Papers by Author: Katsuhiko Inoue

Abstract: 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.

Abstract: The life limit of the standard hardness block of HRC64 made from JIS-SUJ2 steel was evaluated. The HRC64 specimens were annealed at temperatures ranging from 55 to 200
C for about 104 h in maximum. The micro-Vickeres hardness decreased with the annealing time at all temperatures after some incubation periods. Transmission electron microscopy (TEM) showed that the microstructure of the specimen before annealing was composed of primary cementite particles, carbide and martensite matrix. After annealing, fine cementite particles of about few tens of nanometers in size were observed. The decrease in hardness during annealing was caused by the precipitation of the cementite. The life limit keeping the standard hardness value at ambient temperature was estimated to be about 3.5-4 years based on extrapolation of the annealing curves at various temperatures.