Effects of lanthanum and cerium on low temperature plasma nitrocarburizing of nanocrystallized 3J33 steel

Plasma nitrocarburizing of nanocrystallized (NC) 3J33 steel were carried out at 400 and 430 °C for 4 h in a mixed gas of N2:3H2 and different flow rates of rare earths (RE) La and Ce reagents in this paper. Effects of temperature, rare earth addition and its addition amount on the microstructure and hardness of the nitrocarburized layer of NC 3J33 steel were also investigated. Surface phase composition of the nitrocarburized samples was analyzed by X-ray diffraction. Metallurgical structure, La and Ce concentration and microhardness profiles of cross-sectional nitrocarburized samples were studied using an optical microscope, a scanning electron microscope equipped with an energy dispersive X-ray analyzer and Vickers microhardness tester, respectively. The results showed that the surfaces of the nitrocarburized samples were mainly composed of γ′-Fe4N and α′-Fe (a-Fe dissolved with N and C) when the NC 3J33 steel was nitrocarburized at 400 °C. As the temperature was enhanced up to 430 °C, the surfaces consisted of γ′-Fe4N, α′-Fe and low nitrogen compound FeNx (x=0.0324-0.0989), and simple substance La was presented when RE flow rate was 0.1 L/min. The addition of La and Ce into nitrocarburized gas increased the thickness and hardness of the nitrocarburized layers. The samples nitrocarburized at 400 °C with RE flow rate of 0.025 L/min and 430 °C of 0.05 L/min possessed the thickest nitrocarburized layer, highest proportion of nitrides and hardness profile. RE elements could diffuse into the nitrocarburized layer and their concentration increased with temperature. The excess RE impeded the permeation of N, C elements and led to thinner compound layer as well as the diffusion layer.