| Wheel reliability is related to the safety and economy of train transportation.It is hard for the present pearlitic wheels to meet railway development toward higher speed and heavier load in the future.Novel Mn-Si-Cr bainitic steel demonstrates a promising alternative to be a new generation of wheel steels meeting railway development,because of their excellent general properties.Surface quenching plays an important role in getting high performances during production process of above bainitic wheels.However,due to many factors such as high temperature environment and large size,it is difficult to study the surface quenching process in the field.For Mn-Si-Cr bainitic wheel steels,continuous cooling transformation(CCT)and time temperature transformation(TTT)curves were measured.Moreover,kinetics of biaintic and martensitic transformations were studied.Accordingly,a new finite element simulation model about the change of temperature,microstructure and hardness during cooling quenching was built here.At the same time,the preciseness of this model was certified by wheel properties tests.Furthermore,the effect of surface cooling parameters on microstructure and properites of rim section were studied using this model,which could provide a guideline for production parameters adjustment.Related results are as followings:The cooling rate has great influence on the microstructure and properties of the wheel steel.Bainitic transformation happens in a wide range of cooling rates.As-treated microstructure is mainly composed of bainite and martensite.Koistinen-Marburger(K-M)equation can be used to calculate the martensite transformation of above wheel steel during surface quenching,and its transformation rate α is 0.026.Johnson-Mehl-Avrami-Kolmogorov(J-M-A-K)equation is applicable to the calculation of bainitic transformation in the isothermal process for this steel and the activation energy of bainitic transformation(Q),Avrami index(n)and the average value of lnk is 52.17 k J/mol,1.52187 and-8.51094,respectively.A new finite element model of wheel surface quenching was established for this steel by combining many factors including the latent heat,transformation kinetics,transient heat transfer and complex coupling multiphase transition.Thermo-structural behavior of wheel quenching could be predicted using this model and the model simulation shows high reliability and precision in predicting temperature and microstructure distribution.The temperature error between simulated value and the measured distribution is less than 5%.The hardness between predicted value of wheel and detection value of real wheel at different positions is within 2 HRC.These results could provide an effective reference for industrial production.In surface quenching of wheels,the cooling rate difference at different positions leads to the diversity of microstructure,which corresponds to variable properties.The best strength and toughness match is got at a depth of about 20 mm below the tread and the highest volumn fraction of retained austenite content exists at about 30 mm below.The rim microstructure gradually transformed into the granular bainite from the tread to the inside within 50 mm depth below the tread.This model can be used to predict the influence of quenching parameters on wheel properties.Results showed that general properties of bainitic wheel can be optimized by adjusting spraying intensity and spraying time interval in present quenching process.The hardness around the rim can be higher if spraying intensity is increased and the first spraying time is prolonged.In addition,the prediction of this model can provide reference for the hardness match on different wheel and rail service conditions. |
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