Research On The Machining Mechanism Of Rail Milling-grinding Maintenance And Wear & Damage Characteristics Of Machined Wheel/Rail Materials

With the ever-increasing axle load and traffic volume of the trains,the service conditions between the wheel and track have become harsher and more complex,and thus the defects induced by the dynamic wheel/rail interaction are prone to come into being,such as corrugation,head check and spalling.These aforementioned diseases will cause safety hazards to the smooth operation of the trains,therefore it is necessary to remove the rail defects in time.Due to its excellent machining precision and capacity,the rail milling-grinding train has gradually become a kind of significant rail maintenance equipment employed in railway maintenance industry.With the continuous and rapid development of Chinese railway transportation network,the demand for rail milling-grinding trains is increasing day by day.At present,China still mainly relies on the imported rail milling-grinding trains from abroad.There is a lack of systematic research on the machining mechanism of rail milling-grinding maintenance and wear & damage behaviors of machined wheel/rail materials.Based on the rail milling-grinding maintenance,this paper concentrates on the systematic research on the high-temperature dynamic mechanical properties of rail material and the establishment of its constitutive model,the characteristics of cutting force and temperature,the changing laws of machined surface integrity,and the wear & damage behaviors of machined wheel/rail materials,which is of crucial academic value and practical significance to improve the service performance of the wheel and rail,thereby improving the safety,stability and reliability of the train operation.The main research contents and achievements are as follows:(1)The dynamic mechanical properties of U71 Mn rail material over a wide range of temperatures and strain rates were systematically studied.The effects of temperature,strain,and strain rate on the dynamic compressive characteristics of rail material were discussed.Also,the microstructural evolution law was analyzed.Meanwhile,based on the experimental results,the original and modified Johnson-Cook models of the studied material were established,respectively.In addition,the secondary development of the modified Johnson-Cook constitutive model was performed via VUMAT user material subroutine,which provides theoretical and technical support for the analysis of rail machining finite element simulation.The results show that the flow response of rail material is both temperature-sensitive and strain rate-sensitive,and the influence of temperature on the flow response is more remarkable than that of strain rate.The deformation temperature and strain rate exert a significant influence on the microstructure of rail material.The modified JC constitutive model can preferably express the flow behavior of U71 Mn rail material over a wide range of temperatures and strain rates.(2)The characteristics of cutting force and temperature of U71 Mn rail material were studied.Taking the rail milling-grinding application conditions as the design reference,a reliable rail milling-grinding experimental platform was self-developed and established.The rail milling force model and grinding force model were established respectively by orthogonal experimental design,and the reliability of the models was verified by F-test method.Based on the linear heat source method,the temperature field models of rail form milling,form grinding and the joint temperature field model were established respectively.Meanwhile,the finite element models of rail milling and grinding temperature were established and analyzed by utilizing the modified Johnson-Cook constitutive equation of rail material.The results show that the established mathematical models of milling force and grinding force possess high credibility;the mathematical models of temperature field can well reflect the temperature changing law in the process of rail machining;the established finite element models of rail milling and grinding temperature can well reflect the temperature changing trend in the rail machining process.(3)The single-factor experiment method was used to explore the changing laws of the machined surface integrity of U71 Mn rail material,and the corresponding mechanism analysis was also carried out.The specific characterization parameters mainly contain the machined surface roughness and surface morphology,surface residual stress,subsurface plastic deformation layer and work hardening.Meanwhile,the values of the machined surface roughness obtained under different machining parameters were taken as the training samples,and then the surface roughness prediction models of rail milling and grinding were established respectively based on RBF artificial neural network,which can provide an experimental and theoretical basis for rail maintenance,and lay a solid foundation for the research on the wear & damage behaviors of machined wheel/rail materials.The results show that the surface transverse residual stress and surface longitudinal residual stress after rail form milling and grinding all display compressive stresses,and the values of longitudinal residual stress is greater than those of transverse residual stress.Also,different degrees of subsurface plastic deformation and work hardening occur.The morphology of wear debris is dominated by banded shape,accompanied by a small amount of spherical wear debris,and the degree of oxidation of the spherical wear debris is the most serious.The established RBF neural network possesses excellent predictability for the machined surface roughness.(4)Systematic research on the wear & damage behaviors of machined wheel/rail materials was performed.Firstly,U71 Mn rail specimens were ground,and CL60 wheel specimens were turned with various machining parameters.The machined surface microhardness,surface roughness and subsurface plastic deformation layer were examined.Then the sliding and rolling-sliding wear and damage properties under dry conditions were investigated respectively by changing different wheel/rail tribological pairs.The effects of the surface integrity of machined wheel/rail materials on the friction and wear performance were analyzed in terms of friction coefficient,wear amount,subsurface plastic deformation layer and worn surface morphology.The results show that after the sliding contact,the thickness increase degree of the subsurface plastic deformation of the rail blocks is greater than that of the wheel rings.The wear loss of the rail material is much larger than that of the wheel material,and the surface damage of the rail blocks is more serious than that of the wheel rings after sliding contact.After the rolling-sliding contact,the thickness of subsurface plastic deformation of the rail rings is much larger than that of the wheel rings.The wear loss of the wheel material is much larger than that of the rail material,and the surface damage of the wheel rings is more serious than that of the rail rings after rolling-sliding contact.Whether it is sliding contact or rolling-sliding contact,the wheel/rail samples machined with various machining parameters engender different friction coefficients after pairing,and the wheel samples with larger surface roughness generate higher friction coefficients during the wearing test.The surface roughness,microhardness and subsurface plastic deformation layer of the wheel/rail samples after machining exert a comprehensive effect on the wear behavior,and friction pair with appropriate original surface microhardness and roughness generates the smallest amount of wear loss.