Failure analysis of gas turbine first stage blade made of nickel-based superalloy

- The microstructure of the damaged rotor fist-stage blade root is similar to that of new blade.
- The damaged rotor blade has the acceptable initial microstructure.
- SEM images confirm the abnormal conditions. Increasing the service temperature leads to precipitates dissolved in the matrix and re-deposited during shut down of the turbine. Furthermore, the fractography observation confirms local melting in the leading edge.
- Changes of the first-stage airfoil blade microstructure and local melting of blade indicates that the blades were overheated for unknown period of time.

Various degradation mechanisms are characterized in gas turbine rotor blades due to service conditions such as: high temperature and stress. Failure of turbine blade can have the tremendous effects on the safety and performance of the gas turbine engine. This paper investigates a first stage turbine blade failure in a 6.5 MW gas turbine. The blade is made of nickel-based superalloy, and the failure occurred in the airfoils after 6500 h of operation. Several examinations were carried out in order to identify potential failure reasons such as: visual examination, fractography and microstructural characterization used by optical and scanning electron microscopes (SEM) and energy dispersive X-ray (EDX). The precipitated phases morphology (carbides and γ′ (Ni3Al)) changed in the airfoil for example γ′ resolved and re-deposited in addition to decomposition of carbides. Furthermore, the fracture surface exhibits the local melting occurred and re-solidified in the leading edge. From analysis and experimental results of this study, overheating is shown to be the main reason of blade failure.