Improving reliability and lifetime of rejuvenated turbine blades

Abstract

A programme is reported of the rejuvenation of diffusion-coated turbine blades. These blades are usually assessed for consumption of creep life by a metallographic procedure on a sample basis after service and reassembled according to the results. This procedure can result in unsatisfactory reliability levels because of scatter. The objectives of this programme, therefore, are to improve the reliability of sets of reassembled turbine blades by increasing the minimum remaining creep strength, i.e. of blades in the initial stages of tertiary creep, and to eliminate heavily damaged blades with creep cracks concealed by the coating, as well as to improve the life of blades with an assessed remaining service life lower than the time between overhauls. Different rejuvenation treatments have been investigated and checked against the criteria essential for the service behaviour of the blades and the integrity of the material, microstructure, strength, and coating. All these factors were demonstrated to be satisfied by a suitable hot isostatic pressing treatment. The most important result is the evidence that blades with a creep–life consumption of more than 50% can be rejuvenated. Thus, the minimum creep life of used blades can be improved by more than 100%, as tested on a hot–gas rig.

MST/98