Numerical Simulation Research On Carburizing-Quenching Process And Distortion Of High Speed Locomotive Gears

Based on improving the carrying capacity,fatigue resistance and service life of the gears,carburizing-quenching has become the leading heat treatment process for high-performance and hard-surface gears.But due to the long cycle under the high temperature process,there are different temperature changes and phase transformations,which cause complex thermal and transformation stress.Eventually it causes the distortion,affecting the performance and manufacturing cost of the gear.The traditional method of controlling the carburizing-quenching distortion of the gear mainly depends on the test and experience,and because of the many factors affecting the distortion,it is often half the effort,the result is not good and the cost is huge.The control of gear distortion becomes the biggest technical difficulty in the carburizing and quenching process.In this study,based on the technical requirements of the carburizing-quenching process and considering the multi-field interaction such as temperature,phase transformation and stress,the hardness field was added to the analysis model by the numerical simulation method.The main factors such as process,performance and distortion that cause distortion are related to each other,the analysis and calculation model of the carburizing-quenching and its distortion is established.The heat transfer coefficients of the quenching oil and low temperature salt bath medium were obtained by the cooling curve and inverse heat transfer method,and the accurate calculation of the temperature field in the quenching process of the gear was realized.The influence of the water content of the salt bath,the salt bath temperature and the stirring speed on the heat transfer coefficient was analyzed.The results show that the heat transfer coefficient increases with the increase of the water content and stirring speed,and the heat transfer coefficient decreases with the increase of salt bath temperature.With the increase of water content,stirring speed and salt bath temperature,the temperature corresponding the maximum heat transfer coefficient is decreasing,which enhances the cooling capacity and speed of the low temperature region of the salt bath medium.On this basis,the coupling model of temperature,phase transformation and stress was simultaneously established,and an analysis model of oil-quenching and salt-quenching on the carburized distortion sample was established.The distortion performance of the gear material was analyzed by the distortion sample.The thermal and transformation strains were indirectly separated with/without phase transformation consideration,and the distortion process of the distortion sample was analyzed.The influence of process parameters on the distortion was quantified by the orthogonal experiment.The results show that the strain caused by thermal stress plays a major role in the distortion of the sample,and the transformation strain is only a secondary factor.The distortion tendency of 20CrNi2 Mo material is less than that of 17CrNiMo6 material.The carburizing temperature,carbon potential and quenching temperature have a great influence on the distortion of the sample.Finally,by comparison with the oil quenching and salt bath grading quenching of the distorted samples.It was found that the salt bath grading quenching process has a rapid cooling rate at the high temperature region,more fully martensitic grade transformation,and the lower thermal and transformation stress.So it was beneficial to reduce and control the carburizing quenching distortion of the gear.Based on the calculation of the temperature field,the Jominy hardness curves under different carbon contents were obtained by the experimental results,the USS-Atlas formula and JMatPro.The Jominy cooling curves in a specific temperature range were obtained by the temperature field results simultaneously.The Jominy curve model was established for the hardness calculation of the carburizing-quenching.The hardness of the carburizing-quenching workpieces can be calculated based on the carbon content and the cooling time.The model was applied to the calculation of hardness field on the carburized Jominy sample and toothed sample for the 17CrNiMo6 and 20CrNi2 Mo steels.By comparing with the experimental results,it was verified that the model can guarantee good accuracy,and the maximum error is less than 12%.The model does not need to realize phase transformation simulation,and has good simplicity and practicality.Based on the above basic analysis model and combining with the technology requirements of the high-speed locomotive gear,the carburizing-quenching process and its distortion of the gear were analyzed from three aspects of the material performance,process and gear structure.The distortion analysis models on the salt quenching and oil quenching process were applied to the gear ring and gear shaft of the high-speed locomotive,respectively.The distortion law of the gear ring and gear shaft was analyzed and could provide a basis for the distortion control of the process.The results show that on the oil quenching process,there was lumbar drum deformation on the tooth width direction of the gear ring,the maximum expansion of the addendum circle diameter was 0.65 mm,and the expansion of the common normal length was 0.47 mm.Along the tooth width direction,except for the addendum circle diameter near the left end of the shaft was enlarged,and the addendum circle diameter in the other positions of the gear shaft were reduced.The amount of contraction was the largest near the middle of the tooth width.The teeth of the gear ring along the width direction showed a slight saddle deformation trend on the salt quenching process.The maximum expansion of the addendum circle diameter was 0.414 mm,the expansion of the common normal length was 0.32 mm,and the deformation was relatively uniform.By comparing with the distortion measurement data,the simulation accuracy of the addendum circle can be guaranteed.The influence degree and trend of process parameters on the distortion of the addendum circle diameter and the common normal length were simulated and analyzed by the orthogonal test.The results show that the quenching temperature,oil temperature,carburizing potential and quenching holding time had great influence on the two kinds of distortion.Based on the above results,the simulated and optimized carburizing-quenching process parameters and the structure dimensions of the frock were applied to the simulation and actual the carburizing-quenching production of the gear.By comparison with the simulation and measured data under the original conditions,the distortion of the addendum circle was reduced by 12.4% and 8.8%,respectively,and the distortion of the common normal length was reduced by 33% and 53.9%,respectively.Good control effect of the distortion is obtained.