situ transition of Cr-rich cementite to CrN and less amount finer CrN platelets..