Study On Microstructure And Mechanical Properties Of Surface Layer Produced During Plasma Re-nitriding And-Nitrocarbuieizing Of M50NiL Steel

M50NiL steel is selected in this paper which has been mainly applied todriving mediums of high-end equipments such as gears and bearings. Theresearch aims at investigating the microstructure and mechanical properties ofnitrided and nitrocarburized layer of M50NiL steel to obtain optimalnitrocarburizing process. In this paper, the microstructure and properties of thenitrided layer with pure La was investigated. The interaction between La and Nwas analyzed. Plasma rare earth （RE） nitrocarburized orthogonal process designwas formulated. Regression and prediction results of H:N-thickness and RE-thickness were given. The effect of RE on microstructure and mechanicalproperties of plasma nitrocarburized layer were investigated. Microstructure andmechanical properties of depth-related RE nitrocarburized layer were anylized.The influence of temperature, the ratio of H and N and time on the microstructureand mechanical properties of the plasma RE nitrocarburized layers was studied.OM, XRD, SEM and EDS were applied to characterize and analyze themicrostructure of the modified layer, morphologies of the surface and wear scar,elements distributed on the wear scar of the RE nitrocarburized layer. The cross-sectional hardness and wear properties of the RE nitrocarburizd layer were testedand analyzed.The results show that La could attract N during nitrding. La could decreasethe N concentration of the surface, thus making the hardness gradient smooth andregulating the phase structures of the nitrided layer. The RE nitrocarburizedexperiments indicate that RE could increase the C concentration and thickness ofthe nitrocarburized layer and promote the generation of the γ’-Fe4N in the surfacelayer. Meanwhile, RE could help reduce the wear rates of the nitrocarburizedlayer by6.83%, transforming the wear mechanism from severe abrasive wear tomild abrasive wear. Results of stepwise removal experiment show that thesurface layer mainly consists of α’N, γ’-Fe4N and ε-Fe2-3N. The layer12μm、65μm from the surface mainly contains single nitrogen expanded martensitephase α’N, while the layer100μm from the surface consists of a single matensitephase α’-Fe. The wear rate of the layer12μm from the surface is the lowestwhich is only2.466×10^-5mm³N-1m-1and the wear mechanism is mild adhesivewear. The effect of different temperatures on the RE nitrocarburized layer shows that the specimens treated at540℃were not overaged and the thickness of thelayer treated at540℃is133%thicker than that of the specimens treated at460℃. The friction coefficient drops from0.45（treated at460℃） to0.38（treatedat540℃）. The wear rate decreases from5.556×10^-5mm³/Nm（treated at460℃） to3.484×10^-5mm³/Nm（treated at540℃） with a decline of59.5%. The influence ofthe ratio of N and H shows that the thickness of the layer treated with the N-Hratio of2:2is14%thicker than that of the layer treated with the N-H ratio of2:4.The friction coefficient drops from0.53（N:H2:4） to0.47（N:H2:2）. The effect oftime on the RE nitrocarburized layer indicates that the nitrides and carbidesproduced on the surface of the RE nitrocarburized layer grow from tiny anddispersive ones to uniform and dense ones first and then nodular clustersgenerate in process of time. The wear resistance of the RE nitrocarburized layerrises up first and then drops down in process of time.