Fatigue crack propagation behavior of a micro-bainitic TRIP steel

Controlling the grain size of steels is an effective way for tailoring their mechanical properties, such as yield strength, impact toughness, and ductility. In this study, a new industrial thermomechanical treatment was applied to a low-alloyed TRIP-assisted bainitic steel 13MnSiCr7 to achieve a substantial microstructural refinement. In this way the average grain size of the new micro-bainitic steel was decreased from ∼25 μm to ∼5 μm. Fatigue tests were carried out in order to investigate the influence of this new thermomechanical treatment on crack propagation behavior. Besides electron backscatter diffraction (EBSD), vibrating sample magnetometry (VSM), and high-energy synchrotron X-ray diffraction (HEXRD) were used to study the microstructure in the vicinity of the fatigue crack tip. The applicability of each method for detecting the martensitic transformation is discussed. In addition, the contribution of the martensitic transformation to fracture toughness was assessed on the basis of the results obtained by HEXRD.