Effects of rare earths on nanocrystalline for nitrocarburised layer of stainless steel

Solution-treated stainless steel was plasma-nitrocarburised at the precipitation-sensitive temperature with and without rare earth (RE) addition. The nitrocarburised layers were characterised by means of X-ray diffraction, transmission electron microscope, nanoindentation and anodic polarisation test. Experimental results show that the depth of plasma-nitrocarburised layer can be apparently increased from 60 to 75 mu m after RE addition. More importantly, microstructure of stainless steel surface is refined into nano-sized grains after plasma nitrocarburising. At the depth of 20 mu m, the hardness and the modulus of the nanocrystallised layer are as high as 13.6 and 218 GPa. After RE addition, the hardness and the modulus of the nanocrystallised layer increase to 17.5 and 255 GPa, respectively.