Mechanism And Microstructural Evolution On Bainitic Rail Surface Under Wheel-Rail Contact

The formation of White Etching Layer(WEL)and Brown Etching Layer(BEL)on the surface/subsurface of pearlitic rail steels was generally considered as a preliminary process of irregular wear and RCF damages.The failure of conventional pearlitic rail steels is strongly affected by the formation of White and Brown Etching Layers(WEL and BEL)on the rail raceway during service.The WEL/BEL is also called tribologically transformed structure(TTS)layers.In recent years,the bainitic rail steels with excellent strength-toughness balance and resistance against wear and fatigue are promising candidates for the next generation of rail steels.It is generally stated that the hard and brittle WEL/BEL is difficult to generate in the bainitic rails because of low carbon content.However,in this study,we first discovered a unique multilayer heterostructured WEL/BEL on the worn surface/subsurface of the bainitic rail after in-field testing.In this paper,we investigated two types of bainitic rail steels,namely 1280 G and1380G steels,after in-field test.The nature of WEL/BEL was revealed via the multi-scale characterizations on the morphology,microstructure,and chemical composition of the multilayer structure on the surface/subsurface of bainitic rails.With the assistance of analysis on the WEL/BEL structures at different depths,the evolution mechanism of the surface layer structure on worn bainitic rails was discussed.Based on the findings,the differences between the multilayer structures in the two rail steels were uncovered.Meanwhile,the mechanical properties of the multilayer structures were studied by in-situ experiments,which provide a new insight into improving the service performance of bainitic rail steels.Results show that the WEL/BEL in the bainitic rail steel exhibits a multilayer heterostructure with an alternate or irregular distribution.The hardness of WEL/BEL at the topmost surface can reach up to ～1100HV.BEL and WEL have different characteristics,with BEL having higher hardness and smaller grain size than adjacent WEL.The WEL is mainly composed of fine martensite and retained austenite,whereas the BEL is a composite containing nanocrystalline martensite or ferrite,retained austenite,partially dissolved cementite,and oxide particles.The multi-scale chemical analyses reveal that the oxide particles preferentially form in the BEL rather than the WEL.The local selective oxidation promotes the redistribution of alloying elements,generating the Mn,Si,and Cr depletion but C enrichment in the BEL matrix.Furthermore,the analyses on multilayer structures at different depths show that refinement and local oxidation play a significant role in the formation of multilayer structures in bainitic rail steels.Grain refinement and high-temperature oxidation can promote each other so that nanocrystallization and oxidation occur in the same region,thus generating WEL and BEL structures with different grain size.The differences in the multilayer structures of the two rail steels(i.e.,1280 G and1380G rail steels)is discussed,which is attributed to the different cycle numbers experienced by the unit volume in the TTS layer of the two rails.Compared to the 1280 G rail,the more cycles and lower wear rate of the 1380 G rail result in a more sufficient grain refinement and oxidation.Hence,the thickness of the TTS layer is thinner,companied with more uniform structure and elemental redistribution,which could suppress the fatigue crack initiation.In addition,the results of in-situ micro-pillar compression experiments also confirms that the TTS layer of 1380 G rail has a better combination of strength and ductility.