Failure assessment of Nimonic 80A gas turbine blade

Nickel base superalloys are widely used in applications requiring strength at high temperature. During the operation of gas turbines, the blades and other elements of hot gas path suffer service induced degradation which may be natural or accelerated due to different causes. The failure analysis of the 3 MW gas turbine first stage blades made of nickel-base alloy Nimonic 80A is presented. The blade experienced base alloy degradation due to operation at high temperature. Initially the blade was sectioned for metallographic and microhardness test. The microstructure and microhardness was compared in four different blade zones i.e. root, 30%, 60%, and 90% of the total height of the hot region of the blade. On the basis of the observed microstructures and the phases present in the alloy, the main cause of failure was found to be creep damage. Finite element method (FEM) analysis is conducted considering peak loading of centrifugal force and surface (aerodynamic) loads. Based on FEM simulation results, the life of turbine blades is predicted using the Larson–Miller method. Finally to improve the blade life two heat treatment cycles were suggested and applied. The effect of heat treatments on grain size, volume friction of γ′ primary phase, and micro hardness were investigated. A detailed analysis of the coating showed that, the loss of coatings resistant to high temperature is due to oxidation, corrosion, erosion and inters diffusion of coating-substrate, which results in diffusion of alloy elements into the coating.