High-cycle fatigue behavior of T91 steel in air and high oxygen concentration lead-bismuth eutectic at 450 °C

T91 steel is a promising structural material for lead-cooled fast reactors. However, its high-cycle fatigue (HCF) failure mechanisms in liquid lead–bismuth eutectic (LBE) remain poorly understood. This study investigates the HCF behavior of T91 at 450 °C in both air and high oxygen concentration LBE. The results reveal a strong dependence on the service environment. Notably, when the stress amplitude is below 320 MPa, the fatigue life in LBE is longer than that in air. In the high-stress regime, the surface oxide film undergoes rupture, delamination, and spallation, which promote crack nucleation and accelerate fatigue failure. In contrast, in the low-stress regime, the dense oxide film remains intact and exerts both barrier and constraint effects, thereby retarding crack initiation. As a result, T91 exhibits a fatigue life advantage in this regime. These findings highlight the protective role of the oxide scale under low-stress conditions and provide important data for evaluating the long-term service performance of T91 in LBE environments.