Fatigue performance of a wheel steel under rotating bending and ultrasonic axial cycling: Role of elongated sulfides, effects of loading type and stress ratio

• High-cycle and very-high-cycle fatigue of wheel steel: studied at three conditions. • Rotating bending and ultrasonic axial cycling employed for applying R = –1 and 0.3. • Multi-scale characterization reveals microstructural features near crack origins. • Elongated sulfides dominate ultrasonic fatigue in both surface and internal failure. • Failure probabilities quantified in N-S diagram for different load types and ratios. In this paper, a wheel steel, with a relatively low tensile strength and a distinct yield strength, is systematically investigated for high-cycle and very-high-cycle fatigue (failure life N f > 10 7 cycles) under rotating bending (RB, 52.5 Hz) and ultrasonic axial cycling (UAC, ∼ 20 kHz) with stress ratios R = –1 and 0.3. This work provides the first systematic evaluation of a medium-strength wheel steel commonly used in railway applications. All cracks initiated from the specimen surface and failures were limited to the HCF regime in RB tests while UAC tests revealed sulfide-induced failures, occurring both at the surface and in the interior, and extending into the VHCF regime. Microstructural analysis demonstrated nanograin formation and refinement near inclusions under R = –1, and partial cementite dissolution under R = 0.3, indicating stress-ratio-dependent initiation mechanisms. Furthermore, the fatigue limit followed a Gerber-type relation constrained by yield strength, and a probability-based failure map was established to quantify the occurrence of different failure modes. These findings extend the current understanding of VHCF beyond martensitic steels, clarify the role of elongated sulfides in wheel steels, and provide engineering insight into the fatigue reliability of railway wheels.