Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

Development of a Tool for Temperature Estimation From Microstructural Condition of a Nicoraly+Re Coating Applied on the Surface of Gas Turbine Hot Components

[+] Author and Article Information
Claudia Calcagno

Ansaldo Energia,
Corso Perrone 118,
Genoa 16161, Italy

Contributed by the Manufacturing Materials and Metallurgy Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 27, 2013; final manuscript received July 25, 2013; published online October 21, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(1), 012101 (Oct 21, 2013) (7 pages) Paper No: GTP-13-1191; doi: 10.1115/1.4025264 History: Received June 27, 2013; Revised July 25, 2013; Accepted July 28, 2013

The hot gas path components of gas turbines have to withstand to severe conditions in terms of high temperature oxidation, hot corrosion, and creep-fatigue phenomena. The evaluation of components residual life is an important matter for gas turbines producers and the estimation of service temperatures is a key tool for this evaluation. The most diffused methods to estimate service temperatures of gas turbines blades and vanes in Ni based superalloys are related to the microstructural evolution of the dispersed intermetallic phase γ′, Ni3Al. The aim of this work has been the determination of a tool to estimate service temperature on the basis of the microstructural evolutions of a NiCoCrAlY+Re coating. In order to obtain a deep characterization of the coating after exposure at different durations and temperatures, an extensive experimental test program has been planned. Samples of Ni based superalloys, covered by the investigated coating, have been aged in chamber furnaces in the temperature range 700 °C–1000 °C with durations up to 20,000 h. The microstructure of this coating is characterized by β phase, NiAl, which is the Al reservoir, embedded in the matrix, that is constituted by γ′ phase at low temperature and by γ phase over 900 °C. Moreover, electron back scattered diffraction and X-ray diffraction measurements on samples have revealed three classes of secondary phases: the first one has been identified as σ-Cr2Re3, the second one as Cr carbide-Cr23C6 and the third one as α-Cr. σ phase is very abundant at the lower temperatures while it disappears after long exposures at temperatures higher than 900 °C. The σ phase composition is different at different temperatures and the Re content in particular increases with the temperature. Starting from the σ phase composition determined at different temperatures, a tool has been constructed that relates the service temperature to the Re content in the same phase. The new tool has been applied to the analyses of different components. The results of the new method have been compared to those ones obtained with the method based on γ′ features, developed in the past through huge experimental campaigns. The agreement between the two methods is generally good, they can be used in a complementary way due to the fact that the γ′ one seems to be more suitable for high temperature ranges (T > 900 °C) where it gives a reliable estimation, while the σ method is more suitable in the temperature range 700 °C–900 °C.

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Fig. 1

Microstructure of as delivered SiCoat2453 applied by (a) VPS and (b) HVOF

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Fig. 2

Phase identification in the VPS SiCoat2453 on PWA1483SX

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Fig. 3

α-Cr phase inside some β grains with fine and globular morphology in HVOF coating on René 80

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Fig. 4

EBSD of the whole HVOF coating, exposed to 900  °C for 10,000 h: (a) BSE signal; (b) image quality + inverse pole figure; (c) image quality + phase map

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Fig. 5

Microstructure of a HVOF coating exposed at low temperature which shows the presence of high and low Re Cr carbide

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Fig. 6

EBSD Map at high magnification of the VPS coating on Renè80 exposed to 900 °C for 1000 h: (a) BSE signal; (b) image quality + inverse pole figure; (c) image quality + phase map

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Fig. 7

Phase stability as a function of temperature by thermodynamic calculation of the coating composition

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Fig. 8

Normalized Re content (Re content at temperature T / maximum Re content at maximum stable temperature) as a function of temperature in σ phase in tested coupons

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Fig. 9

First stage blade from an Ansaldo GT AE94.3A2 after service

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Fig. 10

Microstructural condition of the metallic coating with its phases and the base material γ′ phase at the leading and trailing edge in the 50% height section of the first stage blade of Ansaldo GT AE94.3A2 after service




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