Alloying Effects on the Microstructure of Fe-1mass%M Binary Alloys Treated by Austenitic Nitriding and Quenching Treatment

Abstract

Surface microstructures were investigated in pure iron and Fe-1mass%M (M = Mn, Cr, Al, Si) alloys gaseous-nitrided at 1123 K and quenching to reveal the alloying effects on surface hardening by nitrogen (N) martensite. Thicker hardened layers with higher hardness than pure iron were obtained in the Mn-added alloys whereas the additions of Si and Al lead to increase the surface hardness with reduction of the hardened layer thickness. On the other hand, adding Cr decreases both the hardness and thickness of the hardened layer. No precipitation of alloy nitride is observed in austenite nor internal ferrite region in Mn-added alloy. Meanwhile, CrN(B1) and AlN(wurtzite) particles are dispersed in ferrite and austenite regions in the Cr- and Al-added alloys, respectively. Unlike those alloys, (austenite+ α-Si₃N₄) lamellar structure is formed in the Si-added alloy followed by martensite transformation of the high temperature austenite during quenching. Phase diagrams of Fe–M–N systems can consistently describe those alloying effects on the microstructure evolution.