Study on depth-related microstructure and wear property of rare earth nitrocarburized layer of M50NiL steel

The quenched M50NiL steel was plasma nitrocarburized at 500 °C with rare earth (RE) addition. The RE nitrocarburized layer of M50NiL steel was removed stepwise (0 μm, 12 μm, 65 μm and 100 μm from the surface) and characterized using SEM equipped with EDS, XRD and microhardness tester respectively. Depth-related wear behavior of the RE nitrocarburized layer of M50NiL steel was investigated using pin-on-disk tribometer. The results show that the surface layer (0 μm from the surface) mainly consists of α′N,C (expanded martensite), γ′-Fe4(N,C), ɛ-Fe2-3(N,C) and a trace of Fe3O4 phase. The phase structure of the inner layers is single α′N,C or α′-Fe. There is a lower steady stage for all the depth-related friction coefficients of the layers due to the generation of the compact oxide film. The layer 12 μm from the surface has the lowest wear rate which is 2.4660 × 10−5 mm3 N−1 m−1. The work hardening effect only occurred on the layer 100 μm from the surface. The wear mechanisms of the layers transformed from mild abrasive and oxidative wear (0 μm from the surface) to severe adhesive and oxidative wear (100 μm from the surface) due to different phase structures and hardness. The oxygen content of the wear scar increases as the flash temperature rises and the hardness decreases when the layer comes inwards. The highest wolfram content for the layer 65 μm from the surface results in the dual role of the wear mechanism and hardness.