Gigacycle fatigue response of tool steels produced by powder metallurgy compared to ingot metallurgy tool steels

Abstract In this work, the gigacycle fatigue response of several tool steel grades, produced by ingot metallurgy and powder metallurgy, has been studied using an ultrasonic frequency resonance testing device. In all cases the S–N curves exhibited a consistent drop up to 10 billion cycles, i. e. there is no fatigue “limit” with these materials. Surprisingly, there was virtually no effect of the composition and hardness of the materials, both for powder metallurgy and ingot metallurgy grades cold work tool steels and high speed steels exhibiting virtually the same S–N curves, however the powder metallurgy steels revealed significantly higher endurance strength levels than ingot metallurgy grades. In the ingot metallurgy tool steel grades, crack initiation started at large primary carbides or carbide clusters, while in the powder metallurgy grades, markedly smaller non-metallic inclusions turned out being the critical defects. Furthermore, it has been shown that it is very important to avoid introducing residual stresses into the specimen surfaces during preparation, since these stresses significantly affect the endurance strength levels and the location of crack initiating sites.