The exact knowledge of the actual airfoil surface metal temperature of advanced GT parts during engine operation is essential with regard to a reliable part lifetime analysis. Typically, in order to assess the airfoil temperatures of service exposed parts, destructive metallographical methods are used, which interpret the superalloy and coating degradation into respective metal temperatures. This traditional metallographical method is well established, however, it provides only limited information on the metal temperatures from few specific cutting locations. This paper describes the development of an innovative approach for the determination of the metal surface temperatures of ex-service turbine blades & vanes using the non-destructive Frequency Scanning Eddy Current Technique (FSECT). The method is basically based on the fact, that MCrAlY coatings, in comparison to the base superalloy material, change their electromagnetic properties in a describable manner with temperature and time. The coated GT part needs to be FSECT measured in the status “ex-service” and after defined reference heat treatments. The respective FSECT signals are then used in order to calculate the actual metal surface temperature. With this method and using an automated robotic system, it is possible to establish accurate and high resolution mappings of the surface temperature distribution over the entire airfoil and platform areas. The exact location of hot spots on the airfoil, which might not be evident with the naked eye, can easily be identified. This new method supports the prediction of local material degradation and lifetime, the optimization of maintenance intervals, and — as a consequence — the establishment of the appropriate reconditioning processes for the ex-service GT parts.

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