Criteria for remaining life estimation and methods for enhancing fatigue resistance of heavy-duty gas turbine bucket metal are based on the analysis of changes in the structure and properties of metal after long-term operation. High-cycle fatigue (HCF) resistance is shown to be a decisive characteristic in the residual life estimation of turbine buckets after operation over 100,000 hours. The tests of the buckets from cast and wrought nickel-based alloys after long-term operation demonstrated decreasing of fatigue strength by up to 25%. The metal structure in operation undergoes notable deterioration mainly in phase redistribution. The size and configuration of metal phases are changing also. It caused the changes in metal properties. The decrease of the bucket fatigue strength correlates with the decrease of metal ductility.
The reconditioning heat treatment resulted in restoring mechanical properties of metal. The fatigue resistance also increased nearly to the initial level.
The influence of operational factors on bucket fatigue strength deterioration has been established. The mechanical damages on bucket airfoil may decrease the fatigue resistance. We found the correlation of endurance limit and damages depth.
The procedures for metal properties recovering and buckets service life substantial extension have been developed. It has resulted in the extension of the buckets service life by up to 50% over the assigned life in gas turbines operated by Gazprom.
Fatigue strength of long term postservice austenitic stainless steel and small fatigue crack growth at high temperature.