Failure and metallurgical defects analysis of IN-738LC gas turbine blades

Abstract

Catastrophic and early failure of third stage blades of a gas turbine was investigated. Some areas on the fracture surface of 4 blades were covered with thick black oxide layer. Quantitative microscopic measurements of primary γ′ and MC carbide phases showed that the blades were not exposed to excessive temperature. Scanning electron microscopy and energy dispersive spectroscopy analysis of oxide layer on fracture surface and stress ruptured specimens within 760–980 °C up to 100 h, and heat-treated specimens were used to calculate the blades temperature. The results revealed that oxide bifilms discontinuities and internal cracks accompanied with shrinkage cavities, nucleation and dendritic growth of γ′ precipitates, and non-uniform microstructure led to locally weakened structures at the bottom of airfoils. Accordingly, uncontrolled parameters in casting were the main reasons for commencing the failure.