Ultrasonic measurements for in-service assessment of wrought Inconel 625 cracker tubes of heavy water plants

The degradation in mechanical properties of Inconel 625 ammonia cracker tubes occurs during the service for long duration in heavy water plants. The present study brings out the possibility of using Poisson’s ratio (derived from measurement of time of flight of ultrasonic waves) in combination with hardness measurements, as an effective non-destructive tool for assessment of in-service degradation of Inconel 625 cracker tubes and qualification of re-solution annealing heat treatment for their rejuvenation. Further, the study also indicates the feasibility of extending the life of some of the tubes beyond the presently followed 120 000 h, before they are taken up for re-solution annealing, without affecting their serviceability. However, further studies are required to identify quantitative criterion for Poisson’s ratio and hardness values, for deciding on the basis for removal of the tubes for rejuvenation.