Microstructural effects on the sub-critical fatigue crack growth in nano-bainite

Bainitic transformation at low temperatures in high carbon steels leads to a dual phase nanostructure of bainitic ferrite and retained austenite. Strengths above 1 GPa can be easily achieved in these steels while also preserving significant amount of ductility. However, in order to be used in most structural applications, microstructure tailoring for high damage tolerance under fatigue loading is of utmost importance. In the current study, we have developed three distinct bainitic microstructures by austempering at 250, 300 and 350 degrees C respectively after similar austenitization treatment. Sub-critical fatigue crack growth investigation for all the specimens has been carried out under ambient environmental conditions. It has been found that the specimen with the coarsest morphology of bainitic ferrite and retained austenite and the highest retained austenite content shows the largest crack growth threshold. Threshold has been found to be reached for the specimens when the crack tip opening displacement attains the respective bainitic lath thickness for different isothermal conditions. In Stage II, the specimens with the largest lath thickness and highest austenite content showed marginally slower crack growth rates than the other specimens. This can be attributed to transformation induced plasticity that is expected to be more severe in the specimens with the highest austenite content. The specimens transformed at the highest isothermal temperature also showed the highest plane strain fracture toughness deduced from the stress intensity value at which the crack propagates catastrophically.