Heterogenous structure and formation mechanism of white and brown etching layers in bainitic rail steel

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle white and brown etching layers (WEL and BEL, respectively) on the rail raceway during service. Bainitic rail steels with an excellent strength–toughness balance are promising candidates for next generation rail steels. It is generally difficult to generate hard and brittle WEL/BEL in bainitic rails because of the low carbon content. This study reports, for the first time, the formation of a unique multilayer heterostructured WEL/BEL in a field-tested bainitic rail. The heterogenous structures of WEL/BEL were characterized using transmission electron microcopy, focused ion beam, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, electron probe microanalysis, atom probe tomography, and nano-indentation techniques. Results demonstrate that WEL is mainly composed of fine-grained martensite or ferrite and retained austenite, whereas BEL is a composite structure containing nanocrystalline martensite or ferrite, retained austenite, cementite, and oxide or O-rich particles. The oxide or O-rich particles are primarily formed within BEL through local oxidation, resulting in secondary partitioning of the alloying elements between the oxidation product and its surrounding matrix. BEL is hard but brittle due to the formation of brittle twinned martensite, cementite, and oxide particles, whereas the adjacent WEL is relatively soft and ductile. Based on these findings, the heterostructured WEL/BEL formation mechanism is preliminarily discussed in terms of inhomogeneous grain refinement, localized oxidation, chemical alterations (including both chemical segregation in the initial microstructure and secondary partitioning of the alloying elements during local oxidation), and phase transformation.