Characterization of creep–fatigue in ferritic 9Cr–1Mo–V–Nb steel using ultrasonic velocity

The microstructural evolution of ferritic 9Cr–1Mo–V–Nb steel, subjected to creep–fatigue at 550 °C, was evaluated nondestructively by measuring the ultrasonic velocity. The ultrasonic velocity was strongly depended on the microstructural changes during creep–fatigue. The variation in the ultrasonic velocity with the fatigue life fraction exhibited three regions. In the first region (within 0.2 Nf), a significant increase in the velocity was observed, followed by a slight increase between the fatigue life fractions of 0.2 Nf and 0.8 Nf and a decrease in the final region. The change of the ultrasonic velocity during creep–fatigue was interpreted in relation to the microstructural properties. This study proposes an ultrasonic nondestructive evaluation method of quantifying the level of damage and microstructural change during the creep–fatigue of ferritic 9Cr–1Mo–V–Nb steel.