Dissimilar Metal Weld and Boiler Creep Damage Evaluation for Plant Life Extension

Abstract

The basic causes of dissimilar metal weld (DMW) failures in boilers are now well understood. Guidelines have emerged for achieving significantly improved DMW life, in terms of operational parameters, weld filler metals, and weld designs. Analytical, as well as experimental, nondestructive techniques have been developed that enable assessment of the condition of DMWs in service so that intelligent run-replace decisions can be made. Techniques for assessing the remaining life of superheater/reheater (SH/RH) tubes are under development. The techniques utilize information on steamside oxide scale growth and dimensional changes on the fireside due to corrosion to compile the temperature–stress profile of operation in the tubes. This information is then combined with standard creep data to estimate the remaining life of the tubes more accurately than was possible heretofore. The potential of a replication technique that allows nondestructive monitoring of creep damage in heavy section pipes has been successfully demonstrated at several utility sites. It has been possible to detect early stages of incipient creep damage and to recommend specific courses of action to affected utilities. Laboratory and field studies currently in progress are expected to result in metallographic and miniature test specimen techniques that would further augment the capabilities of the replication technique, and render it into a more quantitative tool for life assessment of heavy section components.