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69PracticalFailureAnalysisVolume15October2001AnInvestigationoftheDevelopmentofDefectsDuringFlowFormingofHighStrengthThinWallSteelTubesK.M.RajanandK.NarasimhanSubmitted12June2001inrevisedform6August2001Flowformingtechnologyhasemergedasapromising,economicalmetalformingtechnologyduetoitsabilitytoprovidehighstrength,highprecision,thinwalledtubeswithexcellentsurfacefinish.ThispaperpresentsexperimentalobservationsofdefectsdevelopedduringflowformingofhighstrengthSAE4130steeltubes.Themajordefectsobservedarefishscaling,prematureburst,diametralgrowth,microcracks,andmacrocracks.Thispaperanalyzesthedefectsandarrivesatthecausativefactorscontributingtothevariousfailuremodes.Keywordsmetalforming,inclusions,expertsystem,microstructure,surfacefinishPFANF8200156976©ASMInternationalIntroductionFlowformingisapromising,economicalmetalformingtechnologythatcanmeetthechallengingrequirementsofhighspecificstrength,closedimensionaltolerances,andexcellentsurfacefinishdemandedbythedefenseandaerospaceindustries.Therelativelylowtoolingcostandremarkablematerialutilizationduetochiplessmetalformingprovideeconomicdrivers,whiletheabilitytoachievehighstrengthfinishedproductfromlowstrengthstartingmaterialisduetostrainhardening.Flowformingisanincrementalformingprocessthatusesa3Dvariationofbasicrollingprocessesandcombinesrolling,shearing,andbendingintooneoperation.Itissimilartoneitherupsettingnorswaging.Essentiallyapointdeformationmetalformingprocess,flowformingresultsinaparthavingahighlydeformedmicrostructure.Significantincreasesinyieldstrength,ultimatetensilestrength,andhardness,andacorrespondingreductioninductility,accompanytheflowformingprocess.Conventionally,tubesareproducedbyhotextrusionfollowedbydrawingorpilgering.However,itisnotpracticaltohotextrudethinwalltubesbeyondaspecifiedlimit.Drawingisaneasierandlessexpensiveprocessthanextrusiontherefore,athickwalltubeiscoldextrudedandfinishedonadrawbenchorpilgermill.Thedrawingprocessisessentiallyatensileprocess.Microcracksandotherdefectsinsidethematerialtendtopropagateduringthedraw,leadingtofailure.Theareareductionistypicallylimitedto10foreachdrawofahardmaterial,andthetotalreductionmayrequireanumberofannealingcycles.1Anincreaseinthenumberofdrawing/annealingcyclesincreasesthecostofproduction.Itisobviouslyveryexpensivetousedrawingoperationstoproducecomponentsfromhardtoworkmaterials.However,ifdimensionaltolerancesarenotcritical,itisadvisabletousecheapconventionalcolddrawnsteelCDStube.Flowforming,therefore,offersseveraladvantagesoverconventionaltubemakingmethods.TheFlowFormingProcessFlowformingisusedtoproduceaseamlesstubewithtightdimensionaltolerances.Seamlesstubing,theoretically,mayrepresenttheultimateinreliability.2Ametalblankorpreformisformedoverarotatingmandrel.Themetalblankandthemandrelwhicharelockedtogetherrotate,andtheformingrollerfollowsthemandrelatapresetthathasbeenprogrammedintoaCNCflowformingmachine.Thepreformmetalisplasticizedbythelocalapplicationofheavycompressiveforcesexertedbyconicalrollers.Thedeformedmetaltakestheshapeofthemandrel,andproperwallthicknessisachievedbycontrolofthegapbetweentherollersandthemandrel.Flowformingcanbedividedintotwodistinctprocessesforwardflowformingandreverseflowforming.Inforwardflowforming,rollerfeedanddeformedmaterialmovementareinthesamedirection.Theformedmaterialisundertension,andtheK.M.Rajan,ArmamentResearchandDevelopmentEstablishment,Pune411021,India.K.Narasimhan,DepartmentofMetallurgicalEngineeringandMaterialsScience,IIT,Bombay400076,India.Contactemailnaramet.iitb.ac.in.AnInvestigationoftheDevelopmentofDefectsDuringFlowFormingcontinued70PracticalFailureAnalysisVolume15October2001materialundertherollerexperiencescompressivestress.Inreverseflowforming,rollerfeedandmaterialmovementareinoppositedirections.Thematerialundertherollerisinacompressedstatewhiletheformedpartisstressfree.AreverseflowformingtechniquewasusedforthepresentstudyandisschematicallyillustratedinFig.1.ObjectivesThispaperdescribesaninvestigationofthecauseofdefectsandfailuresduringflowformingofthinwallhighstrengthtubesfromAISI4130steel.Theinvestigationincludesstudyofthevariousmetallurgicaland/ormachineparameterslikelytocausetheseproblems.Thecommondefectsreportedforflowformingoftubesarediametralgrowth,prematureburst,buildup,fishscaling,andbellmouthing.3Othertypesofdefectsincludeunevenwallthickness,microcracks,andmacrocracks.4,5AdvancedCNC3rollerflowformingmachinesareabletomanufacturehighqualityprecisiontubes.However,metallurgicalormaterialdefectssuchasinclusionsandnonuniformgrainstructuremayaffecttubequality.Additionally,selectionofunsuitableformingparametersmayalterthequalityofthetubingproduced.Thispaperrelatesthemetallurgicalqualityofthesteeltotheabilitytoachievehighqualitythinwalledflowformedtubing.5,6ExperimentalObservationsBecauseofitsavailability,lowcost,andreasonablygoodcoldformability,SAE4130steelwasselectedforthemanufactureofthinwall,highstrengthseamlesstubesinpressurevesselapplications.Reverseflowformingwasusedforthesepressurevessels.ThedevelopmentworkdescribedinthispaperusedSAE4130steelthatwasnotelectroslagremelted.Theinclusionratingsofthisgradeofsteel,basedonASTME45,areasfollowsSulfidesAluminaSilicatesGlobularoxidesThinThickThinThickThinThickThinThick1.50.52.51.01.51.50.5Thedissolvedoxygen,nitrogen,andhydrogeninthesteel,asdeterminedbyanalysis,was22ppmoxygen,110ppmnitrogen,and3ppmhydrogen.A4axisCNCflowformingmachinewitha3rollerconfigurationwasusedfortheexperimentalwork.AphotomicrographoftheflowformingmachineisshowninFig.2.Thepreformathickwalledstartingmaterialforflowformingwasfabricatedbyforgingbothupsettinganddrawing,piercingwithtaperedpunches,andmandrelforging,followedbyhardeningandtempering.Theflowformingoperationwascompletedinthreepasseswithoutanyintermediateanneal.TheFig.1SchematicdiagramofreverseflowformingprocessFig.2FouraxisCNCflowformingmachinewitha3rollerFig.3TrimmingandcuttingoftensilespecimenfromfullyformedtubeMandrel71PracticalFailureAnalysisVolume15October2001hardnessandpercentagethicknessreductionassociatedwitheachpassareshowninTable1.Theflowformedtube,afterundergoingatotalpercentagethicknessreductionofabout88,wastrimmed,andtensiletestsamplesweretakenfromafullyformedextralengthofthetubeasperASTMA370Fig.3.ThespecifiedandachievedmechanicalpropertiesanddimensionalaccuraciesarepresentedinTable2.ProofPressureTestingTubesthatsatisfiedthespecifiedmechanicalpropertiesanddimensionalaccuracyrequirementsweresubjectedto100hydraulictesting.Thetubesweresubjectedtoapressure10abovethemaximumexpectedoperatingpressureforaboutonemin,thencheckedforpermanentset,ifany,andprematurefailure.Notestingininducedfailureorpermanentsetwasaccepted.Afterpassingthistest,thetubesweresubjectedtobursttesting.BurstPressureTestingOnetubeoutofeachgroupmanufacturedfromthesameheatofsteelandlotofprocessedpreformswasrandomlyselectedandsubjectedtoburstpressuretesting.Theburstpressuretestconfirmsthemarginofsafetyoverthemaximumexpectedoperatingpressure.AfewbursttubesareshowninFig.4.DefectsDefectsintheflowformedtubesmaycausefailureintheprooforthebursttests.Thetypesofdefectsandassociatedfailurescanbecategorizedasmicrocracks,macrocracks,diametralgrowth,ovality,fishscaling,andprematurebursting.Theflowformingprocessoccasionallyintroduceswavinessorbulgesontheouterfaceoftubes.Suchdefectsoccuronlyundercertainworkingconditions.Thisphenomenoniscalledplasticflowinstability.5Kobayashi7analyzedinstabilityinconespinTable2MechanicalpropertiesanddimensionaltolerancesofflowformedtubeMechanicalPropertiesDimensionalTolerancesUTS0.2yieldstrengthEIOvalityStraightnessSurfaceroughnessMPaMPammmmCLAµmSpecified1200min900min6min0.2max0.15maxActual125013509501100780.150.200.10.15N5N6Table1FlowformingsequencewiththicknessandhardnessvariationsineachpassPassNo.InitialthicknessFinalthicknessInitialhardnessFinalhardnessthicknessofPreformmmmmHRCHRCreduction119.509.7521285029.754.87528305034.8752.00303160Fig.4BurstpressuretestedflowformedtubeAnInvestigationoftheDevelopmentofDefectsDuringFlowFormingcontinued72PracticalFailureAnalysisVolume15October2001ningandconcludedthatwrinklingontheprespunflangeoftheconeiscausedbyreliefoftheresidualcompressivestressthatarisesinthetractionfreeflange.Theconespinninginstabilityissimilartothewrinklingphenomenonindeepdrawingoperations.GurandTirosh5studiedtheplasticflowinstabilityintubespinningandfoundthatinstabilityoccursifthecircumferentiallengthofcontactSoftherollerismuchlongerthantheaxiallengthofcontactL,i.e.,iftheratioSLisveryhigh.HighSLratioshelpmaintainacircularcrosssectioninthetube,whilelowratiosleadtogeometricinaccuraciesduetohighconstraintforflowintheaxialdirection.Theseinstabilities,wavinessbulges,andovalityarerelatedtocontrolofpressurevariables,notthematerialqualityand,hence,shouldnotbeconsideredmetallurgicaldefects.CrackingAfullyflowformedtubefromthenonelectroslagremeltgradeSAE4130steelwithatotalreductionof90thicknessoftencrackedafterpressuretestinganddimensionalinspection.Along,sharp,longitudinalthroughwallcrackdevelopedinthetubeasshowninFig.5.Metallurgicalinvestigationofthefailedtuberevealedthepresenceofalargenumberofinclusionsinthematerial.Inclusionsandstringerscreatedlargestressconcentrationsathardparticlematrixinterfaces.ThemechanicalbondbetweentheFig.5ThroughandthroughsharplongitudinalcrackinaflowformedtubeFig.6MicrocracksinflowformedtubeFig.7Sampleindicatinginclusions100Fig.8Flowformedtubewithfishscalesafterfinalpass