Effect Of Nickel-based Catalytic Coating On Gas Nitriding Behavior Of Pure Iron And 304 Austenitic Stainless Steel

As a typical chemical heat treatment method,gas nitriding is widely used to enhance the surface hardness,mechanical properties and corrosion resistance of metal materials.However,the high temperature(>500?),long time(60?80 h)and high energy consumption of traditional gas nitriding lead to the increase of porosity and deformation of nitrided layer,and the precipitation of chromium nitrides from thermal decomposition of nitrided austenite,which reduces the mechanical properties and corrosion resistance of nitrided workpiece and limits the development of gas nitriding technology.Gas nitriding mainly depends on the interface reaction between ammonia gas and the workpiece surface,as well as the effective infiltration of active nitrogen atoms on the workpiece surface which is determined by the process of ammonia adsorption,decomposition and desorption on the workpiece surface.Therefore,the development of new technology to promote the interface reaction on the nitrided workpiece surface,improve nitriding efficiency,reduce energy consumption,and effectively improve the comprehensive performance of nitrided materials,has become the research focus at present.The surface-active enhanced nitriding technology could effectively improve the adsorption and infiltration of active nitrogen atoms on the surface of nitrided workpieces.In order to avoid the adverse effects of the brittle layer brought by traditional gas nitriding,realize the low-temperature and high efficiency nitriding,and develop a new type of catalytic coating which is easy to prepare and high efficiency,the nickel-based catalytic coating was introduced into the gas nitriding process to carry out duplexed surface treatment(short for DST)of metal substrates in this paper.Due to the single composition of high purity iron that could avoid the interference of other elements in the nitriding process,it was regarded as the preferred research object.Subsequently,304 austenitic stainless steel(short for 304ss)was taken as the research object in order to further expand the application of the nickel-based catalytic coating and realize low-temperature nitriding.The micromorphology and phase structure of samples before and after nitriding was characterized using an optical metallographic microscope(OM),scanning electron microscope(SEM,FESEM),transmission electron microscope(TEM),atomic force microscope(AFM)and X-ray diffraction(XRD).The influence of the nickel-based catalytic coating on the hardness,wear resistance and corrosion resistance of the DST workpieces was evaluated.The chemical element state of the samples before and after nitriding was analyzed by X-ray photoelectron spectroscopy(XPS),and the mechanism of nitriding promoted by nickel-based catalytic coating was also discussed.The main findings are as follows:?.Nickel catalytic coatings were prepared on the surface of pure iron,and then gas nitriding was carried out in NH3 medium at 500?,5 h and ordinary pressure.The effects of the micro-scale thickness nickel catalytic coating on the microstructure,surface microhardness and wear resistance of the nitrided layer on pure iron were studied and compared with that of simplex nitrided(short for SN)pure iron.It was found that the DST technology could ensure the thickness of the nitrided layer,reduce the thickness of the brittle compound layer and control the phase composition of the compound layer by controlling the thickness of nickel catalytic coating.The nitrided layer of pure iron was composed of the surface brittle compound layer,and the diffusion layer composed of needle-like ?'-Fe4N and the shorter needle-like nitrides of ?"-Fei6N2.The thickness of needle-like diffusion layers composed of ?'-Fe4N on the DST pure iron could reach 320?m,which was slightly higher than that of the SN pure iron.With the increase of nickel catalytic coating coverage,the phase transformation on the surface of nitrided pure iron was as follows:a small amount of ?'-Fe4N+?-Fe2-3N??'-Fe4N+8-Fe2-3N+a-Fe??'-Fe4N+a-Fe.When the nickel catalytic coating coverage on the surface of pure iron was 34.70×10-3 mg/mm2,the surface microhardness of DST pure iron(?420 HV0.025)was about 4 times that of annealed pure iron(?103 HV0.025),and slightly lower than that of SN pure iron(?500 HV0.025).Compared with annealed pure iron,the wear mass loss of the DST and SN pure iron decreased by 68%and 69%respectively.The wear mechanism of annealed pure iron was mainly characterized by abrasive wear accompanied by adhesive wear and fatigue wear,that of SN pure iron was mainly characterized by adhesive wear,and that of DST pure iron was mainly characterized by adhesive wear accompanied by abrasive wear.The surface microhardness and wear resistance of pure iron were improved by the DST technology,which had a positive effect on the practical use value of pure iron.The ammonia decomposition rate in the exhaust gas from the nitriding process was measured,found that the pretreatment of nickel catalytic coating mainly affected the adsorption and decomposition of ammonia on the surface of the nitrided workpieces.?.Nickel catalytic coating was pre-deposited on 304 austenitic stainless steel,followed by gas nitriding treatment for 6 h with NH3 as the medium at 500? or less and ordinary pressure.Nitriding was carried out at 400? and ordinary pressure for 6 h,the influence of the thickness of the nickel catalytic coating on the microstructure of the DST 304ss substrate was studied and found that the DST 304ss substrate with coating pretreatment for 3 min had the thickest nitrided layer.The effect of nitriding temperature on the nitrided layer structure,microhardness and wear resistance of DST 304ss was studied under the optimal coating pretreatment conditions.In order to expand the application range of nitrided 304ss,GCr15 steel balls(720?800 HV)and Si3N4 ceramic balls(1500?2000 HV)with great differences in hardness were selected as grinding parts.The mechanism of nickel catalytic coating on low-temperature nitriding of 304ss was also discussed.A continuous distribution of nitrided layer was formed on the surface of the DST 304ss substrate,but no continuous nitrided layer was formed on the surface of SN 304ss substrate.The thickness of the continuous nitrided layer(about 6.34 ?m)formed on the substrate surface of DST 304ss at 400?(short for DST-400),but no nitrided layer was formed on the substrate surface of SN 304ss at 400?(short for SN-400).The thickness of the continuous nitrided layer on the near-surface of DST-450 and DST-500 304ss was 24.26 ?m and 69.27 ?m,which was 7.85 times and 2.5 times that of SN-450 and SN-500304ss,respectively.The phase of the DST-400 304ss nitrided layer was mainly nitrogen supersaturated austenite(yN-Fe)and Fe2-3N.The phase of the DST-450 304ss nitrided layer was ?N-Fe,CrN,Cr2N and Fe2-3N.?N-Fe was not observed in the surface layer of the DST-500 304ss nitrided layer,and the main phases were CrN,Cr2N and Fe2-4N.The maximum surface microhardness of DST-400 and DST-450 304ss was 780 HV0.05 and 1450 HV0.05,respectively,which were almost 3.3 and 6.2 times that of untreated 304ss.Compared with the untreated 304ss,the wear volume loss of DST-400 and DST-450 304ss decreased by 75.7%and 89.4%,respectively,when GCr15 was used as grinding parts;the wear mechanism of untreated 304ss substrate was mainly adhesive wear,while that of DST 304ss was mainly adhesive wear accompanied with abrasive wear.When Si3N4 was used as grinding parts,the wear volume loss of DST-400 and DST-450 304ss decreased by about 82.5%and 88%,respectively,compared with that of untreated 304ss;the wear mechanism of untreated 304ss substrate was mainly abrasive wear and fatigue wear,while the high hardness DST 304ss presented slight abrasive wear and adhesive wear.The results indicate that DST 304ss still has excellent wear resistance even in a harsh working environment.?.The Ni/Al2O3 catalytic coating was deposited on the surface of 304ss,then gas nitriding was carried out for 6 h with NH3 as the medium at 450? or less and ordinary pressure.It was found that Ni/Al2O3 catalytic coating pretreatment realized the low-temperature nitriding of 304ss,and the continuous nitriding layer was formed on the surface of DST 304ss even at 350?.Nitriding was carried out at 400? and ordinary pressure for 6 h to determine the optimal preparation process parameters of the catalytic coating.It was found that when the concentration Al2O3 in the plating solution was 30 g/L and electrodeposition time was 3 min,the DST 304ss(short for DST-30Ni/Al2O3-400)had the thickest nitrided layer about 8.5 ?m,and the main phase of it was ?N-Fe and a small amount of Fe2N and Fe3N.The microstructure,microhardness,wear resistance and corrosion resistance of the nitrided layer on DST 304ss were evaluated under the optimal pretreatment conditions.The mechanism of Ni/Al2O3 catalytic coating on the low-temperature nitriding of 304ss was also discussed.The microhardness of duplexed treated surface and nitrided layer on DST-30Ni/Al2O3-400 304ss was 577.92 HV0.05 and 853.66 HV0.05,respectively,which were 2.4 times and 3.5 times that of SN-400 304ss(243.32 HV0.05).The wear volume loss of the duplexed treated surface(3.21×106?m3)and nitrided layer(1.12×106?m3)on DST-30Ni/Al2O3-400 304ss decreased by 92.0%and 97.2%compared with that of SN-400304ss(39.74×106?m3),respectively.The wear mechanism of SN-400 304ss was mainly adhesive wear accompanied by abrasive wear and fatigue wear,and that of DST-30Ni/Al2O3-400 304ss was mainly abrasive wear and adhesive wear.The space resistance of the passivation film formed by the nitrided layer of DST-30Ni/Al2O3-400 304ss was about 5 times that of SN-400 and untreated 304ss.The corrosion potential of the nitrided layer on DST-30Ni/Al2O3-400 304ss was about 45.8 mVscE,which was higher than that of SN-400 and untreated 304ss,and the corrosion current of the former was less than that of the latter.The passivation zone of the nitrided layer of DST-30Ni/Al2O3-400 304ss was wider than that of SN-400 304ss.The corrosion holes with the size of 200?400 ?m appeared on the corrosion surface of SN-400 304ss,while the surface of the nitrided layer of DST-30Ni/Al2O3-400 304ss did not change.The results indicate that Ni/Al2O3 catalytic coating pretreatment improves the microhardness of 304ss,and enhances its wear resistance and corrosion resistance.?.The mechanism analysis shows that the nickel catalytic coating has a positive effect on the adsorption and decomposition of ammonia and the transfer of active nitrogen atoms in the nitriding process,which improves the nitriding efficiency of the substrate.The Ni/Al2O3 catalytic coating prepared by nano-sized Al2O3 powder as a carrier increased the dispersion of Ni,and the presence of the coating improves the specific surface area and catalytic active site on the surface of 304 austenitic stainless steel.The oxygen that adsorbed on the coating surface and provided by Al2O3 combined with the hydrogen of ammonia decomposition to form an O-H bond,which could improve the dehydrogenation ability of ammonia and promote the adsorption and decomposition of ammonia flowing through the surface of the nitrided workpieces.The coating of loose porous structure and the oxygen vacancy generated by part of Al2O3 could provide diffusion channels for the diffusion of active nitrogen atoms to the substrate.The diffusion of metal atoms between the coating and the substrate also could provide more diffusion channels for the active nitrogen atoms to diffuse into the interior of the stainless steel substrate.