EffectsofUltrasonicNanocrystalSurfaceModification(UNSM)onResidualStressState
andFatigueStrengthofAISI304
A.Cherif,Y.Pyoun,andB.Scholtes
(SubmittedJanuary22,2008;inrevisedformJune2,2008)
Theeffectsofanewmechanicalsurfacetreatmentmethod,calledultrasonicnanocrystalsurfacemodifi-cation(UNSM),onnear-surfacemicrostructuresandresidualstressstatesaswellasonthefatiguebehaviorofanausteniticsteelAISI304areinvestigatedanddiscussed.Theresultsarecomparedwithconsequencesofothermechanicalsurfacetreatmentmethodssuchasdeeprollingorshotpeening.
Keywords
AISI304,deeprolling,fatigue,residualstress,shotpeening,ultrasonicnanocrystalsurfacemodification(UNSM),workhardening
1.Introduction
Itiswellestablishedthatthefatiguebehaviorofmetallicmaterialscanbesignificantlyimprovedbyappropriatemechanicalsurfacetreatments.Thisisduetotheformationofmacroscopiccompressiveresidualstressesandworkhardeningatthesurfaceandinnear-surfaceregionscontrib-utingtoinhibitorretardthedamageprocessoffatiguecrackinitiationaswellasoffatiguecrackgrowth(Ref1-5).Differentmethodssuchasdeeprolling,shotpeening,andlaser-shockpeeninghavebeendeveloped.Theirconsequencesonnear-surfacematerialspropertiesaswellasonstrengthandlifetimeunderspecificloadingconditionshaveintensivelybeenstudied(Ref6,7).
Ultrasonicnanocrystalsurfacemodification(UNSM)tech-nologyisanewkindofmechanicalsurfacetreatmenttechnology(Fig.1).ThemainconceptandmechanismofUNSMisasfollows:Atungstencarbideballattachedtoanultrasonicdevicestrikesthesurfaceofaworkpiece20,000ormoretimespersecondwith1000to10,000shotspersquaremillimeter.Thesestrikes,whichcanbedescribedasmicrocoldforging,causesevereplasticdeformationtosurfacelayersand,therefore,inducenanocrystalmicrostructures.However,noinformationaboutresidualstressdistributionscomparedwithconventionalmethodsisavailable(Ref8).Thenanomicrostructuremodificationofsurfacelayerscanimprovestrength(hardness)andductility(toughness)oftheworkpiecesimultaneouslyaccordingtotheHall-Petchrelationship.
Thisprocessalsoimprovessurfaceroughnessandinducescompressiveresidualstressinsurfacelayers,whichwillinturnincreasefatigueresistanceoftheworkpiece.Inthisarticle,near-surfacepropertiesofUNSM-treatedAISI304arecom-paredwiththeresultsofanalogousinvestigationsofshot-peenedanddeep-rolledspecimensandthestabilityoftheproprietiesduringfatigueloadingisstudied.
2.MaterialsandExperimentalProcedures
TheinvestigatedmaterialwasausteniticstainlesssteelAISI304withthechemicalcomposition:0.03%C,0.58%Si,1.14%Mn,0.03%P,0.03%S,18.3%Cr,8.8%Ni,and0.06%N.Thematerialwassolutiontreated,quenched,andwarmrolledtocylindricalbars(diameterof14mm).Themicrostructurewasfullyausteniticwithanaveragegrainsizeof70lm.
Cylindricalfatiguesamples(Fig.2)withadiameterof7mmandagagelengthof15mmwereusedforfatigueteststoobtainS/N-curves.Alltestswerecarriedoutunderstress-controlledtension-compressionloadingatastressratioofR=À1andatroomtemperature.Residualstressdepthprofilesweredeterminedbyx-raydiffraction(XRD)techniqueusingtheclassicalsin2w-methodwithCrKaradiationatthe{220}-planesand(1/2)s2=6.05910À6mm2/Naselasticconstant.Thex-raybeamhadadiameterof1mm.Near-surfaceworkhardeningwascharacterizedbytheFWHMvaluesofthex-raydiffractionpeaks.AllresidualstressandFWHMvaluesweremeasuredinlongitudinaldirectionofthespecimens.ResidualstressandFWHM-depthprofilesweredeterminedbysucces-siveelectrochemicalremovalofmaterialwithoutcarryingoutastresscorrection.Itiswellknownthatinthecaseinvestigatedhereonlynegligibleerrorsoccurwhenmeasuringinnear-surfaceareas(Ref10).
Vickershardnessmeasurementswerecarriedoutonpol-ishedcrosssectionsofthespecimens.
SurfaceroughnesswasanalyzedusingaPerthometerS8P(Mahr).
Martensitecontentwasdeterminedbyastandardprocedureusingx-raydiffractiontechnique.Twointerferencepeaksofausteniteandmartensite,respectively,weretakenintoaccount.
A.CherifandB.Scholtes,InstituteofMaterialEngineering,Univer-sityofKassel,Kassel,Germany;andY.Pyoun,DesignMecha/Mec-hanicalEngineeringDepartment,SunMoonUniversity,AsanCity336-708,Korea.Contacte-mail:echerif@uni-kassel.de.
282—Volume19(2)March2010JournalofMaterialsEngineeringandPerformance
3.ExperimentalResultsandDiscussion
Figure3showsthesurfacetopographyprofileofthespecimensbeforeandafterUNSMtreatment(measuredinlongitudinaldirectionintwodifferentpositions).Itcanbeseenfromthefigurethatthetoolingmarksduetotheturningprocesshavepartiallybeenflattened.Asaconsequence,surfaceroughnessRzimprovedfrom1.7to1.3lm.Figure4showsthemicrographofthesurface.
ResidualstressandFWHM-depthdistributionsweredeter-minedatthreedifferentlocationsinthegagelengthofthespecimen(Positions1-3,center,and±5mm).
TheresultsarepresentedinFig.5and6.CompressiveresidualstressesaswellasincreasedFWHM-valuesofx-raydiffractionpeakswereobservedatthesurfaceandinnear-surfaceregioninallcases.
ThestressvaluesimmediatelyatthesurfacevarybetweenÀ1100MPa(Position3)andÀ635MPa(Position1).Withincreasingdistancefromsurface,residualstressamounts
Fig.1UNSMsystem(Ref8,9)
Fig.2AISI304specimen
Fig.3SurfacetopographyprofilebeforeandafterUNSM-treatedAISI304JournalofMaterialsEngineeringandPerformancedecreaseaswellastheirrangeofvariation.Measurementsalongthecircumferenceofthespecimenshowedonlyasmallresidualstressvariation.
Thethicknessofthesurfacelayerexhibitingcompressiveresidualstressisapproximately0.4mm.
Fig.4SEMmicrographofUNSM-treatedAISI304atthesurface
Fig.5ResidualstressdepthprofilesofUNSM-treatedAISI304atdifferentsurfacepositions
Fig.6FWHMdepthprofilesofUNSM-treatedAISI304atdiffer-entsurfacepositions
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Fig.7S/Nplotofroomtemperature-fatiguedAISI304specimensindifferentsurfacetreatmentconditions(UNSM,deeprolled(Ref11),untreated)Fig.8VickershardnessdepthprofileofAISI304afterdifferentmechanicalsurfacetreatment[DR:deeprolled(Ref11),SP:shotpe-ened(Ref12)]
TheFWHMvaluesinthenear-surfaceregionsincreasefromapproximately0.9°inthebulkto2°atthesurface.
Similardepthdistributionsweremeasuredatallthreemeasuringpositions.However,position3,withmaximumcompressiveresidualstress,showsslightlyhigherFWHMvaluesnearthesurfacethanintheothercases.Correspondingtotheresidualstressdepthdistribution,thethicknessoftheaffectedsurfacelayerisabout0.4mm.
Alimitednumberofspecimenswereavailableforfatiguetests.Consequently,theS/N-curvesshowninFig.7allowonlyaroughestimationofthefatiguestrengthofUNSM-treatedspecimenscomparedtotheuntreatedandthedeep-rolledstate.FromFig.7,onecanconcludethatUNSMtreatmentclearlyimprovesfatiguestrengthandlifetimeinthehighaswellasinthelowcyclefatigueregimecomparedtothestartingconditionwithaturnedsurface,butitisnotaseffectiveasdeeprolling.Onehas,however,totakeintoaccountthatprocessparameters,althoughrealisticallychosen,werenotoptimizedforhighestfatigueperformance,whichlimitsthevalidityofFig.7.
ForamoredetailedassessmentoftheconsequencesofUNSMtreatmentinFig.8to10,characteristicnear-surfacepropertiesmeasuredinthecenterofthegagelengtharecomparedwiththecharacteristicsofshot-peenedordeep-rolledspecimens(Ref7,11,12).Figure8showsthatdistinctdifferencesexistbetweenthemeasuredhardnessdepthdistri-
284—Volume19(2)March2010Fig.9ResidualstressdepthprofilesofAISI304afterdifferentmechanicalsurfacetreatment[DR:deeprolled(Ref11),SP:shotpe-ened(Ref7)]
Fig.10FWHMdepthprofilesofAISI304afterdifferentmechani-calsurfacetreatment[DR:deeprolled(Ref11),SP:shotpeened(Ref7)]
butions.Shotpeeningproducesthehighesthardnessvaluesimmediatelyatthesurface,whereasinthecaseofdeeprolling,athickersurfacelayerisaffected.Itisinterestingtonotethatthisisincontrasttotheresidualstressdepthdistribution(seeFig.9)andFWHMdepthdistribution(seeFig.10),wherespecimenstreatedbyUNSMtechniqueshowthehighestvaluesinnear-surfaceregions.
Thehardnessvaluesincreasefromapproximately220HVintheunaffectedbulkofthematerialupto390HVatthesurfaceafterUNSMtreatment.
Shotpeeningleadstothehighestsurfacehardnessofabout450HV.Theinfluenceofdeeprollingonhardnessachievesadepthofabout0.55mmcomparedto0.3mmafterUNSMtreatmentaswellasofshotpeening;however,hardnessvaluesimmediatelyatthesurfacearelowerthanintheothercases.FromFig.9itisevidentthatresidualstressdepthdistribu-tionsofUNSM-treatedanddeep-rolledspecimensareverysimilarwithhigheramountsafterUNSMtreatmentatthesurfaceandsomewhatloweramountsatgreaterdistancesfromthesurface.Theresidualstressvaluesofshot-peenedspeci-mensareclearlysmaller,andtheaffectedsurfacelayeristhinner.
SimilarobservationscanbemaderegardingthedepthdistributionsofFWHMvalues.Again,UNSMtreatmentleadstohighervaluesatthesurface,andthedeep-rolledspecimensshowthethickestaffectedsurfacelayer.Immediatelyatthe
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Fig.11Martensitecontentdepthprofileafterdifferentkindsofsur-facetreatment[DR:deeprolled(Ref7),SP:shotpeened(Ref7)]
Fig.12ResidualstressdepthprofilesofUNSM-treatedAISI304unloadedandfatiguedtohalfthenumberofcyclestofailure(stressamplitudera=340MPa)
surface,shotpeeningleadstohigherFWHMvaluesthandeeprolling(Ref7;Fig.11).
Asaconsequenceofthesurfacetreatmentsapplied,mechanicallyinducedmartensiteisformedinthesurfacelayerofAISI304.Themeasuredmartensitecontentstronglydependsonthetypeoftheprocessapplied.Shotpeeningleadstothehighesttransformationeffectsatandbelowthesurface.UNSMtreatmentleadstoamartensitecontentofapproximately35%immediatelyatthesurface.Thisvaluecontinuouslydecreaseswithincreasingdistancefromthesurface;exceptforlayersveryclosetothesurface,thevalueisalwayssmallerthanforshotpeeningordeeprolling.
Figures12and13showtherelaxationbehaviorofmacro-scopiccompressiveresidualstressesaswellasofFWHMvaluesofUNSM-treatedspecimensfatiguedtohalfthenumberofcyclestofailure.Oneobservesthatcyclicloadingatastressamplitudeof340MPa(Nf=8000cycles)leadstoareductionofsurfacecompressiveresidualstressesbyapproximately60%.AsimilarresidualstressrelaxationwasalsoobservedbelowthespecimenÕssurface.
FWHMvaluesrelaxtoo,indicatingthatmicrostructuralalterationsoccurinnear-surfacelayersduetothecyclicloadingprocess.However,theworkhardeningeffectofUNSMtreatmentisstillvisibleafterhalfthenumberofcyclestofailure.
JournalofMaterialsEngineeringandPerformanceFig.13FWHMdepthprofilesofUNSM-treatedAISI304unloadedandfatiguedtohalfthenumberofcyclestofailure(stressamplitudera=340MPa)
Fig.14ResidualstressrelaxationatthesurfaceofUNSM-treatedAISI304duringstress-controlledfatigueatroomtemperaturefordifferentstressamplitudes
Fig.15FWHMvaluerelaxationatthesurfaceofUNSM-treatedAISI304duringstress-controlledfatigueatroomtemperaturefordifferentstressamplitudes
Figures14and15showtherelaxationofmacroscopiccompressiveresidualstressesaswellasofFWHMvaluesatthesurfaceofUNSM-treatedspecimensasafunctionofthenumber
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ofloadingcyclesfortwodifferentstressamplitudes.Forthelowerstressamplitudeof340MPa,residualstressremainsstableduringthefirst1000loadingcyclesandthendecreaseuntilfractureoccurs.Ontheotherhand,forastressamplitudeof370MPa,leadingtofracturealreadyafterapproximately1000cycles,acontinuousresidualstressrelaxationisobserved.Forbothloadingamplitudes,FWHMvaluesdecreasewithincreas-ingnumberofcycles,indicatingacontinuousalterationofUNSM-inducedmicrostructuresduringfatigueloading.
4.Conclusions
TheexperimentspresentedinthisarticlewereorientedtowardgainingbasicinformationabouttheeffectsofUNSMtreatmentincomparisonwithmoretraditionalandwell-establishedmechanicalsurfacetreatmentmethodsofausteniticsteelAISI304,i.e.,shotpeeninganddeeprolling.Asaresultofthetests,itisevidentthatthenear-surfacemicrostructuresachievedduringtheUNSMprocessarecomparablewithpropertiesofshot-peened,deep-rolled,orlasershock-peenedlayers.Firstofall,ithastobementionedthatasurfacelayerwithhighcompressiveresidualstressesisformedtogetherwitharemarkablestrain-hardeningeffect.Forthematerialinvesti-gatedinthisworkalsoamechanicallyinducedphasetransformationfromaustenitetomartensitecouldbedetected.Thedifferencesofthedepthdistributionsofthepropertiesmeasuredhavetobeattributednotonlytothecharacteristicfeaturesoftheprocessescomparedbutalsototheindividualprocessparametersapplied.Consequently,nogeneralconclu-sionsshouldbedrawnabouttheeffectivenessofindividualprocessestocreatecompressiveresidualstresses,strainhard-ening,orotherbeneficialeffectsinnear-surfacelayers.Itisratheraquestionofchoosingtherightmethodtogetherwithappropriateprocessparameterstoachieveoptimumconditionsinindividualcases.Itisthereforenotsurprisingthattheconsequencesofcyclicloadingonnear-surfacepropertiesofUNSM-treatedAISI304areinagreementwithobservationsmadeforothermechanicalsurfacetreatmentsaswell.Forthestressamplitudesapplied,apartialrelaxationofstrainharden-ingaswellasofmacroscopicresidualstresscouldbedetected.Altogether,itcanbeconcludedthatUNSMtreatmentgivesrisetoexcellentnear-surfacematerialsmicrostructures.
286—Volume19(2)March2010Becauseoftheoutstandingimportanceofthepropertiesofnear-surfacelayersforstrengthandlifetimeofcomponents,itcanthereforebeaverypowerfultooltoproducehigh-strength,light-weightstructures.
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