EVALUATION OF CREEP DAMAGE PROGRESS BY METALLURGICAL EXAMINATION IN AGED POWER BOILER PRESSURE PARTS

In order to confirm the reliability of non-destructive plant life assessment method by metallurgical examination, creep damage was evaluated in aged power boiler pressure parts operated more than 100 000 h. Then the evaluated creep damage was compared with destructive tests, namely creep rupture test and creep fatigue test, of samples removed from the same positions of components inspected by the metallurgical method. In the metallurgical method, metallurgical degradation was quantified by variation of type and shape of carbide, variation of distribution of alloying elements and nucleation of creep voids. All quantified values but shape of carbide varied systematically during the further operation for 1 or 2 years at all of inspected components. Comparing the quantified metallurgical degradations, with consumed life fraction assessed by destructive tests, relative fraction of M6C carbide, which was quantified type of carbide, and characteristics of Cr and Mo composition spectra, which was quantified distribution of alloying elements, were found to suit the life assessment during first half of life. The creep void density, which was quantified nucleation of creep voids, was found to suit during the latter half. Consequently residual life of aged power boiler pressure parts can be assessed by the metallurgical method. © 1990, The Iron and Steel Institute of Japan. All rights reserved.