Study On The Surface Microstructure Evolution Of Typical Wheel/rail Materials In Wear

Wheel-rail system is the significant running gear system most directly relevant to safety in railway transportation.With the rapid development of heavy load and high-speed railway in China,such issues as wear and contact fatigue between wheels and rails are becoming increasingly obvious,which will directly affect the service longevity of wheels and rails and even pose a threat to the safe and stable operation of trains.However,in the process of wear,since material microstructure and mechanical properties on the surface of wheel-rail treads are constantly are changing,to invsetigate microstructural changes of typical wheel-rail materials during wear is crucial to the design,manufacturing and match of wheel-rail materials.Through the observation of CL60 wheel and U71Mn rail in real operation and the analysis of systematic rolling/sliding wear experiment on CL65 wheel steel and U75V rail steel under laboratory conditions by OM,SEM,T-EBSD and TEM,the microstructrual evolution of typical pearlite wheel-rail material under rolling/sliding wear conditions and the formation mechnism of different types of WELs were systematically investigated.During wheel-rail service period,such factors as brake and creep would cause the temperature on the wheel-rail contact surface to rapidly rise to the temperature needed for austenization.Due to rapid heating speed and extremely short time for heat preservation,the size of austenite grains formed during the above process was considerably fine and small(less than 1?m).Therefore,the martensite formed subsequently during the process of instant cooling also possessed super-fine grains.Since the original cementite wasn't fully dissolved inside austenite and extremely fine and small carbide remained inside the martensite above-mentioned,WELs formed in the process possessed super-high hardness.After the formation of martensite WELs,when the wheel-rail material was used later,martensite continued to deform plastically,and finally inside the surface microstructure,over-saturated a-phase nanocrystalline grains formed.The formation of WELs was caused by the comprehensive action mechanism of thermal and plastic deformation.During rolling wear,due to the impacts of compressive stress and shear stress,the pearilte on the surface underwent plastic deformation:the direction of pearlite lamellae gradually tended to become parallel to that of the wear surface and the spacing between pearlite lamellae also became smaller.When the spacing was less than 100nm,most of cementite began to fracture and dissolve and the ferrite inside the pearlite displayed the feature of subgrain refinement because of plastic deformation.Eventually,bar-shaped ferrite with certain preferential orientation was formed and undissolved,fine and small cementite were scattered in the vicinity of its boundary.Inside hypoeutectoid steel,pro-eutectoid ferrite with low hardness would underwent subgrain refinement due to plastic deformation;some subgrain boundaries were gradually transformed into large-angle boundaries,leading to ferrite refinement.Under the impact of the periodic vibration from the shaft's radial direction,polygonization wear appeared on the surface of the specimens during rolling wear.After the analysis of the specimen's surface morphology,physical dimension,microstructure and properties,what was found was as follows:wave crests were mainly featured by adhesion wear and wave troughs were chiefly marked by fatigue wear.During the wear process,both wave crests and troughs were hardened due to plastic deformation,but their hardening extents and laws were different:(1)the hardness of the surface of wave troughs was higher than that of the surface of wave troughs;(2)the maximum hardness of wave troughs was at the surface,while the maximum hardness of wave crests was at the sub-surfacce;(3)the hardened layer's depth of wave troughs was larger than that of wave crests.Through the investigation of the surface microstructural changes of the CL65 block specimen(in the experiment of ring-block sliding wear)in the wear process,what was discovered was as follows:during the wear process,pearlite on the surface underwent plastic deformation,the lamellar spacing was diminished,great amounts of dislocations formed inside ferrite,and some cementite was fractured and dissolved;plastic deformation continued to increase,cementite began to dissolve considerbaly and became particle-like,dislocations accumulated and tangled and finally dislocation walls were formed;inside ferrite,many short-rod shaped and even equixed-shaped dislocation cells were formed,and then gradually were transformed into low-angle sub-boundries;with the further enlargement of plastic deformation,at the same time when subgrains were further refined,these low-angle sub-boundaries would,through accumulation and annihilation of dislocations,lead the grains to continuously rotate to transform into high-angle boundaries,resulting in the random orientation of grains and causing grains to be refined to the nanometer-sized level.Since the carbide on the surface was fractured,refined and dissolved,the corrosion resistance of the material was improved;therefore,under OM the surface was WELs,whose formation mechanism belonged to deformation mechanism.