A Novel Mechanism of the Formation of White and Brown Etching Layer in Bainitic Rail Steel

Abstract

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle White and Brown Etching Layer (WEL and BEL) on the rail raceway during service. The bainitic rail steels with excellent strength-toughness balance 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, we find a distinct multilayer WEL/BEL composite structure on the worn surface/subsurface of the bainitic rail after in-field testing. The multi-scale characterizations from the microscopic to the atomic scale reveal that the WEL and BEL exhibit different microstructural, chemical, and mechanical features. The oxide particles preferentially form in the BEL rather than the WEL. The local oxidation generates the Mn, Si, and Cr depletion but C enrichment in the BEL matrix. Consequently, the BEL exhibits hard but brittle features due to the formation of brittle twinning martensite, cementite, and oxide particles. In contrast, the WEL is relatively soft and ductile. Under plastic deformation and temperature rise upon wheel-rail contact, the heterogeneous microstructure and chemical micro-segregation in parent material lead to the inhomogeneous microstructural refinement, oxidation, and localized embrittlement, which is responsible for the formation of multilayer WEL/BEL structure in bainitic rail steels.