Influence of hard phase size and spacing on the fatigue crack propagation in tool steels-Numerical simulation and experimental validation

In this study, the fatigue crack growth rate in four different tool steel microstructures (hot rolled, powdermetallurgically processed, as-cast, and carbide-free) is experimentally measured and correlated with hard phase size and spacing, as well as with the roughness of the fracture surface that is created by crack kinking. Numerical simulations of crack growth in carbide-containing microstructures are conducted and investigated. The results indicate a favorable influence of carbides with a larger size and higher degree of roundness, as they create the largest mean free path between the individual carbides at the same hard phase volume content. This facilitates the formation of a plastic zone in the matrix, which dissipates crack energy and reduces the effective stress intensity. In addition, the effect of crack kinking is increased at larger carbide sizes. Concerning practical application, the results suggest that a high degree of deformation is favorable regarding the fatigue growth resistance of tool steels, and that the use of powder metallurgically (PM) grades with small carbides is discouraged, if the lifetime of a tool is mainly controlled by the crack growth rate and not crack initiation.