Abstract
The fatigue behaviour of two ingot metallurgy high speed steels – M2 and M42 / Bõhler S600 and S500, respectively – was investigated up to N_max = 10¹⁰ loading cycles employing an ultrasonic frequency resonance testing system in fully reversed tension-compression mode. Testing was done parallel to the rolling direction. Fatigue failure in the tested cycle number range of 10⁵ to 10¹⁰ cycles occurred at stress amplitudes between 900 and 500 MPa, respectively. Failure was initiated at internal carbide clusters, internal and subsurface non-metallic inclusions, and at primary carbides/carbide clusters located at/near the specimen surface, the latter of which represented the dominant failure mode. The obtained S-N curves revealed a continuous downward slope without any indication for a plateau region, and hardly any difference was observed between the two steels. In case of internal and subsurface failures the fracture surfaces revealed so-called fish-eye patterns formed around the crack origins. For at/near surface failures, half of fish-eye could be observed. In the vicinity of the crack nucleation site a granular area was detected followed by a fracture surface zone showing rather low surface roughness, thus appearing dark in the scanning electron micrographs.

Key words
tool steels, gigacycle fatigue, ultrasonic frequency fatigue testing, non-metallic inclusions, primary carbides clusters, fish-eye patterns

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