Relationship between microstructure and fatigue limit of nanobainitic steels

Abstract:
Despite very high tensile strength up to 2 GPa and high ductility - two properties which would
assume very promising fatigue behaviour - the endurance limit of nanostructured bainitic steels
is often found far beyond expectation. The reason for this often poor fatigue behaviour could not
completely be found until now. In the present thesis interdependencies of the nanobainitic
microstructure and fatigue crack growth behaviour were investigated to find an answer to this
open question. In the beginning it dealed with a whole bunch of different material alloys and
heat treatments in pre tests, which were found to be helpful to get a first glue of expected fatigue
and crack growth behaviour. Results of fatigue tests, crack growth experiments, microstructural
analysis and fracture mechanics evaluation showed a clear relation of crystallographic bainitic
blocks to a growing fatigue crack which was - in the end - summarized in a Kitagawa diagram for
nanostructured bainitic steels. Crystallographic bainitic blocks were found to correlate well with
the critical crack length at which long fatigue crack growth starts as well as with the plastic zone
size in front of a growing fatigue crack. The Kitagawa diagram itself was shown to be a promising
tool for the prediction of fatigue behaviour out of microstructural analysis and crack growth
experiments.