Toward the Application of Multiaxial Failure Criteria: Mean Stress Sensitivity of Carbide‐Rich, High‐Alloy PM‐HIP Tool Steels Under Axial and Torsional Cyclic Loading

Complex‐shaped cold‐work tools often fail due to fatigue. Therefore, the targeted designing of such tools against fatigue fracture under uni‐ and multiaxial cyclic loading is of great economic relevance for industry to increase productivity in metal cold forming. Besides wear resistance, fatigue strength is the most important lifetime‐limiting factor for carbide‐rich, high‐alloy tool steels. When designing tools subjected to fatigue, material properties such as mean stress sensitivity and the ratio of torsional to axial fatigue strength must be considered, alongside further factors influencing the fatigue properties, including the degree of multiaxiality, non‐proportionality, notch effects, or microstructural features. These properties have not yet been sufficiently studied, which makes it difficult to estimate tool service life under real operating conditions. In this work, long‐life fatigue (LLF, ) properties under axial and torsional loading with systematic mean stress variation are presented for AISI M3, D2, and A11 steels. Furthermore, identified crack‐initiating defect properties are correlated with LLF strengths, highlighting the crucial influence of steel cleanliness and intrinsic defect sizes related to the steelmaking process. Material‐specific mean stress sensitivities and Haigh diagrams are provided, enabling verification of empirical estimations for cyclic material properties and continuous fitting approaches, based on an elaborated statistical fatigue database.