Analysis Of Microstructure Evolution For U71Mn Steel At Friction And Wear

The fast train speed and high loads in the railway system reault in more seriously wear and rail corrugation in recent years.The microstructure evolution and wear properties of U71 Mn steel under dry pure sliding conditions by the using of MRH-5A ring-block wear test machine were investigated in this work.The worn surface morphology and microstructure on the sliding surface were analysized by optical microscope,scanning electron microscopy(SEM)and transmission electron microscopy(TEM);the formation and microstructureof WEL were studied mainly.For the ring,wear loss is relatively higher at initial worn stage and it increases slowly in the following experiment.With the increase of cycles,the main wear machanism turns from the combination of oxidation and adrasive wear to obvious fatigue and adhesive wear.With rotating speed raises at 150 N,18000r conditions,wear loss of samples enhances.At 600r/min,18000 r and 100 N,150N,200 N load respectively,wear loss of 150 N have a maximum value.The highest hardness of ring after sliding friction occurs at samples surface and it decreases to matrix hardness with the depth increases from surface.With cycles and rotating speed increase,the surface hardness improves;WEL hardness reaches to 950 HV at 600r/min due to refinement hardening;with growth of load,hardness and thickness of hardened layer increase and hardness of WEL is about 1120 HV,which is contributed to refinement hardening and soliding solution haidening.The microstructure of U71 Mn steel after experiment has two stype: Tribology white etching layer(T-WEL)and plastic deformation layer.WEL which is composed of nanocrystalline α-Fe with fine carbide formed due to severe plastic deformation under the effect of repeating stress,ferrite refined to equiaxed nano-grains and cenmentite broken into pieces and dissolved into ferrite.WEL has formed for running limited cycles at 150 N,600r/min,it increases to 10μm thickness at 18000 r and transverse length decreases with the enhancement of cycles;there is not obvious WEL at 450r/min and the thickness of WEL is around 5μm at 750r/min.At 100 N,150N and 200 N loads,WEL with 10μm thickness distributes unevenly.The present of WEL accelerates crakes development and peels off with the formation of cracks.The microstructure without WEL presents plastic deformation layer,which is characterised by orientation of ferrite and cenmentite lamellar.