Study On Tribo-Fatigue Damage Mechanism And Microstructure Evolution Behaviors Of Wheel Materials

With the rapid development of high-speed railway and heavy-haul railway,wheels are operating under strong friction and multi-field coupling in the wheel/rail rolling contact process.So,the friction,wear and fatigue damage behavior of wheel materials become more and more complicated.As a typical friction fatigue component,it is very necessary to systematically study the behavior of tribo-fatigue damage of wheel materials,establish wear mapping and mechanism mapping of tribo-fatigue damage,which is very necessary to prevent and reduce the fatigue damage of railway wheels.In this thesis,the wheel/rail rolling contact tests with different braking speeds,operating speeds,pre-crack parameters,curve radius,tangential forces and contact stress were performed using a JD-1 wheel/rail simulation facility.The rolling tribo-fatigue damage behaviors of wheel material have been investigated by means of optical microscope(OM),Vickers hardness instrument,laser confocal scanning microscope(LCSM),scanning electron microscope(SEM)and X-ray diffraction analyzer(XRD).From the viewpoint of tribology and material science,the evolution behavior and regularity of the microstructure of the plastic deformation zone during rolling contact have been analyzed by electron backscatter diffraction(EBSD),focused ion beam(FIB)and transmission electron microscopy(TEM).The main accomplished research contents and the chief conclusions are as follows: 1.The effect of experiment parameters on the tribo-fatigue damage of wheel materials in the wheel/rail rolling contact process was elucidatedWith the braking speed increasing,the frictional heat between wheel and rail increases.The aggravation of thermal fatigue cracks as braking speed increasing is due to the decrease of yield strength and other mechanical characteristics of wheel materials from frictional heat.In the role of the corresponding thermal stress and frictional heat,the thermal fatigue cracks are prone to branch and propagate toward the internal material,which will cause serious friction thermal fatigue damage with braking speed increasing.The primary damage mechanism of wheel roller surface transforms from spalling and slight surface fatigue to severe fatigue cracks as the operating speed increases.Fatigue cracks initiate from the wear surface and grow along with the soft ferrite lines within the plastically deformed layer.In the role of bending moment,the cracks mainly initiate on the tips and both sides of the pre-cracks.The cracks propagating directions are not entirely along the original directions of pre-cracks and easily change.The branch cracks initiate from the principle cracks when the cracks propagate to a certain stage.Meanwhile,the propagating mechanism of cracks for wheel materials is the transgranular fracture.The surface oblique cracks are initiated by the combination of tangential force and lateral force on the worn surface of wheel rollers.The cracks mouths are perpendicular to the resultant direction of the surface frictional forces.2.The mechanism mapping of tribo-fatigue damage of wheel materials was establishedThe effects of the tangential force and contact stress on the wear and tribo-fatigue damage behavior of wheel material have been systematically investigated.According to the evolution of wear rate of wheel rollers,the wear diagram of wheel material is established and can be divided into three regions: slight wear region,severe wear region and destructive wear region.Through the analysis of the surface frictional damage morphology of wheel rollers and the average crack length of wheel rollers cross section under each experiment parameter,the tribo-fatigue damage map of wheel material is established and can be divided into three regions: slight fatigue damage region,fatigue damage region and severe fatigue damage region.3.Microstructure evolution of the near-surface plastic deformation zone of wheel materials under rolling contact condition was elucidatedA depth-dependent gradient microstructure of wheel roller is observed: a nanocrystalline region in the topmost surface of 20 ?m thick,a submicrometer region in a depth span of 20 to 45 ?m,a deformed micrometer region at 45-80 ?m deep and an undeformed matrix region with depth more than 80 ?m.In the rolling contact process,the microstructure evolution of wheel material include the following processes: firstly,plastic deformation takes place,the interlamellar spacing of pearlite lamellae diminishes,a small amount of cementite fractures and dissolves and considerable dislocations appeare inside ferrite.Secondly,the dislocations tangle and form dislocation cells and dislocation walls inside ferrite,and the dislocation walls transform into sub-boundaries.Thirdly,the newly-formed ferrite low-angle sub-boundaries are transformed into high-angle boundaries,whose grains are randomly oriented.Finally,under higher strains and strain rates,the refined grains repeatedly undergo plastic deformation and the dislocation cells are formed,the dislocation walls transform into sub-boundaries and the low-angle boundaries transform into high-angle boundaries,until the size of grains reach the stable minimum value and the refinement of grains cease.