Carburizing-nitriding Dual Modification Process And Properties Of M50NiL Steel With High Hardness And Deep Diffusion Layer

M50NiL steel is categorized as a new generation of surface hardened bearing steel,which has been widely used in equipment manufacturing,aviation and navigation industries.In practice,the bearing is required to exhibit superior mechanical properties,such as high fracture toughness,high fatigue life and good wear resistance under high speed,heavy load and high operation temperature conditions.Therefore,the surface treatment of M50NiL steel is indispensable to improve its surface properties.In this study,a principle of high-hardness and highdepth carburizing-nitriding dual modification was proposed to achieve the requirements of service life of M50NiL steel.Gas carburizing,plasma nitriding and carburizing-nitriding dual modification processes were studied.Effect of different processes on microstructures and properties of the modified layers was analyzed.Meanwhile,the modification mechanism involved was revealed.Carbon and nitrogen concentrations in the modified layer can be satisfactorily modeled by employing the model of the growth kinetics of layers and Fick’s diffusion law.The designs of carburizing and nitriding processes were therefore achieved.Based on the characteristics of high-depth of the carburized layer and high-hardness of the nitrided layer,combining with the numerical simulation results of the carbon and nitrogen concentrations profiles,carburizing-nitriding dual modification processes were designed,which are Carburizing（930℃,1.14%Cp,3h+ 910℃,0.85%Ck,3h）+Plasma nitriding（580℃,0.25N₂:0.25H₂,16h）+Quenching（830 ℃）;Carburizing（930℃,1.14%Cp,3h+910℃,0.85%Ck,3h）+Plasma nitrocarburizing（540℃,0.2N₂:0.2H₂:0.15C₂H₅OH,6h）+Quenching（830℃）.Based on the process design of gas carburizing,plasma nitriding and carburizing-nitriding dual modification of quenched M50NiL steel,effect of process parameters on the thickness of the effective hardened layer was studied.The thickness of modified layer of quenched M50NiL steel is significantly increased by gas carburizing.The carburized layer depth increases with the increase of carburizing time.The surface hardness of quenched M50NiL steel is significantly increased by plasma nitriding.The nitrided layer depth increases with the increase of nitriding temperature,N₂:H₂ ratio and time.Carburizing-nitriding dual modified layer shows a great increase in the hardness and thickness in comparison with the gas carburizing or plasma nitriding treatment.The surface hardness and layer depth can be increased to 1007HV0.1 and 940μm,respectively.The surface phase composition of the carburized layer,plasma nitrided layer and carburizing-nitriding dual modified layer of quenched M50NiL steel was characterized and analyzed.Effect of process parameters on the surface phase composition of the modified layer was studied.The ultra-fine microsturcture of the plasma nitrided layer was observed and the nanocrystallization mechanism was discussed.The surface phase composition of the carburized layer mainly contains high carbon martensite α′-Fe,Fe3 C,retained austenite AR,Fe₂O₃ and Fe₃O₄ phases.The surface phases of quenched M50NiL steel plasma nitriding at 490℃ with high N2:H₂ ratio mainly consist of α′-Fe and γ′-Fe₄N and ε-Fe_2-3N.However,the surface phases of quenched M50NiL steel plasma nitriding at 490℃ with low N₂:H₂ ratio mainly consist of α′-Fe and γ′-Fe₄ N without the formation of ε-Fe_2-3N.When the nitriding temperature exceeds 560℃,FeN_0.076 phase would form in surface layer.The surface phase composition of the carburizing-nitriding dual modified layer mainly contains martensite α′-Fe（N,C）,a small amount of Fe₃C,low nitrogen nitride FeN_0.076,Fe₂O₃ and Fe₃O₄.TEM results show that the microstructure of quenched M50NiL steel plasma nitrided at 460℃ is mainly of nanocrystallite and amorphous.However,the degree of grain refinement decreases with the distance from surface.In addition,there is formation of nano-scale γ′-Fe₄N phase in the surface of long time（>8h）nitrided specimen.The mechanism of nanocrystalline in the n itrided layer is that plasma nitriding induces local amorphization in the surface and then amorphous crystallization induces nanocystallization.Moreover,the internal stress in surface layer increases resulting from the effect of the penetration of nitrog en,which makes the dislocation substructures move,amalgamate and rearrange.Finally,the nanocrystalline in the nitrided layer is formed.The property of the carburized layer,plasma nitrided layer and carburizing-nitriding dual modified layer of quenched M50NiL steel was characterized and analyzed.Effect of process parameters on the property of the modified layer was studied.Meanwhile,the modification mechanism involved was revealed.Wear tests show that plasma nitriding,gas carburizing and carburizing-nitriding dual modification can dramatically enhance the wear resistance of M50NiL steel.The main wear mechanism of the quenched specimen is mainly of severe adhesion wear,oxidation wear and fatigue wear.After plasma nitriding,there is no severe plastic deformation in the worn surface and the main wear mechanism is changed into abrasion wear and mild oxidation wear.The main wear mechanism of the carburized layer is fatigue wear and abrasion wear.The friction coefficient of the quenched M50NiL steel can be decreased from 0.40 to 0.34 and the wear rate can be apparently decreased from 0.93×10-3 mg N^-1 m^-1 to 0.03×10-3 mg N^-1 m^-1 by carburizing-nitriding dual modification.The main wear mechanism of the carburizing-nitriding dual modified layer is mild abrasion wear and oxidation wear,exhibiting better wear resistance.Corrosion tests show that plasma nitriding can reduce the corrosion current density and increase the corrosion potential,indicating the improvement of corrosion resistance.The corrosion mechanism is changed from severely general and uniform corrosion to pitting corrosion,and the corrosion resistance of quenched M50NiL steel specimens plasma nitrided at 460℃ is improved with increase in nitriding time.The high-depth hardening mechanism of the carburizing-nitriding dual modified layer is: Firstly,a deep carburized layer is obtained by gas carburizing;then nitrogen element is induced in the carburized layer by plasma nitriding,which can enhance the hardness of the surface layer;and finally the alloy carbides are dissolved and solubilized in the matrix by quenching at high temperature,which enhances the hardness of the modified layer.Meanwhile,the retained austenite in the modified layer and the matrix microstructure are changed to martensite after quenching,which can increase the hardness of the modified layer and core of the M50NiL steel.