Abstract: In order to investigate S–N curve characteristics and subsurface crack initiation behaviour of high speed tool steel tempered at two temperatures in ultra-long life fatigue, cantilever-type rotating-bending fatigue tests are performed in laboratory air at room temperature using hour-glass-shaped specimen. The results show that the testing material can be classified into two S-N curves corresponding to the surface induced fracture and the internal inclusion induced fracture, respectively. The S-N curve for surface induced fracture is at high stress amplitudes and low cycles, while the S-N curve for internal inclusion induced fracture is at low stress amplitude and high cycles. The transition of S-N curve from the former to the later is almost continuous. The S-N curve characteristics are not affected by the tempering temperatures. The fracture origin in the vicinity of an internal inclusion and the microstructure of the material are observed and compared in detail by using SPM, SEM and EPMA , respectively. A granular area exists in the vicinity of the inclusion. Some convex particles in the granular area are the spherical carbide in microstructure. The granular area forms under the threshold stress intensity factor of internal crack propagation, and plays an important role in ultra-long life fa-tigue process.

Key words: Fractography, S-N curve, Surface crack Subsurface crack, Tool steel, Ultralong life fatigue