NATIONMADVISORYCOMMI’rrEEFORAERONAUTICS

TECHNICALNOTE2037

RESIST~CE OFSIXCASTHIGH-TE~EWT~ ALLOYS

TO CRAC~G CAUSEDBY THEWL SHOCK

By M.J.Whit_, R. W. Hall,andC. Yaker

LewisFlightPropulsionLaboratoryClevehd, Ohio

,*...,-

TECHLIBRARYKAFB,NM ___

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*

Illlllllllllllluullflll‘-nn~s~b~”“.+-=NATIONALADVISORYCOl!MITTE4FORAERONAU’rLw-- .-

T!3CHNICALNOTE2037

RESE3TANCEOFSJXCASTHIGH—~ ALLOYS

TOORACKINGCAUSEDBYTHXRMALSHOOK

ByM.J.Whilman,R.W.Hall,andC.Yaker

ImMARY

Aninvestigationwaaundertakentoresistanceofsixcasthigh-temperature

determinealloysto

therelativecrackingcaused

bythermalshook.Thethermal-shockemal.uatfonunitutilizdacontrolledwaterquenchofthesymmetricaledgeofa uniformlyheated,modifiedwedge-shapedspecimen.Thespecimenswereheatedata uniformtemperatureof1750°F for1hourandwaterquenchedat45°F. Thiscyclewasrepeateduntilthermal-shockfailmeoccurred, Theorderofdecreasingresistancetothermal-shockcrackingofthealloyswasS-816,S-590,Vitallium,422-19,X-40,andStellite6.

Theheating-and-quenchingcycleproducedelongationofthequenchededge.Measurementsofthesedeformationsweremadeduringthecyclictests.Thetotalelongationofthequenchededgeatfailurewasfoundtoincreasewiththeresistanceofthematerialtothermalshock.b thisinvestIgation,materialshavingshLLarthermalproperties,suchescoefficientoflineerexpeasion,con-ductivity,andspecificheat,wereshowntohavewidelydifferingresistsncestothermalshock.Metallurgicalexaminationofthealloystructureandstudyofthenatureofcrackpropagationyieldednocorrelationbetweenstructuralcharact~isticsandresistancetocrackingcausedbythermalshock.

Ananalysisofthemannerinwhichthethermal-~hockcrackformedandprogressedintothespecimenandanexaminationofavailabledataonthenotchimpactstrengthofcasthigh-temperatureallogsindicatedthattheremightbea relationbetweennotchimpactstrengthti resistancetocrackingcausedbythermalshock.

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NACATN2037

INTRODUCTION

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TheoperatingconditionsofaircraftgasturbipessubJectcertaincomponentstolargeandsuddentemperaturegradients,whichresultinthermalstressesthatare,insomecases,eithera primeorcontributingcauseofcomponentfailure.Foremmple,ges-turbinebladesaresubjectedtothermalstresseswhentheengineissts&ted,accelerated,decelerated,orstopped.Rapidcoolingoftheturbine-bladeedgesduringdecelerationandstoppingcausescontractionofthesethinsections.Thiscontractionisresistedbytheadjacenthottermetalandasa resultthecooleredgesaresub~ectedtosuddentensilestresses.Rapidheatingproducescom-pressivestressesinthesameareaO.

ObservationsmedeattheNACAMwislaboratoryindicatethatcertaingas-turbinecomponentsfallintensionasa resultofthermalstressesorfail~tercrackshavebeencausedbythesestresses.Theseobservationsitiicatethatonecriterioninselectinga high-temperatureturbinematerialmaybe itsabilitytowithstandtensilestressesproduced.bysuddenlooalizedmolingfrom.anelevatedtemperature.

Previouslaboratoryevaluationsofalloysforhigh-temperatureuseinjetengineshaveconsistedIndeterminingstress-rupture,creep,fat@ue,andcorrosion-resistantproperties.Thefewattemptsthathavebeenmadetoappraisethethermal-shookresist-anceofheat-resistantalloyshavegenerallybeenofa qualitativenature.Anexigencyexistsfora standardmeansofevaluatingthesusceptibilityofgss-turbinematerialstocrackingcausedbythermalshook.

l?he@mary-”purposeoftheinvestigationreportedhereinwastoevaluatetherelativeresistanceofsixhigh-temperaturealloystothermal-shockcracking,whichhereinafteriscalledthermalcracking.

Thethermal~operties,coefficientaPLLneararpanslon,themalconductivity,andspecificheat,havebeenameptedasfactorsrelemnttotheabtlityofmaterialstoresistthermalshock(references1 and2). Experimentalevaluationofthermal-shookresistanceshouldmakepossiblea comparisonoftherelativeimportanceofthermalandmechanicalpropertiesindetermlnhgthermal-cracklngresistanceofalloys.Impactstrengthaswellastensilepro~rtiesareconsiderdbecausethemannerinwhichthetemperaturegradientisinduoedresultsinratesofloadingcon-siderablygreaterthanthoseencounteredinnormaltensiletests. .

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NACATN2037 3.

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Al

Thesixeastalloysinvestigated,S-816,S-590,Vitallium,422-19,X-40,ad Stellite6,havebeenUS* orconsideredforuseingas-turbineandotherhigh-tmperatureapplications.Thealloyspecimenswereuniformlyheatdto1750°F andthenstressedbyproducinga thermalgradientwitha controlledwaterquenchat45°F.Bothspecimencrackinganddeformationprdzoedbythermalshockwereinvestigated.

Metallurgicalexaminationsofthealloysbeforeandafterthermal-shockcyclingweremadetoiLeterminetheqannerofpropa-gationofthecracksanatheeffectoftheshockcycleonthestructureofthealloys.

APPARATUSm PROCEDURE

Resistancetothermalcrackingwasdeterminedforthefollow-ingcastalloys:s-816,s-590,VitKLIium,422-19,X-40s~Stellite6. Thecompositionsofthesealloys,= aete~inedfr~chemicalanalysisoftestspecimensthathadfailed,aregivenintable1. Allsmcimen8mea inthisinvestigationwerecsstatthislaboratory.Theshapeandtheahnensionsofthespecimens,selectedareshowninfigure1. Thisdesignwsachoseninordertoproviaea concentrateionofthermalstressesduringthecoolingor quenchingphaseofthecycle.Quenchingonlythdsmtricaledgeofthespecimenproducesa largetemperaturegradientbetweenthisedgeandtheunquenchedbaaeofthespeoimen.Thethermalstressesresultingfrcmthetemperaturegradientarelargestinthesmallcross-sectionalareaofthequenchesedge.Thisedgeisthereforemostlikelytocrack.A finiteedgewidthof1/32inohwasselectedinpreferencetoa knifeedgeprincipallytominimizetheeffectoxidationmightcontributetofailureoftheeilge.

Thequenching~p~atus(ftg.2)wassoaesignedthatthenarrowedgeofthespecimenwasquenchedina streamofwater,theflowrateandtemperatureofwhichwerecontrollable.InordertominimizeLestconductionfromthespecimentotheholaer&uringquenching,thespecimenwassupportedinsuch amsnnerthatlinecontactwasestablishedbetweentheholaerandthetwocurvedsisesofthespectien(inset,fig.2). Verticaladjustmentofthespeci-menholaerwasPovidedbyfourmachinescrews,whichalsoservedasrigiasupportsfortheholaer.Misallnementofthespecimenwiththeholderwasnegligible.

4 NACATN2037

Theessentialfeaturesoftheflowsystemthroughwhichthequenchingwaterflowsareaisoshowninfigure2. Tapwaterentersatthebaseofthequenohingtank,passesthrougha coolingcoil,andentersthebaseofthequenchingtrough.A horizontalbd’fleabovethewaterinleteliminatesturbulenceandassuresa uniformoverflowofwateralongtheentirelengthofthetrough.ThequenchhgtroughIsequippedwithadrainateachend;thesedrain-ageltnesareclosedduringthe“quenchingoperation.

Thefbwrateofthequenchingwateriscontrolledbyapressure-regulatingvalveplacedintheinletlineofthequenchingtank.Theflowrateismeasuredbyclosingthevalveinthequenohing-troughdrainandmeasuringtherateofflowofwaterfromthesamplingtube.Theflowrateselectedforthequenchwaathehighestatwhichthelevelofwaterabovetheedgeofthetroughwasuniform.Higherflowratesresultedinanunevenor bubblingflowofwaterfromthetrough.Thelevelofthespecimenholderwassoad~ustedthatthesymmetricaledgeofthespeoimenjustcontaotdthewater.

Temperatureofthequenchingwaterwascontrolledbymaintain-inga constanttemperatureinthebathinwhichthecoolingcoilislocated.Thetemperatureofthebathwaaadjustdbyaddingsolldcarbondioxideasrequired.

Thespecimens,placedinV-shapedgroovesnotohedina softrefractorybrickrestingontheheerthofthefurnace,wereheatedina smallwire-woundresistancefurnace.Specialtongs,whichgrippedonlythebaseedgesofthespecimen,wereusedtotransferthespecimensfromfurnaoe toquenchingtrough.

Selectionofexperimentalconditionstobeusedinthethermal-shockcyoleswasbasedonconsiderationof’thefollowtngobjectives:

1.Simulation,asnearlyas~sslble,ofconditionsthatmightbeencounteredinaircraft-enginecomponents

2.Attainmentofcundltionssufficientlyseveretoinsurefailurewithina reasonabletimepericd

3.Avoidanceofhighfurnaoetemperaturesinotiertopreventexcessiveoxidationandchangesinthemicrostructureofthealloys

Onthebesisoftheseconsiderations,a furnacetemperatureof1750°Fwss chosenfortheheatingcycle.Temperaturesinthisrangemaybeencounteredforshortperiodsoftimeingas-turbine

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NACATN2037 5

bladesduringacceleration.Aninertatmspherewasnotusedinthefurnace,whiohfurtherstmulatedengineoperatingconditions.Intheproceduredeveloped,thespecimenwasheldata furnaoetemperatureof1750°1’for1hourtoinsuretemperatureequilibriumthroughoutthespectien.Attheendofthis perid, the sPec~~wassippedinthespeoialtongsandtransferredtothespeoimenholder.Thequenching-troughdrainvalvewasinunediatelyclosed.,causingthe.tier toriseovertheedgesofthetroughontothenarrowedgeofthespeoimen.Thetemperatureoftheq,uemhtigwaterwascontrolledat450+20F andtheflowrateat680+1Oc~biccenttietersperminute.

Whenthespecimenwascool,ftvesremvedfra theholder,theoxidefilmwascarefullyremvedfromthensrrowedge,andthisedgewasmicroscopicallyexainedforcracks.Beoausecraokpropa-gationwasnotIdenticalforallthealloys,an=bitrarycriterionforfailurewasdefineaaspresenoeofanopeningthatextendedaorosstheenttrewidthofthequenchededge.InsomespecimenscraokeprogressedslowlyaorossthewidthoftheWge,whereasinothersoracksextendedaorosstheentirewidthassoonastheyoriginated.C@ing ofthespectmenswascontinuedafterfailure,asdefined,toinvestigate13efo-tionresultingfromthermalcycling.Thenumberofcompletecraokspresentaftereachquenchcyclewas observed.

Inadditiontocausingcraoking,thermalstressescausedwarp-ingofthetestspecimens(fig.3). Inordertodeterminerelattveresistanceofthevariousalloystosuchdeformation,measurementsofthedistortionweremadeaftereverytwoshockcycles.Dis-tortionwasdeterminedbymeasuringtheheightofthese~ntformedbythebaseofthespeoimenandthelineconnectingitsendpoints. SuchmeasurementsweremadeonanopticalcomparatortoanaOCtWaOy of 0.0001Inch.Percentageelongationwascalculatedfromthemeasureddeformationatthetimethefirstcanpletecrackappeared.

E thermaldtifusivityofalloysis to beoalculatecl,the ‘ ,specificheatmustbeknown.Becausespecific-heatvalues forthealloysinvestigatedwereunavailable,cliffusivityvaltzesat3000FwereexperimentallydeterminedusingthemethodofForbes(refer-ence3). Validityofthemethoiwascheckedusingoxygen-freehi~-conauctivity copper,WE 1020steel,and347stainlesssteel.Valuesofcliffustvitycamputedfrqnthemsnufaoturers’datafcmthesematerialswerehigherthantheex~rimentallydete?mindvaluesby8.0’percentforthecopper,9.4percentforthe1020steel,and6.5percentforthe347stainlesssteel.Theerrorin

6 NACATN2037-

experimentallydeterminedcliffusivityvaluesisnotonlyconsistentbutlessthanthepercentagevariationinreportedvalues fen?coefficientoflinesrexpension.InasmuchasthevaluesofbothdiffusivityandcoefficientoflinesrexpansionareusedInoneequation,whtchrelatesphysicalpropertiestoresistancetothezmalcracking,theaccuracyoftheresultsobtainedfromtheequationwillnotbe impairedbqcauseoftheerrorsindlffusivtty.

Uponcompletionofthethemal-crackingexperiments,ametal-lurgicalamminatlonofthefailedspeck.-wasmade.Thespeci-mensweresectionedandexaminedusingstandardmetallographicprocedures.In@icular, studiesweremadeofgrainsize,locationofcrackswithrespecttograinboundaries,andchangesinmicrostructureresultingfromrepeatedheatingaticooling.

KESULTS

A tabulationofcyclestofailureforallthespecimensobservedispresentedintable11inorderofdecreasingresist-anceofthealloystothermalcracking.Figure4 presentsthedeformationsofthevariousmaterialsthroughsuccessivecyclesandfigure5 ~esentsa comparisonofthethermal-shockdefor-mationcharacteristicsofthesixalloys.Thedeformationpercyclewasfoundtodecreasewithsuccessiveshock.cyclesandtovaryconsiderablyamongthealloys.

Thecraoksformastheresultoftensilestressesappliedata veryhighrateofloadingduringthequenchportionofthecycle.Thecracksoriginateatanedgeofthequenchedsurfaceandpro-@essacrossthesurfacewtthrepeatedcycling.ThenumberofoyclesbeforecrackingstartsIsgreaterendtherateofcrackprogressionissmallerforthematerialshavingsuperiorresistancetothermalcracking.

Metallographicexaminationaftercyclingrevealedthatthespecimensallhadapproximatelythesamegrainsize- vaqyingfroma coarsesizeM 100~r squareinchatthecenterofthespecimento1600persquareinchatthequenchededge.Althoughsomeevidencewasfoundindicatingthatincertaininstancesacraokinitiatedata grainboundary,thecrackswerepredominantlytranscrystallineintheirpropagation(fig.6). U general,thestructuresofthevariousmaterialsweres~lar, consistingofcamplexcarbidesina solid-solutionmatrixwithvaryingemountsof agingprecipitate.

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NACATN2037

DISCtESIOl!?OFRESULTS

Withtheapparatusandtheevaluationproceduredevelopedduringthisinvestigation,theresistancetothermalcrackingofheat-resistantalloysmaybedetermined.The~rocedurefacilitatesobtainingthefollowing:

1.GoodreproducibilityofdataresultingfromeasilycontrolledConditions

2.A largevariationincyclestofailureforclifferentalloyspermittingreadydeterminationofrelativeresistancetothermalcracking

3.Deformationdatathatcanbeusedforccmrelationwiththermalandphysicalproperties

Thecompositecurvesofdeformation(fig.5)gf~eanindi-cationoftherelativeresistanceofthealloystodistortioncausedbythermalstresses.Thebehaviorofthematerialsisconsiderablydifferent:S-590endS-616showthemostdeformationpershockcycle,Vitallimand422-19anintermediateamount,andX-40andStellite6 theleast.Althoughalloy422-19hasa deformationmagnittiesWlar tothatofVitallium,theaverage numbera?cyclestofailurefor422-19isaboutthesameasforX-40.Thecurvesarenonlinesrforallthematerialsandttiicatethatinitialshockcyclescausemoredeformationthanensuingshockcycles.A possibleexplanationoftheconcavityofthecurvesisthattheyield@ntofa metalisraistibythestrainhardeningthatresultsfromayplhdthermalstress.U@n repetitionofthesme thermalstress,lessplastfcflowwouldbeexpectedinthemetalbecauseitthenhesa higheryieldpoint.Thel-hourheattngperi~at1750°Fdurtngeachshockcyclemay,however,annealanappreciableportionofthestrainhardening.At%era fewcycles,X-40andStellite6suddenlyfailedanddidnotappreciablydeformwhensubjectedtoadditionalcycles,~obab~ indicatingthatfurtherapplicationofthethermalstressesresultedh extensionofthefracturewithoutfurtherplasticdeformation.

Actualelongation of thenarrowedgeattimeoffailurewascalculatedfromthedeformationmeasurements.Incalculatingtheelongation,thespecimenwsaassumedtobewarpedasanwc ofacircle.Thisassumption=justifiedbycampsrisonsofthecurva-tureoftiespecimensonanopticalcomparatoragainsta circleofsuitableradius.Calculationsoftotalelongationattheswface

8

ofthenarrowedgeatcaseslessthanwould

NACATN2037

failureshowedthattheelongationwasinallhavebeenexpectedina room-temperature

tensiletest.Thisapparentloweringofductilitymayresultfromthehigherrateofloadingofthethermal-shockcycles.

l’romanexaminationoffigure7,thematerialsthatshowthegreatestelongationatfailurealsoresistthegreatestnumberofcyclesbeforefailure,indicatingthata relationexistsbetweenshockductility(theabilitytodeformundersuddenloads)endresistancetothermalcracking.

Thefactorsthatmaycontributetothethemal-crackingresistanceofamaterialmaybeamad intotwoclasses:thermalandmechanicalyoperties.ha previousinvestigation(referenoe1)oftheeffeotsofa temperaturegradtentresultingfromthesuddencoolingofa uniformlyheat~body,theequation

(1)

wasdevelopedfromtheassumptionthat flt/dx03lfi andfromtheequation E = stress/strain.

ThesymbolsusedInequation(1)and.thesubsequentdiscussionaredefinedasfollows:

c heatcapaoity,(Btu/(lb)(%’))

dtjdx thermalgradient

E YOUng’S-UhlS, (lb/sqh.)

Ed ductilitymodulus,(lb/sqin.)

~2 diffusivity,~, (sgftk)

K constant

k thermalconductivity,(Btu/(hr)(sqft)(%/ft))

n numberofcyclestofailure

r’ coefficientofdetermination

‘t tendencytobresk

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9NACATN2037

t temperature,(%’)

x distancefromsurface,(in.)

a coefficientoflinearexpansion,((in./in.)/%)

P density,(lb/cuft)

% elongationattimeoffailure(breakingstrain),(in./in.)

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.

a stress,(lb/sqin.)

‘b breakingstress,(lb/sqin.)

Fornotiuctilematerialsthatbreakwithoutplasticdeformation,theequation~ = E% wasequation(l)inthefollowingmanner:

appreciablecombinedwith

(2)

Intheinvestigate.onreportedinreference1,a correlationwasfoundtoexistbetweenSt and a/liZbfora numberofceramicbcdies.Thesameequationt?anbeconsideredtoapplyto~ter~alsthatshowsomeplaaticductllttybeforefaihrefromthermalshookiftheconventionof“ductilitymcdulus”(reference4)isused.Thisconventionstatesthattheductilitymodulus~ isequaltoultimatestressdividedbybreakingstrain.Theparametera/hZbeonsistsofa thermal~opertyfaotora/h anda mechanicalfactorl/~. Figure7 isa plotonlog-logcoordinatesof 1Aagainstcyclestofatlureforthesixmat alloysandfigure8 L$a s~1~ plotOf O@b 8$ain8tGycl@tof~l~e. Fromthesetwoftgures,usingstandardstatisticalmethods(reference5),theliqesofregressionwerecomputed~ pldted,andthecoefficientsofdeterminationwerecalmlated.Thecoefficientofdeterminationr2 isameasurecfthevarianceinoneoftwovariablesthatisassociatedwiththerelationbetweenthetwovariables.A valueofr2 of1.00indicatesperfectsasociationofonevariblewithanother.Itwasfoundthatfor n aeinst l~b#

!!r . 0.953dfor n againsta%, r2= 0.963.Thisapproachtounityinvaluesof # indicatesthatinbothcasesa straight-lineplotona log-logscale& n against either1/Zb or a/hZb ti

10

justifiable.Theagreementalsoincorrelationbetweenn and

NACATN2037

indicatesverylittleimprovementl~b withtheintroductionofthe

thermalfactoru/h totheindependentvariable.

Itmaythereforebeconcludedthatknowledgeofthethermalproperties,coefficientofltnearexpansion,thermaloonductivlty,endspecificheatisunnecessaryindeterminingtherelativethermal-crackingresistanceofthealloys.A plotOf l/~bagainstn (fig.7)defineswitha highdegreeofaccuracythereeistancetothermalcracking.Thevaluesof h end a usedandthesourceofthe a valuesexelistedintableIII..F’orqualita-tivecomparism,equtio~weredevelopedfromthedataintables11andXIIforcyclestofailureasa functionofthereciprocaloftheelongationatfailured ofthethermalconstanta/h ttiesthereciprocalofelongationatfailure.Theequations,X deter-minedbythemethdofleastsquares~are

-1.318n=

(~)4L98 1

and.

-1.370n=

(%)0.002&

h

Althoughthemagnitudeofthestressescausingfailureofthesespecimensisunlmown,a qualitativeexplanationofcrackformationendprogressionmEVbepostulated.WhentheedgeIssuddenlyquenched,ittendstoc~tractrapids.~is ContritionisrestrainedbythethickerportionofthesPcimen,whichisstillatanelevatedtemperature.Thestressessetupbytheseopposingforcesexceedtheyieldpointofthematerialandplasticflowofthenarrowedgeresults.Althoughthehottermetalunderlyingthenarrowedgehasa loweryieldpoint,thestressesarenot so con-centratedinthisregionbecauseofthelargercross-sectionalarea.Plasticflowofthenarrowedgetendstorelievethestressesinthis~ge butleavesitelongated.Astherestofthespecimencools,itcontractsinenattempttoattainitsoriginalsize,butthiscontractionisrestrainedbytheelongatednarrowedgeandthespecimentheultimatewillresult.

warps.E’thestressesinthestrengthatthetemperatureofInmostcases,cracksstarted

quenchededgeexceedtheedge,crackingattheintersections

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NACATN2037 11

of’thequenchedsurfaceandthecurvedfacesofthespecimenandworkedgraduallyacrossthefaceoftheedgeduringsuccessivecycles.Veriousstagesofthisprocessareappsrentinfigure9.

The@eatestrateofcoolingandthereforethemostdrasticshockconditionserefowi alongtheintersectionsofthequenchedsurfaceandthecurvedfacesofthespecimen.Thisfactorcom-binedwiththepresenceofverysmallroughspots,nicks,andotherflawsthatcemactesstressraisersresultsintheformatimofsmallcracksatthesecorners.Withrepeatedcycling,thesecracksbecomelargeruntiltheyprogresscompletelyacrossthequenchededge(fig.9).

Whenthe relation betweenthe thermal-crackingresistanceandthemechanicalpropertiesofa materialisconsidered,itisneces-saryfirsttoanalyzethenatureofthestresseffectsproducedinthethermalshook.Thesuddenproductionofa temperaturegradientina materialrapidlyproduceshighstresses.Therateatwhichthesestressesareproducedismuchhigherthanthoseencounteredinnormaltensile-testingprocedures.Thethermalstressesso~o-ducedcauserapiddeformationandfinalfailurebycracking.Thenatureofthecrackpropagationinthisinstigationwassuchthatsfterthecracksbegan,thestressededgeofthematerialcanbeconsiderednotched.Thethermal-shockevaluationthereforecon-sistsofrepeatedrapidapplicationsofload,whichproducestressesthatareincreasedbya notcheffectasthespecimensapproachfailure.

Becausethenatureofthethermal-shockcycleissuchthattheedgeisnearbathtemperaturewhiletheheavyPrtionofthespeci-menisnearfurnace temperature,itmaybepostulatedthatthefailureoftheedgeoccursatapproxhnatelyroomtemperature.Ifthisassumptioniscorrect,itwouldbeexpectedthata room-temperaturetestthhtapproxinwtesthestressccmditionsinthermalshockcouldbecorrelatedwiththethermal-crackingresist-anceofamaterial.Ofalltheroom-temperaturetests,theimpacttestmostnearlyresemblesthethermal-shocktestbecauseofthehi@ rateofloadingandpresenceofthenotcheffect.Theimpacttestshouldthereforeyielddatathatcanberelatedtothermal-crackingresistance.TablesIIand111indicatethat,ingeneral,thealloyswiththehighestimpactstrengtharethosethathavethegreatestthermal-crackingresistance.Theimpactstrengthofanalloymaythereforebesomeindicationofitsthermal-crackingresistance.

12 NACATN2037

Theresultsofthemetallurgicalexamination-icatedthatthealloysweresimihzringrainsize.Allthealloysagedduringheating,theamountofprecipitationincreasingwithincreasedtimeattemperature.ThecracksweregenerallytrenscrystallineinpropagationandIttherefcmeappearsthatgrainsizeisanunimportantfactorindeterminingtie’kmal-crackingresistemce.Perhapsthemostprobablereasonforthedlfferencesamongsamplesofeachalloyistheranlcanorientationthatispresentincastcoexse-grdnedalloysofthistype.Becauseallthealloysaresimllarintheirstructuralbehavior,theorderofmeritobtainedinthisinvestigationtightapplyatlowerevaluationtemperatures,thatis,onlythemagnitude(number& cyclestofailure)ofthethermal-crackingresistanceofeaohalloywouldchangewithtem-perature.

SUMMARYOFRESULTS

h investigationwasconductedtodeterminetherelativeresistanceofsixcastheat-resistingalloystothermalcrackingcausedbyrepeatedthermalstresses.

TheorderofdecreasingresistancetothermalcrackingofspeoiallydesignedalloyspectienswasS-816,S-590,Vltallium,422-19,X-40,andStellite6. Thematerialswiththegreatershockductility(theabilitytodeformundersuddenloads)survtvdthegreatermmberofshookcycles.Toofewdata,huwever,wereavail-ableforaccuratequantitativestatmentoftherelation.

Materialehavingsimilarthermalproperties(coefficientofllnearexpansion,thezmalconductivity,andspecifioheat)wereshowntohavewidelydifferentresistancestothermalshock.

Metallurgicalexaminationc&thealloystructuresanda studyofthenatureoforaokpropagationledtotheconchsionthatstructuralcharaoteristlosofthealloyswereinsignificantIndefiningresistancetothemalcraoking.

TheUmltedavailabledataonnotchimpactstrengthofcasthigh-temperaturealloystndioateda possiblerelationbetweennotchimpactstrengthad resistancetothermalcracking.

LewisFlightPropulsion Laboratory,NatfonalAdvisoryCommitteeforAeronautics, .

Cleveland,Ohio,July22,1949.

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NACATN2037

1.Norton,F.H.: Refractories.McGraw-HillBookCo.,Ihc.,2dcd.,1942,p.470.

2.Avery,HowardS.,adl?illcs,CharlesR.: CastHeatResistatAlloysofthe26~lChromim-20%NickelTyw-p~t 1. ~~.A.S.M.,vol.4021948,pp.529-577;discussion,pp.577-584.

3.Cork~JamesM.: Heat.JohnWiley& Sons,Inc.,2dcd.,1942,p.121.

4.LidmanJW.G.~andBobrows@~A.R.: CorrelationofthePhysicalPropertiesofCeramicMaterialswithResistancetoI&acturebyThermalShock.NACATN1918,1949.

5.Heel,PaulG.:IntroductiontohtathematicalStatistics.JohnWiley&Sons,Inc.,1947,pp.78-89.

6.Kittel,J.Howard:ComparisonofCrystalStructuresd 10 .WroughtHeat-ResistingAlloysatElevated.TemperatureswithTheirCrystalStructuresatRoomTemperatures.NACATN1488,1947.

7.Anon.:MetalsHa@book,1948~itioa.Am.Sot.Metals(Cleveland,0.),1948,P.579.

8.Grant,NicholasJ.,Fredrickson,A.F.,andTa@or,M.E.:ASummsryofHeatResistantAlloysfrom120@to18000F.IronAge,vol.161,no.12,March18,1948,pp.73-78.

9.Anon.: SymposiumonMaterialsforGasTurbines.Am.Sot.TestingMaterials(Philadelphia),1946.

NACATN2037

TA6LIEI - COMPOSITIOIVOFAZLOYSEECIMENSASDENRWNED

BYOmMIcmlAlfALYsEs

AlloyS-816S-SWVitallium422-19X-40Stellite6

CICr

L003018.06.3119.17.3 2’?.0.4 25.4.5 25.1.9328.6rJi co Mo w m Fe Si20.8043.804.073.743.51 2.72---20.3219.654.273.823.8025.39---2.7 61.1 5.8 --------1.5 ---15.649.1 6.3 --------1.3 ---11.254.2 ----7.6 ---- .6 0.3.5 62.2 ----5.4 ---- .7 .3

AlloyS-81.6

S-SW

vltdmlnl

422-19

f

PX-40Stellite6

3pedlnenCyclestofailureA 86.B 104H 105A 34D 34E 36D 24w 30VI 6v~ 6v 8

A 7B 6D 7H 7B 2D uE 4G 2Q 2u 2

.

*

!!

.

b .

TABLEIII - HH’81CALEROPERTXB W AIWYS INVISTIGA!I!Q

Alloy Cmrfl.ofent!mf’fUaivity ‘lkmsi.lestrength Percente43e(alarpyV-no’tohd llllear(SQft/br) (1./sllin.) elongation ilnpaot g~ion in2inohes

:~y,*

frm 7@ to duringroc4n-1600°F temperature (f%-lb) S!

((ln~.) “tensiletestNo

700E lmoo 3’ w+

s-816 ‘%.owlo+ bO.16 ‘loo.Oxloa ‘5 b~e4.7fe.53 %12 .0 ‘6.0%.6’0d9.4

s-590 8g.2ox@ bo.16 b~e3.2a9.22‘9.20

Vitallillm ‘8.72x10-6 bo.22 CIO1.3XI.03 a71.o~$ dlo e.9%01.3 f86.2 08.2allo,Q f8.2

422-19 ‘8.54x10-6 bo.20 ’98.M03 d58.3xlo3 a5.o %.5fe,~ 098.1 C59.9 05.0

f~o~ f5.o

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16 NACATN 2037

~Quenched edge

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NACATtd 2037 17

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NACATN 2037 21

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NACATN 2037 23

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