Insights into plasma nitriding behaviour of M50NiL steel with different initial microstructures

Annealed and quenched M50NiL steel specimens were plasma nitrided at 460°C for 4 h under a mixture gaseous supply of 0.05N₂+0.4H ₂ L min^-1 to investigate the effects of the substrate microstructure on the features of nitrided layer, such as microstructure, microhardness and wear resistance. The results show that the nitrided layer is mainly consisted of α-Fe, γ-Fe4N and αN (nitrogen expanded martensite) phases. The surface hardness is significantly enhanced. For the quenched specimens, the nitrided depth decreases with increasing quenched temperature. Compared to the quenched specimens, the nitrided depth of the annealed specimen is thicker under the same nitriding condition. The mechanism may be the variation in the amounts of interstitial carbon atoms and substitution alloying elements in the lattice, and the compressive residual stresses in the substrate. It is also demonstrated that the wear resistance for both the annealed and the quenched specimens can be significantly improved by plasma nitriding treatment.