Failure analysis of a gas turbine nozzle

The failure analysis of the 70 MW gas turbine first stage nozzle made of cobalt-base alloy FSX-414 is presented. The nozzle experience cracks in different vane airfoil sections assisted by base alloy degradation due to an operation at high temperature. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: decreasing of alloy ductility and toughness due to carbides precipitation in grain boundaries; grain growing (coarsening) and vane airfoil stress level. It was found that the vane crack initiation and propagation was drived by thermal fatigue mechanism. The vane crack initiation and propagation were due to fatigue mechanism which was facilitated by grain boundaries brittleness caused by formation of continuous film of carbides on grain boundaries, grains coarsening, and high thermal stress level.