STUDY OF MECHANICAL DEGRADATION OF 9Cr-1Mo-V-Nb STEEL UNDER CREEP-FATIGUE INTERACTION CONDITIONS

Creep-fatigue tests of 9Cr-1Mo-V-Nb steel have been conducted under load control with further study of the steel mechanical degradation via microscopic observation, static-mechanical analysis and ultrasonic evaluation. The tempered specimens exhibited a tempered martensitic structure with a high dislocation density in the lath interior and fine precipitates on the previous austenite grain and martensite lath boundaries. However, the major microstructure changes to the 9Cr-1Mo-V-Nb steel caused by creep-fatigue were the coarsening of Cr23C6 precipitates, the recovery of dislocations due to rearrangement and annihilation, an increase in the martensite lath width, and the formation of cavities. The ultrasonic velocity was observed to increase rapidly within the initial fatigue life fraction (stage I), but the attenuation decreased during this stage. During stage II, there was a slight increase in the ultrasonic velocity and the attenuation subsequently decreased. The ultrasonic velocity decreased and the attenuation increased during the final period (stage III) of fatigue life.