Predicting Subsurface Failure in Case‐Hardened Steel by a Concept of Local Fatigue Safety—Development and Validation of the Concept

ABSTRACT Due to the increased fatigue strength of steels, subsurface failure has become a serious problem. Case‐hardened steels are particularly prone to sub‐surface failure as a result of their gradient of load‐carrying capacity. Three tailored material conditions of 18CrNiMo7–6 + HH (1.6587), representing the case‐hardening layer, were produced by different heat treatments, and subsequently characterized mechanically by tension‐compression fatigue testing. Great attention was paid to the influence of longitudinal and transversal nonmetallic inclusions with respect to the loading direction. Accompanying microstructural characterization comprised X‐ray diffraction, measuring residual stress and retained austenite, as well as fractographical investigations by SEM equipped with EDX. Based on these comprehensive investigations, a concept of local fatigue safety was developed. The concept is based on two novel aspects: (i) a statistical size effect on the basis of highly utilized volumes and (ii) the orientation of nonmetallic inclusions with respect to the loading direction. Two validation geometries were designed to validate these two aspects independently. The prediction of failure load and failure location was accurate and precise. In the future, the concept can be adapted to specific applications, such as case‐hardened gears.