Very high cycle fatigue behaviour of nanostructured bainitic steels

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The research project deals with the failure of nanobainitic steels under very high cycle fatigue (VHCF). Nanobainitic steels are a new class of steels with the potential for high fatigue strength even at very long lifetimes.In general, it can be observed for high-strength steels at very long fatigue loads that the steels fail even at loads below the classical fatigue strength, which was determined for an ultimate number of cycles of 10 million. This late failure is very often accompanied by a changed morphology of the fatigue fracture surface: in the crack initiation area, which in the case of high-strength steels is very often located at non-metallic inclusions, rougher surfaces occur than in the case of shorter lifetimes. This rougher surface is accompanied by a locally refined grain structure, the so-called fine granular area (FGA), which only has grain sizes below 100 nm and is therefore much finer than the original microstructure. This grain refinement is accompanied by a reduction in the threshold value of the stress intensity factor for crack propagation, which, according to the applicant's hypothesis, causes the reduction in fatigue strength.Nanobainitic steels with their hierarchical microstructure show alternating ferritic and austenitic lamellae as smallest microstructural elements, which have a lamella thickness in the range of a few 10 nm and cause a very high static strength of this steel state. So far it has not been investigated whether this fine microstructure is stable under VHCF loadings and thus does not allow FGA formation, which could justify the insensitivity of this high-strength steel class to VHCF failure at reduced stress levels. Therefore, two chemically different nanobainitic steels will be investigated in the project with regard to their VHCF behaviour in two heat treatment conditions each with different fine lamella thicknesses. Besides fatigue testing, the focus here is on the characterisation of the fracture surfaces with regard to possible FGA formation. Depending on the results of this work planned for a first funding period, it will then be possible in a possible second funding period to investigate in more detail which microstructural structural constituents are particularly favourable to increased stability against VHCF loading, whereby more resilient and thus more sustainable steel states could be produced for components. In addition to findings on the further development of steel, the applicant also hopes to gain new insights into the conditions under which FGA are formed, which are still the subject of controversial discussion.