In-situ study the crack initiation mechanism of P92 steel under high-temperature three-point bending fatigue process

Various prior austenite grain size (PAGS) of the P92 specimens were obtained after annealing at 950 °C for different times. In-situ three-points bending fatigue tests were carried out to study the crack initiation mechanisms from 550 °C to 650 °C. Results revealed that the microstructure with large PAGS exhibits better cracking resistance during the fatigue process, which restricts the sliding of prior austenite grain boundary (PAGB) and tends to generate large and sparse cracks. Reducing PAGS promotes the plastic deformation and leads to the fast accumulation of fatigue damage at the initial fatigue stage. Fatigue crack at 550 °C is initiated at the prefabricated notch root, which progresses along a specific route mainly due to the intracrystalline slipping mechanism. However, as the temperature increases to 650 °C, sliding of PAGB is enhanced and it mobilizes a larger region coordinating the plastic deformation, ultimately leads to the opening of the notches. Three typical crack models are established based on the evolution of microstructures both on specimen surface and interior. Moreover, a modified θ projection method is proposed trying to predict the accumulated fatigue damage, of which the physical concepts behind the θ parameters are discussed.