Microstructure deformation and white etching matter formation along cracks

Rolling elements made of 52100 steel were subjected to high austenitizing temperatures to embrittle the steel. The embrittled rolling elements were intentionally given a rougher surface finish and were subjected to bearing tests under rolling-sliding conditions. Post-test material investigations revealed cracks in the raceways and rollers. Detailed electron microscopy investigations showed highly deformed microstructure in the vicinity of the cracks and white etching matter (WEM) along the cracks in the subsurface regions. Intergranularly oriented cracks and associated WEM were observed in the lower subsurface areas of the rollers. In contrast to the cracks formed closer to the surface, the martensitic microstructure in the vicinity of the intergranular cracks did not exhibit the microstructure deformation. The highly deformed microstructure and associated cracking are result of the high surface roughness of the rollers combined with the sliding conditions. The formation of the white etching matter, which was composed of a nanograin structure, is attributed to the crack faces rubbing. These results advance the understanding of damage mechanisms in bearings and the associated microstructural changes in the steel.