Fractographic evaluation of gigacycle fatigue crack nucleation and propagation of a high Cr alloyed cold work tool steel

This work describes investigations into fracture surfaces of AISI D2 type cold work tool steels obtained through fatigue testing in the gigacycle regime, an ultrasonic resonance testing method being employed. Macroscopically, flat fracture surfaces were obtained showing fish-eye type appearance around the crack origins. Primary carbides and carbide clusters turned out to be crack origins at/near the surface and in the core of the specimen. Five zones of crack propagation, exhibiting different surface morphologies, were detected, apparently representing the different stages of fatigue crack growth. The sizes of these crack propagation zones were evaluated and then used for calculation of corresponding stress intensity factors.