Microstructure and mechanical properties of surface layers of 30CrMnSiA steel plasma nitrocarburized with rare earth addition

The pulse plasma nitrocarburizing for 30CrMnSiA steel was conducted at 560 °C for 8 h in mixed gases of N2:3H2 and different flow rates of rare earths (RE) addition. Effects of rare earths (RE) addition in the carrier gas on the surface morphology, phase structure and mechanical properties of the nitrocarburized layer were characterized by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness testing and wear testing, respectively. The results showed that the surface phase structures changed from dual phases ɛ-Fe2-3N(C) and γ′-Fe4N(C) to phase Fe3C and incipient nitrides, and the nitrocarburized surface hardness value decreased slightly from 756 to 681 HV0.1 with the RE addition increasing in the carrier gas, and the corresponding morphology of the nitrocarburized surface was granular nitride group (diameter 0.8–1.5 μm) and compact-fine Fe3C stick and patch (mean size 100–300 nm), respectively. The wear resistance of the experimental steel could be improved remarkably by plasma RE nitrocarburizing. The nitrocarburized layer with Fe3C phase formed in the mixed gases of N2:3H2 and flow rate of 0.5 L/min RE addition showed the lowest friction coefficient and the narrowest wear track.