The relation of microstructural features to the fatigue crack growth and fracture behavior of a high-speed steel

State-of-the-art tool steels, such as high-speed steels, consist of a tempered martensitic matrix and other hard phases embedded within, forming a de facto metal-matrix composite of precipitated hard carbide phases embedded in a softer martensitic metal matrix. The precipitated carbide phases can be subdivided into primary carbides, which precipitate directly from the melt, and small secondary hardening carbides, which precipitate during tempering. During tooling application, most tool failures occur due to material fatigue, where microstructural effects, especially those related to the primary carbide architecture, are not yet fully understood. Therefore, three microstructural variants were developed from a single batch of the same high-speed steel grade to investigate the effect of their microstructure on fatigue crack propagation and fracture behavior. The variants were established solely by different heat treatments and featured i) a high-volume fraction of narrowly-spaced small primary carbides, ii) a high-volume fraction of widely-spaced large primary carbides, and iii) a low-volume fraction of widely-spaced small primary carbides. The primary carbide architecture most suited for long tool lifetimes was identified as featuring small, widely spaced carbides with a highly alloyed matrix.