Microstructure and mechanical properties of M50NiL steel plasma nitrocarburized with and without rare earths addition

• M50NiL steel is nitrocarburized with and without rare earth (RE) addition.
• RE addition can increase the surface hardness and layer thickness.
• RE can increase C, N contents and proportion of iron nitrides in treated layers.
• Plasma nitrocarburizing can significantly improve the wear resistance of M50NiL steel.
• RE addition could further improve the wear properties of the treated layers.

M50NiL steel was plasma nitrocarburized at 500 °C with and without rare earth (RE) addition. The nitrocarburized layers were characterized by optical microscope, scanning electron microscope equipped with an energy dispersive X-ray analyzer, X-ray diffraction, hardness tests and pin-on-disc tribometer. The results show that the RE atoms can diffuse into M50NiL steel surface and change the surface morphology of the nitrocarburized layer. The incorporation of RE atoms increases the surface hardness of the nitrocarburized layer (approximately 130HV0.1 higher), layer thickness (about 14% thicker) and carbon content of the modified layer, respectively. The wear rates of the nitrocarburized specimens are significantly lower than that of the un-nitrocarburized one. The wear mechanisms of the specimens plasma nitrocarburized with and without RE addition are different mainly due to the differences in the surface morphology, the phase proportion and the hardness of the modified layer.