Abstract
A peculiar microstructure formed in a case-hardened steel has been examined by transmission electron microscopy. The microstructure is called a bright etched region (BER) because it appears bright when etched with Nital. BER consists mainly of very fine grains, the average size of which is a few tens nanometers. An amorphous region is observed near a crack which is formed in BER. The observation indicates that, during rolling contact fatigue, a coarse grain of martensite is transformed into polycrystals with very fine grains, and then into an amorphous region, from which a crack initiates.
