| Carburizing and quenching,a crucial heat treatment process for heavy-duty gears,can effectively ensure the surface strength,fatigue resistance,and service life of gears.It has been widely applied in fields such as mining machinery,trains,wind power,and aerospace.With the rapid development of the manufacturing industry,the demand for reliability of heavy-duty gears is increasingly high,and traditional "trial and error" evaluation methods are no longer able to meet the requirements.With the emergence of intelligent manufacturing,the combination of numerical simulation and heat treatment technology can achieve "visualization" of key parameters such as temperature,phase transformation,and stress changes in the heat treatment process.This greatly improves the optimization efficiency of the heat treatment process and effectively predicts the performance of the carburized layer.Therefore,in this study,a multiphysics coupled modeling and analysis of the carburizing and quenching process for 18Cr2Ni4 W lowcarbon alloy steel heavy-duty gears was conducted using COMSOL Multiphysics software.A material parameter database for thermal,mechanical,and phase transformation kinetics of 18Cr2Ni4 W alloy with different carbon contents after carburizing was established through experimental measurement and software calculation.The accuracy of the carburizing and quenching simulation model in predicting carbon concentration and temperature was verified by chemical analysis and thermocouple temperature measurement.The cooling phase transformation model and hardness prediction model were verified by analysis and testing methods such as optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),and microhardness testing.Finally,the stress evolution law and gear distortion state during carburizing and quenching are discussed based on simulation and prediction results.The following are the main conclusions:(1)The analysis of key parameters involved in simulating the carburization process was conducted,with a focus on carbon concentration,and the impact of alloying elements on the model was optimized.Experimental measurements of the carbon concentration in the carburized layer were carried out using mechanical stripping and chemical analysis methods.The accuracy of different diffusion coefficient calculation models for simulating carburization in 18Cr2Ni4 W steel was compared.It was found that the effect of alloying elements on the carbon diffusion coefficient cannot be ignored,especially in steels with high alloying element content.The impact factors of Cr and Ni elements were added to Tibbett’s diffusion coefficient calculation model in this study to increase the accuracy of the carburization simulation.The experimental verification showed that the model with the added impact factors of alloying elements had more accurate predictions of carbon content.(2)Based on experimental measurements of thermodynamic parameters and phase transition data,the critical parameters for modeling the physical field of quenching had been corrected and optimized,resulting in accurate predictions of the phase transition model for the 18Cr2Ni4 W carburized quenching process.The heating expansion curve of the 18Cr2Ni4 W alloy steel was experimentally determined,allowing for the acquisition of austenitization parameters at different carbon contents.The Leblond phase transformation model was utilized to simulate the bainite transformation and,through a new calibration method,obtain the Leblond model phase transition parameters for 18Cr2Ni4 W alloy steel at varying carbon contents.Experimental analysis of the kinetics of martensitic transformation(K-M formula)in the carburized18Cr2Ni4 W steel revealed that the magnitude of the K-M phase transition parameter αis influenced by the alloy composition,decreasing as the carbon content increases.Different quenching temperatures also affect the α parameter.Therefore,when employing the K-M formula to simulate the martensitic phase transformation in carburized alloy steel,the model must thoroughly consider the effects of compositional variations and quenching temperature on the parameters.(3)A finite element calculation model was established for the multi-physics coupling of gear carburizing and quenching(carburizing-temperature-phase transformation-stress-strain field),which enabled the prediction of microhardness and monitoring of stress distribution and evolution process of 18Cr2Ni4 W carburizing and quenching after treatment.The accuracy of predictions for carbon concentration,temperature and phase transformation evolution was validated based on experimental analysis and testing.In terms of hardness prediction,the relationship between carbon content and residual austenite hardness of 18Cr2Ni4 W at different quenching temperatures was summarized based on Maynier’s proposed mixed-phase hardness calculation rule,and a hardness calculation formula after carburizing and quenching was proposed.The formula was embedded into COMSOL to predict the distribution of hardness after gear carburizing and quenching.In terms of stress and strain analysis,stress evolution during gear quenching and post-quenching distortion were analyzed based on simulation results.The simulation results showed that the maximum residual compressive stress did not appear on the surface after gear carburizing and quenching,which was mainly related to the sequence and amount of martensite transformation.The overall distortion trend after gear carburizing and quenching was lumped and rough,and the maximum diameter of the tooth top circle increased by approximately 0.64 mm.This thesis employs a combination of numerical simulations and experimental analyses to investigate the main theories and key parameters involved in the simulation of carburizing and quenching of 18Cr2Ni4 W alloy steel.It provides valuable insights for finite element simulation of heat treatment in this alloy system.The established finite element simulation and prediction model for carburizing and quenching of18Cr2Ni4 W gear based on the optimized key parameters can establish the correlation between process parameters and performance evolution,and provide useful guidance and reference for process adjustment.The thesis has 69 figures,10 tables,and 157 references... |
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