From these background, the authors are developing a non-destructive evaluation technique to measure the degradation of inner materials using ultrasonic velocity. In this technique, the longitudinal wave time-of-flight and averaged shear waves time-of-flight, for which polarizations are orthogonal each other, are measured individually at the same position, and their velocity ratio is calculated. Material degradation is derived from the ratio by referring to a pre-determined master curve of the ratio vs. material degradation.

The technique was applied to valve bolts (12Cr steel) of a steam turbine in an actual power plant and to creep specimens which were loaded and heated under a high temperature condition from 550 to 600.The results are as follows.

1. For creep damaged specimens, the velocity ratio is decreased flatly from 1.814 (undamaged material) to 1.807 (ruptured material).
2. The velocity ratio was related only to creep fracture ratio under any creep conditions. The relations were the same as the valve bolts used in actual plants.

As a result, a replacement plan for creep damaged components can be achieved using ultrasonic velocity.