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Advanced Gas Turbine Materials and Coatings P.W. Schilke GE Energy Schenectady, NY GER-3569G (08/04) © 1995-2004 General Electric Company. All Rights Reserved
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Page 1: 59112614-GER-3569G-Advanced-Gas-Turbine-Materials-and-Coatings.pdf

Advanced Gas TurbineMaterials and Coatings

P.W. SchilkeGE EnergySchenectady, NY

GER-3569G (08/04)© 1995-2004 General Electric Company.

All Rights Reserved

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Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Materials Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Turbine Buckets and Nozzles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Bucket Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Directionally Solidified — GTD-111 Buckets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Equiaxed Buckets — GTD-111 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6IN-738 Buckets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6U-500 Buckets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Future Buckets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Nozzle Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8FSX-414 Nozzles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8GTD-222 Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8N-155 Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Future Nozzle Materials and Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Bucket Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Hot Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9High-Temperature Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10High-Temperature Coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Platinum-Aluminide Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12PLASMAGUARD™ Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Low-Temperature Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Shroud Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Future Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Combustion Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Combustion Liners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Transition Pieces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Turbine and Compressor Wheels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Turbine Wheel Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Turbine Wheel Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Alloy 706 Nickel-Base Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Cr-Mo-V Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1912 Cr Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20A286 Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Other Rotor Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Rotor Developments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) i

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Advanced Gas Turbine Materials and Coatings

Contents (cont’d)

Compressor Blades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Casings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Future Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Additional Sand Castings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Inlet and Exhaust Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Inlet Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Exhaust Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25List of Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

GE Energy ■ GER-3569G (08/04) ii

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 1

Introduction

Advanced GE materials are paving the way fordramatic improvements in gas turbines —improvements that are setting new records ingiving customers the most fuel-efficient powergeneration systems available. Combined-cycleefficiencies as high as 60% are now achievablebecause of increased firing temperature cou-pled with more efficient component and sys-tem designs. Ongoing GE developments nowpromise that the coming decade will witnesscontinued growth of gas turbines with higherfiring temperatures, pressures and outputs.

This paper describes the evolution of solutionsto what used to be incompatible marketdemands: high firing temperatures and longlife, corrosion protection from contaminatedfuels and air, and higher efficiency with fuelflexibility. It concentrates on advances made inthe hot gas path components because they aregenerally the most critical part of the gas tur-bine. Improvements in superalloys and pro-cessing now permit the hot gas path compo-nents to operate in advanced gas turbines fir-ing at increased temperatures for many thou-sands of hours under severe conditions of cen-trifugal, thermal and vibratory stresses. Recentimprovements to compressors and rotors arealso discussed.

GE engineers continue to lead the way inunderstanding and developing materials tech-nology for gas turbines because they can tapknowledge from the laboratories of one of theworld’s most diversified companies, with prod-ucts ranging from aircraft engines to high-technology plastics. They have used theseresources and data collected from more than5,000 gas turbines operating in many climates,and on a wide range of fuels, to verify that thematerials will perform under demanding con-ditions.

Materials Philosophy

The primary philosophy is to build a reliable,efficient, cost-effective machine for the intend-ed service. Whenever possible, standard mate-rials with histories of successful application areused. In many cases, proven technology is uti-lized from aircraft or steam turbine applica-tions. However, many times the uniquerequirements of heavy-duty gas path compo-nents demand special materials and processes.Working with alloy and component suppliersin conjunction with internal GE developmentprograms, alloys and processes have beendeveloped to meet the needs of the gas tur-bine industry.

The first phase of a materials developmentprogram is expensive and time-consuming.First, new ideas and emerging developmentsare screened to select the one or two with thebest potential for satisfying the material designgoals.

Extensive testing follows to ensure that thematerials will perform satisfactorily in heavy-duty gas turbines for tens of thousands ofhours. Long-term creep testing at the expectedoperating temperatures of the material is con-ducted to characterize alloy performance.

Additionally, laboratory evaluations typicallyinclude items such as tensile, rupture, low- andhigh-cycle fatigue, thermal mechanical fatigue,toughness, corrosion/oxidation resistance,production/ processing trials and completephysical property determinations. This phaseof testing can last several years for a new nozzleor bucket material.

After laboratory testing, actual machine-operat-ing experience, the best and final test of a newmaterial, is gained through cooperation withGE customers. Rainbow field tests that containthe material(s) for evaluation are installed incustomers’ machines for side-by-side compari-

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 2

son with the current baseline material.

More than 50 Rainbows have been installedsince the 1950s, covering compressor blade,compressor coating, combustor, nozzle, nozzlecoating, bucket and bucket coating materials.Virtually all of the improved hot gas pathmaterials that we now regard as standard wererain-bow tested in Rainbow rotors prior tointroduction. Typically, the Rainbow materialsare removed and evaluated periodically, thenreplaced with standard parts. Current Rainbowtests include bucket and nozzle coatings, com-bustion components and bucket and nozzlematerials.

The Rainbow rotor tests, including the long-term laboratory tests, constitute the corner-stone of the materials development philoso-phy. They have successfully provided a continu-ous stream of carefully evolved materials andprocesses for GE heavy-duty gas turbines.

After a material has been proven in a Rainbowrotor, producibility is verified through exten-sive first piece qualification tests and pilot lotevaluations. Components must continue tomeet rigorous production non-destructive anddestructive test requirements. Extensive workwith suppliers is completed in order to qualifyparts that use a new material. During this time,trial parts are destructively tested and analyzedto determine that the properties meet therequirements defined by the GE specifications.Hundreds of bucket and nozzle castings andmany wheel forgings have been cut up foranalysis to verify that the processing (startingstock, casting/forging parameters, heat treat-ment, etc.) is correct.

Once a supplier becomes qualified, theprocesses used to make that component are"frozen" for production and can not bechanged without GE approval. Once in pro-

duction, specimens produced with certainforgings and select-ed castings are destructivelytested to ensure specification compliance.Critical rotating components are subjected tonon-destructive inspection techniques such asultrasonics, liquid penetrant, magnetic particleand X-ray examination, depending upon thecomponent. Proof testing is also performed onthe most critical components.

This philosophy of material development andproduction qualification has existed since GEbegan building gas turbines in the 1950s, andit will continue in the future to meet the needsfor improved materials in new and upratedmachines.

Turbine Buckets and Nozzles

ProcessingGE has used investment cast nozzles and buck-ets made by the lost wax technique since themid-1960s. This casting process allows the useof alloys that are difficult to form or machineand provides great design flexibility for inter-nal cooling schemes. For example, ceramiccoring is used extensively in these castings toform air-cooling passages and to provideweight reduction.

Most nozzle and bucket castings used by GEare made by using the conventional equiaxedinvestment casting process. In this process, themolten metal is poured into a ceramic mold ata pressure below 10-2 torr (10-2 mm Hg).Vacuum is used in most cases, except for someof the cobalt alloys, to prevent the highly reac-tive elements in the superalloys from reactingwith the oxygen and nitrogen in the air. Withproper control of metal and mold thermalconditions, the molten metal then solidifiesfrom the surface toward the center of themold, creating an equiaxed structure. To pre-

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 3

vent shrinkage porosity, care is taken to allowproper feeding of molten metal to the castingwhile it solidifies.A variety of investment castbuckets and nozzles has been produced duringthe past 30 years. The examples in Figures 1and 2 indicate the process flexibility in accom-modating design and size variations.

Directional solidification (DS) is also beingemployed to produce advanced technologybuckets. First used in aircraft engines morethan 25 years ago, it was adapted for use inlarge air-foils through the efforts of GEPG andits suppliers several years ago. By exercisingcareful control over temperature gradients, aplanar solidification front is developed in thebucket, and the part is solidified by movingthis planar front longitudinally through theentire length of the part. The result is a bucketwith an oriented grain structure that runs par-allel to the major axis of the part and containsno transverse grain boundaries. The elimina-

tion of these transverse grain boundaries con-fers additional creep and rupture strength onthe alloy, and the orientation of the grainstructure provides a favorable modulus of elas-ticity in the longitudinal direction to enhancefatigue life. More recently, GEPG has workedwith its suppliers to develop large, single-crystalcastings that offer additional creep and fatiguebenefits through the elimination of grainboundaries.

The MS5002C directionally solidified bucketwas the first large land-based gas turbine DSbucket made on a production basis and hasbeen in commercial service since 1989. Figure 3shows three recent examples of directionallysolidified stage 1 buckets: an MS9001FA, anMS7001FA and an MS6001FA. All are etchedto show the directional grain structure.

Secondary operations include electrochemicaland electrodischarge machining, hard-coatingon some components and conventional andcreep feed grinding. These processes and subse-quent coatings for corrosion and oxidation pro-tection are fully qualified for each design toensure that metallurgical quality is maintained,adverse residual stresses are not introduced andoverall properties are not degraded. In addition,dovetails are shot-peened to provide residualcompressive stresses for improved fatiguestrength.

MS7001E1st Stage

MS60011st Stage

MS50021st Stage

MS7001F2nd Stage

MS9001F1st Stage

MS7001F1st Stage

MS3002Uprate

1st Stage

MS5001Uprate

1st Stage

Figure 1. Investment cast buckets

MS70012nd Stage

MS50011st Stage

MS9001F3rd Stage

MS60011st Stage

MS3002Uprate

1st Stage

Figure 2. Investment cast nozzles

MS9001FA MS7001FA MS6001FA

Figure 3. Directionally solidified buckets

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Advanced Gas Turbine Materials and Coatings

GE Power Systems ■ GER-3569G ■ (08/04) 4

Bucket Materials

The stage 1 bucket must withstand the mostsevere combination of temperature, stress andenvironment; it is generally the limitingcomponent in the machine.

Since 1950, turbine bucket material tempera-ture capability has advanced approximately850°F/472°C, approximately 20°F/10°C peryear. The importance of this increase can beappreciated by noting that an increase of100°F/56°C in turbine firing temperature canprovide a corresponding increase of 8% to13% in output and 2% to 4% improvement insimple-cycle efficiency. Advances in alloys andprocessing, while expensive and time-consum-ing, provide significant incentives throughincreased power density and improved efficien-cy.

Figure 4 shows the trend of firing temperatureand bucket alloy capability. The compositionof the new and conventional alloys discussed isshown in Table 1. The increases in bucket alloytemperature capability accounted for themajority of the firing temperature increase

until the 1970s, when air cooling was intro-duced, which decoupled firing temperaturefrom bucket metal temperature. Also, as themetal temperatures approached the1600°F/870°C range, hot corrosion of bucketsbecame more life-limiting than strength untilthe introduction of protective coatings.

During the 1980s, emphasis turned toward twomajor areas: improved processing to achievegreater bucket alloy capability without sacrific-ing alloy corrosion resistance; and advanced,highly sophisticated air-cooling technology toachieve the firing temperature capabilityrequired for the new F generation of gas tur-

2600 1400

1300

1200

1100

1000

900

700

1950 1960 1970 1980 1990 2000

800

2400

2200

2000

1800

1600

1400

1200

Tem

p. °F

Tem

p. °C

Year

LegendFiring TempBucket MaterialRupture Capability

1.4 kg/cm2 x 10-3 (20 ksi)(100,000 hrs)

SteamCooling

AdvancedAir CoolingConventional

Air Cooling

U500

IN738

DSGTD-111

GTD-111

SCAlloys

RENE 77(U700)

Figure 4. Firing temperature trend andbucket material capability

BUCKETSU500 18.5 BAL 18.5 4 3 3 0.07 0.006RENE 77 (U700) 15 BAL 17 5.3 3.35 4.25 0.07 0.02IN738 16 BAL 8.3 0.2 2.6 1.75 3.4 3.4 0.9 0.10 0.001 1.75GTD111 14 BAL 9.5 3.8 1.5 4.9 3.0 0.10 0.01 2.8

NOZZLESX4 0 2 5 1 0 BAL 1 8 0.50 0.01X4 5 2 5 1 0 BAL 1 8 0.25 0.01FSX414 28 10 BAL 1 7 0.25 0.01N155 21 20 20 BAL 2.5 3 0.20GTD-222 22.5 BAL 19 2.0 2.3 1.2 0.8 0.10 0.008 1.00

COMBUSTORSSS309 23 13 BAL 0.10HAST X BAL 1.5 1.9 0.7 9 0.07 0.005N-263 20 BAL 20 0.4 6 2.1 0.4 0.06HA-188 22

22

22 BAL 1.5 14.0 0.05 0.01TURBINE WHEELS

ALLOY 718 19 BAL 18.5 3.0 0.9 0.5 5.1 0.03ALLOY 706 16 BAL 37.0 1.8 2.9 0.03Cr-Mo-V 1 0.5 BAL 1.25 0.30A286 15 25 BAL 1.2 2 0.3 0.25

0.250.08 0.006

M152 12 2.5 BAL 1.7 0.3 0.12COMPRESSOR BLADES

AISI 403 12 BAL 0.11AISI 403 + Cb 12 BAL 0.2 0.15GTD-450 15.5 6.3 BAL 0.8 0.03

COMPONENT Cr Ni Co Fe W Mo Ti Al Cb V C B Ta

Table 1. High-Temperature Alloys

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 5

bines. (See Figure 5.) The use of steam coolingto further increase combined-cycle efficiencieswill be realized in the 1990s.

All GE gas turbine bucket alloys are vacuum-cast, nickel-base superalloys that are strength-ened through solution and precipitation-hard-ening heat treatments. Figure 6 shows the stressrupture strength of these alloys and the super-alloys used for nozzle applications: GTD-222,FSX-414 and N-155. This comparison is pre-sented in the form of the stress required forrupture as a function of a parameter thatrelates time and temperature (the Larson-Miller Parameter).

This parameter is one of several importantdesign parameters that must be satisfied toensure proper performance of the alloy in abucket application, especially for long servicelife. Creep life, high- and low-cycle fatigue,thermal fatigue, tensile strength and ductility,

impact strength, hot corrosion and oxidationresistance, producibility, coatability and physi-cal properties must also be considered.

Directionally Solidified—GTD-111BucketsThe stage 1 bucket material currently in pro-duction is directionally solidified GTD-111. Thisis the same as GTD-111 equiaxed except fortighter control on the alloy chemistry. Thisbucket material is currently being used on the6FA, 7FA and 9FA turbines, and on the 6B, 9EC,7EA and on the 5/2C and D and 3/2J upratedturbines. DS GTD-111 is also being applied tostage 2 and stage 3 buckets of the 7FA and 9FAgas turbines.

As discussed earlier, the use of directionallysolidified GT-111 results in a substantialincrease in the creep life, or substantialincrease in tolerable stress for a fixed life.Figure 7 shows the advantage of directionallysolidified GTD-111 compared to equiaxed.This advantage is due to the elimination oftransverse grain boundaries from the bucket,the traditional weak link in the microstructure.

1000

500

460

40

10

3

2

1

600 700 800 900

1200°F

°C

1400 1600

IN-738

GTD-111U-500

GTD-222

FSX-414

N-155

Stre

ss K

SI

Kg/

cm2

x 10

-3

Temp.100,000 Hrs

Life

BucketsNozzles

Figure 6. Stress rupture comparison –bucket and nozzle materials

Figure 5. Advanced air-cooling technology

•Increased Tensile Strength: ~ 25%•Increased Tensile Ductility: ~ 100%•Increased Fatigue Strength: ~ 900%

(Strain Controlled)• Increased Impact Strength: ~ 33%•Increased Creep Strength: ~ 22°C (40°F)

Creep Advantage Temp °C

Creep Advantage Temp °F

Stre

ss K

SI

Stre

ss K

G/C

M2

x 10

-3

40

30

20

10

0

3

2

1

010

20 30 40 50 60

15 20 25 30 35

Benefits GTD-111 (DS)

Figure 7. Directionally solidified GT-111 vs. equiaxed

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 6

In addition to improved creep life, DS GTD-111 possesses more than 10 times the straincontrol or thermal fatigue compared toequiaxed GTD-111. The impact strength of DSGTD-111 is also superior to that of equiaxed,showing an advantage of more than 33%.

Equiaxed Buckets — GTD-111GTD-111, the basic alloy used for both DS andequiaxed applications, was developed andpatented by GE in the mid-1970s. It possessesabout a 35°F/20°C improvement in rupturestrength in the equiaxed form, compared toIN-738. GTD-111 is also superior to IN-738 inlow-cycle fatigue strength. At the same time,GTD-111 has corrosion resistance comparableto IN-738, the acknowledged corrosion stan-dard for the industry.

The design of this alloy was unique in that itutilized phase stability and other predictivetechniques to balance the levels of critical ele-ments (Cr, Mo, Co, Al, W and Ta), therebymaintaining the hot corrosion resistance of IN-738 at higher strength levels without compro-mising phase stability. The same principles thatwere used to enhance the castability of IN-738were also incorporated into GTD-111.

A substantial effort was made to "tune in"GTD-111 so that it could be used to make highquality investment cast buckets. During thisphase of the process/alloy development, alarge number of trial parts were made, repre-senting the span of bucket sizes envisioned. Atfirst, trials were confined to stage 1 parts, butmore recently this has expanded, and GTD-111 is now being used in the larger, latter-stagebuckets. During all of these producibility trials,buckets were made, non-destructively tested,and many were cut up to determine proper-ties. These evaluations provided the feedback

required for optimizing the processing of theseparts.

IN-738 BucketsIN-738 has been the stage 1 bucket material onall models built between 1971 and 1984, whenGTD-111 was introduced. In addition, IN-738has been used in more recent years as thestage 2 bucket material in the three-stageMS6001, MS7001 and MS9001 models. IN-738is notable as being one of a very small class ofmodern superalloys that has an outstandingcombination of elevated temperature strengthand hot corrosion resistance. The balance ofthese two properties was optimal for heavy-dutygas turbine applications. It was specificallydesigned for application in a land-based gasturbine, as opposed to aircraft use. IN-738 wasthe first cast bucket material used by GE in theheavy-duty gas turbines that had not seen priorservice in aircraft gas turbine applications.

IN-738 was first developed by the InternationalNickel Company, but its chemistry was subse-quently modified by GE to improve its castabil-ity. This, together with considerable work onmodifying the casting techniques, them-selves,enabled the commercial adoption of an alloythat otherwise would have been classed asnearly impossible to cast in large sizes. Thiswork enabled the successful application of IN-738 over the past 20 years in GE gas turbines.Indeed, this alloy is now used throughout theentire heavy-duty gas turbine industry.

U-500 BucketsMany of GE’s stage 3 gas turbine buckets arecurrently made of U-500, an alloy that wasused for stage 1 buckets in the mid-1960s. LikeIN-738 and GTD-111, this alloy is a precipita-tion-hardened (gamma prime), nickel-base

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 7

alloy. It is currently being applied to the latterstages of buckets in selected gas turbine mod-els.

Future BucketsWith the introduction of DS GTD-111, a com-mercial reality, development efforts are nowfocusing on single-crystal processing andadvanced DS alloy development. Single-crystalairfoils offer the potential to further improvecomponent high-temperature materialstrength and, by control of crystal orientation,can provide an optimum balance of properties.

In single-crystal material, all grain boundariesare eliminated from the material structure anda single crystal with controlled orientation isproduced in an airfoil shape. By eliminating allgrain boundaries and the associated grainboundary strengthening additives, a substantialincrease in the melting point of the alloy can be

achieved, thus providing a correspondingincrease in high-temperature strength. Thetransverse creep and fatigue strength isincreased, compared to equiaxed or DS struc-tures. GE Aircraft Engines has been applyingsingle-crystal bucket technology for more than10 years in flight engines. The advantage of sin-gle-crystal alloys compared to equiaxed and DSalloys in low-cycle fatigue (LCF) is shown inFigure 8. GE is currently evaluating and Rainbowrotor testing some of these single-crystal alloysfor application in our next generation gas tur-bines.

The continuing and projected temperaturecapability improvements in bucket materialcapabilities are illustrated in Figure 9. Togetherwith improved coatings, these new bucketmaterials will provide continued growth capa-bility for GE gas turbines in the years to come.

50

40

30

20

10

0IN738

Equiaxed Equiaxed

Rel

ativ

e Li

fe

DS DS SC SCGTD-111 GTD-111 2nd Gen 1st Gen 2nd Gen

Figure 8. Bucket alloys — LCF life

1650 900

880

860

840

8201980 1985 1990 1995 2000

1600

1550

1500

Year of Introduction

Temp

°C°F

GTD-111

D.S. GTD-111

SingleCrystal

Rupture Temp20 KSI (100,000 Hrs)(1.4 Kg/cm2 x 10-3)

Figure 9. Continuing improvements in bucket materials capability

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Nozzle MaterialsStage 1 nozzles (GE terminology for the sta-tionary vanes in the turbine) are subjected tothe hottest gas temperatures in the turbine,but to lower mechanical stresses than the buck-ets. Their function is to direct the hot gasestoward the buckets and they must, therefore,be able to withstand high temperatures andprovide minimal gas turning losses. The noz-zles are required to have excellent high-tem-perature oxidation and corrosion resistance,high resistance to thermal fatigue, relativelygood weldability for ease of manufacture andrepair, and good castability. Latter-stage noz-zles must also possess adequate creep strengthto support themselves and the attacheddiaphragms from the external casing.

FSX-414 NozzlesThe current alloy used for all production stage1 nozzles and some latter-stage nozzles is FSX-414, a GE-patented cobalt base alloy. Cobalt-base alloys generally possess superior strengthat very high temperatures, compared to nickel-based alloys. This alloy is a derivative of X-40and X-45, both of which were also developedby GE and first used in the 1960s. FSX-414contains less carbon than X-40 to enhance weld-ability, and more chromium to improve oxida-tion/corrosion resistance. Long-life tests in asimulated gas turbine combustion chamber havedemonstrated a two- to three-fold increase inoxidation resistance compared to X-40 and X-45.This improvement permitted an increase in thefiring temperatures of approximately100°F/56°C for equivalent nozzle oxidation life.

GTD-222 NozzlesThe latter-stage, nickel-based nozzle alloy,GTD-222, was developed in response to theneed for improved creep strength in somestage 2 and stage 3 nozzles. It offers animprovement of more than 150°F/66°C in

creep strength compared to FSX-414, and isweld-repairable.

An important additional benefit derived fromthis alloy is enhanced low-temperature hot cor-rosion resistance. By tailoring the alloy to pro-vide an optimum combination of creepstrength and weldability, a unique GE-patentednickel-base alloy was created to satisfy thedemands of advanced and uprated GE gas tur-bines. This alloy is vacuum investment cast andhas exhibited good producibility. Rainbow noz-zle segments were fabricated from GTD-222and have shown excellent performance aftermore than 40,000 hours of service. This nozzlealloy is now being used in the 6FA, 7FA, 9FA9E, 9EC and 6B machines.

N-155 NozzlesN-155, also referred to as Multimet, is an iron-based alloy chemically similar to the S-590 usedin early bucket applications. It is more readilyavailable, possesses better weldability than S-590 and is used in the latter-stage nozzles ofthe MS3000 and MS5000 series of turbines.

Future Nozzle Materials and CoatingsFSX-414 nozzle material has been extremelysuccessful since the 1960s. However, because ofthe continuous increase in turbine operatingtemperatures, developmental programs havebeen initiated to bring advanced nozzle alloysinto commercial production. The first of theseprograms resulted in the introduction of GTD-222 for latter-stage nozzles. In the stage 2 noz-zle application, GTD-222 is coated with an alu-minide coating to provide added oxidationresistance to this component. Another pro-gram is directed toward the evaluation andmodification of currently used Aircraft Enginealloys with improved high-temperaturestrength and high temperature oxidationresistance.

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Bucket Coatings

Bucket coatings are required to protect thebucket from corrosion, oxidation and mechan-ical property degradation. As superalloys havebecome more complex, it has been increasing-ly difficult to obtain both the higher strengthlevels that are required and a satisfactory levelof corrosion and oxidation resistance withoutthe use of coatings. Thus, the trend towardhigher firing temperatures increases the needfor coatings. The function of all coatings is toprovide a surface reservoir of elements thatwill form very protective and adherent oxidelayers, thus protecting the underlying basematerial from oxidation and corrosion attackand degradation.

Experience has shown that the lives of bothuncoated and coated buckets depend to alarge degree on the amount of fuel and aircontamination, as well as the operating tem-perature of the bucket. This effect is shown inFigure 10, which illustrates the effect of sodium,a common contaminant, on bucket life at1600°F/871°C. The presence of increased lev-els of contaminants give rise to an acceleratedform of attack called hot corrosion.

In addition to hot corrosion, high-temperatureoxidation and thermal fatigue resistance havebecome important criteria in the higher firing

gas turbines, as shown in Figure 11. In today’sadvanced machines, oxidation is of concernnot only for external buckets surfaces, but alsofor internal passages such as cooling holes.

Hot CorrosionHot corrosion is a rapid form of attack that isgenerally associated with alkali metal contami-nants, such as sodium and potassium, reactingwith sulfur in the fuel to form molten sulfates.The presence of only a few parts per million(ppm) of such contaminants in the fuel, orequivalent in the air, is sufficient to cause thiscorrosion. Sodium can be introduced in anumber of ways, such as salt water in liquidfuel, through the turbine air inlet at sites nearsalt water or other contaminated areas, or ascontaminants in water/steam injections.Besides the alkali metals such as sodium andpotassium, other chemical elements can influ-ence or cause corrosion on bucketing. Notablein this connection are vanadium, primarilyfound in crude and residual oils, and lead,most frequently resulting automobile exhaustemissions or as a transportation contaminatefrom leaded gasolines.

There are now two distinct forms of hot corro-sion recognized by the industry, although theend result is the same. These two types arehigh-temperature (Type 1) and low-tempera-ture (Type 2) hot corrosion.

Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 9

Percentilesfor Commonly

Used Fuels

60

50

40

30

20

10

00.5 1.0 1.5 2.0

Nat. Gas:True Distillates:Treated Ash Forming:

Life

= T

hous

and

Hour

s

Equivalent Sodium (Fuel, Air, Water Mix), ppm

50% 90%50%

50%

90%

90%

IN738 + PtAl Coating

IN738 Uncoated

U700 Uncoated

Figure 10. Effect of sodium on bucketcorrosion life

1970 1980 1990 2000Year

Coat

ing

Life

Req

uire

men

ts

Oxidation

Thermal Fatigue

Hot Corrosion

Figure 11. Bucket coating requirementsand coating evolution

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High-temperature hot corrosion has beenknown since the 1950s. It is an extremely rapidform of oxidation that takes place at tempera-tures between 1500°F/816°C and1700°F/927°C in the presence of sodium sul-fate (Na2SO4). Sodium sulfate is generated inthe combustion process as a result of the reac-tion between sodium, sulfur and oxygen.Sulfur is present as a natural contaminant inthe fuel.

Low-temperature hot corrosion was recognizedas a separate mechanism of corrosion attack inthe mid-1970s. This attack can be very aggres-sive if the conditions are right. It takes place attemperatures in the 1100°F/593°C to1400°F/760°C range and requires a significantpartial pressure of SO2. It is caused by lowmelting eutectic compounds resulting fromthe combination of sodium sulfate and someof the alloy constituents such as nickel andcobalt. It is, in fact, somewhat analogous to thetype of corrosion called Fireside Corrosion incoal-fired boilers.

The two types of hot corrosion cause differenttypes of attack, as shown in Figures 12 and 13.These are metallographic cross sections of cor-roded material. High-temperature corrosionfeatures intergranular attack, sulfide particles

and a denuded zone of base metal. Low-tem-perature corrosion characteristically shows nodenuded zone, no intergranular attack, and alayered type of corrosion scale.

The lines of defense against both types of cor-rosion are similar. First, reduce the contami-nants. Second, use materials that are as corro-sion-resistant as possible. Third, apply coatingsto improve the corrosion resistance of thebucket alloy.

High-Temperature OxidationMetal oxidation occurs when oxygen atomscombine with metal atoms to form oxidescales. The higher the temperature, the morerapidly this process takes place, creating thepotential for failure of the component if toomuch of the substrate material is consumed inthe formation of these oxides. Figure 14a showsthe microstructure of a coated bucket that hasseen about 30,000 hours of service. At the tem-peratures seen in this region of the airfoil, nosignificant oxidation attack of the coating canbe seen.

By contrast, Figure 14b shows the microstruc-ture of the same type of coating, which hasbeen severely attacked after about the samelength of service. At the higher temperatures,

Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 10

Sulfide Spikes

25µ

Figure 12. Hot corrosion (high-temperature type)

Low-Temp.Corrosion Scale

25µ

Figure 13. Hot corrosion (low-temperature type)

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 11

which must have been present in the Figure 14bcase, insufficient aluminum was available inthe coating to maintain a protective oxide atthe surface, and oxygen was able to diffuseinto the interior of the coating structure whereit formed discrete, discontinuous, aluminumoxide particles. This phenomenon is known asinternal oxidation. Such a situation quicklydepletes the coating of its available aluminum,rendering it non-protective.

At the higher temperatures, >1650°F/>899°C,relatively rapid oxidation attack of some mate-rials can occur unless there is a barrier to oxy-gen diffusion on the metal surface. Aluminumoxide (Al2O3) provides such a barrier.Aluminum oxide will form on the surface of asuperalloy at high temperatures if the superal-loy’s aluminum content is sufficiently high.Thus, the alloy forms its own protective barrierin the early stages of oxidation by the creationof a dense, adherent aluminum-oxide scale.

However, many high-strength superalloys inuse today cannot form sufficient protectivescales because the compositional requirementsfor achieving other properties, such as highstrength and metallurgical stability, do notallow for the optimization of oxidation/corro-sion resistance in the superalloy itself.Therefore, most of today’s superalloys mustreceive their oxidation protection from special-ly engineered coatings.

High-Temperature CoatingsHigh-temperature coatings are used where thetemperatures of the components exceed theinherent oxidation resistance of the material.Considerable development has occurred dur-ing the past 20 years in the field of high tem-perature coatings. The result has been amarked increase in the capability of these coat-ings to resist not only hot corrosion attack overlong periods of time, but high-temperatureoxidation as well. GE heavy-duty coatings avail-

SubstrateSoundCoating

Figure 14b. Photomicrograph of a coating on a bucketmaterial showing internal oxidation ofcoating (dark particles)

Substrate SoundCoating

Figure 14a. Photomicrograph showing soundmicrostructure of a coated bucket thathas been in service

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GE Energy ■ GER-3569G (08/04) 12

able today have lives that are 10 to 20 timeslonger than the first-generation coatings undera wider diversity of corrosion and oxidizingconditions.

GE has used two basic classes of coatings duringthe past 25 years. The first class used was a diffu-sion-style coating called platinum aluminide(PtAl). The second class is an overlay-style coat-ing such as PLASMAGUARD™ GT-29 IN-PLUS™.

The development of each of these coating sys-tems was in response to field needs. The plat-inum aluminide was the original heavy-dutycoating and addressed corrosion problemsseen by a large segment of the fleet in the1960s. It doubled the corrosion life of theuncoated IN-738 buckets of that time. ThePLASMAGUARD™ GT-29 coating improvedthat corrosion resistance by a further 50%.That same high level of hot-corrosion resist-ance is kept in the more recent PLASMA-GUARD™ GT-29 PLUS, which also has sub-stantially more oxidation resistance, asrequired by the more advanced machines.PLASMAGUARD™ GT-29 IN-PLUS™ is a two-layer coating, with the top layer also applied tothe internal surface of the bucket. Most recent-ly, GT-33 IN-COAT™ and IN-PLUS™ have

been developed and applied to the stage 1buckets of the higher firing temperaturemachines, such as the 7FA and 9FA machines.This coating possesses even greater high-tem-perature oxidation capability than the GT-29IN-PLUS™.

A comparison of stage 1 bucket coatings isshown in Figure 15, while a more detaileddescription of each is in the following sections.

Platinum-Aluminide CoatingsAll stage 1 buckets have been coated since thelate 1970s. Up until mid-1983, the coating usedby GE on most stage 1 buckets was a platinum-aluminum (PtAl) diffusion coating. This coat-ing was selected over the straight aluminidecoatings because it provided superior corro-sion resistance both in burner test rigs and infield trials. The platinum-aluminum coating isapplied by electroplating a thin (0.00025inch/0.006 mm) layer of platinum uniformlyonto the bucket air-foil surface, followed by apack diffusion step to deposit aluminum. Thisresults in a nickel-aluminide coating with plat-inum in solid solution or present as a PtAl2phase near the surface.

The platinum in the coating increases theactivity of the aluminum in the coating,

Oxidation Corrosion

Comparative Resistance to:

PtAl

GT-29

GT-29 PLUSGT-29 IN-PLUSTM

GT-33 IN-PLUSTM

Cracking

Figure 15. Comparative resistance in types of coatings

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enabling a very protective and adherent Al2O3scale to form on the surface.

A Rainbow example of comparative corrosionon PtAl-coated and uncoated IN-738 buckets,run side-by-side in the same machine undercorrosive conditions, is shown in Figure 16. Thetwo buckets were removed for interim evalua-tion after 25,000 hours of service. This unitburned sour natural gas containing about3.5% sulfur and was located in a region wherethe soil surrounding the site contained up to3% sodium.

The uncoated IN-738 bucket has penetrationextending 0.010 to 0.015 inches (0.25 to 0.4mm) into the base metal over most of thebucket surface. The coated bucket generallyshows no evidence of base metal hot corrosionattack, although some of the bucket areasshowed coating thinning. Only at some verysmall locations on the leading edge of thecoated bucket was the coating breached andthen to only a depth of 0.001 to 0.002 inches(0.025 to 0.05 mm).

PLASMAGUARD™ CoatingsThe latest GE-developed and patentedPLASMAGUARD™ coatings are now GE’s stan-dard stage 1 bucket coatings — GT-29 PLUS™and GT-33 PLUS™ for solid buckets; GT-29

IN-PLUS™ and GT-33 IN-PLUS™ for cooledor hollow vaned buckets.

PLASMAGUARD™ coatings are examples ofoverlay coatings and differ from diffusion coat-ings, such as the platinum-aluminum coatings,in one major respect. At least one of the majorconstituents, (generally nickel) in a diffusioncoating is supplied by the base metal. An over-lay coating, on the other hand, has all the con-stituents supplied by the coating itself. Theadvantage of overlay coatings is that more var-ied corrosion resistant compositions can beapplied since the composition is not limited bythe base metal composition, nor is thicknesslimited by process considerations.

PLASMAGUARD™ coatings are applied by theVacuum Plasma Spray (VPS) process in equip-ment especially designed to apply this coatingin a uniform and controlled manner to GEbuckets. In this process, powder particles ofthe desired composition are acceleratedthrough a plasma jet to velocities higher thanthose achievable through atmospheric plasmaspray methods. (See Figure 17.) The solidifica-tion of the powder onto the airfoil results in amuch stronger coating bond than can beachieved by using conventional atmosphericplasma spray deposition because of the higherparticle speeds and the cleaner, hotter sub-

Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 13

Figure 16. Stage 1 turbine buckets: coated anduncoated IN-738; 25,000 service hours

Figure 17. PLASMAGUARD™ GT-20 coated shroud

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GE Energy ■ GER-3569G (08/04) 14

strate. In addition, higher coating densitiesand soundness are achievable using the VPSapproach.

The first production VPS facility was installedin Schenectady during the early 1980s. Thisfacility has been augmented by a newer, highercapacity and more-automated VPS facility inthe gas turbine manufacturing plant inGreenville, South Carolina. (See Figure 18.) Thisfacility has the capability to coat the latterstage buckets with PLASMAGUARD™ coatingsand to provide refurbishment capability forused buckets.

Extensive laboratory corrosion testing was per-formed on candidate PLASMAGUARD™ coat-ing compositions in the late 1960s to the early1970s. This led to the selection of GT-29 as theoriginal PLASMAGUARD™ coating because itsatisfied the field need for superior hot corro-sion resistance, compared to the original PtAlcoating. This laboratory testing was confirmedby field experience in Rainbow rotors thatwere installed in the mid-1970s. More than40,000 hours of satisfactory turbine operationhave now been accumulated on this coating, asshown in Figure 19.

In the mid-1980s, GE found that more oxida-tion resistance was required for the higher fir-ing temperature gas turbines, generally above1950°F/1065°C in air cooled machines and

above 1750°F/954°C in uncooled machines.(See Figure 11.) This led to the introduction ofthe patented PLASMAGUARD™ GT-29 PLUScoating that combines the demonstrated hotcorrosion protection of GT-29 with a substan-tial increase in oxidation protection. Theenhanced oxidation protection offered by GT-29 PLUS is gained from an increased alu-minum content in the outer region of thecoating matrix. In service, the higher alu-minum content of the GT-29 PLUS forms amore oxidation-protective aluminum oxidelayer that greatly improves the high tempera-ture oxidation resistance. PLASMAGUARD™GT-29 IN-PLUS™ was introduced for advancedcooled and hollow vaned buckets. This coatingincludes a diffused, aluminum-rich layer onthose inner passages, cooling holes and sur-faces to protect against oxidation that wouldotherwise occur.

Recently, a new PLASMAGUARD™ coatinghas been developed and Rainbow tested in sev-eral gas turbines and has shown excellentdurability after more than 24,000 hours ofservice. This new coating, GT-33, was designedto provide more oxidation resistance and more

VPSCoating

25µ

Figure 19. VPS coating after more than 40,000 hoursturbine exposure – pressure face

Figure 18. VPS production facility

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Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 15

resistance to cracking than the GT-29 composi-tion. This coating may also be used with anouter layer enriched with aluminum to providemaximum long-term life. PLASMAGUARD™GT-33 is currently being introduced in the Fclass machines.

Coatings for bucket refurbishment have alsobeen introduced recently. These coatings,known as EXTEND-COAT™, are based uponthe GT-29 and GT-33 PLASMAGUARD™ com-positions and were developed to be applied toserviced hardware. Several GE Service Centershave been qualified to apply these coatings forthe service market.

Low-Temperature CoatingsLow-temperature coatings find their greatestneed in latter stage buckets and in stage 1buckets of machines that run a substantial por-tion of their duty cycle at part load.

For instance, the stage 3 buckets of the 7FAand 9FA machines are currently coated with adiffused chromide coating which, although notsuitable for higher temperature stages, willimpart substantial protection against both cor-rosion and oxidation at the lower tempera-tures of this part. In addition, a PLASMA-GUARD™ GT-43 coating composition hasbeen developed, after an extensive laboratorycorrosion rig and mechanical testing program,for use in severe low temperature corrosionapplications. This GE-patented coating, alsoapplied by the same VPS process, has shownexcellent performance in Rainbow rotors, con-firming its laboratory corrosion resistance.

Shroud CoatingsNew gas turbine models such as the 6FA,7FA and 9FA operate at considerably highertemperatures than previous heavy-duty gasturbines. Therefore, to provide a durablestage 1 bucket stationary shroud component,

PLASMAGUARD™ GT-20 is being used tocoat the surface of this high temperature,inner shroud component. (See Figure 20.) Thiscoating was developed and has been usedextensively by GE Aircraft Engines on its flightengine shrouds. It provides an extremely oxi-dation-resistant surface and a rub-tolerantcoating in the event that the bucket blade tipsrub against the stationary shroud.

Future CoatingsCoating development work is continuing atGE, aiming at further improvements to theoxidation- resistance and thermal fatigue resist-ance of high-temperature bucket coatings. Inaddition to these environmentally resistantcoating development efforts, work is alsounderway to develop advanced thermal barriercoatings (TBCs) for application to stationaryand rotating gas path components. By carefulprocess control, the structure of these TBCsmay be made more resistant to thermal fatigueand their lives greatly extended. Rainbow rotortesting of some of these coatings is currently inprogress.

Combustion HardwareThe combustion system is a multiple-chamberassembly composed of three basic parts: the

Figure 20. 7FA PLASMAGUARD™ GT-20 coatedshroud

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GE Energy ■ GER-3569G (08/04) 16

fuel injection system, the cylindrical combus-tion liner and the transition piece. Driven bythe ever-increasing firing temperatures of thegas turbines and the need for improved emis-sions control, significant development effortsare being made to advance the combustionhardware of heavy-duty gas turbines. Whatwere originally simple parts in early gas tur-bines are now highly complex pieces of hard-ware with sophisticated materials and process-ing requirements.

The primary basis for the material changes thathave occurred has been increased high temper-ature creep rupture strength. These materialchanges had to be done while maintaining satis-factory oxidation/corrosion resistance. An indi-cation of the strength improvement is shown inFigure 21, which compares the creep rupturestrength of the three material classes now inuse. Nimonic 263, the most recently introducedalloy, is some 250°F/140°C stronger than theoriginal AISI 309 stainless steel. Hastelloy-X,which was used in the 1960s through the early1980s, is intermediate in strength between thetwo.

Combustion LinersTwo major changes have occurred since theoriginal AISI 309 stainless louver cooled liners:the adoption of Hastelloy X/RA333 in the1960s, and the adoption of the slot-cooledliner in the early 1970s. This slot-cooled designoffers considerably more liner cooling effec-tiveness, and, from a materials standpoint,presents a new area of processing challenges.Fabrication is primarily by a combination ofbrazing and welding. Earlier liners, on theother hand, were made using a welded con-struction with mechanically formed louvers.

As firing temperatures increased in the newergas turbine models, HS-188 has recently beenemployed in the latter section of some com-bustion liners for improved creep rupturestrength.

In addition to the base material changes, theuse of a thermal barrier coating (TBC) oncombustion liners of advanced and upratedmachines has been incorporated. TBCs consistof two different materials applied to the hotside of the component: a bond coat applied tothe surface of the part, and an insulating oxide

CHARACTERISTICS• 15—25 Mil (380—640 Micron) Thickness• Insulating — Porous• Plasma Sprayed in Air• Two Layers

— Bond Coat — NiCrAlY— Top Coat — YTTRIA Stabilized Zirconia

ADVANTAGE• Reduced Metal Temperature of Cooled Components• 8—16°F (4—9°C) Reduction Per Mil (25.4 Micron)

of Coating

Figure 22. Thermal barrier coatings

450

1009080706050

40

30

20

10

800 900 1000 1100 1200 1300 1400 1500

500 550 600 650 700 750 800

N263

Hast X

309SS

7.06.05.04.03.0

2.0

1.0

Temp. °C – 100,000 Hrs. Life

Temp. °F – 100,000 Hrs. Life

Str

ess

KS

I

Kg/

cm2

x 10

-3

Figure 21. Rupture comparison, N-263 vs.Hallestoy-X vs. 309SS

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GE Energy ■ GER-3569G (08/04) 17

applied over the bond coat. The total thicknessused is 0.015 to 0.025 inch (0.4 to 0.6 mm).Characteristics and advantages of the TBCs areshown in Figure 22, and the microstructure anda coated liner are shown in Figure 23. The pri-mary benefit of the TBCs is to provide an insu-lating layer that reduces the underlying basematerial temperature and mitigates the effectsof hot streaking or uneven gas temperaturedistributions. These coatings are now standardon a number of GE gas turbines and havedemonstrated excellent performance in pro-duction machines.

Transition PiecesAlthough less complicated than the liners, thetransition pieces have probably been morechallenging from a materials/processes stand-point. Therefore, new materials have tended tobe first introduced on the transition piece.

From a design standpoint, significant improve-ments have been made on advanced modelsthrough the use of heavier walls, single-pieceaft ends, ribs, floating seal arrangements, selec-tive cooling, etc. These design changes havebeen matched by material improvements.Initial 1950s transition pieces were made fromAISI 309 stainless steel. In the early 1960s,nickel base alloys Hastelloy-X and RA-333 wereused in the more limiting parts. These alloysbecame standard for transition pieces by 1970.

In the early 1980s, a new material, Nimonic263, was introduced into service for transitionpieces on the MS7001 and MS9001 models.This material is a precipitation-strengthened,nickel-base alloy with higher strength capabili-ty than Hastelloy-X. It was extensively tested inthe Gas Turbine Combustion DevelopmentLaboratory and successfully tested in Rainbowcombustion hardware. Nimonic 263 transitionpieces have accumulated more than 25,000hours of successful experience in MS7001machines. The Nimonic 263 material is beingphased into the higher firing temperature gasturbine models and will be used in futureuprated machines.

Since the early 1980s, TBCs have been appliedto the transition pieces of the higher firingtemperature gas turbine models and to uprat-ed machines. Field experience over thousandsof hours of service has demonstrated gooddurability for this coating on transition pieces.

A recent improvement has also been made toincrease the wear resistance of some transitionpieces in the aft end or picture frame area.Cobalt-base hard coatings applied by thermalspray have been tested in field machines andthe best spray has been shown to improve thewear life of sealing components by more thanfour times. The selected coating, calledExtendor™, is available for many of the cur-rent gas turbine models to extend the wear lifeof these components. This improvement intransition piece seal wear is now also beingincorporated into many of the new productionmachines.

Turbine and Compressor WheelsThe rotor design of all GE heavy-duty alloy gasturbines is a bolted construction made up offorged compressor and turbine wheels, distancepieces (junction between compressor and tur-

Top Coat

Bond Coat

Liner Coating Microstructure

Figure 23 Thermal barrier coated liner,Hallestoy-X vs. 309SS

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GE Energy ■ GER-3569G (08/04) 18

bine), spacers (between some of the turbinewheels) and stub shafts. The most critical com-ponents in the rotor are the turbine wheelsbecause of the combined conditions of elevatedtemperatures and the requirements for strengthand toughness. Further, unlike the aircraft gasturbine, these wheels are of very large diameterand section thickness. For this reason, extensiveuse of steel wheels has been made in heavy-dutygas turbine designs. This has been made possi-ble by the lower compressor pressure ratios(i.e., lower compressor discharge temperatures)and by using long shank buckets, permittinglower temperature operation of the dovetailedperiphery of the wheels. With increasing firingtemperatures, the incorporation of air coolingof wheels has also extended the application ofsteel wheels.

With the advent of the advanced 7FA, 9FA and6FA type machines with much higher firingtemperatures and compressor ratios, it hasbeen necessary to utilize a nickel-base wheelmaterial, Alloy 706, for the turbine wheels andspacers of this machine. The use of this materi-al provides the added temperature capabilityrequired to meet the current 7FA and 9FA fir-ing temperature requirements now and in thefuture.

A full range of testing evaluation is requiredduring wheel material development, as is thecase with bucket and nozzle partition materi-als. For instance, tensile and creep/ruptureproperties, metallurgical stability, inspectability,fracture mechanics characteristics and pro-ducibility on a commercial scale are amongthe aspects that must be considered and evalu-ated. A complete test evaluation of forgings isrequired for process qualification of eachmaterial and supplier encompassing the sec-tion size involved.

Turbine Wheel ProcessesAll of the turbine wheels currently manufac-tured for GE designs are produced either fromvacuum arc remelted (VAR) or electroslag re-melted (ESR) material, or from ladle refined,vacuum-degassed steel. In the VAR process, anelectrode is arc-melted under vacuum into awater-cooled copper crucible. ESR is somewhatsimilar, with remelting done under a speciallyformulated slag. Both result in a very low levelof inclusions and chemical segregation andquite uniform structure because of the shallow,molten pool present throughout the formationof the ingot.

Control of microstructure and properties inthe bore region of low sulfur (less than0.005%), vacuum degassed steel, is achieved tothe same level as required for VAR/ESR steel.This is done by controlling the amount andmorphology of sulfide inclusions. In the caseof the nickel-base alloy, Alloy 706, control ofmicrostructure and properties in this materialstarts by using triple-melted ingotsVIM/VAR/ESR to achieve a very high qualityingot that is homogeneous and free of harmfulphases.

Following melting, wheels are either open-dieor closed-die forged, depending on the capa-bility of the forging supplier. Alloy steel wheelsare quenched and tempered to provide thecorrect properties, while austenitic nickel baseand iron base wheels are strengthened by anaging heat treatment somewhat analogous tothe heat treatment given to buckets.

Following heat treatment, all wheels are fullyultrasonically inspected to stringent standards.Mechanical testing of rings removed from theturbine wheels, including room temperatureand hot tensile tests, impact tests, fatigue testsand rupture tests, where required, are per-

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formed to verify that all design requirementsare met.

Each turbine wheel is spin-tested prior to itsinstallation into a rotor in a facility such as theone shown in Figure 24. Alloy steel wheels,because of the brittle-to-ductile transition tem-perature phenomenon, are first spun cold toverify the absence of critical size defects. Allwheels, including the cold spun steel wheels,are hot spun at bore stresses slightly abovetheir yield strength to induce residual com-pressive stresses in the bore region. These spin-ning operations, in addition to the stringent,non-destructive testing performed both beforeand after spin testing, provide maximum assur-ance against brittle fracture in service.

TURBINE WHEEL ALLOYS

Alloy 706 Nickel-Base AlloyThis nickel-based, precipitation-hardened alloyis the newest to be used in turbine wheel appli-cation. It is the 7FA, 9FA, 6FA and 9EC turbinewheel and spacer alloy, and it offers a very sig-nificant increase in stress rupture and tensileyield strength compared to the other wheelalloys. (See Figures 25 and 26.) This alloy is simi-lar to Alloy 718, an alloy that has been used forwheels in aircraft turbines for more than 20years. Alloy 706 contains somewhat lower con-

centrations of alloying elements than Alloy718, and is there-fore possible to produce inthe very large ingot sizes needed for the large7FA and 9FA wheel and spacer forgings. (SeeFigure 27.)

Cr-Mo-V AlloyTurbine wheels and spacers of most GE singleshaft heavy-duty gas turbines are made of 1% Cr - 1.25% Mo - 0.25% V steel. This alloy isused in the quenched and tempered conditionto enhance bore toughness. Stress rupturestrength of the dovetail region (periphery) iscontrolled by providing extra stock at theperiphery to produce a slower cooling rateduring quenching. The stress rupture proper-

140 10.0

8.0

6.0

4.00 100

200 400 600 800 1000

200 300

IN-706

A-286

M-152CrMoV

Temp.°C

Temp.°F

0.2%Yield

Strength

Stre

ss K

SI

400 500

120

80

60

100

Stre

ss k

g/cm

2 x

10-3

Figure 26. Tensile yield strength comparison(turbine wheel alloys)

Cr-Mo-V

450

4.0

2.0

200

100

60

20

0 0

800 900 1000 1100 1200

500 550 600

M-152

A-286IN-706

14.0

6.0

°C

°F

Temp.100,000 Hr.

Life

Stress

KS

I

Kg/

cm2

x 10

-3

Figure 25. Stress rupture comparison (turbinewheel alloys)

Figure 24. Spin test facility (Greenville plant)

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GE Energy ■ GER-3569G (08/04) 20

ties of this alloy are shown in Figure 25.

12 Cr AlloysThis family of alloys has a combination ofproperties that makes it especially valuable forturbine wheels. These properties include goodductility at high strength levels, uniform prop-erties throughout thick sections and favorablestrength at temperatures up to about900°F/482°C.

M-152 alloy is a 2% to 3% nickel-containingmember of the 12 Cr family of alloys. Initially,it was and still is used on the MS5002 machineas a replacement for A286. It features out-standing fracture toughness, in addition to theproperties common to other 12 Cr alloys. M-152 alloy is intermediate in rupture strength,between Cr-Mo-V and A286 alloy, and has high-er tensile strength than either one. (See Figure25.) These features, together with its favorablecoefficient of expansion and good fracturetoughness, make the alloy attractive for use ingas turbine applications.

A286 AlloyA286 is an austenitic iron base alloy that hasbeen used for years in aircraft engine applica-

tions. Its use for industrial gas turbines startedabout 1965, when technological advancesmade the production of sound ingots suffi-cient in size to produce these wheels possible.Since that time, some 1,400 MS3002 produc-tion wheels have been placed in service.

As knowledge of the capabilities of M-152increased, production of the MS5002 wheelswas switched from A286 to M-152. A286 is cur-rently being introduced into the new 9EC tur-bines as part of a composite aft shaft.

Other Rotor ComponentsAll of the other rotor parts are individuallyforged. This includes compressor wheels, spac-ers, distance pieces and stub shafts. All aremade from quenched and tempered low-alloysteels (Cr–Mo–V or Ni–Cr–Mo–V) with thematerial and heat treatment optimized for thespecific part. The intent is to achieve the bestbalance of strength, toughness/ductility, pro-cessing and non-destructive evaluation capabil-ity, particularly when it is recognized that someof these parts may be exposed to operatingtemperatures as low as -60°F/-51°C.

All parts are sonic and magnetic particle test-ed. Many last-stage compressor wheels arespun in a manner analogous to turbine wheelsas a means of proof testing and imparting boreresidual stresses. This last-stage compressorsteel is probably the next most critical rotorcomponent after the turbine wheels.

Rotor DevelopmentsThe most recent major rotor developmenteffort that has been underway at GE is thedevelopment of an Alloy 718 turbine rotor forthe next generation of gas turbine machines.This effort required close cooperation betweenGE, and its superalloy melters and large forg-ing suppliers to conduct the solidification and

Figure 27. 7FA IN-706 turbine forging

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GE Energy ■ GER-3569G (08/04) 21

forging flow studies, the necessary sub-scalewheel forging experiments and the extensivemechanical and physical property determina-tions necessary to bring into production a newwheel material.

This development effort has resulted in theproduction of the largest ingots ever made andforged into high quality qualification turbinewheel and spacer forgings. Concurrent withthe process development effort was an effort todevelop new non-destructive techniques toinspect these turbine forgings to greater levelsof sensitivity than ever before possible. Thesenew ultrasonic inspection techniques are beingapplied to all the Alloy 706 and the Alloy 718turbine forgings to ensure an even greaterlevel of confidence in these high strength forg-ings.

Additional development efforts continue toimprove the current processing of other forg-ings by working with our suppliers on the fur-ther optimization of properties and forgingquality. In-process, non-destructive evaluationof all rotor components continues to beemphasized as a critical aspect to producequality forgings.

Compressor BladesCompressor blading is variously made by forg-ing, extrusion or machining. All productionblades, until recently, have been made fromType 403 or 403 Cb (both 12 Cr) stainlesssteels. During the 1980s, a new compressorblade material, GTD-450, a precipitation hard-ened, martensitic stainless steel, was intro-duced into production for advanced anduprated machines, as shown in Table 1. Thismaterial provides increased tensile strengthwithout sacrificing stress corrosion resistance.Substantial increases in the high-cycle fatigueand corrosion fatigue strength are alsoachieved with this material, compared to Type

403. Superior corrosion resistance is alsoachieved due to its higher concentration ofchromium and molybdenum. Compressor cor-rosion results from moisture containing saltsand acids collecting on the blading. Duringoperation, moisture can be present because ofrain, use of evaporative coolers or condensa-tion resulting from humid air being accelerat-ed at the compressor inlet. Moisture may bepresent in the compressor during operationup to between stage 5 and stage 8, where itusually becomes warm enough to prevent con-densation. When the turbine is not in opera-tion, the compressor can still become wet ifmetal temperatures are below the local dewpoint. (This can happen to units stored inhumid environments.) The chemistry of thismoisture deposit on the blading determinesthe severity of the corrosion phenomenon.

In the early 1960s, GE first experienced corro-sion pitting on bare 403 in oil platform appli-cations when several machines developed pitsand failed compressor blades. Generally, theservice time on these machines ranged from20,000 to 60,000 hours. As a result of this expe-rience, GE adopted NiCd coating for use inselected applications, and later for all compres-sor blades in the "wet" stages (normally up tostage 8). However, because of recent, morestringent EPA requirements, this coating hasnow been replaced by a new GE developedand patented coating called GECC-1. This newaluminum slurry coating has a protectiveceramic top layer that provides improved ero-sion resistance. (See Figure 28.) This coating hasaccumulated more than 100,000 hours of fieldtesting and has shown to be equal to or betterthan conventional aluminum slurry coatings incorrosion protection and substantially better inerosion resistance. This coating has beenapplied by GE Service Shops as a refurbish-ment coating for several years and is now

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Advanced Gas Turbine Materials and Coatings

being applied to all new units. All IGVs andthe first three stages of rotating and stationaryairfoils in the compressor will be made fromGTD-450; the next five stages will be made ofGTD-450 for the F class machines and GECC-1coated AISI 403 or 403 Cb for the othermachines. The rest of the blading will be AISI403 or 403 Cb uncoated. This change will pro-vide GE’s machines with better corrosion anderosion protection and eliminate cadmiumfrom the environment.

GTD-450 is a precipitation-hardened, marten-sitic stainless steel with excellent aqueous cor-rosion resistance. Laboratory tests have shownthat GTD-450, in very acidic salt environments(pH~4), possesses excellent resistance to pit-

ting. These test results, shown in Figure 29,indicate that uncoated GTD-450 without acoating is equivalent or better than Al or NiCdcoatings for acidic corrosion resistance. Fieldexperience of more than 48,000 hours hasconfirmed the excellent corrosion resistance ofuncoated GTD-450. These tests have alsoshown that conventional aluminum slurry coat-ings can suffer erosion damage and leave sig-nificant areas of the blading unprotected.Therefore, in machines where erosion may beexperienced, GECC-1 on 12 chromium blades,or uncoated GTD-450, is recommended. TheGTD-450 material should not be used coated,as coating will decrease fatigue life.

CasingsFor all models except the F-technologymachines, the entire "tube" surrounding thegas turbine rotor is composed of a series ofcast iron castings bolted together end-to-end.The castings (inlet and compressor) at the for-ward end of the machines are made of grayiron, while those at the aft end (discharge andturbine shell) are generally made of ductileiron or, in some, steel castings or fabrications.The excellent castability and machinabilityoffered by cast iron makes it the obviouschoice for these somewhat complex parts that

GE Energy ■ GER-3569G (08/04) 22

GTD-450 Bare

Al Slurry Coatings

NiCd + Topcoats

NiCd

Bare

0 2 4 6 8 10Worst Best

Figure 29. Acidic laboratory tests

Figure 28. GEEC-1 compressor blade coating

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have close tolerances. Cast iron is less prone tohot tears and shrinkage problems than caststeel. Experience has also shown it to provide ahigher degree of dimensional stability duringshop processing.

Although stress is important in determiningwhich of the two types of cast iron (gray orductile) is used in the castings, operating tem-perature is of prime importance. Gray iron isgenerally limited to applications where temper-atures do not exceed 450°F/239°C, ductileiron to applications no greater than650°F/343°C. In the case of gray iron, GE usesa type that has a minimum tensile strength of30 ksi (2.1 kg/cm2 x 10-3), similar to ASTM-A48, Class 30. Ductile iron, on the other hand,is a ferritic type [60 ksi (4.2 kg/cm2 x 10–3)TS, 40 ksi (2.8 kg/cm2 x 10–3) YS, 18% E1],similar to ASTM-A395. The 7FA and 9Fmachines utilize ductile iron for the inlet andcompressor casing and a fabricated CrMo steelcombustion wrapper and turbine shell. Morerecently, cast 2 1/4 Cr - 1Mo steel is being intro-duced into the F-technology machines for thecombustion wrapper and turbine shells.

Future MaterialsAdvances in ductile iron have been made inlaboratory trial castings that will enable thismaterial to be extended to higher temperatureapplications. These trial heats have shown thecapability to extend the useful temperature ofthis material by 100°F/56°C. This developmentprogram is now in the Rainbow field trialphase and will most likely find application inadvanced and uprated GE gas turbines.

Additional Sand CastingsIn addition to the casings, several other largecomponents, such as bearing housings, innerbarrels, support rings and diaphragms in thestator section of the turbine, are produced

from sand castings. Cast iron is again usedwhere possible; however, where higher temper-ature or planned welding is encountered, steelis employed. For example, Cr-Mo-V has beenused for support rings where temperaturesreach 1000°F/538°C, and carbon steel hasbeen used for bearing housings requiring weldfabrication.

Quality is a key factor in the successful opera-tion of any part, and sand castings are noexception. From the conceptual stage, qualityis built into these parts. Foundry personnel arecalled in early in the design stage to providethe best possible castability consistent withfunctional requirements. Before any casting isgranted production approval, a process mustbe found that produces three consecutive cast-ings meeting rigid X-ray inspection require-ments. Once such a process is found, it is pre-cisely documented and must be followed forall subsequent production. Recently, a sonicscreening procedure was developed to supple-ment X-ray inspection. It was designed toreduce inspection time and increase coveragewhile maintaining strict standards of castingintegrity.

In addition to the X-ray/sonic monitoring ofcasting visual examinations, magnetic particleinspection and, in the case of bearing hous-ings, leak tests, are always employed. All thesecombine to provide a very comprehensivequality check on sand cast components.

Inlet and Exhaust Systems

Inlet SystemsThe inlet system environment is ambient airwith low velocity air flow over interior surfaces.Materials of construction are generally low car-bon steel, including the inter baffles used overacoustic material to reduce the noise level. Inselected marine environments, a corrosion-

Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 23

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resistant steel may be used for these interiorbaffles. Standard protection practice for theinlet system is an inorganic zinc primer paintand/or galvanizing. External finish coats areapplied by the customer.

Exhaust SystemsThe stack construction consists of low carbonor low alloy steel structural members andsheets that are protected from the elevatedtemperature exhaust gases by 409 stainlesssteel. Further up the stack, silencers that con-sist of acoustical material encapsulated in per-forated 409 stain-less steel are used to reducethe noise level to low values.

On stacks, standard protection is an inorganiczinc primer, a paint with an excellent combina-tion of corrosion resistance and temperaturecapability. For better weathering resistanceand high temperature performance, thisprimer is being topcoated with an aluminumsilicone paint on wing, cowl, plenums andplenum expansion joint surfaces in the factory.All other exhaust system surfaces are primedwith an inorganic zinc primer and topcoatedin the field with the same high temperaturealuminum silicone paint for maximum corro-

sion protection.

The introduction of the inorganic zinc primermentioned above for inlet and exhaust systemsprotection was the result of a high-temperaturepaint test program. Various types of paint sys-tems were tested at temperatures between400°F/204°C and 1000°F/538°C for twomonth-long periods. These tests differed fromprevious high-temperature paint tests in thathumidity exposures were inserted betweenthermal cycling exposures. Humidity expo-sures were introduced to provide a betterassessment of the effects of weathering andhumidity combined with cyclic heating.

The tests yielded useful information. Best over-all results were obtained with a system consist-ing of inorganic zinc primer top coated withthe standard aluminum silicone paint. This sys-tem satisfactorily survived all exposures includ-ing 1000°F/538°C tests. All systems employinginorganic zinc primer were sacrificially protec-tive in salt spray exposures.

While the excellent weathering characteristicsof the inorganic zinc primers are well-estab-lished, these tests additionally confirmed theirhigh temperature cycling capability.

Advanced Gas Turbine Materials and Coatings

GE Energy ■ GER-3569G (08/04) 24

SummaryThe purpose of this paper has been to describe some of the materials currently being used in GEgas turbines and to verify our commitment to continued GE leadership in material and processdevelopment. The activities described in this paper are by no means complete. Major materialsdevelopment work is underway at GE to provide a continuous stream of new and improved materi-als for gas turbine application to meet our customers’ needs for the most efficient gas turbines.GE’s intent is to provide the materials necessary for the advancement of turbine firing temperatureswhile maintaining the high levels of unit reliability, availability and maintainability.

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GE Energy ■ GER-3569G (08/04) 25

List of FiguresFigure 1. Investment cast buckets

Figure 2. Investment cast nozzles

Figure 3. Directionally solidified buckets

Figure 4. Firing temperature trend and bucket material capability

Figure 5. Advanced air cooling technology

Figure 6. Stress rupture comparison — bucket and nozzle materials

Figure 7. Directionally solidified GT-111 vs. equiaxed

Figure 8. Bucket alloys — LCF life

Figure 9. Continuing improvements in bucket materials capability

Figure 10. Effect of sodium on bucket corrosion life

Figure 11. Bucket coating requirements and coating evolution

Figure 12. Hot corrosion (high-temperature type)

Figure 13. Hot corrosion (low-temperature type)

Figure 14a. Photomicrograph showing sound microstructure of a coated bucket that has been inservice

Figure 14b. Photomicrograph of a coating on a bucket material showing internal oxidation of coat-ing (dark particles)

Figure 15. Comparative resistance in types of coatings

Figure 16. Stage 1 turbine buckets: coated and uncoated IN-738; 25,000 service hours

Figure 17. PLASMAGUARD™ GT-20 coated shroud

Figure 18. VPS production facility

Figure 19. VPS coating after more than 40,000 hours turbine exposure — pressure face

Figure 20. 7FA PLASMAGUARD™ GT-20 coated shroud

Figure 21. Rupture comparison, N-263 vs. Hallestoy-X vs. 309SS

Figure 22. Thermal barrier coatings

Figure 23. Thermal barrier coated liner, Hallestoy-X vs. 309SS

Figure 24. Spin test facility (Greenville plant)

Figure 25. Stress rupture comparison (turbine wheel alloys)

Figure 26. Tensile yield strength comparison (turbine wheel alloys)

Figure 27. 7FA IN-706 turbine forging

Figure 28. GECC-1 compressor blade coating

Figure 29. Acidic laboratory tests

List of TablesTable 1 High-temperature alloys

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