Microstructure And Properties Of Surface Layer Of AISI 4140 Steel Plasma Nitrided And Oxynitrided

Glow discharge plasma nitriding is a chemical heat treatment technology.High-energy nitrogen ions/atoms were obtained by glow discharge and then penetrate into the material to strengthen the surface properties of steel.The plasma nitriding and oxynitriding treatment were studied in this subject.4140 steel was selected as the experimental material.Nitrogen and hydrogen were selected as the gas source for nitriding,and plain air was selected as the oxygen source for oxynitriding.The phase structure,cross-sectional morphology,surface structure,mechanical properties,and corrosion properties of 4140 steel through nitriding/oxynitriding were tested and analyzed using optical microscope,scanning electron microscope,X-ray diffractometer,Vickers hardness tester,friction and wear tester,electrochemical test and other instruments.The phase composition and microscopic morphology of the nitrided layer and the oxynitrided layer of 4140 steels were compared.The phase composition of the nitrided layer includes?_N,Fe_2-3N and Fe₄N phase.The oxynitrided layer consists of?_N,Fe_2-3N,Fe₂O₃ and Fe₃O₄ phase.When the nitriding temperature increases,the?_Nphase is transformed into a nitride phase.The surface of the PN520-4 sample is mainly Fe_2-3N phase,and the surface of the PN560-4 sample are mainly Fe_2-3N and Fe₄N phase.The main phase of the PON0.15 sample is Fe_2-3N and iron oxide phase.As the proportion of oxygen increases,the surface phases are?_N,Fe_2-3N and iron oxide phase.The surface roughness of nitrided sample increases with the increase of the nitriding temperature.The nitrogen content on surface increases first and then decreases with the increase of the nitriding temperature.The thickness of the oxide layer on the surface of oxynitrided sample is about 3?4?m.The nano Fe_2-3N phase is uniformly distributed in the oxide layer.The Fe₃O₄ phase content of the PON0.175sample is the highest.When the oxygen ratio is low,the oxide layer structure is loose.When the oxygen ratio increases,the oxide layer thickness becomes thinner.The mechanical properties of the nitrided layer and the oxynitrided layer of 4140steels were compared.Plasma nitriding and oxynitriding both significantly increase the hardness of surface layer of samples,and the surface hardness was about 1000HV0.1.When nitriding temperature rises from 400^oC to 520^oC,the surface hardness increases and the hardness gradient becomes gentler.When the temperature rises to560?,the hardness decreases due to the decrease of surface nitrogen concentration,and the phase changes from Fe_2-3N to Fe_2-3N phase and Fe₄N phase.The PN1:1sample showed the best microhardness distribution.The layer thickness increase and the hardness gradient become gentler with the nitriding time increases.The hardness distribution of PON0.15 and PON0.25 sample showed a sharp drop in hardness distribution.PON0.175 and PON0.20 sample have a better gradient hardness distribution.Compared with the nitrided sample,the hardness of the subsurface layer of the oxynitrided sample is lower,because the nitrided sample has a higher proportion of Fe_2-3N phase.The surface nanohardness of the oxynitrided sample is lower than that of the nitrided sample,which is determined by the oxide layer with lower hardness.The friction and wear properties of the nitrided layer and the oxynitrided layer of4140 steels were compared.The friction coefficient of the untreated samples fluctuates obviously,and the wear mechanism is mainly adhesive wear,accompanied by oxidation wear.The friction coefficient of the nitrided sample is high,and the oscillation is obvious.The nitriding treatment cannot weaken the adhesive wear of steels.The wear mechanism is still adhesive wear and slightly oxidative wear.The wear rate of nitride sample is an order of magnitude lower than that of the untreated sample.The PN520-4 sample has better wear properties,and the wear rate is 96.3%lower than that of the untreated sample.The friction coefficient of the 4140 steel oxynitrided sample is lower than that of the untreated sample,which proves that the oxide layer can weaken adhesive wear of steel.In comparison,the friction coefficient and wear rate of the oxynitrided sample is lower than that of the nitrided sample,because a dense oxide layer is formed on the surface.The main wear mechanisms of the oxynitrided layer are abrasive wear and oxidative wear.The PON0.175 sample has the best wear performance,which is 47.3%lower than the nitrided sample in wear rateThe corrosion resistance of the nitrided layer and the oxynitrided layer of 4140steels were compared.Plasma nitriding can improve the corrosion resistance of the material.When the ratio of nitrogen to hydrogen is 1:1 for the PN520-4 sample,the i_corr is the smallest,p_corr is the largest,the?E value is the largest,and its corrosion resistance is the best.Because the surface phase is mainly Fe_2-3N phase.The addition of oxygen improves the corrosion resistance of the sample,which is mainly due to the formation of Fe₃O₄ phase on the surface of the oxynitrided sample,which has a dense phase structure and high electrochemical stability.When the ratio of nitrogen to hydrogen to air is 0.3:1:0.875,it exhibits the best corrosion resistance.