Analysis of Spherical Carbides Formed in Chromium Added Hypereutectoid Bearing Steels

Microstructures in a bearing steel, JIS SUJ2, have been observed and analyzed in detail by means of transmission electron microscope and synchrotron radiation diffraction in SPring-8. Spherodized carbides in the steel are generally recognized as spherical cementite particles, but some of them have been clearly shown to be M23C6 type of carbide in this study. The shapes and sizes of these two types of carbides are similar to each other. On the martensitic matrix of the steel, it is believed to be stable at relatively low temperature, but it is also shown to be decomposed to form cementite by prolonged aging at 383 K, which provides a reason for decrease in hardness in standard hardness blocks, previously reported by the authors.