Abstract
Gigacycle fatigue characteristics of high speed tool steel, JIS SKH 51, was investigated under cantilever-type rotary bending fatigue tests in an open environment at room temperature. As a result of fatigue tests, S-N curve was clearly classified into two types of fracture modes by the different crack origins. One fracture mode occurred at the surface inclusion in the region of short fatigue life and high stress amplitude level. The other was at the subsurface inclusion in the region of long fatigue life and low stress amplitude level. The transition of crack initiation site from surface to subsurface appeared at about 2×104 cycles and the stress amplitude for the transition was affected by the tempering temperature of the specimen. On the other hand, difference on fatigue life for internal fracture mode between specimens treated by different tempering temperature did not observed. Fatigue fracture mechanisms were discussed through the detailed observation of subsurface crack initiation site.
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