Crack Growth Behavior and Evaluation of Material Degradation for Serviced First--Stage Nozzles of Gas Turbine

摘要

Crack growth behavior in serviced first-stage nozzles of an 80 MW class gas turbine was studied. Crack growths in the outer wall and trailing edge were mainly observed in the nozzles inspected in this research. Cracks propagated along the dendrite or grain boundary. Precipitation of chromic carbides was also observed, and it seemed to affect crack growth. The small punch method which is a destructive test using a small l0 mm square and 0.5 mm thick specimen, was suitable for evaluating the degradation of the gas turbine nozzle. Crack growth behavior was also analyzed from inspection data. Crack growth in nozzles operated in DSS mode was more severe than that in WSS mode. These cracks were identified as the thermal fatigue cracks that propagate by temperature variations at start-up and shut-down of the turbines. The crack propagation in the outer wall has a tendency to arrest when the crack length reaches about half width of the wall. This crack arrest will be caused by thermal stress releasing with crack growths. A statistical trend analysis based on 3σ gave a conservative assessment result and was found to be a reasonable assessment method for the gas turbine nozzle cracking.