CREEP DAMAGE OF LOW-ALLOY STEELS AFTER LONG-TERM EXPOSURE AND ITS EVALUATION BY REPLICA METHOD

The intended operation time prolongation of the key components of fossil-fuelled power plants working in the creep conditions over 200,000 or even 250,000 hours (far beyond their original design lifetime) must be accompanied by thorough and extensive monitoring of the current material state and particularly creep damage. The extent of cavitation damage is routinely tested in-situ by using replica method and its quantification is based on a practice stated in VGB-TIN 507, NORDTEST NT TR 170 or NORDTEST NT TR 302 that further develop the original Neubauer's classification. Although the evaluation of cavitation damage based on the measurement of the number of cavities and/or creep micro-cracks is a routine activity, what is still in question is the depth of the damaged area and the corresponding material properties through the wall thickness. Mutual correspondence between the creep damage evaluation performed in accordance with NORDTEST NT TR 302 and VGB TIN 507 was carried out in a pipe bend made of a low-alloy 0.5Cr-0.5Mo-0.3V steel which was creep exposed at 525-540 degrees C for more than 225,000 hours and the relation between creep damage at the outer surface of the pipe bend (as a typical result of replica testing) and mechanical properties of the whole pipe bend were discussed in the paper with reference to the residual life assessment.