Very high cycle fatigue mechanism of carbide-free bainite/martensite steel micro-alloyed with Nb

In this study, the carbide-free bainite/martensite (CFB/M) steel was micro-alloyed with Nb (CFB/M-Nb). The very high cycle fatigue (VHCF) behaviors of this CFB/M-Nb steel were investigated by ultrasonic fatigue test. Especially, different microstructures of the CFB/M-Nb steel were deliberately designed to disclose the VHCF failure mechanism. In addition, the effect of hydrogen in the formation of the optical dark area (ODA) was analyzed, which was thought to control the formation of ODA by concentrating around interior inclusions. Results show the hydrogen cannot be considered to dominate the formation of ODA. Mostly important, “soft or coarsely soft structure induced fatigue crack” should be responsible for the primary VHCF failure mechanism of CFB/M-Nb steel.