Ferrite and Spheroidized Cementite Ultrafine Microstructure Formation in an Fe-0.67 Pct C Steel for Railway Wheels under Simulated Service Conditions

The microstructure development of a high-carbon steel (0.67 pct C) for railway wheels as they are affected by rolling contact with rail tracks and by cyclic frictional heat from braking is studied in the vicinity of the contact surface by scanning electron microscopy (SEM) and electron backscattered diffraction. An ultrafine microstructure consisting of ferrite grains with a size of less than 1 μm and spheroidized cementite particles is formed in the region up to 100 μm below the contact surface. The generation of low-angle sub-boundaries associated with the rearrangement of accumulated dislocations involved in continuous recrystallization of ferrite microstructure contributes to the microstructure refinement at temperatures lower than A1 temperature (1000 K). Fine spheroidized cementite particles with uniform distribution obstruct the migration of high-angle grain boundaries, by which the dislocation density is maintained sufficiently high for the formation of sub-boundaries. The formation of a texture, corresponding to the surface texture typically formed in low-carbon steels by hot rolling without lubrication at ferritic temperatures, is observed in the ultrarefined microstructure region. The results drawn from this study strongly indicate the occurrence of “in-situ microstructure control” under service conditions.