Microstructure and Properties of the Contact Surface Layer of U20Mn Bainitic Rail Steel under Rolling/Sliding Conditions

Within this layer, fatigue cracks were observed to preferentially initiate and propagate along the metal flow lines, which delineate the direction of severe shear deformation. Detailed microstructural analysis revealed that the surface transformation under cyclic rolling/sliding contact is predominantly governed by intensive dislocation activity. The motion and reorganization of dislocations progressively subdivided the original bainitic ferrite laths, leading to the formation of dislocation cells.

These cells subsequently evolved into new grain boundaries with high misorientation angles, culminating in significant grain refinement. Quantitative assessment showed that the number of grains per unit area increased remarkably—by a factor of about seven compared to the untested matrix. This refinement process was accompanied by substantial work hardening, resulting in a microhardness increase of 18.58% relative to the base material.

Furthermore, advanced microstructural characterization techniques confirmed the development of a distinct shear deformation texture aligned parallel to the {111} crystallographic planes in this refined region. Energy-dispersive X-ray spectroscopy (EDS) also indicated noticeable carbon enrichment at the newly formed grain boundaries, suggesting carbon migration during the deformation process. A comprehensive analysis of these findings confirms that the microstructural evolution in the near-surface region of U20Mn bainitic rail steel under rolling/sliding conditions is the direct and primary factor governing its wear resistance and contact fatigue performance.

The stability and evolution of this surface-affected layer are intrinsically linked to, and constrained by, the initial multiphase microstructure, which consists of bainitic ferrite laths, retained austenite films, martensite, and martensite-austenite (MA) islands. The insights gained from this work contribute to a more fundamental mechanistic model for RCF damage in advanced rail steels and provide valuable guidance for the future development of bainitic steels with enhanced service life and reliability under demanding railway operations.