Research Article Microstructural Changes during High ...downloads.hindawi.com/journals/amse/2016/1745839.pdf · Research Article Microstructural Changes during High Temperature Service

Research ArticleMicrostructural Changes during High Temperature Service ofa Cobalt-Based Superalloy First Stage Nozzle

A Luna Ramiacuterez1 J Porcayo-Calderon2 Z Mazur1

V M Salinas-Bravo1 and L Martinez-Gomez34

1 Instituto de Investigaciones Electricas Reforma 113 62490 Cuernavaca MOR Mexico2CIICAp Universidad Autonoma del Estado de Morelos Avenida Universidad 1001 62209 Cuernavaca MOR Mexico3Instituto de Ciencias Fısicas UniversidadNacional Autonoma deMexico AvenidaUniversidad sn 62210 CuernavacaMORMexico4Corrosion y Proteccion (CyP) Buffon 46 11590 Ciudad de Mexico DF Mexico

Correspondence should be addressed to J Porcayo-Calderon jporcayocgmailcom

Received 30 October 2015 Revised 7 March 2016 Accepted 8 March 2016

Academic Editor Amit Bandyopadhyay

Copyright copy 2016 A Luna Ramırez et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Superalloys are a group of alloys based on nickel iron or cobalt which are used to operate at high temperatures (119879 gt 540∘C)and in situations involving very high stresses like in gas turbines particularly in the manufacture of blades nozzles combustorsand discs Besides keeping its high resistance to temperatures which may approach 85 of their melting temperature thesematerials have excellent corrosion resistance and oxidation However after long service these components undergo mechanicaland microstructural degradation the latter is considered a major cause for replacement of the main components of gas turbinesAfter certain operating time these components are very expensive to replace so themicrostructural analysis is an important tool todetermine themode of microstructure degradation residual lifetime estimation and operating temperature andmost important todetermine the method of rehabilitation for extending its life Microstructural analysis can avoid catastrophic failures and optimizethe operating mode of the turbine A case study is presented in this paper

1 Introduction

Gas turbines blades are manufactured mainly with nickel-based and cobalt-based superalloys During the commercialoperation of gas turbines which are part of a power stationblades and other components of turbine are subject tonatural wear and damage due to various causes which caninterrupt continuous operation The source of damage maybe metallurgical or mechanical and is manifested in theequipment operation such as a decrease in the availabilityreliability and performance and an increase in the risk offailure Also after a prolonged service moving blades andnozzles show a decrease in metallurgical characteristics sothe creep strength fatigue impact and corrosion resistancedecrease There are different factors which influence lifetimeof themain components of a gas turbine including design andoperating conditions but it is the latter that has an impacton the lifetime of these components Generally for most gas

turbines operating conditions are very severe The followingfactors have great effect operation environment (high tem-peratures fuel and air contamination solid particles etc)high mechanical stresses (due to centrifugal forces vibratoryand flexural stresses etc) and high thermal stresses (due tothermal gradients)

The phenomena described above do not operate inisolation typically there are two or more factors being activesimultaneously causing reduction of blades or nozzle lifetimeunder the following damage mechanism [1 2] that is creepthermal fatigue (low cycle fatigue) thermomechanical fatigue(high cycle fatigue) corrosion and oxidation erosion orforeign object damage (FOD)

The type of damage or degradation which occurs ingas turbine blades and nozzles after prolonged servicemainly includes external surfaces damage (corrosion oxi-dation cracks foreign object damage erosion and fretting)and internal damage of microstructure such as 1205741015840 phase

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2016 Article ID 1745839 7 pageshttpdxdoiorg10115520161745839

2 Advances in Materials Science and Engineering

coarsening grain growth micro void growth in grain bound-aries carbide precipitation and brittle phase formation

Surface damage produces dimensional deteriorationgenerating loss of the bladenozzle original dimension result-ing in increased stress and turbine efficiency reductionDuring operation the material microstructure is affected byhigh temperature combined with high stresses However theextent of deterioration differs due to the following factorstotal service time and operation history (number of startupsshutdowns and trips) gas turbine operation condition (tem-perature rotational speed mode of operation ie base loador cyclic duty) and manufacturing alterations (grain sizeporosity alloy composition heat treatment)

Then a brief description of the Ni-base and Co-base basesuperalloys is givenThese alloys are used in the manufactureof critical turbine components (moving blades nozzles com-bustors and transition ducts) of stationary gas turbines Febased or Ni-Fe based superalloys are not mentioned becausetheir use in gas turbine critical components is not common

Ni-Based Alloys The nickel-base alloys are the more complexand the most widely used for the hottest components ofgas turbines (eg gas turbine first stage blades) In the heattreated condition superalloys represent a composite materialconsisting of several intermetallic phases linked by a metalmatrix The major phases present in these superalloys are[3] as follows gamma matrix (120574) Ni-based austenitic phase(FCC) usually containing a high percentage of solid solutionelements such as Co Cr Mo and W gamma prime (1205741015840)which is Ni

3(Al Ti) based intermetallic phase Carbides

generally types M6C and M

23C6which tend to precipitate

into grain boundaries topologically closed packed (TCP)type phases such as 120590 120583 and Laves which precipitate afterprolonged high temperature service

These alloys can be classified into solid-solution hardenedalloys and precipitation hardened alloys or gamma prime (1205741015840)alloysThe formermay be forged or cast contain few elementsforming 1205741015840 particles but are hardened by refractory elementssuch as tungsten and molybdenum and carbide formationand also contain Cr to impart corrosion resistance (oxida-tion) and Co to give microstructural stability Precipitationhardened alloys can also be forged or cast In addition to 1205741015840particle formation as the main hardening mechanism alsoincorporates elements such as tungsten (W) molybdenum(Mo) tantalum (Ta) and niobium (Nb)

Co-Based Alloys Cobalt-based superalloys (eg X 40 X 45and FSX-414) are primarily used in themanufacture of all firststage nozzles and in some turbines are used in the last stagedue to their good weldability and hot corrosion resistanceThese alloys have higher strength at high temperatures thanNi-based alloys and also have excellent resistance to thermalfatigue oxidation and corrosion [4]These alloys have cobaltas the principal alloying element with significant amounts ofnickel and chromium and smaller amounts of tungsten andmolybdenum niobium tantalum and sometimes iron Theyare mainly hardened by carbide precipitation Alloys harden-ing by carbide precipitation contain between 04 and 085carbon Such superalloys consist of an austenitic matrix (fcc)

and a variety of precipitated phases such as primary carbides(M3C2 M7C3 and MC) and coarse carbides (M

23C6) and

GCP types phases (geometrically compact phases) such as12057410158401015840 and 120578 (Ni

3Al) and TCP (topologically close packed) type

phases (120590 120583 119877 or 119871) (Cr Mo)119909(Ni Co)

119910[5]

2 Superalloys Microstructural Degradationduring High Temperature Service

There are several microstructural degradation mechanismsoccurring in superalloys used in the manufacture of hotsection components of gas turbines (nozzles moving bladesand combustion chambers) The most common degradationmechanisms are aging and coarsening of the 1205741015840-phase trans-granular precipitation growth of carbides in grain bound-aries brittle phases precipitation and growth of cavities dueto creep

Coarsening and Aging of 1205741015840 Phase The size and shape ofthe 1205741015840 phase in nickel-based superalloys are not stable afterlong periods of operation at high temperatures Howeverafter a heat treatment this phase is very near to equilibriumwith the 120574 matrix and therefore there is little additionalprecipitation or growth of this phase from the supersaturatedmatrix Nevertheless some particles may grow by a diffusionmechanism [6] that is the average particle radius increaseswith aging time 119905 This is represented by the followingequation

1199033

(119905) minus 1199033

(0) = 119870119905 (1)

where 119903(0) is the average radius of the particle at 119905 = 0 119903(119905) isthe average radius of the particle in time 119905 and119870 is the kineticconstant which depends on temperature Various studies [78] on 1205741015840 growth phase in Ni-base superalloys and Fe-Ni-Alalloys have confirmed that growth obeys the law described in(1)

Changes in morphology of the 1205741015840 phase modify themechanical properties of the materialrsquos component sincephase 1205741015840 is intended to act as a barrier to dislocationmovement slowing creep consequently resistance to thisfailure mechanism is greatly diminished [9] In commercialsuperalloys the 1205741015840 phase changes from spherical to cuboidalshape although most of the particles have an intermediateform Aging is revealed as an increase in average particlesize The 1205741015840 phase can be identified in the microstructure asparticleswhose shape is irregular and larger [10 11]The shapeof this phase in a nondegraded and degraded condition canbe observed in Figures 1(a) and 1(b)

Morphology and Degeneration of MC M23C6 and M6C Car-bidesThe role of carbides in superalloys is complex carbidesseem to prefer the grain boundaries as a site location in Ni-base superalloys while in Co-base and Fe-base superalloysappear to precipitate intragranularly [3] The most commoncarbides in all Ni Fe-Ni and Co-base superalloys are basicMC M

23C6 and M

6C and seldom M

7C3[12] The most

stable carbide found in Ni-base and Co-base superalloys isthe MC type where M represents the element Ti A fraction

Advances in Materials Science and Engineering 3

120574998400

(a)

120574998400

(b)

Figure 1 Blade root micrograph of (a) gamma prime (1205741015840) without degradation and (b) gamma prime (1205741015840) with moderate degradation after24000 h in service IN738LC superalloy

M23C6

Figure 2 M23

C6

carbides precipitated in grain boundariesIN738LC superalloy after 24000 h of service

of Ti can be replaced by Nb Ta W and Cr depending onthe alloy composition In Co-based superalloys containingW WC carbide is dominant [13] This carbide generally hasa pseudo cubic or script shaped figure it precipitates asdiscrete particles distributed heterogeneously through thealloy in intragranular or transgranular locations The sourceof carbon needed in the heat treatment of these alloys istaken from theWC In the course of a prolonged service MCprimary carbides decompose into secondary carbides rich inchromium (M

23C6) MC carbide decomposition occurs by

diffusion of carbon into the 120574 matrix and 1205741015840 phase resultingin the formation of M

23C6carbides near the matrix-interface

[14] as shown in Figure 2 The MC decomposition can bestated by the reaction

MC +120574

1205741015840997888rarr M

23C6+ 1205741015840 (2)

or

(TiMo)C + (NiCrAlTi)

997888rarr Cr21Mo2C6+Ni3(AlTi)

(3)

The above reaction occurs at a temperature of approximately980∘C (1800∘F) but has also been observed at a temperatureof about 760∘C (1400∘F) [15 16] The M

23C6carbide has a

significant effect on the superalloys properties Its critical

location (grain boundaries) increases the rupture strengthinhibiting grain boundary sliding However failure by breakmay be originated by fracture of these particles

Phase Topologically Close Packed (TCP) Superalloys havehigh levels of refractory elements such as Mo W Re Ru andTa in order to increase creep and crack resistance [17 18]These elements function as solid-solution enhancers of boththe 120574 matrix and the 1205741015840 phase Re is a strong hardener itprecipitates mainly in the 120574 matrix and apparently slowsdegeneration of the 1205741015840 phase High amounts of refractoryelements make the superalloy prone to form TCP phases the120590 phase being the most common in Ni-base superalloys [19]It has been shown that the formation of these phases has adetrimental effect on the creep rupture life of superalloysThese phases increase the strain rate of both conventional andsingle crystal superalloys [20 21] Other detrimental effectson superalloys are a decrease in ductility impact resistanceand thermal fatigue

3 Case Study Degradation in Service of a GasTurbine First Stage Nozzle Segment

The nozzle segment (the complete wheel comprises 16 seg-ments with two blades per segment) of the first stage of a gasturbine serves to rotate and direct the flow of hot gas to therotating turbine with themost favorable incident angleThereis no centrifugal load on the nozzle segment The combina-tion of bending loads a thermal gradient caused by coolingof the nozzle results in high stationary operating stresseson the nozzle [22] The first stage nozzle may experiencedamage mechanisms such as creep fatigue-creep oxidationcorrosion andmechanical damage during its service life [23]The microstructural evaluation is one of the most importanttools in assessing the current condition of the nozzle segmentfor its correlation with the service conditions experienced bythe componentThemicrostructural evaluation can point outstrategies for repair andor heat treatments for rejuvenationand recover the mechanical properties and extend the usefullife of the alloy

The evaluated component is a segment of the first stagenozzle of a 60 MW gas turbine gas inlet temperature to


Vanes

Flow channel

Leading edge

Pressure side

(a)

Leading edge

Trailing edge

Zone D934∘C

Zone C907ndash934∘C

Zone B560∘C

Zone A693∘C

(b)

Figure 3 (a) General view of the nozzle vane (b) Analysis regions and internal temperature distribution on the nozzle vane transversalsection in the cutting plane at 50 height (section of maximum temperature)

Table 1 Chemical composition of FSX-414 superalloy (wt)

Alloy C Cr Ni Co W Fe BFSX-414 025 29 10 52 75 10 001

the turbine is 1086∘C The full nozzle consists of 32 vanesand is cooled by air extracted from the compressor dischargeThe microstructural evaluation was performed after 54000operating hours in mode of base load The nozzle is madeof a conventional cobalt-based FSX-414 superalloy by meansof conventional investment casting (equiaxial grains) andwithout coating its chemical composition is shown in Table 1The gas turbine operates with natural gas An overview ofthe nozzle segment (two vanes) is shown in Figure 3(a) thevanes have cooling passages on the pressure surface and inFigure 3(b) the different operating temperature zones areindicated on a section of the nozzle block The maximumservice temperature (Figure 3(b)) is recorded in the leadingedge of the nozzle blade and the temperature distributionwasobtained by numerical analysis using Computational FluidDynamics (CFD) with the code Star CDV 3150 [24] Figure 4shows some cracks detected near the cooling cavities of thenozzle close to the trailing edge

Microstructural Characterization of Nozzle BladeThe nozzlemicrostructure was evaluated at a zone corresponding toa height of 50 of the flow channel on the low and hightemperature sectionThe characterization included grain sizeand carbide precipitation In order to evaluate the extentof damage in the superalloy the microstructure in the lowtemperature zone (zone B) was compared with the high

Cracks

Trailing edge

Figure 4 Cracks on the nozzle vane near the internal shroud closeto the trailing edge

temperature region (zone D) The microstructure of the lowtemperature zone can be taken as a reference or initial condi-tion of the alloy because at that temperature microstructuralchanges are insignificantThemicrostructure of the low tem-perature zone is shown in Figure 5 this consists of equiaxedgrains of the 120574 phase matrix (Figure 5(a)) and at highermagnification (Figure 5(b)) dispersed carbide particles in thegrain boundaries and matrix can be observed Figure 5(c)shows the unit area quantized to determine the percentageof precipitatesThismicrostructure is characteristic of cobalt-base superalloys [25ndash28]

Table 2 shows the grain size and the volume fraction atthe different zones in the nozzle pressure side in cross sectionVolumetric fraction of carbides in each area was determined


100120583m

(a)

Carbides

50120583m

(b)

10120583m

(c)

Figure 5 (a) General microstructure (b) magnified microstructure of the low temperature zone (zone B) and (c) minor precipitation ofcarbide particles

Grain growth

200120583m 100120583m

Figure 6 Grain growth in the high temperature (zone D)

Table 2Quantitativemicrostructure in different zones of the nozzlevanes

Microstructuralparameter Zone A Zone B Zone C Zone D

Temperature [∘C]693 560 907 934

Average grain size[120583m] 313 54 401 531

Volume fraction ofcarbides [] 736 072 1022 1296

taking into account the area ratio of carbides in 120583m2totalmeasured area also in 120583m2

As shown in Table 2 the extent of deterioration of thesuperalloy (grain growth and higher amount of precipitates)

depends directly on the metal temperature The micrographsin Figure 6 show a larger grain size for the area where thetemperature is high (zone D) and the micrograph of Figure 7shows a higher amount of precipitates (area D) comparedto the ldquocoldrdquo or reference see Figure 5 The grain size ratiobetween area A (693∘C) and the high temperature zone D(934∘C) is 06 and the grain size ratio between the referencearea and the high temperature area is 01 The growth ofgrain size (coarse growth) is one of the main symptomsof microstructural worsening of nozzlersquos material This isexplained because the material is exposed to gas at hightemperature and velocity

The nozzle microstructural investigation revealed thepresence of a continuous band of precipitated carbides ingrain boundaries and a rise in the volume fraction of carbidesup to 50 This occurs because of the transformation ofM6C carbides to M

23C6carbides Carbide transformation is


Secondary carbides

100120583m

Figure 7 Precipitated carbides in grain boundaries of FSX-414superalloy at intermediate temperature zone (area D)

encouraged by the high operating temperature of the nozzlemainly at the blade leading edge the latter type of secondarycarbides takes place abundantly in the Co-base superalloyswith more than 5 Cr [29] Precipitation of these secondaryphases in grain boundaries reduces material creep resistance

This dense and continuous network of carbides observedmainly in the area D reduces the ductility and toughness ofthe alloy by up to 30 of its initial value and may facilitateinitiation and propagation of cracks due to grain boundariesbrittleness all this leads to decreasing the useful life ofthe alloy Additionally such grain boundary precipitationreduces the material creep resistance [1 27] The microstruc-tural characterization of FSX-414 superalloy revealed that thegrain size increased considerably see Figure 6 This may alsoreduce the material fatigue life [1 30] Also the average grainsize increment in the vane body reduces alloy fatigue life [1]

An analysis of thermal stress was performed which isnot indicated in this work but the results showed that themaximum tension stresses at steady-state were located nearthe cooling holes and blade profile on the pressure side of thenozzle

In addition because the gas turbine nozzle is a fixedcomponent its operational stresses are generated only by thegas flow pressure and by thermal loads due to temperaturegradients through the nozzle elements These stresses andtemperature gradients cause fatigue damage during transientand steady-state operation this thermal stress induces theinitiation and propagation of cracks

From the metallurgical evaluation carried out and cracksdetected by nondestructive testing the nozzle segment ana-lyzed is a candidate for repair It is noteworthy that there arevirtually no limits for their rehabilitation by welding becausethis component remains fixed during operation and is notexposed to concentrated mechanical stresses caused by thecentrifugal force

Repair may include use of conventional welding andorbrazing subsequently applying a postweld heat treatmentincluding a solution heat treatment at a temperature of 1150∘Cfollowed by rapid cooling and then an aging cycle at a

temperature of 980∘C followed by cooling In the event thatthe nozzle blocks have no coating and in order to decreasethe effect of elevated temperature on the microstructure ofthe blades the use of a thermal barrier coating (TBC) shouldbe considered to improve corrosion and oxidation resistance

4 Conclusion

Amicrostructural study to determine the extent of damage interms ofmicrostructural deterioration can be used to identifyand therefore determine the type of repair (heat treatmentwelding conventional or brazing) to the nozzle block seg-ment Comparing deterioration parameters discussed aboveand the temperature distribution over the nozzle block adirect relationship between the magnitude of damage of thesuperalloy and the metal temperature can be establishedTherefore metallurgical analysis ofmain components of a gasturbine is a very useful tool that provides information neededto make decisions about the possibility of repair establishrisk of fracturing and evaluate the operating conditions ofthe equipment Consequently metallurgical characterizationshould be incorporated into maintenance schedules It isnoteworthy to mention that any metallographic study shouldbe complemented by a stress and temperature distributionanalysis in order to corroborate or determine the nozzlefailure mechanism

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

Financial support from Consejo Nacional de Ciencia yTecnologıa (CONACYT Mexico) (Projects 196205 159898and 159913) is gratefully acknowledged

References

[1] Z Mazur V M Cristalinas Navarro A Hernandez Rossetteet al ldquoDesarrollo de herramientas para prediccion de vidautil residual de alabes de turbinas de gasrdquo Internal ReportII4311887002P07 IIE Cuernavaca Mexico 2003 (Spanish)

[2] X Wu W Beres and S Yandt ldquoChallenges in life predictionof gas turbine critical componentsrdquo Canadian Aeronautics andSpace Journal vol 54 no 2 pp 31ndash39 2008

[3] C T Sims N Stoloff and W C Hagel Superalloys II High-Temperature Materials for Aerospace and Industrial PowerWiley-Interscience New York NY USA 1987

[4] N R Muktinutalapati ldquoMaterials for Gas Turbinerdquo httpwwwintechopencomdownloadpdf22905

[5] H L Bernstein ldquoMaterials issues for users of gas turbinerdquo inProceedings of the 27th Texas AM Turbomachinery SymposiumCollege Station Tex USA September 1998

[6] A Baldan ldquoProgress in Ostwald ripening theories and theirapplications to nickel-base superalloys Part I Ostwald ripeningtheoriesrdquo Journal of Materials Science vol 37 no 11 pp 2171ndash2202 2002


[7] H Li L Zuo X Song Y Wang and G Chen ldquoCoarseningbehavior of 120574 particles in a nickel-base superalloyrdquo Rare Metalsvol 28 no 2 pp 197ndash201 2009

[8] H J Dorantes-Rosales N Cayetano-Castro J J Cruz- RiveraV M Lopez-Hirata JL Gonzalez-Velazquez and J Morena-Palmerın ldquoCinetica de engrosamiento de precipitados coher-entes en aleaciones base Hierrordquo Suplemento de la RevistaLatinoamericana de Metalurgia y Materiales SI 2 pp 637ndash6452009

[9] J Liburdi P Lowden D Nagy T R De Priamus and SShaw ldquoPractical experience with the development of superalloyrejuvenationrdquo in Proceeding of the ASME Turbo Expo Power forLand Sea andAir (GT rsquo09) pp 819ndash827Orlando FlaUSA June2009

[10] P S Kotval ldquoThemicrostructure of superalloysrdquoMetallographyvol 1 no 3-4 pp 251ndash285 1969

[11] Z Mazur A Luna-Ramırez J A Juarez-Islas and A Campos-Amezcua ldquoFailure analysis of a gas turbine blade made ofInconel 738LC alloyrdquo Engineering Failure Analysis vol 12 no3 pp 474ndash486 2005

[12] Z G Yang J W Stevenson D M Paxton P Singh and KS Weil ldquoMaterials properties database for selection of high-temperature alloys and concepts of alloy design for sofc appli-cationsrdquo Tech Rep PNNL-14116 US Department of Energyunder Contract DE-AC06-76RL01830 2002

[13] G P Sabol and R Stickler ldquoMicrostructure of nickel-basedsuperalloysrdquo Phys Status Solidi vol 35 no 1 pp 11ndash52 1969

[14] G Lvov V I Levit and M J Kaufman ldquoMechanism ofprimary MC carbide decomposition in Ni-base superalloysrdquoMetallurgical and Materials Transactions A Physical Metallurgyand Materials Science vol 35 no 6 pp 1669ndash1679 2004

[15] H Kitaguchi ldquoMicrostructure-property relationship in ad-vanced Ni-based superalloysrdquo in MetallurgymdashAdvances inMaterials and Processes Y Pardhi Ed chapter 2 InTech RijekaCroatia 2012

[16] V P Swaminathan andN S CheruvuCondition and RemainingLife of Hot Section Turbine Components Is Essential to InsureReliability Materials Engineering Department Gas TurbineMaterials 1997

[17] Z-X Shi J-R Li S-Z Liu X-G Wang and X-D Yue ldquoEffectof Ru on stress rupture properties of nickel-based single crystalsuperalloy at high temperaturerdquo Transactions of NonferrousMetals Society of China vol 22 no 9 pp 2106ndash2111 2012

[18] A C Yeh and S Tin ldquoEffects of Ru on the high-temperaturephase stability of Ni-base single-crystal superalloysrdquo Metallur-gical and Materials Transactions A Physical Metallurgy andMaterials Science vol 37 no 9 pp 2621ndash2631 2006

[19] R Darolia D F Lahrman and R D Field ldquoFormation oftopologically closed packed phases in nickel-base single crystalsuperalloysrdquo in Superalloys 1988 S Reichman D N Duhl GMaurer S Antolovich and C Lund Eds The MetallurgicalSociety 1988

[20] A Volek R F Singer R Buergel J Grossmann and Y WangldquoInfluence of topologically closed packed phase formation oncreep rupture life of directionally solidified nickel-base super-alloysrdquo Metallurgical and Materials Transactions A PhysicalMetallurgy and Materials Science vol 37 no 2 pp 405ndash4102006

[21] M V Acharya andG E Fuchs ldquoThe effect of long-term thermalexposures on the microstructure and properties of CMSX-10 single crystal Ni-base superalloysrdquo Materials Science andEngineering A vol 381 no 1-2 pp 143ndash153 2004

[22] R Viswanathan Damage Mechanisms and Life Assessment ofHigh Temperature Components ASM International MaterialsPark Ohio USA 1989

[23] T J Carter ldquoCommon failures in gas turbine bladesrdquo Engineer-ing Failure Analysis vol 12 no 2 pp 237ndash247 2005

[24] STAR-CD Version 315A Methodology Computational Dynam-ics 2002

[25] H Kazempour-Liacy S Abouali and M Akbari-GarakanildquoFailure analysis of a repaired gas turbine nozzlerdquo EngineeringFailure Analysis vol 18 no 1 pp 510ndash516 2011

[26] R Bakhtiari and A Ekrami ldquoMicrostructural changes of FSX-414 superalloy during TLP bondingrdquo Defect and DiffusionForum vol 312ndash315 pp 399ndash404 2011

[27] H L Bernstein R C McClung T R Sharron and J MAllen ldquoAnalysis of general electric model 7001 first-stage nozzlecrackingrdquo Journal of Engineering for Gas Turbines and Powervol 116 no 1 pp 207ndash216 1994

[28] G M Shejale ldquoMetallurgical evaluation and condition assess-ment of FSX 414 nozzle segments in gas turbines by metallo-graphic methodsrdquo Journal of Engineering for Gas Turbines andPower vol 133 no 7 Article ID 072102 2011

[29] W H Jiang X D Yao H R Guan Z Q Hu and WH Jiang ldquoSecondary carbide precipitation in a directionallysolified cobalt-base superalloyrdquo Metallurgical and MaterialsTransactions A vol 30 no 3 pp 513ndash520 1999

[30] J A Daleo K A Ellison and D H Boone ldquoMetallurgicalconsiderations for life assessment and the safe refurbishmentand requalification of gas turbine bladesrdquo Journal of Engineeringfor Gas Turbines and Power vol 124 no 3 pp 571ndash579 2002

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coarsening grain growth micro void growth in grain bound-aries carbide precipitation and brittle phase formation

Surface damage produces dimensional deteriorationgenerating loss of the bladenozzle original dimension result-ing in increased stress and turbine efficiency reductionDuring operation the material microstructure is affected byhigh temperature combined with high stresses However theextent of deterioration differs due to the following factorstotal service time and operation history (number of startupsshutdowns and trips) gas turbine operation condition (tem-perature rotational speed mode of operation ie base loador cyclic duty) and manufacturing alterations (grain sizeporosity alloy composition heat treatment)

Then a brief description of the Ni-base and Co-base basesuperalloys is givenThese alloys are used in the manufactureof critical turbine components (moving blades nozzles com-bustors and transition ducts) of stationary gas turbines Febased or Ni-Fe based superalloys are not mentioned becausetheir use in gas turbine critical components is not common

Ni-Based Alloys The nickel-base alloys are the more complexand the most widely used for the hottest components ofgas turbines (eg gas turbine first stage blades) In the heattreated condition superalloys represent a composite materialconsisting of several intermetallic phases linked by a metalmatrix The major phases present in these superalloys are[3] as follows gamma matrix (120574) Ni-based austenitic phase(FCC) usually containing a high percentage of solid solutionelements such as Co Cr Mo and W gamma prime (1205741015840)which is Ni

3(Al Ti) based intermetallic phase Carbides

generally types M6C and M

23C6which tend to precipitate

into grain boundaries topologically closed packed (TCP)type phases such as 120590 120583 and Laves which precipitate afterprolonged high temperature service

These alloys can be classified into solid-solution hardenedalloys and precipitation hardened alloys or gamma prime (1205741015840)alloysThe formermay be forged or cast contain few elementsforming 1205741015840 particles but are hardened by refractory elementssuch as tungsten and molybdenum and carbide formationand also contain Cr to impart corrosion resistance (oxida-tion) and Co to give microstructural stability Precipitationhardened alloys can also be forged or cast In addition to 1205741015840particle formation as the main hardening mechanism alsoincorporates elements such as tungsten (W) molybdenum(Mo) tantalum (Ta) and niobium (Nb)

Co-Based Alloys Cobalt-based superalloys (eg X 40 X 45and FSX-414) are primarily used in themanufacture of all firststage nozzles and in some turbines are used in the last stagedue to their good weldability and hot corrosion resistanceThese alloys have higher strength at high temperatures thanNi-based alloys and also have excellent resistance to thermalfatigue oxidation and corrosion [4]These alloys have cobaltas the principal alloying element with significant amounts ofnickel and chromium and smaller amounts of tungsten andmolybdenum niobium tantalum and sometimes iron Theyare mainly hardened by carbide precipitation Alloys harden-ing by carbide precipitation contain between 04 and 085carbon Such superalloys consist of an austenitic matrix (fcc)

and a variety of precipitated phases such as primary carbides(M3C2 M7C3 and MC) and coarse carbides (M

23C6) and

GCP types phases (geometrically compact phases) such as12057410158401015840 and 120578 (Ni

3Al) and TCP (topologically close packed) type

phases (120590 120583 119877 or 119871) (Cr Mo)119909(Ni Co)

119910[5]

2 Superalloys Microstructural Degradationduring High Temperature Service

There are several microstructural degradation mechanismsoccurring in superalloys used in the manufacture of hotsection components of gas turbines (nozzles moving bladesand combustion chambers) The most common degradationmechanisms are aging and coarsening of the 1205741015840-phase trans-granular precipitation growth of carbides in grain bound-aries brittle phases precipitation and growth of cavities dueto creep

Coarsening and Aging of 1205741015840 Phase The size and shape ofthe 1205741015840 phase in nickel-based superalloys are not stable afterlong periods of operation at high temperatures Howeverafter a heat treatment this phase is very near to equilibriumwith the 120574 matrix and therefore there is little additionalprecipitation or growth of this phase from the supersaturatedmatrix Nevertheless some particles may grow by a diffusionmechanism [6] that is the average particle radius increaseswith aging time 119905 This is represented by the followingequation

1199033

(119905) minus 1199033

(0) = 119870119905 (1)

where 119903(0) is the average radius of the particle at 119905 = 0 119903(119905) isthe average radius of the particle in time 119905 and119870 is the kineticconstant which depends on temperature Various studies [78] on 1205741015840 growth phase in Ni-base superalloys and Fe-Ni-Alalloys have confirmed that growth obeys the law described in(1)

Changes in morphology of the 1205741015840 phase modify themechanical properties of the materialrsquos component sincephase 1205741015840 is intended to act as a barrier to dislocationmovement slowing creep consequently resistance to thisfailure mechanism is greatly diminished [9] In commercialsuperalloys the 1205741015840 phase changes from spherical to cuboidalshape although most of the particles have an intermediateform Aging is revealed as an increase in average particlesize The 1205741015840 phase can be identified in the microstructure asparticleswhose shape is irregular and larger [10 11]The shapeof this phase in a nondegraded and degraded condition canbe observed in Figures 1(a) and 1(b)

Morphology and Degeneration of MC M23C6 and M6C Car-bidesThe role of carbides in superalloys is complex carbidesseem to prefer the grain boundaries as a site location in Ni-base superalloys while in Co-base and Fe-base superalloysappear to precipitate intragranularly [3] The most commoncarbides in all Ni Fe-Ni and Co-base superalloys are basicMC M

23C6 and M

6C and seldom M

7C3[12] The most

stable carbide found in Ni-base and Co-base superalloys isthe MC type where M represents the element Ti A fraction


120574998400

(a)

120574998400

(b)


M23C6

Figure 2 M23

C6







MC +120574

1205741015840997888rarr M

23C6+ 1205741015840 (2)

or



(3)


23C6carbide has a








Vanes

Flow channel

Leading edge

Pressure side

(a)

Leading edge

Trailing edge

Zone D934∘C


Zone B560∘C

Zone A693∘C

(b)






Cracks

Trailing edge





100120583m

(a)

Carbides

50120583m

(b)

10120583m

(c)


Grain growth

200120583m 100120583m




Temperature [∘C]693 560 907 934









Secondary carbides

100120583m









4 Conclusion


Competing Interests


Acknowledgments


References







































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




120574998400

(a)

120574998400

(b)


M23C6

Figure 2 M23

C6







MC +120574

1205741015840997888rarr M

23C6+ 1205741015840 (2)

or



(3)


23C6carbide has a








Vanes

Flow channel

Leading edge

Pressure side

(a)

Leading edge

Trailing edge

Zone D934∘C


Zone B560∘C

Zone A693∘C

(b)






Cracks

Trailing edge





100120583m

(a)

Carbides

50120583m

(b)

10120583m

(c)


Grain growth

200120583m 100120583m




Temperature [∘C]693 560 907 934









Secondary carbides

100120583m









4 Conclusion


Competing Interests


Acknowledgments


References







































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Vanes

Flow channel

Leading edge

Pressure side

(a)

Leading edge

Trailing edge

Zone D934∘C


Zone B560∘C

Zone A693∘C

(b)






Cracks

Trailing edge





100120583m

(a)

Carbides

50120583m

(b)

10120583m

(c)


Grain growth

200120583m 100120583m




Temperature [∘C]693 560 907 934









Secondary carbides

100120583m









4 Conclusion


Competing Interests


Acknowledgments


References







































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




100120583m

(a)

Carbides

50120583m

(b)

10120583m

(c)


Grain growth

200120583m 100120583m




Temperature [∘C]693 560 907 934









Secondary carbides

100120583m









4 Conclusion


Competing Interests


Acknowledgments


References







































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Secondary carbides

100120583m









4 Conclusion


Competing Interests


Acknowledgments


References







































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



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Research Article Microstructural Changes during High ...downloads.hindawi.com/journals/amse/2016/1745839.pdf · Research Article Microstructural Changes during High Temperature Service