Influences of Manufacturing-Related Microstructural Variations on Fatigue in Carbide-Rich Tool Steels

In cold-work applications, tool steels with high carbide contents are used as cutting and stamping tools. The tool service life is limited by wear resistance and fatigue strength. The relationship between manufacturing-related microstructural influences and fatigue strengths of tool steels has not yet been adequately investigated. To investigate these influences on high-cycle fatigue (HCF) strength (N_G = 10⁷), rotating bending tests are performed on AISI D2 and AISI M2/M3. Raw materials are produced by conventional ingot casting and subsequent hot working (HW) as well as in a powder metallurgy (PM) process with hot isostatic pressing (HIP) and forging. Herein, a statistically validated correlation of process-related defect size and the resulting fatigue strength is presented. Both PM steels show significantly higher HCF strength than the HW steels. Critical defects in PM appear to be exclusively small oxide inclusions. In contrast, fatigue cracks in HW are typically initiated by the fracture of large, blocky eutectic carbides. The main factor influencing HCF strength is defect size. Other critical features of the microstructure include matrix hardness, circularity, and defect type. Improvements in fatigue strength can be obtained by reducing the size of fracture mechanical defects, inclusions for PM, and eutectic carbides for HW microstructures.