Study On Composite Process Of Plasma Nitriding And Post-Oxidation With Hollow Cathode Assisted

Plasma nitriding treatment can improve the hardness,wear resistance and corrosion resistance of structural steel.Nevertheless,the corrosion and wear resistance can be further improved by oxidation after nitriding,this is mainly due to the formation of Fe3O4 oxide layer on top of the compound layer.The Fe3O4 have a compact structure,lower friction coefficient and the chemical stability.Thus,the key point in post-oxidation process is encourage formation of pure Fe3O4 phase and avoid formation of Fe2O3 phase.In this study,the 40Cr steel were plasma nitrided and post-oxidation in the home-made ion-nitriding furnace.In the process of plasma nitriding,the same process parameters were carried out.In the process of post-oxidation,different process parameters were carried out,the mechanism of post-oxidation will be studied by changing the post-oxidation process parameters,including temperature,gas ratios,time.By using the optical microscope,X-ray diffraction analysis,scanning electron microscope,Laser confocal microscope testing instruments,charactering multiple layer thickness,phase composition and microhardness,and evaluating the corrosion resistance and wear resistance.The study presents that 40Cr steel of plasma nitriding and post-oxidation can form a superficial oxide layer on the subsurface compound layer and nitrogen diffusion layer beneath the compound layer.The oxide layer is composed of Fe3O4 and Fe2O3 and the thickness is about 0.7-1.9?m.The ratio of the two oxides is determined by the post oxidation process parameters.Under the condition of excessive oxygen content and higher temperature,Fe2O3 will appear in the oxide layer.After the treatment of post-oxidation,the hardness of surface become lower and the nitrogen diffuses deeper compare with nitriding treatment.The highest amount signal-phase Fe3O4 of formed after post-oxidation at 540 ? for 1h under O2/NH3 ratio of 1:3,the optimal wear and corrosion resistance is obtained at this condition.Besides,the paper studied the feasibility of the post-oxidation treatment in air atmosphere based on the above study.The result shows that there is no obvious distinguish after post-oxidation under air/NH3 ratio of 9.2:1 and O2/NH3 ratio of 1:3.