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

Metallurgical Evaluation and Condition Assessment of FSX 414 Nozzle Segments in Gas Turbines by Metallographic Methods

[+] Author and Article Information
Girish M. Shejale

Quality Control Department, Masaood John Brown, P.O. Box 11931, Dubai, United Arab Emiratesgshejale@mjbi.com

J. Eng. Gas Turbines Power 133(7), 072102 (Mar 22, 2011) (6 pages) doi:10.1115/1.4002682 History: Received May 17, 2010; Revised May 25, 2010; Published March 22, 2011; Online March 22, 2011

Gas turbine components such as nozzle segments, buckets, transition pieces, and combustion liners experience damages such as creep, fatigue, high temperature oxidation, and corrosion. The reliability, availability, and efficiency of high temperature gas turbine parts are based on condition assessment and remaining life analysis. These gas turbine components are normally repaired and refurbished after stipulated operating hours. The decision on the extent of repairs is based on various inspection stages. Among various methodologies of condition assessment, metallography followed by microscopic evaluation has gained wide acceptance since it is cost effective, quick, and reliable. Extensive in-house efforts have been put forth in this field in the development of improved techniques of metallography for accurate determination of material degradation and condition assessment. Experimental studies on frame 6, first stage nozzle segment (FSX 414—cobalt based alloy) were conducted to assess the condition of the nozzle segment by using a laboratory electropolishing technique for metallographic preparation. Sections taken from the nozzle segment were electropolished and examined in light optical microscope (LOM) and scanning electron microscope (SEM). It is concluded that the improved electropolishing technique is effective in assessing creep-fatigue, thermal fatigue, and hot corrosion damage. Based on this, the condition of the nozzle segment is assessed. Typical results of frame 6, first stage nozzle segment are presented and discussed.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Photograph of the nozzle segment showing the locations at trailing edges and outer sidewall for microexamination

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Figure 2

Photograph of the nozzle segment showing the location at inner sidewall for microexamination

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Figure 3

Schematic of the laboratory electropolishing setup

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Figure 4

Photomicrograph of the trailing edge section; oxidation and hot corrosion is observed on the grain boundaries (arrows); unetched

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Figure 5

Photomicrograph of the trailing edge section showing the cooling hole. Cracks are evident at the cooling hole (arrows). Cracks appear to be typical of thermal fatigue; unetched.

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Figure 6

Photomicrograph of the trailing edge section at the same location as shown in Fig. 4; oxidation and hot corrosion is evident along the grain boundaries (arrows); etched

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Figure 7

Photomicrograph of the trailing edge section showing the hot corrosion attack from the cooling hole surface. Crack is evident at the cooling hole surface (arrow). Oxide products are evident within the crack. This crack is typical of thermal fatigue; unetched.

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Figure 8

Photomicrograph of the trailing edge section; cooling hole surface displays oxidation and hot corrosion (arrow); unetched

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Figure 9

Photomicrograph of the trailing edge section at the same location as shown in Fig. 8; note the oxide and hot corrosion penetration in the metal (arrows); etched

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Figure 10

Photomicrograph of the trailing edge section (crack area) away from the surface showing voids (arrows); the voids are possibly casting pores; unetched

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Figure 11

Photomicrograph of the trailing edge section showing the general microstructure; coarse carbides are evident at the surface; etched

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Figure 12

Photomicrograph of the inner sidewall at crack section. The crack has propagated along the interdendritic grain boundaries (arrow). Cracking is typical of thermal fatigue; etched.

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Figure 13

Photomicrograph of the inner sidewall section showing the general microstructure; microstructure at the surface displays coarse carbides (arrows); etched

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Figure 14

Photomicrograph of the outer sidewall section showing the crack at the surface. The section displays the presence of weld metal. Crack is along the fusion zone. Crack morphology is of thermal fatigue type; etched.

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Figure 15

Photomicrograph of the outer sidewall section showing the general microstructure; etched

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Figure 16

SEM micrograph of the trailing edge section (crack area) at the cooling hole surface. Oxidation and corrosion products are evident. Voids are evident within and along the grain boundaries. Arrows show voids along the grain boundaries; etched; original magnification: 400×.

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Figure 17

SEM micrograph of the trailing edge section (good area) at the surface showing the oxidation and hot corrosion attack; etched; original magnification: −3000×

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Figure 18

SEM micrograph of the inner sidewall section at the surface showing oxidation and hot corrosion attack along the grain boundaries (arrows); etched; original magnification: −6000×

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Figure 19

SEM micrograph of the trailing edge section (good area) at the surface showing the oxidation and hot corrosion attack (arrows); etched; original magnification: −3000×

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Figure 20

SEM micrograph of the inner sidewall section at the surface showing oxidation and hot corrosion attack along the grain boundaries; etched; original magnification: −3000×

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Figure 21

EDS representative spectrum performed on the trailing edge section at the crack location

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