In cold work applications, tool steels with high carbide contents are mainly used as cutting and stamping tools. The efficiency of cold forming processes depends on the tool life, which is limited by wear resistance and fatigue strength. Forming tools often fail due to fatigue fracture since fatigue design is usually not performed. Microstructural and statistical understanding of the underlying, fatigue strength influencing factors, which result directly from manufacturing, is still lacking.
To investigate the influence of manufacturing on the fatigue strength, rotating bending tests of AISI D2 (X155CrVMo12-1) and AISI M2 (HS6-5-2/-3) were carried out. Powder-metallurgical (PM) and melt-metallurgical (MM) production via conventional ingot casting followed by forging were investigated. Since in case of ingot casting, the subsequent forging produces carbide bands, which cause anisotropy, longitudinal and transverse direction were tested separately. This study presents for the first time a statistically full evaluated comparison and correlation of manufacturing with generated defect sizes, corresponding failure mechanisms and resulting fatigue strengths.
Both PM microstructures show significantly higher fatigue strengths than MM, the latter showing only 5–10% higher fatigue strength under longitudinal compared with transversal testing. Critical defects in PM are exclusively small, spheroidal, oxide inclusions at which crack initiation occurs through interfacial detachment. In contrast, crack initiation of MM in both longitudinal and transverse directions occur mainly through fracture of large, block-like eutectic carbides or larger carbide accumulations. In addition, crack initiation can be detected in transverse directions at elongated inclusions. Statistical fracture-surface-analysis reveals that the primary factor influencing fatigue strength is the defect size. Secondly, matrix hardness plays a crucial role. Circularity and defect type represent tertiary factors. To improve fatigue strengths, the defect sizes should be focused. For PM in particular, purity alone is of importance. For MM, firstly the size of primary and eutectic carbides is essential.