Effects of uncontrolled combustion in a power plant gas turbine on fracture surface features and microstructural evolutions of a nickel-based superalloy blades

Abstract

The present work investigates the fracture surface features and microstructural evolutions of nickel-based superalloy (Udimet 500) blades after uncontrolled combustion and overheating in a power plant gas turbine. In normal operation of the turbine, flameout occurred, but fuel continued to flow to hot section of turbine. The injected fuel failed to ignite as intended, but combusted upon contact with hot surfaces, including blades and the exhaust duct. This led to an explosion and the turbine failed after 753 equivalent operating hours (EOH) in normal operation. The majority of the damage was in the second-stage blades, primarily due to their proximity to the exhaust duct. The explosion in the exhaust duct led to the dynamic fracture of the second-stage blades and an increase in temperature, which caused the microstructural degradation of the airfoil part of the blades. The microstructure of the blades was investigated by an optical microscope (OM), a field-emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS). The results revealed that the most of microstructure deterioration of the blades occurred in the airfoil of the blades and the microstructure of the root of the blades did not deteriorated since the airfoil experienced a higher temperature in comparison with the root. The degeneration of MC carbide to M23C6 carbide was observed at the airfoil grain boundaries, where M represents Cr and Mo. Furthermore, the morphology of γ' precipitates in the airfoil changed to cauliflower-morphology by growing and splitting mechanisms. The spalling nature of the fracture surface suggested that the dynamic fracture, which is produced by interaction between shock waves, as well as occurred. The failure of gas turbine blades was a consequence of uncontrolled combustion in the turbine section, leading to dynamic fracture and deterioration of the microstructure in the airfoil parts of the blades.