ABSTRACTA corrosion- and creep-resistant austenitic stainless steel

has been developed for advanced recuperator applications. Thisfully austenitic alloy is optimized for creep strength whileallowing the formation of a chemically-stable external aluminascale at temperatures up to 900°C. An alumina scale eliminateslong-term problems with the formation of volatile Cr oxy-hydroxides in the presence of water vapor in exhaust gas. Thefirst batch of commercially fabricated foil was produced with acomposition selected from prior laboratory creep and oxidationresults. The results for ~80 and ~105µm thick foil are comparedto the prior laboratory-fabricated foils and other commercialcandidates. Results from initial creep testing at 750°C showcomparable creep strength to other commercial Fe-base foilcandidates. Laboratory exposures in humid air at 650°-800°Chave shown excellent oxidation resistance for this composition.Similar oxidation resistance was observed for sheet specimensof the first set of alloys exposed in a modified 65k Wmicroturbine for up to 6,000h.

INTRODUCTIONA current focus of the U.S. Dept. of Energy’s Combined

Heat and Power (CHP) program is to increase the efficiency andperformance of gas turbines while decreasing the cost per kW.Turbines are especially well-suited for CHP systems wheresystem efficiency can exceed 80% by using the waste heat fromthe turbine for process heat, steam generation and/or heatingand cooling [1-4]. For small gas turbines (

as a major factor in degrading the oxidation resistance of allchromia-forming steels. The surface oxide can react to form avolatile oxy-hydroxide[15,16]:

1/2 Cr2O3(s) + 3/4 O2(g) + H2O(g) = CrO2(OH)2(g) (1)

Consumption of the limited Cr reservoir present in an 80-100µm thick foil is greatly accelerated by the combination ofoxidation (parabolic kinetics) and volatilization (linearkinetics). At the long exposure times required in service(minimum 30-40kh recuperator lifetime), the Cr loss rate isdominated by the linear volatilization kinetics[17-20]. Moredetails about the parameters, such as gas velocity, that effect thekinetics can be found elsewhere[16,19].

For recuperators operating at >600°C, replacement alloysin use include Nb-modified Fe-20Cr-25Ni (composition similarto alloy 709), alloy 120 and, for larger turbines, Ni-base alloy625 [21-22]. All of these alloys have higher Cr and Ni contentsthan Type 347 stainless steel, Table I, which results in betteroxidation resistance. However, all of these alloys rely on theformation of a Cr-rich oxide, and thus are degraded by the oxy-hydroxide volatilization described above. Significant Cr losseshave been observed in foils exposed in laboratory testing for 10-20kh [18,23]. From these results it appears clear that, for thenext generation of advanced recuperators operating at 700°C orhigher requires more oxidation resistant alloys.

It is well known that alloys that form protective Al-richoxides are more resistant to water vapor because of the higherstability of α-Al2O3[16]. Alumina-forming alloys and coatingsare standard in the hot section of gas turbines[24]. An earlyrecuperator material study evaluated the creep and oxidationresistance of commercial alumina-forming alloys[25]. Thoseresults showed that no wrought Fe-base alumina-forming alloyhad sufficient strength for the application. Aluminum additionsto austenitic steels tend to stabilize the weaker ferritic phaseresulting in very high creep rates[26,27]. To address thismaterial capability gap, a new class of Fe-base creep-resistantalumina-forming austenitic (AFA) alloys has been developedbased on careful control of the composition on: (1) a macro-level to maintain a fully-austenitic alloy and (2) a micro-level tonucleate stable precipitates that provide creep strength withoutinhibiting the formation of a protective alumina scale[27-30].The AFA alloy properties are particularly well-suited for thin-walled recuperators for both microturbines and small gasturbines. As progress continues on the development of this classof alloy, current results are presented on the creep and oxidationbehavior of commercially-made 80 and 105µm (3.2 and 4.2mil)AFA foils. The composition was selected based on priorlaboratory creep and oxidation results as well as field exposuresin a modified 65kW microturbine.

EXPERIMENTAL PROCEDUREThree ~13kg vacuum induction melted (VIM) heats of AFA(F1-F4) were cast by Carpenter Technology Corp. as tapered 10cm x 10 cm x 22 cm long ingots. A ~180kg VIM heat of F4 wassubsequently cast for commercial processing to foil. Alloycompositions are shown in Table 1. Further details of thelaboratory-scale processing were provided previously [23].Commercial pieces of ~18cm wide foil were fabricated withthicknesses of ~380, 105, 80µm (15, 4.2, 3.2mil) wereproduced. An example cross-section is shown in Figure 1.Typical grain sizes for foils are given in Table 1. The averagegrain size of the specimens was calculated using the mean linealintercept length, counting between 150-600 grains.

Creep specimens were machined by electro-discharg emachining. The creep specimens were 114mm (4.5 in.) longwith a gage length of 25.4mm (1 in.) and 6.35 mm (0.25in)wide. The shoulders were approximately 16mm wide. Pads ofthe same thickness as the foil were spot-welded on the shouldersfor re-enforcement and 3.17mm (0.125in.) diameter pin holeswere cut in the shoulders for gripping. The extensometer was arod-in-tube type that transmitted extension out of the hot zoneof the furnace to averaging LVDT sensors. Because of the smallsection areas, specimens were dead loaded. Creep testing wasperformed in laboratory air at 677°C with 117MPa load and at750°C with 100 MPa load to induce rupture in a reasonablyshort time.

Foil oxidation coupons (~12 mm x 18 mm x ~100 µm) weretested in the as-annealed surface condition and (~25 x 25 x0.9mm) sheet specimens were polished to a 600 SiC grit finishfor exposure in a 65 kW microturbine at Capstone TurbineCorp.. This turbine has been modified to produce exhausttemperatures of ~720°C and has been detailed elsewhere [22].For laboratory exposures, all of the specimens were cleaned inacetone and methanol prior to oxidation. Exposures were 100 hcycles at 650°, 700° or 800°C and mass changes were measuredafter every cycle using a Mettler-Toledo model XP205 balance,with an accuracy of ±0.01 mg/cm2. The amount of waterinjected was used to calibrate the water content at 10±1 vol.%for these experiments. Up to 40 specimens were positioned inalumina boats in the furnace hot zone so as to expose the

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Figure 1. Light microscopy of polished cross-sections ofcommercial AFA F4

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specimen faces parallel to the flowing gas. As a furtherevaluation of the oxidation resistance, sheet coupons of the F1-F4 compositions were placed in the hot gas path of a modified65kW microturbine at ~720°C [22]. After exposure, specimenswere Cu-plated and sectioned for metallographic analysis.

RESULTSSelection of F4 Composition

Figure 2 shows the polished cross-sections of sheetspecimens of the three initial AFA alloys exposed in the 65kWmicroturbine at ~720°C for 6,004h. The F1 composition withonly 20%Ni showed relatively poor oxidation resistancecompared to F2 and F4.

Figure 3 shows the tensile creep behavior of the threecompositions. The shorter lifetime for AFA F2 can be attributedto the lower C content (0.05%) in this alloy, Table 1. The Nb:Cratio was too low, likely resulting in a low fraction of the NbCstrengthening precipitates[31]. A slightly higher creep rate wasobserved for AFA F1 compared to F4. However, one problemwith this comparison is that the grain sizes of the three foilswere not the same, Table 1. Nevertheless, the F4 compositionappeared to have the best combination of creep and oxidationresistance and was selected for a larger heat.

Laboratory Oxidation TestingFigures 4-6 compare the performance of specimens of the

AFA foils at 650°, 700° and 800°C in humid air to specimens ofone of the advanced recuperator materials, commercial alloy120 foil. One difference was that the commercial alloy 120 foilhad a bright annealed (oxide-free) finish. In contrast, the

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c

b 10µm

a 10µm

Figure 2. Light microscopy of polished cross-sections ofsheet specimens after 6,004h at 720°C in microturbine

exhaust. (a) AFA F1, (b) AFA F2 and (c) AFA F4.

Cu plating

AFA-F1

AFA-F2

AFA-F4

Cu plating

surface oxide

FeOx

internal oxidation

Figure 3. Tensile creep behavior of laboratory-made AFAfoils with strain % plotted versus exposure time at 677°C

(1250°F) in air with a load of 117MPa (17ksi).

Figure 4. Specimen mass change for commercial 347 and120 foil (80-100µm thick) compared to AFA foils during

100h cycles in humid air at 650°C.

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laboratory-made and commercial AFA foils had a residual oxidelayer (tarnish) from the final anneal that likely resulted in lowerinitial mass gains. At 650°C, the specimen of commercial type347 foil exhibited a high mass gain after ~1kh of exposure dueto the formation of thick, Fe-rich oxide nodules on the surface,

Figure 4. The higher-alloyed 120 foil specimens have shownprotective behavior for >30kh at this temperature. However, thealloy 120 foil specimens began to show a linear mass loss after~4kh of exposure, Figure 4. The measured specimen masschange reported in Figures 4-6 is the summation of oxide scalegrowth, oxy-hydroxide volatilization and spallation of the scale:

ΔMspecimen = ΔMoxide growth-ΔMvolatile-ΔMspall (2)

There was no evidence of scale spallation from these specimens,so that oxide growth and oxy-hydroxide volatilization weresummed in the mass gain data. The net mass loss thus can beattributed to the loss of Cr due to volatilization of CrO2(OH)2.being greater than the mass gain due to oxide growth.

The exposures times for the commercial AFA foil couponsare relatively short at each temperature compared to the alloy120 and laboratory-made AFA foil specimens. However, theyshow a similar trend as the laboratory-made F4 foil at eachtemperature. One commercial F4 coupon initially showed ahigher mass gain at 650°C. This may be due to an imperfectionor burr on the foil specimen and longer exposures will indicateif there is cause for concern about the oxidation resistance. Thelaboratory-made F4 foil has reached 5 kh and showed noindication of mass loss. In general, the mass changes are so lowfor the AFA specimens that the data reflect the sensitivity of thebalance, ±0.01mg/cm2.

At 700°C in humid air, the volatilization and oxidationprocesses are faster and the AFA foil specimens have clearlydifferentiated from the alloy 120 specimens, which showed asteady mass loss after ~1kh, Figure 5. The laboratory made F4foil specimen has passed 9kh without showing any mass loss.Figure 6 shows the mass change at 800°C. Foil specimens of F1and F2 showed earlier failure at this temperature while the F4specimen has been exposed for almost 10kh, without indicationof rapid oxidation. In contrast, the alloy 120 foil specimens(dashed lines in Figure 6) experienced first a mass gain due tooxide growth, then a mass loss due to volatilization and finallya more rapid mass gain as thick Fe-rich oxide nodules formedbetween 6-8kh. Several of the alloy 120 specimens stoppedbefore 10 kh. Prior work reported almost 50% of the Crreservoir had been consumed from the metal after 10kh at800°C in wet air [23]. For the commercial AFA F4 foilspecimens, one specimen was stopped after 1,000h at eachtemperature for characterization, while additional specimens arebeing exposed for longer times.

Laboratory Creep TestingFigure 7 shows creep results for the commercial AFA foil

materials in laboratory air compared to the laboratory-madeAFA foils and several commercial foils. The 750°C/100MPaconditions have been used in prior studies to rapidly comparecreep behavior of candidate alloy foils. The finer grain size(27µm) in the commercial foil can likely explain the slightlyhigher creep rate observed for the commercial foils compared to

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Figure 6. Specimen mass change for commercial alloy 120foil (80µm thick) compared to AFA foils during 100h cycles

in humid air at 800°C.

Figure 5. Specimen mass change for commercial alloy 120foil (80µm thick) compared to AFA foils during 100h cycles

in humid air at 700°C.

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the laboratory F4 foil. Two curves for alloy 120 are shown,indicating a range of creep behavior for this material. Exactprocessing information was not available for these differentheats so it is not possible to address in this study the range increep behavior observed. The AFA material is much strongerthan type 347 foil and is initially comparable to the commercial20-25Nb foil (based on alloy 709). However, the 20-25Nb foilhas shown a longer rupture life and is currently in test, alongwith an alloy 120 foil specimen. Further heat treatment orcomposition modifications may be needed to improve the creepstrength of AFA F4, if that is needed. Additional testing at677°C (see Figure 3) will be conducted because thattemperature is closer to the actual operating temperature.

Further AFA alloy development is still in progress. The F4composition was selected for commercialization based on itssuperior oxidation resistance and adequate creep strength,relative to other A FA compositions. Further compositionmodifications are being explored, such as a higher C content incomposition F2, that could produce a better combination ofproperties.

CONCLUSIONThe development of creep- and oxidation-resistant

alumina-forming austenitic steels for advanced recuperatorapplications has progressed to the evaluation of the properties ofthe first batch of commercial foil. This AFA composition wasselected based on laboratory and engine exposures of threecandidate compositions. Initial results on the commercial foil

has shown promising oxidation and creep results at 650°-800°C.These evaluations will continue in order to compare the long-term behavior of the AFA foils to other commercial alloys thathave been exposed for up to 30 kh. Larger castings of AFA F4composition have been made, portions of a 4,500kg and a 900kgingot are now being prepared to fabricate the next batches ofcommercial foil in the correct size and specification forrecuperator air cell fabrication, so that foil folding and weldingevaluations can be conducted.

ACKNOWLEDGMENTSThe author would like to thank M. Stephens, J. Moser, T.

Brummett and H. Longmire at ORNL for assistance with theexperimental work, and P. Maziasz for comments on themanuscript. This research was sponsored by the U.S.Department of Energ y, Office of Energy Efficiency andRenewable Energy Commercialization and Deployment andIndustrial Technologies programs.

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Copyright © 2011 by ASME

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