| Railway plays an increasingly important role in the rapid development of China's transportation industry that endeavors to design trains that are capable of bearing heavy loads with high speed.While the key to realize this goal lies in the lightweight structure of components.The hollow railway shaft can be assumed as a pivotal part in creating railway axle's lightweight structure for it can not only meet the intensity requirements of the axle,but also greatly reduce the unsprung weight to see that trains operated in a stable and safe way.Therefore,hollow railway shafts are widely utilized in high speed trains.Currently,hollow railway shaft is chiefly forged from hollow blank,a method featuring high cost and 65%-73% material utilization ratio,of which the core technology is mastered by developed countries as Japan,Germany and France.The hollow railway shafts of China's highspeed trains,at present,are majorly imported products,which,to some extent,hinders the development of high-speed railway's high-end equipment manufacturing industry in the country.The Multi-wedge Synchrostep Cross Wedge Rolling(MSCWR)can be considered as the best technique applied in rolling large long axle parts,characterized by reducing the roller surface of mould and the size of the main part of device.To study rolling hollow railway shaft utilized in high-speed trains by MSCWR is of theoretical significance and engineering value for promoting the development of the country's high-speed railway equipment manufacturing industry.The paper,taking hollow railway shaft made of 25CrMo4 as its subject,has studied the mechanical behavior and dynamical structural change of 25CrMo4,low-alloy steel,with the help of Gleeble-3500 thermal testing machine,and built the steel's high-temperature rheological constitutive model,dynamic recrystallization kinetic model and dynamic recrystallization grain size model via metallographic analysis.According to the experimental data and results of theoretical prediction model,the established model can accurately display the flow characteristics and microstructure evolution of 25CrMo4 when it is undertaken rolling.The author has summarized the rotation condition and flattening condition of hollow shaft rolled by MSCWR by means of establishing mechanical model,and deduced the steady rolling condition for hollow railway shaft's multi-wedge synchrostep rolling considering the roller's flattening condition,as a result,putting forward the unstable flattening rule of hollow railway shaft's cross wedge rolling;imported constitutive model made of 25CrMo4 to the software DEFORM to build a rigid-plastic finite element model that shows the heat,stress and coupling occurred during the metal forming process;accomplished the numerical simulation of hollow railway shaft rolled by multi-wedge synchrostep rolling,to obtain the distribution and change rule of stress,strain,temperature and tissue field in the process of rolling,and expound the deformation mechanism of hollow railway shaft rolled by multi-wedge synchrostep rolling.Meanwhile,researches have been carried out to analyze the effects of different process parameters on ellipticity and wall thickness uniformity,so as to explain the rule of non-elliptical,wall thickness uniformity when it is undergoing multi-wedge synchrostep rolling,as well as the process parameters selected to ensure wall thickness uniformity.Moreover,the paper has analyzed the relation between the grain size and distribution of the roller and process parameters,and expounded the microstructure evolution of hollow railway shaft rolled by MSCWR.The 1:5 scale model established by H630 cross wedge rolling mill further has verified the reliability of the results obtained in this paper with experimental data relatively tallied with theoretical results.The paper provides theoretical guidance for applying MSCWR in rolling the hollow railway shaft utilized in high-speed train and developing homegrown core technology. |
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