The creation of sub surface White Etching Cracks in rolling bearings

This report describes the development of a laboratory test rig for creating sub surface White Etching Cracks in roller bearings, the test specimens and the parameters applied during testing and finally electron microscope and nanomechanical characterization of the cracks and the adjacent microstructure. The test rig configuration allows for performing tests at a temperature range from -40 °C up to 100 °C, under contact pressures reaching up to 1.9 GPa and 6000 RPMs. A plasma discharge can be initiated in the lubricating film through the DC current applied between the test specimens. The test specimens were deep groove ball bearing type FAG 61908, 62/40 x 12 mm with a standard low alloy through hardening bearing steel of type 100C6 with a martensitic matrix. After testing microcracks were found located at 80 - 120 µm under the raceway surface both in the inner and outer ring of the bearings. The cracks were connected to elongated MnS inclusions with crack lengths between 20-50 µm. TEM investigations revealed a ferritic nanocrystalline microstructure in the range 20 – 70 nm on each side of the cracks near the crack tip ends. Nanomechanical indentations located in an area of nano-sized ferritic grains indicated that the hardness is somewhat lower than the martensitic matrix.