We performed metallurgical, hardness and chemical testing and analysis to determine the cause of failure on two third stage blades on a gas turbine which failed after 11,00 hours in service.
The first one exhibited only grinding damage and chipped corners, while the second had fractured. One corner of the fractured surface exhibited a dark black oxide layer, while the rest of the surface exhibited a light grey coloration. The texture of the fracture appeared to be different between the oxidized and non-oxidized sections.
Therefore the fracture appeared to have occurred in two stages, with the dark oxidixe portion being the first. This is an area of stress concentration for the loaded blade. The failure occurred at the trailing edge of the blade.
The fracture surface of the broken turbine blade was examined using a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectrometer (EDS). No evidence of creep or fatigue was observed at any location.
A quantitative chemical analysis of the broken blade was taken. The chemistry of the broken blade was found to be similar to a wrought nickel-chromium superalloy Rene 80.
The broken blade appeared to have failed as the result of impact damage or overload that occurred in two distinct stages. The evidence indicates a brittle crack initiated in the base of the blade that did not propagate all the way through the thickness of the blade. This primary fracture existed for some time and developed a thin black oxide layer. At some time later, the crack developed all the way through, possibly as a result of overload. Stress concentration at the leading edge likely contributed to the crack initation.
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