Failure Analysis of Second Row-Nozzles Based on FSX-414 Cobalt Alloy in a Gas Turbine Unit

In this study, the failure analysis of the second row-nozzles of one of the gas turbine units was investigated. The vanes alloy was FSX-414 cobalt base alloy. The visual evaluations indicated that the leading-edge region of about 8 vanes was completely damaged, and needed to be replaced. The microstructural evaluation was studied by optical microscopy (OM) and scanning electron microscopy (SEM). The results showed that the primary carbides such as M3C2 and M7C3 were dissolved and the secondary carbides with M23C6 composition were formed continuously and coarsely on the grain boundaries. Also, the crack networks were created in the grain boundaries which were propagated throughout the sample thickness. The X-ray diffraction (XRD) and energy disperse spectroscopy (EDS) analyses indicated that hot oxidation and sulfidation have occurred on the leading-edge surface. It seems that the increasing temperature due to the asymmetry of the combustion line had caused such damage.