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69PracticalFailureAnalysisVolume1(5)October2001AnInvestigationoftheDevelopmentofDefectsDuringFlowFormingofHighStrengthThinWallSteelTubesK.M.RajanandK.Narasimhan(Submitted12June2001；inrevisedform6August2001)Flowformingtechnologyhasemergedasapromising,economicalmetalformingtechnologyduetoitsabilitytoprovidehighstrength,highprecision,thinwalledtubeswithexcellentsurfacefinish.ThispaperpresentsexperimentalobservationsofdefectsdevelopedduringflowformingofhighstrengthSAE4130steeltubes.Themajordefectsobservedarefishscaling,prematureburst,diametralgrowth,microcracks,andmacrocracks.Thispaperanalyzesthedefectsandarrivesatthecausativefactorscontributingtothevariousfailuremodes.Keywords:metalforming,inclusions,expertsystem,microstructure,surfacefinishPFANF8(2001)5:69-76©ASMInternationalIntroductionFlowformingisapromising,economicalmetalformingtechnologythatcanmeetthechallengingrequirementsofhighspecificstrength,closedimen-sionaltolerances,andexcellentsurfacefinishde-mandedbythedefenseandaerospaceindustries.Therelativelylowtoolingcostandremarkablematerialutilizationduetochiplessmetalformingprovideeconomicdrivers,whiletheabilitytoachievehighstrengthfinishedproductfromlowstrengthstartingmaterialisduetostrainhardening.Flowformingisanincrementalformingprocessthatusesa3-Dvariationofbasicrollingprocessesandcombinesrolling,shearing,andbendingintooneoperation.Itissimilartoneitherupsettingnorswag-ing.Essentiallyapointdeformationmetalformingprocess,flowformingresultsinaparthavingahighlydeformedmicrostructure.Significantincreasesinyieldstrength,ultimatetensilestrength,andhard-ness,andacorrespondingreductioninductility,accompanytheflowformingprocess.Conventionally,tubesareproducedbyhotextru-sionfollowedbydrawingorpilgering.However,itisnotpracticaltohotextrudethinwalltubesbeyondaspecifiedlimit.Drawingisaneasierandlessexpen-siveprocessthanextrusion；therefore,athickwalltubeiscoldextrudedandfinishedonadrawbenchorpilgermill.Thedrawingprocessisessentiallyatensileprocess.Microcracksandotherdefectsinsidethematerialtendtopropagateduringthedraw,leadingtofailure.Theareareductionistypicallylimitedto10%foreachdrawofahardmaterial,andthetotalreductionmayrequireanumberofanneal-ingcycles.1Anincreaseinthenumberofdrawing/annealingcyclesincreasesthecostofproduction.Itisobviouslyveryexpensivetousedrawingoperationstoproducecomponentsfromhard-to-workmater-ials.However,ifdimensionaltolerancesarenotcritical,itisadvisabletousecheapconventionalcolddrawnsteel(CDS)tube.Flowforming,therefore,offersseveraladvantagesoverconventionaltubemakingmethods.TheFlowFormingProcessFlowformingisusedtoproduceaseamlesstubewithtightdimensionaltolerances.Seamlesstubing,theoretically,mayrepresenttheultimateinreliabil-ity.2Ametalblankorpreformisformedoverarotatingmandrel.Themetalblankandthemandrel(whicharelockedtogether)rotate,andtheformingrollerfollowsthemandrelatapresetthathasbeenprogrammedintoaCNCflowformingmachine.Thepreformmetalisplasticizedbythelocalap-plicationofheavycompressiveforcesexertedbyconi-calrollers.Thedeformedmetaltakestheshapeofthemandrel,andproperwallthicknessisachievedbycontrolofthegapbetweentherollersandthemandrel.Flowformingcanbedividedintotwodistinctprocesses:forwardflowformingandreverseflowforming.Inforwardflowforming,rollerfeedanddeformedmaterialmovementareinthesamedirec-tion.Theformedmaterialisundertension,andtheK.M.Rajan,ArmamentResearchandDevelopmentEstablishment,Pune-411021,India.K.Narasimhan,DepartmentofMetallurgicalEngineeringandMaterialsScience,IIT,Bombay-400076,India.Contacte-mail:naramet.iitb.ac.in.AnInvestigationoftheDevelopmentofDefectsDuringFlowForming(continued)70PracticalFailureAnalysisVolume1(5)October2001materialundertherollerexperiencescompressivestress.Inreverseflowforming,rollerfeedandmater-ialmovementareinoppositedirections.Thematerialundertherollerisinacompressedstatewhiletheformedpartisstressfree.Areverseflowformingtechniquewasusedforthepresentstudyandissche-maticallyillustratedinFig.1.ObjectivesThispaperdescribesaninvestigationofthecauseofdefectsandfailuresduringflowformingofthinwallhighstrengthtubesfromAISI4130steel.Theinvestigationincludesstudyofthevariousmetal-lurgicaland/ormachineparameterslikelytocausetheseproblems.Thecommondefectsreportedforflowformingoftubesarediametralgrowth,prematureburst,buildup,fishscaling,andbellmouthing.3Othertypesofdefectsincludeunevenwallthickness,microcracks,andmacrocracks.4,5AdvancedCNC3-rollerflowformingmachinesareabletomanu-facturehighqualityprecisiontubes.However,metallurgicalormaterialdefectssuchasinclusionsandnonuniformgrainstructuremayaffecttubequality.Additionally,selectionofunsuitableformingparametersmayalterthequalityofthetubingpro-duced.Thispaperrelatesthemetallurgicalqualityofthesteeltotheabilitytoachievehighqualitythinwalledflowformedtubing.5,6ExperimentalObservationsBecauseofitsavailability,lowcost,andreasonablygoodcoldformability,SAE4130steelwasselectedforthemanufactureofthinwall,highstrengthseam-lesstubesinpressurevesselapplications.Reverseflowformingwasusedforthesepressurevessels.ThedevelopmentworkdescribedinthispaperusedSAE4130steelthatwasnotelectroslagre-melted.Theinclusionratingsofthisgradeofsteel,basedonASTME45,areasfollows:SulfidesAluminaSilicatesGlobularoxidesThinThickThinThickThinThickThinThick1.50.52.51.01.51.50.5·Thedissolvedoxygen,nitrogen,andhydrogeninthesteel,asdeterminedbyanalysis,was22ppmoxygen,110ppmnitrogen,and3ppmhydrogen.A4-axisCNCflowformingmachinewitha3-rollerconfigurationwasusedfortheexperimentalwork.AphotomicrographoftheflowformingmachineisshowninFig.2.Thepreformathickwalledstartingmaterialforflowformingwasfabricatedbyforging(bothupsettinganddrawing),piercingwithtaperedpunches,andmandrelforging,followedbyhardeningandtempering.Theflowformingoperationwascompletedinthreepasseswithoutanyintermediateanneal.TheFig.1SchematicdiagramofreverseflowformingprocessFig.2Four-axisCNCflowformingmachinewitha3-rollerFig.3TrimmingandcuttingoftensilespecimenfromfullyformedtubeMandrel71PracticalFailureAnalysisVolume1(5)October2001hardnessandpercentagethicknessreductionassoc-iatedwitheachpassareshowninTable1.Theflowformedtube,afterundergoingatotalpercentagethicknessreductionofabout88%,wastrimmed,andtensiletestsamplesweretakenfromafullyformedextralengthofthetubeasperASTMA370(Fig.3).ThespecifiedandachievedmechanicalpropertiesanddimensionalaccuraciesarepresentedinTable2.ProofPressureTestingTubesthatsatisfiedthespecifiedmechanicalprop-ertiesanddimensionalaccuracyrequirementsweresubjectedto100%hydraulictesting.Thetubesweresubjectedtoapressure10%abovethemaximumexpectedoperatingpressureforaboutonemin,thencheckedforpermanentset,ifany,andprematurefailure.Notestingininducedfailureorpermanentsetwasaccepted.Afterpassingthistest,thetubesweresubjectedtobursttesting.BurstPressureTestingOnetubeoutofeachgroupmanufacturedfromthesameheatofsteelandlotofprocessedpreformswasrandomlyselectedandsubjectedtoburstpressuretesting.Theburstpressuretestconfirmsthemarginofsafetyoverthemaximumexpectedoperatingpressure.AfewbursttubesareshowninFig.4.DefectsDefectsintheflowformedtubesmaycausefailureintheprooforthebursttests.Thetypesofdefectsandassociatedfailurescanbecategorizedasmicro-cracks,macrocracks,diametralgrowth,ovality,fishscaling,andprematurebursting.Theflowformingprocessoccasionallyintroduceswavinessorbulgesontheouter-faceoftubes.Suchdefectsoccuronlyundercertainworkingconditions.Thisphenomenoniscalled“plasticflowinstabil-ity.”5Kobayashi7analyzedinstabilityinconespin-Table2MechanicalpropertiesanddimensionaltolerancesofflowformedtubeMechanicalPropertiesDimensionalTolerancesUTS0.2%yieldstrength%EIOvalityStraightnessSurfaceroughness(MPa)(MPa)(mm)(mm)CLA(µm)Specified1200(min)900(min)6(min)0.2(max)0.15(max)-Actual1250-1350950-11007-80.15-0.200.1-0.15N5-N6Table1FlowformingsequencewiththicknessandhardnessvariationsineachpassPassNo.InitialthicknessFinalthicknessInitialhardnessFinalhardness%thicknessofPreform(mm)(mm)(HRC)(HRC)reduction119.50%9.7521285029.75%4.87528305034.875%2.00303160Fig.4BurstpressuretestedflowformedtubeAnInvestigationoftheDevelopmentofDefectsDuringFlowForming(continued)72PracticalFailureAnalysisVolume1(5)October2001ningandconcludedthatwrinklingontheprespunflangeoftheconeiscausedbyreliefoftheresidualcompressivestressthatarisesinthetraction-freeflange.Theconespinninginstabilityissimilartothewrinklingphenomenonindeepdrawingopera-tions.GurandTirosh5studiedtheplasticflowinstabilityintubespinningandfoundthatinstabilityoccursifthecircumferentiallengthofcontactSoftherollerismuchlongerthantheaxiallengthofcontactL,i.e.,iftheratioS:Lisveryhigh.HighS:Lratioshelpmaintainacircularcross-sectioninthetube,whilelowratiosleadtogeometricinaccuraciesduetohighconstraintforflowintheaxialdirection.Theseinstabilities,wavinessbulges,andovalityarerelatedtocontrolofpressurevariables,notthemater-ialqualityand,hence,shouldnotbeconsideredmetallurgicaldefects.CrackingAfullyflowformedtubefromthenon-electroslagremeltgradeSAE4130steelwithatotalreductionof90%thicknessoftencrackedafterpressuretestinganddimensionalinspection.Along,sharp,longi-tudinalthroughwallcrackdevelopedinthetubeasshowninFig.5.Metallurgicalinvestigationofthefailedtuberevealedthepresenceofalargenumberofinclusionsinthematerial.Inclusionsandstringerscreatedlargestressconcentrationsathardparticlematrixinterfaces.ThemechanicalbondbetweentheFig.5ThroughandthroughsharplongitudinalcrackinaflowformedtubeFig.6MicrocracksinflowformedtubeFig.7Sampleindicatinginclusions(100×)Fig.8Flowformedtubewithfishscalesafterfinalpass