The subject of this research project is the failure of nanobainitic steels under very high cycle fatigue (VHCF) loading. Nanobainitic steels represent a new class of steels with the potential for high fatigue strength even at very long service lives. In general, high-strength steels sometimes fail at very long fatigue stresses below the classic fatigue strength. This late failure at low stresses is often associated with a changed morphology on the fatigue fracture surface: In the crack initiation area, which is often located at non-metallic inclusions in high-strength steels, rougher surfaces occur than at shorter lifetimes. This rougher surface correlates with a locally refined grain structure, known as the fine granular area (FGA), which consists exclusively of grain sizes below 100 nm and is therefore much finer than the original microstructure. This grain refinement lowers the threshold value of the stress intensity factor for crack propagation, which according to our hypothesis results in the reduction of the fatigue strength. The hierarchical microstructure of nanobainitic steels consists of alternating ferritic and austenitic lamellae with thicknesses of only a few tens of nanometers, which provides these steels with very high static strength. Whether this fine microstructure remains stable under VHCF loading and thus prevents the formation of FGA has not yet been investigated. Such stability could explain an insensitivity of this high-strength steel class to VHCF failure at reduced stress amplitudes. As part of the project, nanobainitic steels in three different heat treatment conditions will be examined regarding their VHCF behavior. In addition to fatigue testing, the focus lies on characterizing the fracture surfaces with regard to potential FGA formation. 

Depending on the results of this work, the microstructural components that contribute to increased stability against VHCF stress can be investigated in more detail at a later stage. This could result in more resilient and sustainable steel states for components. Furthermore, in addition to findings relevant for steel development, new insights into the formation conditions of FGAs are expected - an issue that remains the subject of ongoing debate.