NDT methods for revealing anomalies and defects in gas turbine blades.docx

NDT methods for revealing anomalies and defects in gas turbine blades.docx

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NDTmethodsforrevealinganomaliesanddefectsingasturbineblades

NDTmethodsforrevealinganomaliesanddefectsingasturbineblades

Author:

J.Pitkänen

VTTManufacturingTechnology

Espoo,Finland

Co-authors:

T.Hakkarainen,H.Jeskanen,P.Kuusinen,K.Lahdenperä&P.Särkiniemi

VTTManufacturingTechnology

M.Kemppainen

TechnicalUniversityofHelsinki

M.Pihkakoski

HelsinkiEnergy

Espoo,Finland

Contact

ABSTRACT

Energyproducedbygasturbinesisincreasingallovertheworld.Oneimportantpartinthesegasturbinesistheturbineblading.Improvedbladesandvanesarebeingintroducedwhiletheindustryalsousestheoldgenerationofbladetypes.ApplicableNDTmethodsareneededforallbladetypes.ManyNDT-methodscanbeappliedinlaboratoryenvironment.Thisoverviewdescribesavarietyofmethodsforbladeinspectionsuchasdyepenetrant,eddycurrent,radiographyandultrasonicmethods.Someofthesemethodsareconsideredonthebasisoftheexperiencegainedinmeasurements.OnenewUT-methodisdescribed,easilytransferableforpracticalinspectionsonsite.AwideapertureVTTprobehasbeendevelopedforthepurpose.Withthisspecialprobedifferentbladepropertiescanbemeasured,suchascoatingthickness,detectionandsizeofcracks,delaminationsaswellasmechanicalproperties.

INTRODUCTION

ThelifetimeofGT-bladesisveryimportanttoutility.Thelifetimeofparts,wherethetemperatureishighismostcritical,becauseofthermalstresses,corrosion,oxydationanderosion.Thesefactorsreducethelifetimeofablade.Thelifetimecanvarysuddenlyinuse,becauseofmanyunforeseenincidentsoccuringinenvironmentofthebladesare.Alsosomevariationinproductionorinrepaircandeterioratethematerialsandinsametimeshortenthelifetimeofthecomponent.

DyepenetranttestingistheNDTmethodmostfrequentlyusedforinspectinggasturbinebladesandvanes.Itisoftenrecommendedtousefluorescentdyepenetrantsforinspection.Aftersometimeinservicethebladesurfaceisoftencorroded.Inthesecasesfluorescentdyepenetrantisnotrecommendedandnormaldyepenetrantismoreusable.Thedrawbackofdyepenetranttestingisitssuitabilityonlyforsurfaceopeningcracks.Subsurfacedefectsarenotdetected.Whenacrackisfoundwithdyepenetranttesting,itisnotpossibletoestimateitsdepth,andhereothermethodsareneeded.

Therearemanymethodswhichcanbeusedfordetectionofcracks,ageinganddegradationofacoating,andforthicknessmeasurement.Thermalmethodscanbeusedformeasurementofwallthicknessanddetectionofnearsurfaceanomalieslikedelaminations（Aladin1996,Carletal1998a）,problemsincoolingchannels（Carl1998b）.

IthasbeenobservedthatforinstanceinMCrAlY-coatingsthepermeabilityinbasematerialaswellastheincoatingarechangingduringthelifetimeofacomponent.InMCrAlY-coatingthedecreaseofCr-contentand/orβ-phasechangethemagneticpropertiestomoreferromagnetic.Eventhoughtheeffectisnotlarge,itcanbedetectedinsomecases（Czechetal1998）.Czechetalmeasuredthemagneticpermeabilitywithacoilinconjuctionwithpermanentmagnets.Sotheycouldestimatethelifetimeofcoating.ThisisalsopossiblewiththeeddycurrentmethoddevelopedbyENEL（Antonelli1998a,b）.ThemethoddevelopedbyENELcanalsotbeusedforthedeterminationofthecoatingthicknessofMCrAlY-coatings.

EDDYCURRENTTECHNIQUES

Eddycurrenttechniquehasbeenusedfordetectingandsizingofacrack.Itisbesttocalibratewithacalibrationblockmadefromthesamematerialastherealcomponent.Thismeansmorecostsbutgivescertainlymostreliableresults.Ludwigetal（1998）havereportedthatopencracksandtightcrackscanbeseparatedfromeachotherwitheddycurrenttechnique.

Howeverfromacrackwhichgoesthroughgrainboundariesisclearlymoredifficulttoestimatethesizeofacrack.ThiswasaccordingLudwigetal（1998）problemtheestimatethelifetimeofacrackedblade.Figure1showseddycurrentsignalsfromthreecracksthatwerefoundwithdyepenetranttesting.

a）3surfacebreakingcracksonaturbineblade

b）Eddycurrentsignalsfromcracks.

Fig1:

Eddycurrentsignalsfromthethreecracksshownintheblade.

Measuringthethicknessofacoatingissimplewitheddycurrenttechnique,iftherearenostrongchangesinpermeability.Inadditionthegeometryofabladecancomplicatethemeasurement.Figure2showsthethicknessmeasurementfromafirststageX45vane.Thecoatingwasnonconductingceramicmaterial.Inthiscasethemeasurementcanbeeasilytocarriedoutwithconventionaleddycurrenttechnique.Moredemandingistomeasuremetalliccoatings,withagoodconductivelikeMCrAlY-coatings.Thepointforthicknessmeasurementcalibrationhastobechosenverycarefully,seefigure2,becauseastrongvariationisobservedinbasematerialdependingonbasemateriallocationinblade.Thisofcoursehasaneffectonthemeasurement.ThemethoddevelopedbyENEL（Antonellietal,1998）doesn'tneedcalibrationblockstocarryoutthicknessmeasurement.

Fig2:

ECthicknessmeasurementofthecoatingfroma1.stagevane,materialX45withceramiccoating.

Duringthelifetimeofamaterialpropertiescanvarydrasticallyasshowninfigure3.Inthesecasesthestrongeffectfromthepermeabitilitycannotbecorrectedeasily.InthiscasethecoatingfromIN738bladeisdamaged.Thedamagedareaisgivesasimilarsignaltoferriticsteel.Whileintheundamagedareathemeasuredsignalwasmorelikeonefromstainlesssteel（AISI316）.Thechangesofthematerialpropertiesonthesurfaceofablademaketheinspectionmoredifficultbothinthecaseofcracksandinmeasuringthethicknessofthecoating.

Fig4:

ThicknessmeasurementprincipleThetimeofflightofleakyRayleighwavevariesaccordingtothethicknessofthecoating

ULTRASONICTECHNIQUES

Ultrasonictehcniquesusedforturbineblademeasurementarenormallyappliedtodetectcracksintheshoeoftheblade.InthiscaseweconsideronlyleakytheRayleighwavetechnique,whichcanbeutilizedforturbineblademeasurementwithcontactultrasonicprobedevelopedbyVTTManufacturingTechnology.ThismethodhasbeenstudiedbyKauppinen（1997）.Withthisprobeitispossibletomeasurethicknessofthecoating,figs4and5.

Fig5:

ThicknessmeasurementwiththetimeofflightofleakyRayleighwavefromaMCrAlY-coatedturbineblade

Thedevelopedprobeisalsosuitablefordetectionofcracks,iftheleakyRayleighwaveispenetratingintothebasematerial.IncasewhereleakyRayleighwavedoesn'tpenetrateintothebasematerial,itisstillpossibletousenormalRayleighwave.TheleakyRayleighwavedecaysrapidlywhencracklikedefectarepresent.Inthefigure6.weseethatwithleakywavemodeitiseasytodetectevensmallcracksdownto50μm.Indepththeresolutiondependsonthefrequency（wavelength）oftheprobe.

Fig6:

（a）MCrAl-coatedGT-blade,inwhichtheEDM-notchdepthsvarybetween50μm-500μm（b）ultrasonicB-scanimageinpitch-catchmode.（c）UltrasonicB-scanimageinpulse-echomodefromthebladewithEDM-notches

Fig7:

MaterialX45ismeasuredwithcontactwideapertureprobeisshowninfig.7.Thecenterfrequencyoftheprobewas12MHz.TheplateisshowninupperpictureandC-scanresultinlowerpicture

Sametechniqueisusedforthermalfatiquecrackdetectioninfig7.Asitisclearlyseentwocracksinthisplatecanbedetectedwiththiscontactprobe.Theplateisabout3mmthickandoneofthecracksisextendingthroughoutthewall（inthemiddleoftheplate）.

Otherapplicationpossibilitiesfordevelopedprobetypearedetectionofdelaminations,measurementofmaterialwavevelocityandelasticity.

RADIOGRAPHICTECHNIQUES

Radiographicmethodscanbedividetonormalthroughwallmeasurementwithfilmorwithrealtimeradioscopy,X-raydiffractionmethod,comptoneffectmeasurement,X-raytomography,neutrontomography,neutronradiography,positronannhilation.VTThasusedrealtimeradioscopyequipmentformeasuringturbinebladesandHelsinkiUniversityofTechnologyhascarriedoutsomeX-raydiffractionmeasurements.

InX-raymeasurementtheradiationdecaysdifferentlyinvariousmaterialsanddiscontiniuties.Thechangesareaffectedbydensityvariation,thicknessvariation,variationincompositionofthematerialandfromlackofmaterial（corrosion,cracks）.Infigure8therealtimeradioscopyequipmentisshown.Theinspectedobjectislocatedbetweenthesourceandtheimageintensifier.Theobjectisradiatedandintheimageintensifiermeasuredradiationisdigitizedthroughvideocard.ThisinformationcanbesavedontheharddiscorCD-rom.ThesemeasurementhasbeencarriedoutwithX-raytubeof160kV.

Fig8:

ThePC-realtimeradioscopysysteminevaluatingradiographicresults

Fig9:

X-raypicturefromtheinsidestructuresofthegasturbineblades

Fig10:

Coolingchannelcrossing,whichcouldcausefromtheedm-manufacturingofrthechannels.

Inpicturescracks,pores,geometricalthicknesses,materialdensityvariationscanbedetected,figure9.Especiallycoolingholesareclearlyseeninthepictures,figure10.Poreshavebeendetectedmostlyinradiographymeasuments.Thesizesofthoseporesareaboutsameasthedimensionsofthecoolingchannels.Theblocksinthecoolingchannelscancauseextrastresstoturbineblade.Thecoolingofthebladeischangeddrasticallyandthetemperaturedistributiononthebladeschangedcanrapidlydamagetheblade.

ResidualstressesweremeasuredfromaturbinebladematerialIn939.ThemeasuringinstrumetwasXSTRESS3000（St