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Rémi HUSSON, Jean-Yves DANTAN, Cyrille BAUDOUIN, Serge SILVANI, Thomas SCHEER,
Régis BIGOT Evaluation of process causes and influences of residual stress on gear distortion
-CIRP Annals- Manufacturing Technology - Vol. 61, p.551-554 - 2012
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Evaluation
of
process
causes
and
influences
of
residual
stress
on
gear
distortion
Re´mi
Husson
a,b,*
,
Jean-Yves
Dantan
(2)
b,
Cyrille
Baudouin
b,
Serge
Silvani
(3)
a,
Thomas
Scheer
a,
Re´gis
Bigot
ba
Renault–PowertrainProcessEngineeringDivision,FRCTRB02280,67,ruedesBonsRaisins,92500Rueil-Malmaison,France
b
ArtsetMe´tiersParisTech–LaboratoiredeConceptionFabricationCommande(EA4495),4,rueAugustinFresnel,57078METZCedex03,France
1. Introduction
The inherent phenomena of the manufacturing processes involve some variations of product properties. In the case of gearbox,theyaffecttheproductfunctionalitiesandquality[1,2]. Toimproveproductquality,itisimportanttoknowthecausality betweenProcessKeyCharacteristicsandProductProperties.This paperfocusesontheheattreatmentwhichisappliedtoimprove mechanicalpropertiesandtomatchspecifications.Heattreatment alsohassideeffectssuchasgeometricalvariationsonglobaland localscales[3].
Distortionisrelated toseveral causes. Thisstudyfocuseson residualstress,whichisoneofthesefactors,anditsinfluenceson distortionafterheattreatmentareevaluatedthroughexperiments andsimulation.
2. Stateoftheart
Distortion during heat treatment is due to these major phenomena:
spatial and temporal heterogeneity of temperature during heatingandquenchingwhichleadstoheterogeneousdilatations
[4];
timingofphasetransformations,forexamplefromausteniteto martensite[4,5];
transformationinducedplasticity(TRIP)[6];
decrease ofyieldstrength whentemperatureincreaseswhich causesstressreliefbyplasticdeformation[4].
Distortionafterheattreatmentcanbeseenastheresultofa distortionpotentialgraduallystoredintothematerialallthrough the process [7,8]. Each manufacturing step contributes to the distortionpotentialwhichisphysicallyrelatedtophysicalcarriers. Carrierspresentdependenciesoneachother,asshowninFig.1[9]. Inthisstudy,themanufacturingprocessincludescoldforging, machiningandheattreatment.First,thereliefofresidualstress inducedbyforgingaffectsdistortion[10].Then,duringmachining steps,turningandgearhobbingmodifyresidualstressdistribution and distortion [11,12]. Consequently, amongst all carriers of distortionpotentialdescribedinFig.1,thisstudymainlydealswith residualstress.
3. Objective
Theglobalaimofthisprojectistoimprovetheunderstandingof gearboxshafttoothdistortionduringheattreatment.Experiments and simulationsarecarriedoutwithasecondary gearboxshaft comingfromautomotiveindustry.
Ithasbeenobservedthatteethdistortionisaconsequenceof sizeandshapechangesofthebase-body[3].Moreover,metrology softwaredoesnotalwaysapplytheindependenceprinciple.Thus, previous works proposed a waytodistinguish localand global deformations in measurement analysis[13,14]. As a result, we considerglobalandlocalgeometryindependently.
Thefirstgoalofthepresentstudyistoprovetheinfluenceofthe carrier ‘‘residual stress’’ [9] on heat treatment distortion. The secondobjectiveistoobserveifdeformationbehavioursonlocal and global scales are related. Thirdly as manufacturing chron-ologiessometimesdependsonproductionplants,theimpactofthe manufacturing chronology on residual stress is investigated by simulation.Theoverallobjectiveistoknowmoreaboutresidual stress:itsconsequencesongeometryafterheattreatmentandits causestowardsmanufacturinghistory.
ARTICLE INFO Keywords: Heattreatment Deformation Residualstress ABSTRACT
In theautomotive industry, heattreatment ofcomponents is implicitlyrelated to distortion. This phenomenonisparticularlyobviousinthecaseofgearsbecauseoftheirtypicalandprecisegeometry. Evenifdistortioncanbeanticipatedtoanextentbyexperience,itremainscomplextocomprehend.This paperpresentsanapproachtoestimatethedistortionbasedontheideaofadistortionpotentialtaking intoaccountnotonlygeometrybutalsothemanufacturingprocesshistory.Thentheideaisdeveloped throughsimulationandexperimentsincludingannealingtounderstandtheimpactofresidualstresson geardistortioninanindustrialcasestudy.
* Correspondingauthorat:Renault–PowertrainProcessEngineeringDivision,
4. Residualstressexperiments
4.1. Experimentalprocedure
First inordertoobserve theinfluenceofresidualstressafter machiningonheattreatmentdistortion,behavioursofvariouslots areexperimentallycompared,asshowninFig.2.Aftermachining, gearboxshaftsareseparatedinthreedifferentlots.LotAisnot stressrelieved,lotBisstressrelievedduring4hat2008CandlotC isstressrelievedduring4hat6008Cunderanargonatmosphere. Residualsurfacestressesinaxialandtangentialdirectionsare measuredbyX-raydiffractionon3shaftsectionsatstages1and2. Materialcontainsmorethan95%offerriticsteelthereforeX-ray diffractionappearsas a thin andintense peak.Precision of the measuringtakesintoaccount curve-fittingerrorand reproduci-bilityerror.Itisestimatedto30MPa.
Bymeasuringresidualsurfacestressesbeforeandafterstress relieving,theinfluenceoftemperatureandtheefficiencyofstress relievingarequantified.Onaglobalscale,theorientationandnorm ofshaftbendingareconsidered.Then,teethsurfacegeometryis measuredbyaCoordinateMeasuringMachinededicatedtogear metrology.
Finallywehypothesizethatamongstallcarriersofdistortion, temperature,chemicalcomposition/segregationsand microstruc-ture/grainsizearesupposedidenticalforthethreelotsatstages1, 2and3respectively.
4.2. Residualsurfacestresses
The residual surface stress levels in tangential and axial directionsaftermachiningarescatteredandvaryapproximately from 310MPa to 370MPa, as visible in Fig.3.Lot Bresidual surfacestress(notrepresented)hasbeenmeasuredonlyonsection 3anditslevelisalmostthesameasaftermachining.Therefore,the 4h–2008Ctreatmentisnotefficientenoughtodecreaseradically residualsurfacestress.Takingintoaccountuncertainty,difference
aresignificantbetweenlot AandC. Infact,lotCresidualstress levelsdecreasetolessthan150MPainabsolutevalue,whichisless than 20% of the rupture stress of the studied steel. Thus, the residualstresslevelishypothesizedasnotsignificantafterthe4h – 6008Cstressrelieving.
4.3. Globaldistortions
First,circularrunoutismeasuredasafunctionofaxialangle. Radial lubricationholesvisible inFig. 2areused asanangular reference.In themeasuringplan, shaftis considered as an off-centered circle. For each shaft, identificationof eccentricity by curve-fitting is made on 4 sections. Then orientation and magnitudeofbendingisdeducedforeachshaft.
Thus,bycomparingglobaldistortionaftermachiningatstage1 andafterheattreatmentatstage3,bendinghasthetendencyto orientate towards the vertical direction which matches to lubrication holes, see Fig. 4. Moreover, bending increases in magnitude.Thisdistortionoftheshaftisdescribedasa‘‘banana distortion’’becauseofitsbendprofile.Actually,wedonotobserve anydifferencebetweenthe3lotsatstage2,thereforewedonot presenttheresultshere.Stressrelievinghasnoinfluenceonthe bending,ineitherorientationormagnitude.
4.4. Localdistortions
Bycomparisonbetweenstages1and2,lotBdistortionsarevery low on chosenteethcharacteristics,as visiblein Fig.5. In fact, variations are smaller than 2
m
m, which is the measuring uncertainty of the measuringmachine. Therefore, at this point gear distortion is considered as nullfor these criteria and the 2008Ctreatmentdoesnotappearasinfluentonresidualstressor geardistortion. On the contrary, lot Cworkpiecesshow higher variations,especiallyforfHbwhichisthehelixslopedeviationasdefinedin[1].Thetotalcumulativepitchdeviationontheleftflank Fp_lefthasalsoagreatermeanforlotCthanlotB.Thedistortionis
low for the profile slope deviation fHa [1]. Finally, the 6008C
treatment leadstodistortionsthat areveryclosetousualheat treatmentdistortions.
Duringheat treatment,lotA showshigherdistortionsinthe mean,especiallyontotalcumulativepitchdeviationFpforboth
flanksandonthehelixslopedeviationfHb,asshowninFig.6.LotB
workpieces have scattered results, therefore lot B has high standard deviation. Stressrelieved workpieces from lot Chave weakerdistortionsthanthosefromlotA.Therefore,4h–6008C stressrelievingleadstoweakerdistortion.Foralllots,distortions areveryweakinthecaseofprofileslopedeviationfHa.
4.5. Experimentanalysis
Theexperimentpresentedhereallowsustodeducetwomain results:
Residualstress level hasbeen decreased by the6008C – 4h heating. During heat treatment, stress relieved shafts have
Casting Rolling, Forming Soft machining Heat treatment Hard machining
Chemical composition/ segregations Microstructure / grain size
(Residual) stress Temperature Mechanical History
(Target) geometry
Interdependencies
Carriers of distortion potential
Fig.1.Processchainpriortooperational behaviourandcarriersofdistortion
potentialofatypicalcomponentforthetransmissionindustry[9].
Fig.2.Experimentalplan,3differentlots(A–C)aresetupaftermachining.LotAisnotstressrelieved,lotBisstressrelievedduring4hat2008CandlotCisstressrelieved
lower gear distortion than shafts that have undergone no stressrelieving.Nevertheless,bendingbehaviourisidenticalfor alllots.
Lot B shafts do not undergo gear distortion during stress relieving,thereforetheykeptthesamegeometryaslotAshafts. However,heattreatmentdistortionsarelowerinthemeanforlot B than lot A. Therefore, independently from geometry the ‘‘residualstress’’and/or‘‘mechanicalhistory’’carriersof distor-tionpotentialhaveaninfluenceongeardistortion.
As seen in Fig. 1 [9], dependencies exist between carriers of distortion potential. In the aforementioned experiment, the implicit hypothesis of identical geometry, temperature, chemical composition/segregations, microstructure/grain size hasbeenassumedforthe3lotsatstage2.However,mechanical history has not been considered. The next section will focus on the influence of mechanical history onthe residual stress state.
5. Residualstresssimulation
5.1. Simulationprocedure
In order to observe the influence of mechanical history on residualstress,twodifferentvirtualmanufacturingprocesseshave beencomputedbyFEMsimulationon‘‘Forge2009’’1
inthecaseof a Z axis 20MnCr5-steel cylinder (length: 100mm, diameter: 30mm).
Theinitialstate:first,+700MParesidualstressinZZdirection fortheinsideand 600MParesidualstressinZZdirectionforthe outsideisapplied.Initialvaluesof 600MPaand700MPabefore balancinghavebeenchosenarbitrarily.Infact,theideaistoapply highresidualstressvaluestoensurethevisibilityofresults.
Then,ageometry/residualstressbalanceoperationtakesplace insimulation.
Process1:theshaftisdrilled(12mm-throughhole)thenturned (2mm-depthcutting).
Process2:theshaftisturned(2mm-depthcutting)thendrilled (12mm-throughhole).
MachiningoperationsinForge1
donotimpactresidualstress butonlyremovematerial.Afterthat,residualstressisbalanced. Drillingandturningareidenticalandleadtothesamegeometrical removal.Temperature,microstructureandchemicalcomposition areconsideredashomogeneousduringsimulations.Consequently, theonlydifferencebetweenbothprocessesistheirhistory.
5.2. Simulationresults
Twoprocessesareappliedinparallel,drillingontheonehand andturningontheotherhand.Residualstresslevelsareobtained by simulation. Therefore, uncertainty can be due to numerical errorsandduetoadifferencebetweenmodelsandexperiments. Nevertheless,thegoalhereistoobserveifresidualstresslevelsare significantlydifferent;highprecisionisnotrequired.
Finally by material removal and residualstress balance,the residualstresslevelsofbothpartsdecrease.Nevertheless,residual stressdistributionsaredifferent.Drilling–turningprocessleadsto greatermagnitudesof ZZdirectionresidualstress,from 20to
Fig.3.Residualsurfacestressesontangential(a)andaxial(b)directionsfor3
sectionsof2shaftsfromlotAand2shaftsfromlotC,representedwithstandard
deviationplusuncertainty. (a)Residualsurfacestresstangential direction,(b)
residualsurfacestressaxialdirection.
–0.02 –0.01 0 0.01 0.02 –0.03 –0.02 –0.01 0 0.01 0.02 0.03 –0.02 –0.01 0 0.01 0.02 –0.03 –0.02 –0.01 0 0.01 0.02 0.03 (a) (b)
Bending Amplitudes and Orientations (mm) lubrication hole
Fig.4.Angularorientationofbendingaftermachiningatstage1(a)andafterheat
treatmentatstage3(b).Crosssection(orthogonaltotheshaftaxis)ofworkpieces
comingfromlotsA,BandC.Bendingorientationandmagnitudeareidentifiedfrom
runoutmeasurements.
Fig.5.Geardistortionsfor1shaftfromlotB:4h–2008Cstressrelieving(SR)and1
shaftfromlotC:4h–6008Cstressrelieving,representedwithuncertainty.Fp:total
cumulativepitchdeviation,fHb:helixslopedeviation,fHa:profileslopedeviation.
Fig.6.GeardistortionsduringheattreatmentforlotsA(5shafts),B(5shafts)andC
(3 shafts) represented with standard deviation plus uncertainty. Fp: total
300MPa(seeFig.7).Decreasinggradientishighwithinthefirst hundredthsof millimetre from theinside of theshaft. Such a distributionis inherited fromtheinitial state and heart disap-pearance.Infact,duringdrilling,theheartofthepartdisappears, removinghigh levelsof residual stress.Thus, after this manu-facturingstep,positivestressareasaresmaller,andthenthepartis balanced.Onthecontrary,theturning–drillingprocessgenerates smallervariations.Thiseffectcanprobablybeexplainedbymore progressiveresidual stressbalances withinthepart during the turning–drillingprocess.
5.3. Simulationanalysis
During this simulation, high residual stress levels and significantmachiningstepshavebeenchosentoprovidevisibility. Accordingly,significantdifferencesareobservablebetweenboth finalparts.Geometricallyspeaking,themanufacturingstepshave beenidentical;historywastheonlyvariableunderconsideration. Thus,process chronologyappears tohavean influenceon final residualstress.
6. Conclusion
Distortion potential is dependent on various factors of influence,alsoknownascarriersofpotential.Inthisstudy,due tothemanufacturingprocessandstateoftheart,thefocushas beenonresidualstress.
The experimental plan improves the understanding of the consequencesofresidualstressonheattreatmentdistortion:
Greater magnitudes of residual stress lead to higher tooth distortions. Residualstress is an influentcarrier of distortion potential.
On the other hand, the influence of residual stress on the orientation and magnitude of shaft bending has not been observed. Therefore, influences of the residual stress on distortion arehighon a localscale butnot on a globalscale. No relationshiphasbeenobservedbetween thetwoscalesof distortion.
Simulation revealed the influence of mechanical history on residualstress.Dependenciesexistbetweenthosetwocarriersof potentialdistortionatasignificantlevel.
Asaresult,residualstresshastobetakenintoaccountinthe manufacturingprocessincludingheattreatment.Residualstressis a highlyimportantcarrierofdistortion,especially forthetooth surface.Asafirstconsequence,takingintoaccountresidualstress in manufacturingprocessesdesign and process chronologyis a way to limit distortion. Concretely, a solution to decrease distortioncouldbetoapplystressrelievingbeforeheattreatment. Inthiscase,distortionoccurringduringstressrelievingshouldbe anticipated or corrected by machining. Secondly, once critical manufacturingstepsandrequiredresidualstressdistributionare identified,cuttingconditionscanbeupdated.Therefore,residual stresswillbetakenintoaccountin-process.
A future work will be to study the consequences of stress relievingbeforeheattreatmentinmoredetail.Ithasbeenobserved that decreasingresidual stresslevelsis a waytodecrease heat treatment distortion. Nevertheless, residual stress also has an impacton thefatiguestrengthoftheworkpiece.Bothgeometry and mechanical characteristics are product properties that are influenced by additionof stress relieving in themanufacturing process.
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Fig.7.ResidualstressinZZdirectionfordrilling–turningprocessshaftandturning–
