NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

In the last years ENEL (the Italian National Electric Power Authority) started a program forthe installation of large heavy-duty gas turbines for electricity production. The high inlet gastemperatures needed to increase the efficiency of gas turbines cause severe corrosion / oxidationphenomena on their hot components, which are therefore protected with high-temperature coatings;the turbine blades refurbishment is largely dependent on the coating integrity and evolution withservice. CISE and ENEL started a research program finalized to the study of a new electro-magnetictechnique suitable for NDT on plasma-sprayed turbine blades (1). It relies on a dedicated hardwareproviding frequency-scanning eddy-current data, and on a model-based data interpretation; themodel describes a plane electromagnetic wave interacting with a multi-layered conductor, each layerof which is characterized by a conductivity / permeability value and by a thickness. The inverse modelapplied to the experimental data gives the values of the layer parameters.

Fig.: Non-destr. parametervsbeta-phase depletion

The model-based dataanalysisallows calibration-free NDT; this is a key feature, as such a procedure is in principle easily applicablewhenever a material can be modeled as a multilayered conductor without the need of calibrationsamples. The technique has been applied to several NDT problems, relative to quality assurance andservice damage evaluation; applications presented in this paper refer to: a) quality control ofnon-serviced MCrAIY coatings on Ni-base superalloys; b) service damage evaluation of MCrAIYcoatings; c) TiCuIn anti-fretting coating thickness measurement on TiAIV blades. The coatingthickness values are compared to optical microscopy measurements; for problem b) the aluminum-phase content of the coating is related to the ratio between its electrical conductivity and magneticpermeability (see figure). Results show the reliability of the technique as a wide-range NDT tool formetallic coatings.

1. G. Antonelli, M. Ruzzier, F. Necci, ASME Technical Paper 97-GT-001, presented at TurboExno '97. Orlando. FL. 2-6 June 1997: to be Published in the ASME Transactions.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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