Microstructure and mechanical properties of martensitic steels after ultrafine-crystallized drilling were investigated. Nano- (at top surface) and submicron-crystalline structures (UFG structure) were formed near drill-hole surface at more than Ac1 by ultrafine-crystallized drilling. The nanocrystalline structure has extremely high hardness and high thermal stability. These characteristics of nanocrystalline structure were similar to those formed by ball milling and shot peening. The tensile property of specimen with UFG structure near the specimen surface showed the similar deformation behavior to that without UFG structure. The fatigue life of specimen with surface UFG structure increased independently of the existence of residual compression stress as compared to that without UFG structure.
