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

Investigation of Stress Assisted Grain Boundary Oxidation Cracking in MAR-M002 High Pressure Turbine Blades

[+] Author and Article Information
Austin Selvig1

Department of Mechanical Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canadaaselvig@connect.carleton.ca

Xiao Huang

Department of Mechanical Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canada

Mike Hildebrand, David Stek

Dawn Operations Centre, Union Gas Limited, P.O. Box 1180, Dresden, ON, N0P 1M0, Canada

1

Corresponding author.

J. Eng. Gas Turbines Power 133(8), 082101 (Apr 08, 2011) (8 pages) doi:10.1115/1.4002821 History: Received July 05, 2010; Revised July 07, 2010; Published April 08, 2011; Online April 08, 2011

Modern superalloys have enabled high pressure turbine (HPT) blades in gas turbine engines (GTE) to operate at higher temperatures. Unfortunately, the complexity of these materials can make it difficult to understand the failure mechanisms of these blades. HPT blades made of the nickel-based superalloy Mar-M002 have been found to suffer from stress assisted grain boundary oxidation (SAGBO) cracking. HPT blades removed from an RB211-24C aeroderivative industrial GTE were sectioned, and the cracks and microstructure were studied using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). No cracks were found on the external surface of the blade, which had been coated with an oxidation resistant material. Surface irregularities were found along the walls of the inner cooling channels throughout the entire blade. Larger SAGBO cracks were observed to be near the lower 25% span of the blade and had initiated from the surfaces of the cooling channels. SEM/EDS analyses showed that these cracks had large amounts of alumina and hafnium-rich particles within them. It is evident that these cracks occurred in locations where the combination of high stress and high temperature led to higher rates of oxygen diffusion and subsequent oxidation of grain boundary carbides. Hafnium carbide precipitates along the grain boundaries expanded as they converted into hafnium oxide, thus further increasing the stress. It is envisaged that this increase in stress along the grain boundary has caused the cracks to initiate and coalesce. Based on this observation, it is believed that the inner cooling channels of these HPT blades could benefit from the application of an oxidation resistant coating in order to prevent or delay the formation of these cracks.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Location of cuts and sample labeling, e.g., A29 is the sample from section a, radially cut through cooling channels 2 and 9; (b) location of cooling channels

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Figure 2

Grain boundary with small precipitates (sample taken from blade root region)

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Figure 3

The gamma prime morphology from samples (a) R1 (ROOT), (b) C29 (50–75% span), (c) D29 (75–100% span), and (d) S10 (shroud)

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Figure 4

Gamma prime morphology from the 25% transverse specimen (each figure represents the microstructure taken from the location indicated in the middle insert)

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Figure 5

SEM micrographs of various cracks: (a) large crack in sample A29, (b) medium sized crack found in A38, (c) small crack found in A38, and (d) small crack found in film cooling hole of B10

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Figure 6

Surface irregularities ((a) and (b)) near precipitates on uncoated side of A38 and (c) on root versus ((d) and (e)) the coated side where no cracks are forming and (f) an example of precipitate formation on the uncoated side without surface irregularity

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Figure 7

EDS mapping of crack observed in sample A29; (a) BSE image, (b) Al, and (c) O

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Figure 8

Locations of EDS spot analyses of large crack in A29: (a) near surface, (b) past tip, and (c) small crack in A38

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Figure 9

Cracking mechanism flow chart

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