Evaluation of creep damage by ultrasonic velocity measuremets

The influence of creep damage, i.e. creep cavitation, on the ultrasonic (US) velocity has been investigated for low alloy ferritc steels. Velocity measurements have been carried out on laboratory creep specimens tested up to 23.000 h. The results are compared to metallographic examinations and density measurements as well as the theoretical predictions. It is found, that US velocity strongly correlates with the volume fraction of creep porosity. The detection limit is approximately 0.1 % for longitudinal waves and 0.2 % for shear waves. Additional measurements using surface waves have been perforemd on creep damaged components removed from service in order to evaluate the practical applicability of the velocity technique. Results indicate the creep damage such as pipe bends may be detectable at the stage of linked micropores.