Microstructure and corrosion resistance of quenched AISI 4140 steel plasma nitrided and nitrocarburised with and without rare earths

Abstract

The quenched AISI 4140 steel specimens have been pulse plasma nitrided and nitrocarburised with and without rare earth addition at 560°C for 4 h. The microstructure, microhardness and corrosion resistances of surface layers for the nitrided/nitrocarburised specimens were characterised using optical microscopy, X-ray diffraction, microhardness test, scanning electron microscopy and anodic polarisation tests in 3·5%NaCl solution respectively. A significant hardened layer bearing compound and diffusion layer can be obtained when the quenched steel (α′-Fe) is plasma nitrided/nitrocarburised at this experimental condition, and the compound layer mainly consists of γ′ (N/N,C) and ϵ (N/N,C) phases. The results show that the rare earth addition favours the formation of ϵ (N,C) phase in the surface layer. The compact compound layer with more ϵ (N,C) phase can be obtained by plasma rare earth nitrocarburising of the quenched AISI 4140 steel and has much higher corrosion potential, pitting potential and longer passive region than other compound layers.