Study On The Microstructure And Performance Evolution Of Wheel/rail Materials In Wear

With the rapid development of high-speed and heavy-load railway in China,wear failure and fatigue damage of wheel/rail become more prominent,reducing the service life of wheel/rail materials and severely affecting the safety of railway transportation.Analyzing wheel/rail failure mechanisms,improving comprehensive properties of wheel/rail materials,and prolonging wheel/rail service life are the key scientific and technical problems that urgently need to be studied and solved in railway industry.Through the observation of the actual wheel/rail after service and the wheel/rail materials under laboratory conditions by SEM and TEM,the microstructural and property evolution of several types of commonly used wheel/rail materials and bainitic wheel/rail materials under different wear conditions are analyzed and investigated,the formation mechanism of polygonization wear(Poly-W)and its effect mechanism on wear process are revealed,and the formation process and evolution mechanism of tribological white etching layer(T-WELs)and martensite white etching layer(M-WELs)of different wheel/rail materials during wear process are compared and analyzed.Under the condition of high-speed and heavy-load service,the severe plastic deformation pearlite is the main microstructure of wheel/rail tread;because the relatively high wheel flange/rail edge slip rate leads to higher strain of the microstructure on the surface layer,the TWELs and even the M-WELs tend to form.The microstructures inside the T-WELs are mainly nanoscale ferrite grains and fine cementite particles,and the ones inside the M-WELs are primarily composed of twin martensite,a small amount of residual austenite and some undissolved cementite particles.The fatigue spalling of the M-WELs is mainly divided into three stages.First,since the M-WELs does not undergo plastic deformation easily due to the high strength,crack initiation occurs in the region of stress concentration during wear process: 1)at the wedge angle of the boundary between the M-WELs and the matrix pearlite;2)on the surface of the M-WELs.Then,the cracks at the interface between the M-WELs and the matrix propagate into depths along the interface,while the cracks on the surface of the M-WELs propagate into depths inside the MWELs.Eventually,the cracks initiated and propagated from the surface of the M-WELs and the ones initiated and propagated from the surface interface between the M-WELs and the matrix converge,resulting in deep spalling at the local region of the M-WELs.During the process of two-disc rolling wear experiment occurs Poly-W.According to the surface morphologies and profile variations,the evolution process can be divided into the incubation stage in which macro-wear morphologies display periodic changes(the height difference between crest and trough is rather low),the development stage in which Poly-W occurs in the local region of the specimen(the height difference between crest and trough ranges from several micrometers to tens of micrometers)and the mature stage in which Poly-W occurs all over the circumference of the specimen(the height difference between crest and trough is tens of micrometers).During the process of two-disc rolling wear experiment,that Poly-W appears on different wheel/rail materials corresponds to the coupling effect of mechanical vibration and wear,and wear resistance and conditions of the worn surface determine the uneven wear degree of the specimens,which affects the formation and development of Poly-W.Due to additional load caused by mechanical vibration and higher slip rate on the trough,the wear mode changes from oxidation wear to fatigue wear;compared with the crest,the trough possesses higher surface hardness and thickness of the hardened layer,and its microstructure has higher thickness of the deformation layer and more obvious grain refinement.During sliding wear,the processes where different forms of ferrite are refined to ultra-fine grains are different and affected by the second phase.As shear strain increases,through dislocation multiplication and motion,particle-shaped ferrite grains with the diameter of several tens of micrometers experience the following processes: plastic deformation,the formation of ferrite grains and subgrains with the diameter of several micrometers,the formation of submicrometerized-lamellar-shaped microstructure,the formation of submicrometerized-lathshaped microstructure and the formation of short bar-shaped fine grains.Finally,short barshaped fine grains undergo again the formation of smaller lath-shaped ferrite,subgrain segmentation and the formation of fine grains,until grain refinement reaches a stable state where equiaxed grains with several tens of nanometers are formed.Lamellar-shaped eutectoid ferrite and lath-shaped bainitic ferrite undergo respectively the subsequent refinement processes after the formation of submicrometerized-lamellar-shaped microstructure and the formation of submicrometerized-lath-shaped microstructure;in addition,affected by cementite and martensite transformed from residual austenite,ferrite grains finally obtained are finer.Through the experiment conducted on bainitic wheel/rail steels with similar chemical composition,it is revealed that their wear resistance mainly depends on hardening ability.Higher content of residual austenite contributes to coordinating plastic deformation on the surface layer of bainitic steels,leading to larger strain on the surface layer and finer grains inside the surface layer;meanwhile,more martensite are formed because austenite undergoes martensite phase transformation during the deformation.These two impacts caused by higher content of residual austenite increases the hardness of the surface layer,thus enhancing the wear resistance of the bainitic wheel/rail steels.