Abstract
Gigacycle fatigue behaviour of wrought cold work tool steel of AISI D2 type (DIN Nr. 1.2379) was studied employing an ultrasonic fatigue testing system operating under fully-reversed tension-compression mode (R = -1) at 20 kHz up to N_max = 10¹⁰. The work presented here focused on the effect of anisotropy on the material fatigue behaviour. Primary carbides (clusters) located near/at the surface were found to be fatigue crack initiation sites. Fracture surfaces revealed rather flat surface morphology and around the crack origin half of so-called fish-eye was obtained. A granular area (GA) was observed surrounding the carbide particles, followed by a very flat zone. The obtained S-N data showed considerable anisotropy of the fatigue behaviour. Specimens with their axis perpendicular to the rolling direction revealed about 150 MPa lower fatigue endurance strength at 10¹⁰ loading cycles compared to samples with axis parallel to the rolling direction. Detailed investigation of fracture surface was performed in order to evaluate fatigue crack nucleation and propagation.

Key words
tool steels, gigacycle fatigue, material anisotropy, carbide clusters, ultrasonic fatigue testing

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