Failure Analysis of the Various Sides of a Second-Stage Gas Turbine Nozzle Made of FSX-414 Cobalt-Based Alloy

Second-stage gas turbine nozzles are an important part of the gas turbine due to their performance in hot temperature conditions. In this investigation, service-induced failure analysis and microstructural variations of different sides of a second-stage nozzle in a 100 MW gas turbine made of FSX-414 cobalt-based alloy were comprehensively studied to determine the conditions and mechanisms of the failure process. The primary carbides M3C2 and M7C3 types were converted to secondary carbides M23C6 type by phase transformation process and formed a continuous layer in the grain boundaries owing to high-temperature operation conditions. A network of cracks was created in the grain boundaries, leading to a fracture in the vane of the nozzle. The continuity and density of the produced carbides decreased ductility and toughness of the nozzle. The results demonstrated that hot oxidation and sulfidation occurred in the leading-edge zone. The aggregation and asymmetry of the combustion line and, consequently, the increasing temperature were the reasons for the damage. Some macrocracks were observed by visual inspection on the surface of the nozzle owing to exposure to high temperatures and placement in the path of the hot gas with high pressure. Based on the results, the continuous and coarse brittle M23C6 carbides on the grain boundaries were the major cause of the failure due to the crack growth caused by stress corrosion. These carbides provided appropriate paths for the growth of the crack through grain boundaries, leading to failure after the operational out-of-service condition of a second-stage gas turbine nozzle. Operational in over-hot conditions and an inappropriate filtration system were the main reasons for the operational-induced thermal fatigue failure process of the second-stage gas turbine nozzles. Based on the results, oxidation and microstructural variations were the main causes of the failure process of the second-stage gas turbine nozzles.