Carburizing And Nitriding Duplex Treatment And Strengthening Mechanism Of 20Cr2Ni4A Steel

Gear plays a vital role on the equipment transmission process and is an important basic component.Most of the transmission equipment fails due to failure of gear,so the high performance of the gear plays an important role on prolonging the overall service life of the equipment.There are many ways of failure of gear,such as pitting,breakage and wear.20Cr2Ni4A steel is one of the main steel types of transmission gears.It has high strength and toughness,hardenability,good contact fatigue resistance,excellent comprehensive mechanical properties,and low cost.It is often used to make transmission gears for heavy-duty vehicles.The surface hardness of the 20Cr2Ni4A steel gear treated by the traditional process cannot meet the service life need of heavy equipment.This project aims at the needs of 20Cr2Ni4A steel for high hardness,deep penetration layer,excellent wear resistance and corrosion resistance.Basing on the thermodynamic and kinetic model of thermal expansion infiltration,carburizing and nitriding duplex treatment is designed.Basing on the deep carburization.Plasma nitriding and quenching process realize the hardening,wear and corrosion resistance of the surface layer,while maintaining the high hardness of the core of the material.First,the thermodynamic software Thermo-Calc and the database TCFE9 are used to calculate the equilibrium phase diagram of 20Cr2Ni4A steel,which provides a theoretical basis for the formulation of heat treatment temperature and atmosphere.The carbon concentration distribution model of the modified layer after carburization of20Cr2Ni4A steel and the prediction model of the hardness of the modified layer were constructed as the basis for the design of thermal expansion kinetics.A model of the nitrogen concentration distribution of the nitriding diffusion layer of 20Cr2Ni4A steel was constructed,and the plasma nitriding process was designed according to the simulation results.Furthermore,using metallography and XRD to study the influence of process parameters on the structure,it is found that the phase composition of the carburizing and quenching layer of 20Cr2Ni4A steel is mainly expanded martensiteα’_c-Fe phase,accompanying by a small amount of Fe₅C₂ carbides and Fe₃O₄ oxides.The phases of the carburizing layer after long-term and short-term carburization do not change.After long-term carburization,the carbon content of the expanded martensiteα’c-Fe in the modified layer increases,and the diffraction peak shifts to the left.The nitriding surface phase composition is mainlyα’_N,ε-Fe_2-3N andγ’-Fe₄N phases.With the increase of nitriding temperature,the nitrogen-containing martensite phase of the sample gradually decreases,the content ofγ’-Fe₄N phase increases,and the content ofε-Fe_2-3N phase decreases;with the extension of time,the content ofγ’-Fe₄N phase increases and the content ofα’_N phase decreases;with the increase of nitriding and hydrogen ratio,the content ofε-Fe_2-3N phase in the modified layer increases.The phases of the carburizing and nitriding duplex composite modified layer are mainlyγ’-Fe₄N andε-Fe_2-3（C,N）phases and a small amount of martensiteα’-Fe（C,N）phases.After quenching,theγ’-Fe₄N andε-Fe_2-3（C,N）phases are transformed intoα’-Fe（C,N）phases.With the increase of the quenching temperature and the extension of the holding time,the composition of the surface phase does not change.Secondly,the hardness,wear resistance and corrosion resistance of the modified layers of carburizing,nitriding,carburizing and nitriding duplex treatment and quenching are characterized.After 20Cr2Ni4A steel is carburized,the effective hardened layer thickness increases significantly,and the hardness first increases and then decreases.The modified layer of the long-term carburizing treatment has a lower volume wear rate and better corrosion resistance.The hardness and thickness of the carburizing and nitriding duplex composit modified layer are significantly higher than that of a single carburizing or nitriding modified layer,but the hardness of the core is significantly reduced due to the tempering effect of the nitriding process.After carburizing and nitriding duplex treatment,the volume wear rate of the modified layer increased slightly,but the corrosion resistance is significantly improved.After quenching,the surface hardness and core hardness are significantly improved.The volume wear rate of samples after quenching under different conditions has little change;the corrosion resistance after quenching is significantly lower than that before quenching.The strengthening mechanism of the carburizing and nitriding duplex treatment is clarified.The gas carburizing obtains a deeper infiltration layer,and the plasma nitriding obtains a carburizing and nitriding duplex composite layer of a certain thickness.After quenching,the carbide and nitride are re-dissolved to produce a solid solution strengthening effect.In the modified layer,the retained austenite and the matrix structure undergo martensite transformation,resulting in a large amount of martensite structure,which improves the hardness of the modified layer and the core.Finally,it is concluded that the first two-stage gas carburizing,strong infiltration for480 minutes at 930°C and a carbon potential of 1.1%,followed by diffusion at 910°C and a carbon potential of 0.85%for 290 minutes,and then decaburizing at 850°C for 20minutes,then plasma nitriding for 8 hours at 500°C with a nitrogen-to-hydrogen ratio of0.2:0.2,and finally quenching at 850°C for 30 minutes is the best treatment.The surface hardness reaches 859HV_0.1,which is 174HV_0.1 higher than carburizing and is close to nitriding.The thickness of the nitriding layer reaches 1774μm,which is 324μm thicker than carburizing and 1536μm thicker than nitriding,and has good wear resistance,which has important practical value.