PECULIAR MICROSTRUCTURE FORMED BY ROLLING -CONTACT FATIGUE IN BEARINGSTEELS

A peculiar microstructure formed in a case-hardened steel has been exa mined by transmission electron microscopy. The microstructure is calle d a bright etched region(`BER) because it appears bright when etched w ith Nital. BER consists mainly of very fine grains, the average size o f which is a few tens nanometers. An amorphous region is observed near a crack which is formed in BER. The observation indicates that, durin g rolling contact fatigue, a coarse grain of martensite is transformed into polycrystals with very fine grains, and then into an amorphous r egion, from which a crack initiates.