Investigation of the damage mechanisms during very high cycle fatigue (VHCF) of a tempered carbon steel

Abstract

The study is an overview of recent investigations dealing with the fatigue damage in the HCF and VHCF regime of 0.5C-1Cr martensitic steel (German designation: 1.7228) in a tempered condition. The experimental part is focussed on the microstructural characterization regarding crystallographic orientation, reconstruction and grain size distribution of prior austenite grains as well as metallographic investigations. Electrolytically polished cylindrical bulk specimens have been loaded under fully reversed (R=-1) loading condition using an ultrasonic fatigue testing machine (f=20000Hz) and an electro-mechanical resonance fatigue testing machine (f=95Hz). By means of a light microscope attached to the testing systems, fatigue crack initiation and propagation were correlated to the material microstructure and the specimens fatigue life. The application of a fast and high resolution thermography camera system in combination with the resonance testing system allows to use the heat dissipation determined on the surface of the fatigue specimen as a measure of specific fatigue damage. The tested specimens have been carefully investigated by means of scanning electron microscopy (SEM) in combination with automated electron back-scatter diffraction (EBSD) and energy dispersive X-ray (EDX) analysis. It was found that under HCF and VHCF loading conditions crack initiation is caused by slip band formation between the martensitic laths structure, which leads to microcrack initiation and propagation. The propagation of microcracks is sensitive to changing crystallographic orientations when crossing a grain boundary. In case of run-out specimens (109 cycles) microcracks of the length of several microns have been found that were blocked by prior austenite grain boundaries which act in this case as effective barriers against microcrack propagation.