Influences of Microstructure on the Crack Growth Behavior of High-Speed Steels

The excellent mechanical properties of tool steels, and high-speed steels in particular, are the result of their microstructure and the complex interplay of the phases therein. For high-speed steels, the relevant microstructural features include the martensitic matrix, primary carbides precipitated directly from the melt, and secondary hardening carbides formed during the tempering process. While the relationship of these features to static mechanical properties is relatively well understood, the connection between, for example, primary carbides and fatigue failure under cyclic loading conditions typical of actual applications remains unclear. This thesis summarizes research aimed at furthering the understanding of how an HSS microstructural composition affects its fatigue behavior. All investigations were performed on one HSS grade subjected to various heat treatments to establish systematic variations in microstructure at the same composite hardness, thereby eliminating the effects of hardness and chemical composition on fatigue performance. The HSS¿s fatigue performance was assessed for the propagation of both short and long cracks, whereby ¿short cracks¿ refers to cracks for which crack tip shielding mechanisms that reduce the load seen at a crack tip are not yet fully active, as opposed to long cracks. Moreover, the applicability of the current state-of-the-art short crack fatigue testing was verified using a newly developed testing method that measures the propagation of application-relevant, microstructurally small, and shallow cracks. The performed research demonstrated the positive impact of retained austenite, if stabilized using a quenching-tempering-partitioning procedure, on the short crack fatigue behavior of HSS. Moreover, the thesis also identified the importance of an optimized primary carbide spacing combined with a pronounced degree of secondary hardening carbide precipitation.