Influence of hard phase morphology on the fatigue crack propagation in tool steels

Carbide morphology in tool steels can differ significantly in dependence of the processing route, ranging from an eutectic carbide network in cast materials to dispersed hard phases in powder metallurgically manufactured grades. The carbide morphology influences the resistance against fatigue crack growth, as the formation of the carbides determines the length of tough steel matrix ligaments, which counteract the crack growth by plastic deformation. In this study, the fatigue crack growth in four different tool steel microstructures (hot rolled, powdermetallurgically processed, as-cast, carbide-free) is experimentally measured and compared to numerical simulations of crack growth in carbide-containing microstructures. The results indicate a favorable influence of larger carbides, as they create the largest mean free path between the individual carbides. This facilitates the formation of a plastic zone in the matrix, which dissipates crack energy and reduces the effective stress intensity. In addition, crack deflection is increased with larger carbide sizes