6. Failure analysis and recommendations
Examination of the failed 1st stage compressor blade #1 revealed cracks initiated from the anomaly region and propagated towards airfoil midchord under moderate load levels until final tensile overload separation occurred. Boroscopic inspection records did not show any SEA0400 of corrosion or pitting marks on the first stage blades. The service environment is not considered a corrosive environment. Looking at the inlet duct filters, the status was good and induced FOD is discarded from the root cause analysis.
Deep investigation of the anomaly region displayed morphological features of intergranular separation, and a golden coloration indicating that the fracture at loop of Henle region was produced by tensile overload at temperature above 1000 °°F, most likely during the forging process. The microstructure at the anomaly region showed a much higher density of Niobium carbide precipitates. This suggested that the billet used for the blade forging contained a localized carbon-rich area that originated during casting of the billet, which led to the formation of large amounts of brittle carbides along the grain boundaries and subsequently resulted in internal grain-boundary cracking during forging.