Numerical Simulation And Experimental Study On Carburizing-quenching Process Of 18CrNiMo7-6 Alloy Steel

Gear steel usually use heat treatment process of carburizing and quenching to obtain good comprehensive mechanical properties that hard surface and tough core.The process has been applied in many important fields such as mechanical manufacturing,automotive and aerospace.In the context of“industrial 4.0”and intelligent manufacturing,numerical simulation of heat treatment has greatly improved people's ability to control and understand the heat treatment process due to the rapid development of computer numerical methods.The accuracy of numerical simulation is not only supported by advanced basic theory,reasonal mathematical model and efficient numerical calculation methods,the accuracy and integrity of material parameters also play a key role.Therefore,this work was based on the influence of carbon content on the material parameters of carburized and quenched steel,18CrNiMo7-6 samples with different carbon content in different depths of carburizing layer were prepared.And the database of mechanical properties and phase transformation kinetic parameters of 18CrNiMo7-6 carburized samples were established by optical microscopy,X-ray diffraction,scanning electron microscopy and thermal simulation and mechanics performance testing,etc.At the same time,DEFORM-HT software was used to numerically simulate the carburizing and quenching process,and the carbon gradient,microstructure,residual stress field and hardness field of the carburized and quenched samples were experimentally verified.Finally,based on the weighted average calculation method of mixed phase hardness,the hardness model of retained austenite was proposed.The main work is as follows:?1?The finite element models of the carburizing and temperature field in carburizing process of were established.The austenization parameters(A_c1 and A_c3)of the temperature field with different carbon content were obtained by heating expansion curve.Then the effect of two diffusion coefficient models on the numerical simulation accuracy of carburizing process of 18CrNiMo7-6 alloy steel was compared.The first is the function D?T,C?considering temperature and carbon content,and the second is the function D?T,M,C?considering temperature,alloying elements and carbon content.The carbon content distribution along the depth of three samples?1mm,2mm and 4mm?was measured by mechanical stripping method.The experimental data and simulation results were compared to analyze the influence of two diffusion coefficient models on carburizing simulation accuracy.?2?The material data of 18CrNiMo7-6 alloy steel used for numerical simulation of quenching after carburizing are measured.In order to consider the effect of carbon atom on material data,solid carburizing,vacuum diffusion and cryogenic treatment were used to prepare the martensite samples with different carbon contents.Expansion experiment was carried out with DIL805D/A/T dilatometer,and the phase transformation kinetic parameters such as parameters of martensite transformation mathematical model?K-M formula?,thermal stress and microstructure stress parameters were obtained.The mechanical properties parameters?elastic modulus,plastic modulus and yield stress?of different microstructures?austenite and martensite?at different temperatures were obtained by tensile test,and the functional relationship between material parameters,temperature,microstructure and stress was established.?3?A finite element model of temperature field,microstructure field and stress field coupling in quenching process of 18CrNiMo7-6 carburized steel was established.The changes of hardness,residual austenite volume fraction and residual stress along the depth of 2mm carburizing layer sample after carburizing and oil quenching process were measured by stripping method.The evolution of temperature field,microstructure field,hardness field and residual stress field in 18CrNiMo7-6carburizing steel was simulated and forecast by DEFORM-HT software.The simulation results were compared with the experimental results to analyze the precision of quenching simulation.Finally,according to the experimental hardness,carbon content,volume fraction of residual austenite,and employing the weighted average calculation method of mixed phase hardness,the empirical formula for the relationship between residual austenite hardness and carbon content?when carbon content higher than 0.55wt%?was revised and proposed.