Failure analysis of a third stage gas turbine blade

This paper investigates a third stage turbine blade failure in the 150 MW unit of a thermal power plant. This primary event caused extensive damage to the unit, i.e. rupture of all blades present in the third and subsequent stages. The blade is made of nickel-based superalloy, Inconel 738, and the blade failure occurred at approximately 22,400 operating hours (25,600 equivalent operating hours) after a major overhaul. Several examinations were carried out in order to identify the failure’s root cause: visual examination, SEM fractography, chemical analysis, micro-hardness measurement, and microstructural characterization. The fracture on the turbine blade is located at the top fir tree root and the fracture surface exhibits two characteristic zones: the first zone shows slow and stable crack growth with crystallographic faceted cracking and striation formation, and the second shows interdendritic fracture, typical of final stage failure. From the examinations carried out, it was possible to identify the cause of the primary failure. The identified fracture mechanism was high cycle fatigue originated by fretting on the fir tree lateral surface (i.e. fretting fatigue).