4D observations of rolling contact fatigue processes by laminography using ultra-bright synchrotron radiation

Since rolling contact fatigue (RCF) is known to be affected by non-metallic inclusions, the RCF crack initiation and propagation mechanisms in high-strength steels were observed using ultrabright synchrotron radiation laminography. The material of the sample was a high-carbon chromium bearing steel, which intentionally contained a high concentration of sulfur to enable the observation of crack initiation from MnS inclusions. To conduct an RCF test nearby the experimental hatch of a beam line of a synchrotron radiation facility, a special RCF testing machine was developed. Fatigue tests were interrupted to conduct laminography. The fatigue life for flaking depended on the length, width, and orientation of inclusions. For every length and orientation of inclusions, vertical cracks, whose faces were perpendicular to the rolling direction, first appeared, then horizontal cracks, whose faces were parallel to the sample surface, were formed after the vertical cracks reached a critical length. The initiation life of the vertical cracks and horizontal cracks depended on the length, width, and orientation of the MnS inclusions. The depth of horizontal crack initiation site, however, was almost independent of these parameters. The mechanism of the flaking process, which was directly observed by laminography using synchrotron radiation, was completely different from that previously supposed from observations by conventional microscopy.