carriedfunny

THESTORYOFSHOTPEENINGTHEAUTHORistheChiefMetallurgistoftheBelmont,CaliforniaplantoftheWesternGearCorporation.
HeisagraduateoftheUniversityofCaliforniain1950andholdsaBachelorofSciencedegreeinPhysicalMetallurgy.
HeisamemberoftheAmericanSocietyforMetalsandtheAmericanElectroplatersSociety.
Mr.
LeghorndesiresthatduecreditbegivenMr.
JohnO.
Almenwholenttechnicalassistanceandadviceinthepreparationofthispaper.
TheSocietyreceivedpermissiontopublishthispaper,gratis,throughthecourtesyofCaptainE.
E.
Sprung,U.
S.
Navy,Retired.
GEORGELEGHORNthatdrovethespearheadintothearmorinonecaseandcausedthearmortobendthespearheadinanother.
Itwastheplainoldfactthatthereliablearmorerknewfrompracticalteststhatcoldhammeredweaponsandbreastplateswereharderandstrongerthanthosewhichhadbeenplacedinafire.
Theintuitivemystic,ontheotherhand,wassoenthralledbythesupposedmeritsoffirethathislastoperationwastoheatthemetalandthusannealorsoftenit.
Also,theunreliablearmorermayhavebeensmart--butlazy.
Heobservedthatthelongerhehammeredthecopper,theharderitbecametoformittothedesiredshapeandheknewthatbyheatingit,hecouldrestorethesoftnessandductilityandthusmakehisjobeasier.
Byannealingjustprevioustothefinallighttapsofthehammer,heleftthearmororspearheadsoftandprobablycostthelifeofhiscustomer.
Ifcopperishammeredtoomuch,itcanbecomebrittleandsubjecttocracking;however,ittakesalotofhammeringtodothis.
Itdoesshow,nonetheless,thatthearmorerhadtouseexperiencedjudgmentinformingaspearheadfromaroughblockofcopper,First,hehadtofiretreatit(anneal)tomakeitsoftandductileandreceptivetohammerforming.
Henexthammeredittoroughformandthenreannealedittoremovethehammerhardnesspriortothefinalhammeringofthespearpointtoitsdesiredshapeandstrength.
Knowingjusthowfarheshouldcarryhisfirsthammerformingandannealingsothatfinalshapingbyhammerwouldgivetheweaponexcellenthardnessandstrengthwithoutbrittleness,calledforalotofexperienceandjudgmentonthepartofthearmorer.
SHOTPEENINGisameansofcoldworkingthesurfaceofmetalpartsbymeansofahailorblastofroundmetalshotdirectedagainstthesurface.
Itisequivalenttoamyriadofsmallhammerblowsimpingedovertheentiresurfaceindentingthesurfaceandcausingplasticflowandworkhardeningofthesurfacemetal.
Thisworkhardeningofthesurfacemetalincreasesitstensilestrengthandyieldpointandasmallpercentageofthebeneficialresultsofshotpeeningareattributabletothiseffect.
Thegreaterbenefit,byfar,however,resultsfromthefactthatthesurfacemetalisupsetandputintocompression;thesurfacecompressivestressisextremelybeneficialinobviatingfatiguefailures.
Whatfatiguefailuresareandhowshotpeeninghelpstoeliminatesuchfailureswillbedealtwithlateron.
Shotpeeningisafairlyrecentrenovationofaveryancientartandinordertohaveaclearconceptionofwhatoccursinthepeeningprocess,itmightbewelltogobackinhistoryandconsidersomeoftheearlyaspectsofcoldworkingor,asitwouldhavebeenreferredtoinoldendays,hammerhardening.
IntheIliad,theGreekchronicler,Homer,referstocopperbreastplatesandspearheadsandverypoeticallydescribeshowsomespearpointspenetratedthebreastplateswitheasewhileothersmerelybentonstrikingandfelltothegroundwithoutmorethandentingthearmorofthewearer.
Theconnotationofhiswritingisthatsomemysticforcehadpermittedthespearofonewarriortopenetratethebreastplateofhisenemywhile,intheothercase,thearmorofonewouldbendandturnasidethespearofhisenemy.
Believeme,itwasn'tthepowerofpositivethinkingA.
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JournalNovember1957653LEGHORNAmodernmetallurgistwouldmeasuretheincreaseinhardnessandstrengthofthecopperformeasuredamountsofcoldworkingandthendetermine,byactualcontrolledtests,whichpercentageofcoldworkgavetheoptimumresultsfortheuseintended.
Ifthishappenedtocomeouttosay205reductionofmetalthicknessbycoldrolling,themetallurgistwouldthenspecifya20%coldrollreductionofthecopperandtheQualityControlSupervisorwouldsetupaninspectionproceduretoassurethatthiswasadheredto.
Asyoucansee,wehaven'tmadeanybasicchangesovertheancientarmorers'methods,butwehavedevelopedstandardmethods(coldrollinginthisinstance),standardteststodeterminejusthowmuchcoldworktogivethepart,andstandardsofqualitycontroltoassurereproductibility.
Modernman'sachievement,inthisandmanyotherancientarts,hasbeentotaketheguessworkoutoftheoperation.
Anotherhistoricaluseofcoldworkingwhichismoredirectlyanalogoustomodernshotpeeningwastheoldtimeblacksmith'sartofhammerpeeningthetensionsideofcarriagesprings.
Hefoundoutthatifhebenttheflatcarriagespringinthesamemannerthatitwouldbebentaspartofaloadedcarriageandthenballpeenedtheconvex(tension)sidewhilebent,hecouldimprovethelifeofthespringsevenforgreaterloadsorwhensmallerspringswereused.
Today,wecallthis"strainpeening"andsomeoftheincreasesinservicelifeofpartssotreatedarephenomenal.
Theoldtimeblacksmithmerelyknewfrompastexperiencethatitworkedsohewentaheadanddidit.
Hisshotwasasbigastheballontheballpeenhammerheused.
Thestrikingintensitydependedonthemassofthehammerandthestrengthofhismuscles,modifiedofcourse,bywhatevergoodjudgmenthehadmanagedtoaccumulatefrompastexperience.
Hispeeningratewasoneindentionatatimesoonewouldsuspectthatalargeballwasusedtocoverasmuchareaaspossibleperblowandthussavesometime.
Anycraftsmanwhowasproudofhisworkwouldaddthepeeningoperation;however,hecouldhardlyaffordtouseallofhisdayjusttoimproveafewsprings.
Hereagain,wehaveaddedbutlittletotheartasusedbytheblacksmith.
Wehave,however,greatlyincreasedthescopeofusageandourknowledgeastowhyofitseffect.
Morethanthat,wehavesetupstandardmeansfordoingthesamethingwithahailorblastofsmall,wellroundedshotofspecifichardness-wehavedevelopedmeansofspecifyinghowtodoagivenjobandwehavesetupcontrolstoinsureduplicationofresults.
MACHINESFORSHOTPEENINGMachinesforshotpeeningfallintotwodistinctcategories.
Oneisthewheeltypemachinewhereinabladedwheelrotatingathighspeedisgravityfedwithpeeningshotandspraysthisshotontotheworkbytheslingingactionoftheblades.
Thesecondistheairblasttypemachinewherein654A.
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November1957theshotisimpelledontotheworkbymeansofablastofpressurizedair.
Thewheelhastheadvantageofemittingaterrifichailofhighspeedshotandthusgreatlyshorteningthepeeningtimenecessarytogainthedesiredresult(archeight).
Duetothehighspeedandgreatvolumeofshotthrown,theworkmustnecessarilybeconfinedandfixturedinacabinet.
Theresultingwearontheimpellerblades,cabinetwallsandfixturescanbeexcessive.
Fixturingisalsomoredifficultthanwithairblastmachinesandthecabinetlimitsthesizeoftheparttobepeened.
Largewheeltypemachinesareinitiallyveryexpensivebutagreatmanyareusedwherethereissufficientproductionofthesametypesofitemstoconstantlyfeedthroughthemachine.
Thewheeltypeofmachineisgenerallycheapestforlargevolumeproduction.
Theairblastmachinesvaryastomethodoffeedingtheshotintotheairblast.
Insome,theshotissuckedupbythevacuumcausedbytheaspiratingactionoftheblast;inotherstheshotisgravityfedintotheairstreamandsometypeshavepressurizedairtofeedtheshotintotheairblastatthesamepressure.
Suchmachinesareusuallyofthecabinettype,althoughagoodmanyareutilizedinasealedroom.
Thegreatadvantageoftheairblastmachineisitsversatility.
Thenozzlecanbedirectedontotheworkasrequiredinsteadofhavingtofixtureandpositiontheworkinawheelblastwhichcannotbedirected.
Forpeeningindividualpartsorshortrunsofthesamepart(sayupto50parts),theairblasttypesarefarsuperiorandcheapertooperatethanthewheeltypes.
IwouldliketomentionherethesuperioradvantagesoftheVacu-Blastmethodforsometypesofwork.
Thisappliesparticularlytoworkwhichisroundinnatureandwhichmustbepeenedonthe'periphery.
Exampleswouldbelargegearstobeshotpeenedontheteethonly,orlongtorsionshaftsorpipeswhicharetobepeenedoveritsentiresurface.
DuetotheshotpickupprincipleincorporatedintheVacu-Blastdesign,itisnotnecessarytocarryouttheoperationinacabinetorsealedroom.
Althoughasmallamountofshotwillaccumulateonthefloor,itusuallyislessthanwillescapefromasupposedlysealedcabinetofawheeltypemachine.
ThemostimportantfeatureoftheVacu-Blastpickupnozzleistheeasewithwhichitcanbeappliedtothetypeofworkmentioned.
Alargegear,forinstance,canbesetuponatrunionandrotatedwhiletheblastnozzleistraversedacrossthegearface.
Variationsoftraversingspeed(avariablespeedmotorshouldbeused)andblastnozzleairpressurewillgiveacorrectcombinationofarcheightandcoverage.
(Note:Thesetermswillbedefinedlater.
)Inalikemanner,alongtorsionbaroroilwelldrillpipecanbesurfacepeenedmerelybyrotatingtheshaftorpipeoncentersandtraversingtheblastnozzlealongthelengthofthework.
Hereagain,thespeedoftraverseandblastairpressurescanberegulatedtogivethedesiredcoverageandarcheight.
THESTORYOFSHOTPEENINGLEGHORNTHESTORYOFSHOTPEENINGPeeningshotcanbepurchasedassuchfromagoodmanycompanies.
Itiswellroundedandscreenedtodefinitesizeranges.
Itusedtobeassumedthattheshothardnesshadtobeequaltoorgreaterthanthehardnessofthemetalbeingpeened.
Underthesecircumstances,theonlyshotavailabletopeencarburizedpartsortoolsteelshavingahardnessof60Rcwaschilledironwhichhasahardnessofabout63Rc.
However,ithasbeenfoundthatheattreatedsteelshothavingahardnessof46Rcisjustaseffective,asfarasincreasingfatiguestrengthisconcerned,asischillediron.
Steelshotismoreexpensiveinitiallythanchillediron;however,thebrittlenatureofchilledironcausesittobreakupveryrapidlyand,infact,ithasbutonefourthtoonesixththelifeofsteelshot.
Chilledironalsopresentsamajorseparationproblemsinceshotpeeningsspecificationsrequirethatbrokenshotbeseparatedandeliminatedfromthepeeningshot.
Steelshotisthereforemuchcheaperinthelongrun;itiscleanertohandleandcausesmuchlesswearoncabinetwalls,wheels,nozzlesandfixtures.
Cutwireshotisanothermaterialusedforpeening;howeversinceitismanufacturedbyclippingoffshortlengthsofsteelwire,itis,initially,notroundandhassharpedges.
Peeningspecificationsrequirethatthistypeofshotbeblastedagainstscrapsteeluntilitroundsoutandthisprocess,besidesbeingtimeconsuming,decreasestheshotlife.
Itissuggestedthatroundsteelshotbeusedforallpeeningoperationsonsteelparts.
Bypurchasingit,assuch,muchtimeandtroublewillbesaved.
ItmightbewelltoaddherethatanumberofauthoritiesintheshotpeeningfieldprefercutwiretocaststeelshotandJohnAlmenstatesthatcutwirecanbeusedascutonhardmetalssuchascarburizedpartsandthathepersonallyprefersclippedwiretoso-calledsteelshotforpeeningsteelhardenoughtoresistseriousnotching.
Clippedwirehastheadvantageofuniformityofmass(duetoclippingoffdefinitelengthsfromwiresofuniformdiameter)thatcannotbeachievedbyscreeningcaststeelshot.
Conditioned,roundedbypreviousblastingagainstahardenedsurface,clippedwireshotcanbepurchasedanditiscommonpracticetopeenwiththismaterialandtomakesmalladditionsoftheunconditionedclippedwireshotastheoriginalshotloaddiminishesduetowearordragout.
Actually,thesizeuniformityclaimissomewhatobviatedbythefactthattheshotwearsandbecomessmallerinuseandthelowerlimitofsizeisfinallydependent,aswithcaststeelshot,ontheefficiencyofthedustremovalsystem.
Someinterestingexperimentshavebeencarriedoutconcerningtheeffectonarcheightwhichoccursbyuseofnewconditionedclippedwireshoteachparticleofwhichhasadefinitemass.
Asamatterofidlespeculation,onecannothelpbutwonderatthemassofresearchdatathatwouldbeavailableonduplexing(theuseoftwoormoreshotsizesindifferentratiosastoquantityofeach)ifshotwasavailableonlyinpreciselydefiniteincrementsofsize.
Thisvariablemayormaynotbeimportantandagreatdealofresearchremainstobedoneonit.
Nonferrousmetalscanbepeenedwithballsofnonferrousmetals,ornonmetallicmaterialssuchasglass,plasticsandthelike.
Generallyspeaking,aluminumwouldbepeenedwithaluminumshot.
JohnAlmenstatesthat"somenonferrousmetalscanbedamagedbypeeningwithsteelorironshotbecauseseriouselectrolyticcorrosionmayresultfromembeddedironparticlesandthatthisisparticularlyseriousinmagnesium.
"Itmightbewelltonotethatsoftmetalssuchasmagnesiumandaluminumcanbeeffectivelypeenedwithglassbeads.
Inpeeningstainlesssteelwithsteelorironshot,careshouldbeexercisedtomakesurethatanyembeddedshotdustisremovedfromthesurfaceofthepartbyapostchemicaltreatment(passivating)sincesuchparticlescanmateriallydecreasetheresistanceofstainlesssteeltocorrosion.
Itis,perhaps,anottoowellknownfactthatstainlesssteel,thesurfaceofwhichhasembeddedironparticlesfromsuchsourcesasanironcontaminatedgrindingwheelorironoxideparticlesfrombuffingcompounds,ispronetorusting.
Manypuzzlingoccurrencesofrustinginstainlesscanbeattributedtothiseffectandagoodpostpassivationtreatmentcanobviatethetrouble.
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November1957655SHOTPEENINGVARIABLESPeeningrateandarcheightattainedcanvarywithshotsize,shothardness,shotspeed,quantityofshotthrownpersecondandangleofimpact.
Shotsizeisspecifiedashavingadiameterwhichisonehalf,orless,theradiusofthesmallestfilletorsurfaceirregularityofthecriticalareabeingpeened;therefore,ifaparthavingaonesixteenthinchradiusfilletistobeshotpeened,thelargestshotthatshouldbeusedwouldhaveadiameterofonethirtysecondofaninchorless.
Shothardnessforpracticallyallsteel(ferrous)partscanbe45-50Rc.
Inotherwords,around,heattreatedsteelshotcanbepurchasedandusedeffectivelyforallpeeningoperationsonsteelregardlessofhardness.
Speedofshotcanbechangedbyregulatingtheairpressureor,inthecaseofawheel,thewheelspeedshouldbeadjusteduntilthearcheightandcoveragerequiredcanbeattained(tobediscussedlater).
Quantityofshotthrownisimportantonlywithrespecttotheamountoftimeittakestodoapeeningjob.
Ifyoucanthrowalotofshotatadefinitespeed,thejobcanbefinishedinashortertime.
(Morepeenedpartsperhour).
Theshotblastshouldstrikethesurfacebeingpeenedasnearlyverticallyaspossible;however,itisseldomthatthiscanbeaccomplishedinactualpracticesincetheblastmaybedirectedatagrooveorfillet.
ItiscommonpracticetopeensuchsurfacesasthoughthebottomonlywasbeingTYPESOFSHOTUSEDFORPEENINGTHESTORYOFSHOTPEENINGLEGHORNtreatedandtoaccepttheresultsduetopeeningsomeareasatanangletotheshotblast.
Inotherwords,itisn'tworriedabouttoomuchexceptinverycriticalareas.
Inverycriticalparts(thosewhicharesubjecttorapidfatiguefailures)everyattemptshouldbemadetodirecttheshotblastperpendiculartothesurfacesubjecttothehighestloading(thepointwheretheparthasbeenfailing).
Somereductioninshoteffectivenesscanoccurduetoshotreboundwhentheblastisdirectednormaltoasurface.
Thisisparticularlytrueincentrifugalmachines.
Forpurposesofstandardization,qualitycontrolandreproducability,someformofmeasurementmustbesetupforanyspecificprocessand,fortheprocessofshotpeening,thestandardofmeasurementisarcheight.
ThismeansofmeasurementwasdevisedbyJohnAlmenwhileattheResearchLaboratoriesDivisionofGeneralMotorsandtheteststripusedalongwiththedialgageformeasurementarecalledtheAlmenStripandAlmenGage.
Thistestisbasedonthefactthatasheetofmetalwhichiscoldworkedononesideonly(asbyshotpeening)willdeform,duetothecompressivestressessetuponthecoldworkedside,andformaboworarc(hence--archeight)withthepeenedsidebeingconvex.
Themorethepeeningthegreaterthearcing,uptoacertainpoint.
Almenstandardizedthesheetofmetalused,bothastosireandphysicalcharacteristicsasfollows:656A.
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November1957steelblockwillgivethestripthesamearcheightasahardenedblockforthesameshotpeeningexposure;however,afteradozenormoretestexposures,thesoftsteelblockwillbebadlyindentedandmayneedreplacing.
Almenblockscanbepurchasedbut,inapinch,asoftsteelblockcanbemadeupforuse.
DimensionsandshapesoftheAlmenblockandgagecanbefoundintheSAEManualonShotPeening.
Inuse,thisblock,withsecuredAlmenstrip,issolidlyplacedatthesamepositionwithrespecttotheblastingnozzleorwheelastheworkitsimulates.
Inthecaseofagear,theAlmenblockwouldbeattachedtoadummyorscrapgearofthesamedimensionsastheonetobepeened.
Theattachmentcanbemadebyuseofthesamescrewsthatholddownthestrip.
Forshortruns,aflatwouldmerelybemilledontothegearsurfaceandthisflatwouldbedrilledandtappedforthehold-downscrews.
TheAlmenstripwouldthenbeattacheddirectlyontothemilledsurfaceforpeeningtrials.
ThethingtorememberisthatyouarepeeningtherootsofthegearteethsotheAlmenstripshouldbeatthesamedistancefromthenozzleastheroot.
Theshotblastwouldbedirectedverticaltotherootsurfaceandpeeningwouldtakeplaceasthegearrotatedonaspindlethuspeeningallofthegearteeth.
Inthecasewheretheshotpattern(areaofshotblastcoverageonthepeenedsurface)didnotcoverthefulllengthofthegearteeth,thenozzlewouldhavetobetraverseddownthelengthofthetoothwhilethegearisrotatingundertheblastthusassuringthatallofthetoothsurfacesarepeened.
Itissometimesconvenient,forfieldwork,totackweldaflangeontotheAlmenblocksothattheblockcanbe"C"clampedontoanykindofajury-riggedframesoastoholditatthenecessarydistancefromtheblastnozzle.
Inotherwords,adummygeardoesn'thavetobeafullgearbutmerelyanextensionfromtherotatingdevice(asanarm)ontheendofwhichtheblockcouldbeattached(bytackweldingorscrews)sothattheAlmenstripisatthesamedistancefromtherotatingcenteraswouldbetherootsofthegearteeth.
Imademention,previously,ofthenecessarydistanceoftheworkbeingpeenedfromthenozzleorifice.
Thisdistancecanbeanythatisrequiredtodothejob.
Thethingtorealizeisthatthecloserthenozzle,themoreintenseandrapidisthepeeningaction;however,theshotpatternisverylimitedinareaandthisnecessitatesmovingtheworkorthenozzleorbotharoundsoastocoverallofthesurfacetobepeened.
If,bymovingthenozzle3or4inchesfartherawayfromtheworksurfacetheblastpatternwouldcoverallofthesurfacetobepeened,thenthiswouldbethesimplestprocedure;here,however,itwouldbenecessarytoprolongthepeeningtimetoachievethenecessaryarcheightandcoverage.
(Coverageisthepercentageofpeenedsurfaceactuallyindentedorstruckbytheshot.
If5percentofthesurfaceoftheAlmenstripusedinthetestrun,ortheactualsurfacepeened,wasunindentedbytheshotthenthecoveragewouldbe95%).
STANDARDSOFMEASUREMENTFORSHOTPEENINGThesestripsofSAE1070coldrolledspringsteelareheattreatedto44-50Rc.
andmusthaveaflatnessof.
0015archeightasmeasuredontheAlmenGage.
Itshouldbenotedthatpracticallyallteststripspurchasabletodayareabout46-48Rc,andwithin0.
0005archeight.
ThesestripsaremountedonanAlmenBlock(Stripholder)whichisusuallycarburizedandhardenedormadeofheattreatedtoolsteeltogivesolidbackupforthestripand,primarily,topreventdamagetotheblockbycontinuousexposuretotheshotstream.
ItshouldbenotedthatasoftLEGHORNTHESTORYOFSHOTPEENINGBeforegoingontowhywewouldpeentoacertainarcheightandhowwewouldsetuptoaccomplishthis,Iwishtoclarifyexactlywhataspecificarcheightmeansandthe"A"and"C"strips.
SupposewehaveaGovernmentorotherspecificationthatreads"Thepartwillbeshotpeenedonareaasshowninaccompanyingprintto.
016A2.
"ThismerelymeansthattheAlmen#2(2for#2gage)gagewillbeused(thereusedtobeanAlmen#1gagebutitsusehasbeendiscontinuedsothereisactuallyonlyonemeasuringgagetoworryabout).
TheAstandsfortheAlmenAteststripandthe.
016istheactualthousandthsofaninchthatthedialgagepointcanadvanceduetothecurvatureoftheAstripbypeening.
Thedialofthegagereadsinunits,as1,2,3,4,5etc.
,buttheseareactuallythousandths.
Therefore,.
016A2meansa16readingonanAlmenAstripwhenanumber2gage(theonlygageinuse)isused.
Aspecificationof.
010C2meansa10readingonaCstripmustbeattained.
Sincethenumber1gagehasbeendiscontinued,youwilloften-timesfinddesignationsof.
018Aorsay.
012C.
Thenumber2gageisinferredinsuchacase.
YoumayevenseespecificationsasI4Aandthismerelymeans.
014A2.
TheAstripisuseduptoarcheightsof.
024A2andtheCstripabovethisreading.
Thereisadirect7to2ratiobetweenAandCstripreadings,thatis.
an.
024A2readingwouldbeequivalentto2/7(.
024)or.
007C2.
TheAstripisusedbetween.
004A2and.
024A2andtheCstripfrom.
007C2to.
020C2.
Shotpeeningintensitiesabove.
016C2areseldomifeverrequiredandmostpartswouldbeoverpeenedatthisarcheight.
Onethingtorememberinmeasuringarcheightisthatthesmoothorconcavesideofthepeenedstripshouldbeplacedagainstthegagesothatthesurfaceroughnesswillnoteffectthereading.
Apublishedroughguideastowhatarcheighttopeenpartstoisbasedonthethicknessofthepartandistabulatedasfollows:intensitytoreachdeepuniformmetal.
"Thetoleranceonpeeningtoaspecifiedarcheightis±.
002,therefore,aspec.
of.
016A2means.
014A2to.
018A2and.
010C2wouldbesatisfiedbyanarcheightof.
008to.
012C2.
Iwishtopointoutherewhyshotpeeningspecificationsaresometimessohaphazardlyapplied.
Thereisacorrectscientificwaytoarriveatanarcheightspecificationforanyparticularpartandtheprocedureistoshotpeenaseriesofgroupsofthesamepart(usually3or4toagroup)todefiniteintensitiesinsteps.
Thepartsofeachgroupwouldbepeenedtothesamearcheightandthegroupswouldbepeenedtosay.
006A2,.
010A2,.
014A2,etc.
Thesepartswouldthenberuntodestructioninfieldorsimulatedfieldtestsandthearcheightconformingtothegroupwhichgavethelongestservicelifewouldbespecified.
Thedrawbackhereistheextremeexpenseofsuchtestsandthatitmustbedoneoneverycomponentwhichispronetoservicefailuresincethepeeningintensitywhichgivesoptimumlifeononepartwillnotgenerallygiveoptimumlifeonanotherpart.
Tocounter-balancethiswehavethefactorthatshotpeeningapartbyguesswillgenerallygiveaveryadvantageousincreaseinfatiguelife,anditisreadilyrealizedthatitismuchcheaperinthegreatmajorityofcasestopeenbetween.
012and.
018A2andaccepttheincreasedserviceliferesultingfromthistreatmentthantotryandgetoptimumresults.
.
Withrespectto"guesspeening"deeppeeningwillreducethe"hazard,"ifany,ofthisprocess.
Someparts,however,havebeenscientificallytestedtoestablishtheoptimumshotpeeningconditionsandanymanufacturerwhohasexpendedsuchalargeamountofmoneytoestablishthisarcheighthasabonafiderighttodemandthathispartsbeshotpeenedundercontrolledconditionsandtoprecisearcheights.
Onthisbasis,andthebasisthat.
allmanufacturerswantqualitycontroltoguaranteetheuniformityoftheirproduct,wewilldiscussthemethodofcontrolledshotpeeningtomeetadefinitespecification.
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November1957657ThicknessofPartArcHeight1/16".
004A21/8.
008A1/4.
014A3/8.
018A1/2.
021A5/8.
007C.
008C7/8orgreater.
010CInactuality,80%ofallshotpeeningisheldintherangeof.
010A2to.
020A2andevenfilletsof"radiusona4"diametershaftareusuallypeenedto.
018A2.
JohnAlmengivesconsiderableinsightintotheartofadjudgingspecificationsforpeeningbyhisstatementthat"inpeeningclean,uniformparts,itisnotnecessarytousehighintensitiessinceallthatisneededistoavoidsurfaceweakness.
Roughforgedorotherparts,whichmaybedecarburizedorscaledshouldbepeenedwithsufficientSHOTPEENINGTOSPECIFICATIONSThefirststepistoprovideafixturetosupporttheAlmenblockwithitsteststripsothatitspositionsimulatesthecriticalsurfacestobepeened.
Second,aguessismadeastothepeeningconditionsthatwillbenecessarytomeetthespecification.
Asanexample:sayS330tru-steelshotisusedatanairpressureof50poundsandthenozzleispositioned3inchesfromthesurfacetobepeened.
Asaspecificexample,letusassumethattheworkisamaindriveshaft3"indiameterwithanintegralcouplingflange8"indiameterononeendandthattheareatobepeenedistheradius(fillet)wheretheshaftbecomestheflangeasshowninsketch.
Assume.
016A2isthespec.
(Seefollowingdrawing):THESTORYOFSHOTPEENINGLEGHORNAdummyfixture(showninlowerpartofdrawing)wouldbemadeupwhichwouldrigidlyholdanAlmenblocksothatthedistancefromthecenterofrotationtotheteststripwouldbethesameasthedistancefromtheaxisoftheshafttopointAwhichishalfwayupthefillet.
Theblockwouldbepositionedsothatitssurfacewasatrightanglestotheblaststreamwhendirectlyunderit(45°angletotheaxisofrotation).
ThefactthattheendsoftheAlmenstripwillcomeclosertotheblastnozzleduetotheblockbeingflatinsteadofroundisoverlookedsincetestsarebasedonthisarbitrarypositioninginthefirstplace.
Thepositionoftheblockshouldbethesameaswasoriginallyusedinderivingthespecifiedarcheight.
Ifnoderivationtestswereoriginallyrun,thepositioningdescribedwouldbeused.
Thirdly,usingthesetupdescribedashotpeening(archeight)curveisdetermined.
Thisisdonebyrunning(unpeened)newAlmenstripsforspecificperiodsoftimesuchas5,10,15,20,25,35,45,60,90and120seconds.
ThearcheightoneachstripistakenontheAlmengageandthesearcheightsaregraphedagainstthecorrespondingtimeasshown.
Sucharcheightcurvesareverycharacteristic.
Theygoupsteeplyatfirstthenrapidlychangetoamuchmoregentleslope.
ThepointX,justtotherightoftheslopechangeiscalledthesaturationpointandthisisthedegreetowhichsuchshotpeeningshouldbecarried.
Sayourfirsttrygaveusanarcheightatthesaturationpointof.
020A2.
Wewouldthendecreasetheairpressuretosomelowervalue,say40poundsandrerunthecurve.
Ifthesaturationpointonthegraphofthisruncameoutas.
013A2,wewouldincreasetheairpressureto,say45poundsandrerunthetest.
Saythesaturationpointcameoutat.
017A2.
Sincethespec.
is.
016A2(.
014-xO18A2)wewouldbeintoleranceandwewouldthenexaminetheAlmenstripat.
017A2tomakesurethatthecoveragewasasspecifiedorbetter.
Coverageisusuallyspecifiedat90%minimum.
Ifthetimeforthe.
017A2stripwas25secondsat45poundspressure,wewouldpolishthesidetobepeenedonanew658A.
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Journal,November1957FATIGUEFAILURESLEGHORNAlmenstripandthenshotpeenitatthissetting.
Thestripwouldberemovedandexaminedunderamagnifyingglassandanestimateofthecoverage(indentedsurface)made.
Innearlyallcasesthiswillbeabove90%atthesaturationpoint.
Ifitisnot,wewouldrepeattheforegoingtestsusingasmallersizeshotandwouldfindthatwecouldgetabove90%coveragebyusingsay40poundspressureandapeeningtimeof45secondstogiveusasaturationpointarcheightof.
016A2.
Wewouldthenbereadytostartcontrolledproductionpeeningtospecification.
Usingthesmallersizeshot,ashaftwouldbeloadedintotherotationfixtureandpeenedfor45secondsat40poundspressure.
Itwouldbewelltopointoutatthispointthat,whereasourpeeningtimehereis45seconds,itwouldtakeatleastfourminutesofpeeningtimetooverpeenand,eventhen,itwouldnotbedamagingbutwouldmerelystarttodetractfromthebeneficialincreaseinfatiguelifealreadygained.
Itisoftentimesthepractice,therefore,topeenbeyondsaturationtosomepoint(Z)onthecurvethusgivingassurancethatthesaturationpointhasbeenreachedandcompletecoverageeffected.
Indoingthis,wehaveincreasedthepeeningtimeso,topreventthearcheightfromgoingoverspec(.
018A2)wedecreasetheairpressureslightlytocompensate.
Imightaddthatthisisfairlycommonpractice;however,withalittleexperience,youwillfindthatpeeningtosaturationisnotdifficultandthisshouldbethetechniqueused.
Wehaverepeatedlystatedthatshotpeeninggreatlyextendsthefatiguelift,ofmetals.
ThedefinitionoffatiguetakenfromtheASM"MetalsHandbook"isasfollows:"thetendencyofametaltobreakunderconditionsofcyclicstressingconsiderablybelowtheultimatetensilestrength.
"ToquoteRobertBurns,"TheBestlaidplansofmiceandmengangaftaglae.
"Herewehaveacaseofmankindusinghisscientificknowledgetofindoutthestrengthofsteelinp.
s.
i.
,calculatetheloadingproducedonasteelmemberinamachineandthenverywiselydesigningthememberoutofsteelsothatitwouldtakealloftheloadingitwouldget.
Beingofacautiousnature,hemadethememberatleast50%strongerthanhistestsandcalculationssaidithadtobe.
Thisisknownasa"factorofsafety"andisaclearacknowledgementthatmankindisn'ttoosureofhimself.
It'sroughonadesignengineertofindoutthathisliberalpadofprotectionwasn'tliberalenoughinthelongrun.
Themachineperformedbeautifullyforawhile.
Thenthememberbusted;thoughthis"failurebyfracture"usuallyhadtheaggravatinghabitofoccurringaftertheguaranteeperiodhadpassed.
Itiseasytovisualizetheamountofsoul-searchingandrecriminationthattookplaceintheengineeringworld.
Itwasn'tjustonemachinethatbrokebutalotofthem.
TheengineersknewsomethingwaswrongandbegantowonderTHESTORYOFSHOTPEENINGifmaybethestaticloadingteststheyhadperformedsometiculouslyonbarsofsteelmightbeinapplicablewithrespecttomachineinmotion.
Tocutitshort,thisprovedtobetherightanswerandalotofresearchresultedinthefatiguediagramasfollows:10,000,000cyclesonthisdiagramisconsideredasanindefinitelifespan.
Uptoacertainstress(horizontal--nofailureline)youcouldcyclestressapartover10,000,000timesanditwouldnotfail.
Abovethislineyoucouldcycleitforsay200,000cyclesandthenitwouldbreak.
Atastillhigherstress(lessthantheultimatetensilestrengthasdeterminedbystaticloading)thepartcouldfailin1000cyclesorless.
Theresultis,thatforreciprocating.
rotatingorvibratingparts,wenolongerdesignonthebasisofstatictestsbutonthebasisoffatiguetests.
Partswhichfailbyfatiguearefairlyeasilyrecognizedbothbytheoriginofthefailureandtheclamshellmarkingsoverapartofthefracturedsurface.
Suchfailuresgenerallyoriginateatastressriserwhichcanbeafillet,hole,keyway.
forginglap,punchmark,seam,corrodedareaorsteelhavingvariationsinstructure(ashardspots).
Failureoriginatesatoneofthesepointsandacrackgraduallyworksintothepart.
Itshouldberealizedthatthepartstillhasamplecross-sectiontosustaintheloadingunderstaticstressesand,therefore,doesnotfailuntilthecross-sectionalareahasbeenreducedtoapointwhereitcannolongersustaintheappliedload.
Itthenfailssuddenly.
Thepart(area)whichletsgoallatoncehasacrystallineappearance,whereas,theareawhichfailedoveraperiodoftimehasaclamshellappearanceresultingfromtherubbingactionofthesurfacesoneachothera,illustratedinthisdiagram:A.
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November1957659THESTORYOFSHOTPEENINGLEGHORNHOWSHOTPEENINGINCREASESFATIGUESTRENGTHANDLIFEShotpeening,ineffect,raisesthedemarcationlineonthefatiguediagramasshownabove.
ThisdiagramisfromtheSAEManualonshotpeeningandis,ofcourse,idealizedfordemonstrativepurposes.
Itcanbeseen,thatforthestress(load)atpoint1,theservicelifeofthepartwouldbealmostdoubledandatpoint2,forthesameservicelife,theloadonthepartcouldbeincreasedbyabout40%.
Point4wouldallowanincreaseofloadingofabout50!
%f"withoutfailureandapartthatwouldfailwithinsay200,000cyclesatpoint3wouldlast10,000,000cyclesormoreifpeened.
Itcanreadilybeseenfromthisdiagramwhysomepartswhenshotpeenedcanhaveahundredfoldincreaseinservicelifeovercriticallyloadedunpeenedparts:WHYSHOTPEENEDPARTSRESISTFATIGUEFAILURE660A.
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November1957sideonly,althoughthisismerelyconjecture.
Thefactremainsthatthesurfaceofaworkingpartisextremelycriticalfatiguewise.
Underbeamloading,asshowninfollowingdiagram,theconvexsideofthepartisundertension(undesirable)andtheconcavesideisundercompression(desirable):Thesamepart,whenshotpeenedoverallintheunloadedcondition,hasitssurfaceincompressionduetothecoldworkingandplasticupsettingofthesurfacemetalbythehailofshot.
Sincethesurfaceisincompression,thecoreisintensiontobalanceouttheforcesasshowninfollowingdiagram:Inabout90%ofthecases,fatiguefailuresoriginateatthesurfaceofthepartatsomepointwhichactsasastressraiser.
Afewfailuresoriginateatsubsurfacedefectssuchasslaginclusions,porosities,flakes,orthelike,inthesteel.
Thisisparticularlythecasewherethesurfaceisincompressionduetoshotpeening,nitriding,carburizing,orthelike,andwherethepartisveryhighlyloaded.
Subsurfacefatiguefailuresarerareandneednotbedealtwithfurther.
Thesurfaceistheweakestpartofastructuresinceitisherethatimperfectionsthatactasstressraisersoccurand,alsoduetothefactthat,underbeamloading,thetensilestressesinthepartareatamaximum.
Evenso,theseexplanationsdonotwhollyaccountforthesurfaceweaknessofapartunderfatigueconditions.
Itmaybeduetothefactthatsurfacegrainsaresupported(buttressed)fromtheinnerIftheshotpeenedpartisbeamloaded,thestresswhichresultsisanadditivecombinationofthestressesduetobeamloadingandtheinitialstressesduetoshotpeeningasshowninfollowingdiagram.
LEGHORNHESTORYOFSHOTPEENINGTheresultantstressshownasaheavyblacklineshowsthatthesurfaceofthepartisstillincompressionevenontheconvexsideofthebeamloadedpartandfatiguefailuresdonotoccur!
inacompressedarea.
Thecenterisunderahightensilestress;however,itisnowherenearasdetrimentalhereasitwouldbeatthecriticalsurface.
Wecanloadthebarbeamwiseuntiltheconvexsurfaceofthepeenedpartisintensionbutthistensilestresswouldbefarlessthanitwouldbe.
ifthepartwerenotpeenedandwouldprobablybewellwithinthefatiguestrengthofthematerial.
Thesurfacecompressivestresssetupbyshotpeeningcounteractsthetensilestressessetupbybeamloadingandthusbringsthesurfacemetalintothesafetyareaofcyclicstressloading.
OTHERUSESOFSHOTPEENINGAsmentionedbefore,shotpeeningisprimarilyusedtoincreasethefatiguelifeofcyclestressedparts;however,thereareagoodmanyotherfieldswhichcanderiveuniquebenefitsfromshotpeening.
Thesewillbelistedanddiscussednumerically.
1.
Asanaidinpreventionofstress-corrosioncracking.
ThisformoffailurewasoriginallycalledSeasonCrackingand,whereasithasexistedaslongasmankindhasworkedwithmetals,itwasattackedasametallurgicalproblemafterBritishtroopsinIndiabegantoexperiencearashoffailuresofbrasscartridgecases.
Thesecaseswereinspectedonarrivalandweresupposedlysound;however,duringtheseasonalwetweathercharacteristicoftheIndianmonsoons,thecartridgecaseswouldsplitopenwithoutevenbeingfired.
Metallurgicalsleuthingrevealedthatthesecaseswerecolddrawnandthatsurfacetensilestressesweresetupduringtheprocess.
Itwasalsofoundthatmostmetalsandbrass,inparticular,wouldreadilycrackwhensuchpartswereexposedtocorrosivemediaorconditionssuchasthehot,humidmonsoonclimateofIndia.
Thesolutiontotheproblemwastoannealthecasestorelievethedamagingsurfacetensilestresseswhichwerelockedinbymechanicalworkingofthemetal.
JohnAlmengivesanotherversionofthename.
SeasonCracking,totheeffectthatbrasscarriagelampshadatendencytosplitopenduringtheearlysummerseasonandsubsequentinvestigationrevealedthatitwasduetothecorrosiveattackofammoniageneratedbybarnyardmanurepiles.
Asmentioned,annealingwillobviatethisformoffailure;however,shotpeeninghasalsobeenusedtocorrectthedefect.
Shotpeeningcounteractsthesurfacetensilestressesbysuperimposingsurfacecompressivestresseswhicharenotsubjecttocrackingundercorrosivecondition.
2.
Asanaidineliminatingfret-corrosionfailures.
Metalpartswhichfret(periodicallyruborimpact)againsteachotherundercorrosiveenvironmentaresubjecttopittinganddisintegration.
Sincesurfacecompressivestressesinpartssubjecttosuchfailureswillgreatlyamelioratethecondition,shotpeeningisresortedtoasacorrectivemeasure.
Thisisparticularlytrueonaircraftparts.
3.
Straighteningofpartsbyshotpeening.
AnAlmenstripbends(arcs)duetothecompressiveforcesandmetalupsettingcausedbypeening.
Thissameeffectcanbeusedtostraightenpartswhicharefairlythinorquitelargecomparedtotheircross-section.
Forinstance,largefairlythin("thick)ringgearswhichwereconsiderablyoutofroundhavebeenselectivelyshotpeenedtobringthemwithina.
003"outofroundtolerance.
Asanexaggeratedexample,wewillusepeeningtoroundoutasteelhoopwhichisellipticalinshape.
Automobileaxleshaftscanbestraightenedoutbypeeningafterheattreatingwithlifeincreasesofmorethanahundredfold.
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Peen-forming.
Peenstraighteningisactuallypeenforming;however,intheaircraftindustry,wholewingsectionsaregivencamber(wingcurvature)byselectiveshotpeening.
YoucouldvisualizetheoriginalplateasbeingalargeAlmenstripwhichisshotpeenedatavariableintensityoveritssurfacetogiveavariationincurvature.
Thisformingcouldbedonebydies;however,theconvexsurfaceofthepartwouldhaveadetrimentalsurfacetensilestresswhichwouldbepronetofailurebyfatigueorstress-corrosionattack.
Byshotpeening,thepeened(convex)surfaceisputintocompressionbytheplasticupsettingofthesurfacemetalwhiletheopposite(concave)surfacealsoissurfacecompressive(concavesideofabentpartisincompression).
Sawmanufacturersandsawfilersformcircularsawsbypeening.
Averyoldart.
Theydonotusesmallareasofimpactbuthammershavinglargeradiiofcurvature.
Anothersuchuseisthehammeringofpistonringstomakethemopenup.
Bydoingthis,theyhavetobeclosedin,compressed,tofitintothecylinderbarrelandassumetheiroriginaltruecircleshapewhensocompressed.
Weretheseringsmerelyflexbentoutward,theywouldyieldattheweakestpointandwouldnotresumeatruecircleformonbeingpressedintothecylinder.
5.
Shotpeeningasacorrectivemeasureforgrindingdefects.
Groundmetalsurfacesaregenerallyinasurfacetensilestateduetoupsettingofthesurfacemetalbytheheatgeneratedinthisprocess.
Severegrindingwithaloadedordensewheelwillgeneratesomuchsurfaceheatandupsettingthatthesurfacewillactuallysplit,oncooling,duetotheexceedinglyhightensilestressessetup.
Thesegrindingcrackslookmuchlikethecrazepatternonasun-bakedsheetofmudandarecausedbythesameforces.
Asasheetofmud,leftfromadriedpondorpuddle,dries,itshrinksduetoevaporationofitscontainedwater.
Thisshrinkagesetsuptensilestressesandthemudsheetdevelopsacrackpatternwhenthesestressesexceedthecohesiveforcesbetweenthemudparticles.
662A.
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November1957Shotpeeningcounteractsthesedeleterioussurfacetensilestressesbysuperimposingahighersurfacecompressivestress.
Belowisaroughgraphofsomeexperimentalresultsconductedongroundandshotpeenedspecimens.
6.
Shotpeeningofpartspriortochromeplatingtocorrectlossoffatiguestrengthinherenttochromeplatedparts.
Chromiumplatinggreatlydecreasesthefatiguelifeoftheplatedpart;however,byshotpeeningthesurfacestobechromeplatedpriortotheplatingoperation,thisdefectcanbeobviated.
Onthenextpageisagraph,basedonexperimentalwork,whichillustratesthis.
7.
Peeningasameansoftestingtheadhesionofelectroplates.
Sucharticlesassilverplatedaircraftbearingsrequireexcellentadhesionoftheelectroplatedmetaltothebasemetalandthisadhesioncanbecheckedonacontrolbasisbyshotpeeningaselectedpercentageofthesilverplatedbearings.
Ifadequateadhesionisnotpresent,theshotpeeningwillcausethesilverplatetoblisterandpeelatthedefectiveareas.
8.
Shotpeeningasameansofporositycorrectionincastings.
Smallporesincastingscansometimesbesealedbytheplasticflowofthesurfacemetalduetoshotpeening.
Itisnoteffectiveforlargepores.
9.
Shotpeeningtogiveanoilretentivelubricatingsurface.
Thisisaveryvaluabletechniqueandhasbeenlargelyoverlookedbytheengineeringprofession.
OneGermanautomobilehascylinderbarrelswhichhavebeenshotpeenedandthenchromeplated.
Theindentationscausedbytheshotpeeningareoilretentive.
Duetotheincreaseinfatigueproperties,thisprocessisprobablysuperiortotheporouschromeplatingofcylinderbarrels.
10.
Shotpeeningcanbeusedtoroughenandexpandslightlyundersizedpartstocreatetightfits.
Knurlingisusedtoaccomplishthesamething.
11.
Decarburizedsteelsand,particularly,partswhichareonlypartiallydecarburized,canbeimprovedbyshotInthisdevice,oneroll(B)rotatesatagreaterspeedthantheother(A)andsuchmaterialsaslinoleumscraparefedthroughthetynesforshreddingsothatthematerialcanbereprocessed.
Duetothedifferenceinspeedoftherollsandtheresistanceofthescraplinoleumagainstthetynes,theshreddertynesofrollBareflexed(beanloaded)inaclockwisedirection(Up-atthepointofcontactX)whilethoseofrollAareflexeddown(alsoinaclockwisedirection).
Ifthen,withreferencetothepointX,thetynesofrollBareflexedupandpeenedontheundersideandthoseofrollAareflexeddownandpeenedonthetopside,wewillhaveappropriatelystrainpeenedthetynesandgreatlyLEGHORNTHESTORYOFSHOTPEENINGpeening;however,shotpeeningwillnotimproveadecarburizedparttothepointwhereitisasgoodasaproperlyheattreatedpart.
12.
Ultrasonicactivatedparts,suchascuttingtools,whicharesubjecttoextremecycling(suchatoolexpandsandcontracts27,000timespersecond),canbegreatlyenhancedlifewisebyshotpeening.
Thisisacaseoffatiguepropertyimprovement.
13.
Anothercasewhichnicelyillustratestheimprovementinfatiguelifeduetoshotpeeningisitsuseonoverloadprotectiondevices.
Amachinewhichwassubjecttojammingandbreakageofintegralpartsduetooverload,wasgivenoverloadprotectionbymachiningafilletinthedriveshaftsoastocausetheshafttoshearatthefilletwhenajamoccurred.
Thisfilletservedthepurposeintendedbutfailedrapidlybyfatigue.
Byshotpeeningthefillet,thefatiguefailureswereeliminatedandtheoverloaddevicefunctionedeffectively.
14.
Deburringbyshotpeening.
Deburringcansometimesbeeffectivelyandeconomicallyaccomplishedbyshotpeening.
Apartwhichistobecarburizedorhardenedcan,insomecases,beleftundeburredandbehardenedbyshotpeening.
Thepeeningblastsawaythehardenedburrsandtheincreaseinfatiguestrengthresultingfromthepeeningmorethanoffsetsanydetrimentalsurfaceeffectscausedbytheburrsbeingbrokenaway.
15.
Somepartsareshotpeenedasameansoftexturingthesurfaceforestheticpurposes.
Iwouldliketomentionstrainpeeningatthispoint.
Thishaspreviouslybeenreferredtowithrespecttothehammerpeeningofcarriageleafspringsontheconvexsidewhilesubjecttoabendingloadandtestsshowthatitisparticularlyadvantageousinthespringfieldforbothcoilandleaftypes.
Ifacoilspring,subjecttocompressiveloadinginservice,iscompressedtoabout80%ofthetensilestrengthinajigandthenshotpeened,itsresultantfatiguelifecanbeincreasedsometwentyfoldoverthelifeexpectancythatwouldbeachievedifitwereshotpeenedtothesameextentintheuncompressedstate.
ThefatiguelivesofanidenticalsetofSprings,oneofwhichwasunpeened,onenormallypeenedandonestrainpeenedwouldbeoftheorderofmagnitudeof1to10to200.
Itshouldbeborneinmindthatanyflexedpartisessentiallyaspringwhetherusedforthatpurposeornotandthatbyflexingthepartunderajigimposedloadinthesamedirectionthatitwouldbeflexedinserviceandthenshotpeeningtheconvex(tensionstressed)side,wearestrainpeeningthepartanditsfatiguelifewillbegreatlyenhanced.
Aspecificexampleinmindwouldbetheuseofstrainpeeningtoprolongthelifeofshreddertynes.
Thesetynesaremerelyroundheat-treatedsteelspines,amultitudeofwhichprotrudefromtheperipheriesoftworollsasshowninsketch.
A.
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November1957663THESTORYOFSHOTPEENINGLEGHORNimprovedthelifeofapartwhichwouldotherwisebeverypronetofailure.
Inactualpractice,thesetyneswouldbeflexloadedandpeenedinaholdingjigandtheninsertedintotheholdingholesoftheshredderrolls.
Carewouldhavetobeexercisedtoseethatthetyneswereproperlypositioned.
AtthepointMonrollA,thepeenedsideofthetynewouldbetotheleftsincethistynewouldbeflexeddownwardattheshreddingpointX.
ThetynesatpointNofrollBshouldalsohavethepeenedsidetotheleftsince(atpointX)thetynewillbeflexedupward.
Analternativeandfarsimplermethodwouldbetoshotpeenthetynesoverallorcoldworktheirsurfacesinatumblingbarrel.
Thiswouldnotgivethelifeadvantageofstrainpeeningbutwouldgiveavalidlifeincreaseand,atthesametime,permitthetynestobeassembledatrandom.
ThereAremanyinstanceswhereonefactor(thegreaterlifeincreaseduetostrainpeening)willoutweightheother(simplicityofaccomplishmentandeaseofassembly)andviceversa.
Forinformativepurposes,wewillmentionsomespecificarticleapplicationsofshotpeening.
1.
HamiltonStandardusesshotpeeningonpropellerhubstoincreasethefatiguelife.
2.
Intheaircraftindustry,boltheadswhicharesubjecttohighloadingandvibrationarepeenedinthefilletbetweentheheadandtheshank.
3.
Starterrotorstheteethofwhichbrokeoutat45,000rpmduetofatigue,wereshotpeenedandgaveexcellentserviceatspeedsinexcessof45,000rpm.
4.
Otheritemsonenginessuchaspistonrods,tappets,shafts,rockerarms,etc.
wereshotpeenedintheircriticalareas(fillets,anglesorpointswherethepartschangefromheavytothincrosssection)toimprovetheirfatiguelife.
5.
Thefollowinggraphshowstheliferesultsonsomelatchspringsgivenvarioustreatments.
6.
Intheoilwellindustry,suchitemsasdrillpipe,suckerrods,mandrels,swivelpins,cutterbodies,etc.
arepeenedtogiveextendedlife.
7.
Torsionbars--byshotpeeningandpresetting,thelifeoftorsionbarscanbeincreasedfromabout30,000to250,000cyclesoraneightfoldincrease.
8.
Gears,whethercarburized,quenchedandtemperedorannealed,areshotpeenedtoincreasefatiguelife.
Oneexampleisthatofacasehardenedgear,stressedto80,000psiinservice,whichhaditscyclelifeincreasedfrom200,000to30,000,000cyclesasaresultofshotpeening.
9.
Marinepropellers-shotpeenedtopreventcavitationandfatigue.
10.
Airplanepropellers--peevednearhubendtoincreaselife.
11.
Propellerhubs--peevedonconeseatbearingareatopreventfretcorrosioncracking.
12.
Exhaustpipesarepeenedtoimprovefatiguelifeandthisgiveseffectiveimprovementeventhoughthepartreachesatemperatureof850°F.
Withrespecttohightemperatures,ithasbeendemonstratedthatbyshotpeeningnotched(notchservesasastressriser)specimensofNimonic80at.
008A2intensitya30%improvementinfatigueliferesultedevenaftertempering(heating)theshotpeenedpartsfor50hoursat1000°F.
Ahighstrengthalloysteelshowsastrengthlossofonly1000psiaftershotpeeningandtemperingfor100hoursat700°F.
13.
Linkbeltchainrollersgainaneffectivelifeincreasebyshotpeening.
14.
Polelinehardwaresuchaseyeboltsandturnbucklescanbeimprovedbypeening.
15.
Brittlefailurefromlowtemperaturecanbeavoidedbyshotpeening.
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LEGHORNTHESTORYOFSHOTPEENINGADDITIONALNOTESONSHOTPEENINGWHICHAREWORTHREMEMBERING1.
Exceptfortheverysmallestsizesofshot,airpressuresover50psishouldnotbeused.
2.
Inmostcases,theincreaseinfatiguelifeitnotparticularlyinfluencedbyshotsize;however,somerecenttestshaveshownthatbiggershotmaycutfatiguestrength.
Itisthereforeadvisabletousethesmallestshotthatwillgivethecorrectarcheightandcoverageatthesaturationpoint.
3.
Careshouldbetakennottooverpeenmetals(suchascopper,selfhardeningmanganesesteelsandsometypesofstainlesssteels)whichworkhardenrapidly.
4.
Surfacefinishpriortoshotpeeninghasagooddealofinfluenceonfatiguelife.
Shotpeeningactuallytendstoroughenfinefinishes;however,thisrougheningeffectisnotdetrimentalduetotheincreasedfatiguestrengthwhichresults.
5.
ThenewSAEnumberingsystemforshotsizesisbasedonhighandlowlimitscreening;however,thenumberfollowingtheSisactuallythenominalsizeoftheshotintenthousandths.
Forexample,thelargestsizeshot(S-1320)hasanapproximatenominalsizeof.
1320"diameter.
S-330bas.
033"diameterandthesmallestsizeshot(S-70)hasanominalsizeof.
007"diameter.
6.
Improvedfatiguelifeofsteelpartsincreasesasthehardnessofthepartsincreaseandshotpeeningbecomeslesseffectiveasthemassofthepartincreases.
7.
Shotpeeningislesseffectiveonstraightpush-pulltypesofcyclingthanforothercycleinducedstresses;however,itoftentimesrenderslifeimprovementeveninthiscase.
Don'tbeafraidofshotpeening.
Considerablematerialhasbeencoveredinthistreatisetodemonstratethehowandwhyofshotpeeningtocontrolledspecifications;however,thereisanothersideofthestoryanditconcernstheshotpeeningofpartsbyoperatorjudgmentonly.
Inspiteofthefactthatthereisacorrect(scientific)waytoestablishtheproperarcheightandpeeningconditionstogiveoptimumfatiguelifeimprovementtoaspecificpart,thefactstillremainsthatthismethodisexceedinglytimeconsumingandexpensive.
Itisseldomjustifiedexceptinthecasewherethecostcanbeproratedoveraverylargeproductionvolumeorwherethepartissopronetofailurethatitisabsolutelynecessarytoestablishtheoptimumconditionsforfatiguelifeimprovement.
Itcanhonestlybesaidthatmostshotpeeningspecificationsarebased,notonexperimentalresults,butonthebasisofaconservativeguessbyapersonwithsomebackgroundknowledgeinthisfield.
Thepercentageofimprovementinthefatiguelifeofcycledpartsissoconsiderable,andthenumberofsuchcaseswhereitworkseffectivelyissofavorable,thatoneisjustifiedinassumingthatacycledpartwillbebenefitedifitispeenedunderthefollowingconditions:1.
airpressuresusedarebelow50psiexceptfortheverysmallestshot,2.
coverageisfairlycompleteasjudgedbyeye,3.
asmall(S70toS170)tomedium(S-230toS-460)sizeshotisusedand4.
careistakenthatthecriticalareasbeingpeenedarefairlyuniformlyandcompletelycovered.
Thoughtheseconditionswillseldomgivemaximumimprovementtothepart,theywillalmostinvariablyimprovethepart.
SupposeforexamplethatatriphammeronEmachinewasconstantlyfailing.
Thiswouldprobablybeduetofatigueandshotpeeningtheareawherefailuresoccurredcouldveryprobablyremedythefailuresor,atleast,greatlydecreasethem.
Thepeeningprocedurewouldbetoestablishthecriticalareasonthepartand,withanozzlepressureof40-50psi,directablastofS-110toS-230steelshotagainstthecriticalareasuntilthecoverageseemedcompletebyvisualexamination.
Ifthecriticalareaswerenotreadilyapparent,thepartwouldbeshotpeenedoverallexceptforareaswhicharemachinedtoafittolerance.
Suchmachinedareaswouldbestoppedoffbypluggingholes(asthreads)orboreswithrubber(orevenwood)plugsandoutersurfaceswouldbecoveredwithrubberadhesivetape(scotchorfrictiontapecanbeused).
Thereislittledoubtbutthatthepartwouldbeimprovedandthatthisisavalidoperatingprocedure.
Iftherewereaconsiderableproductionoftheseparts,theywouldbefixturedandpeenedundercontrolledconditionstogivethecustomertheassuranceofuniformityofqualityinhisproducttowhichheisentitled.
Bywayofsubstantiatingthismethodofapproach,letusexaminesomeofthebasicideasbehindit.
1.
Thereisnothingmysteriousastowhyshotpeeningissobeneficialinobviatingfatiguefailures.
Itcreatessurfacecompressivestressesinthepartandthereisawealthofevidencetoshowthatfatiguefailuresdonotoccurwheresuchstressesexist.
Evenplateglassismadeimmeasurablystrongerwhenprocessedsoastocreatesurfacecompressivestressesalthough(duetothebrittlenatureofglass)thisisdonebythermalupsetting.
Othernonmetallicmaterials(asrubberandplastics)canaccruelengthenedservicelifebyinducingsuchbeneficialsurfacestressesinthem.
2.
Wehavealreadymentionedtheancientuseofhammerpeeninganditiswelltonotethattheshotsizeinthisinstancewasthesizeoftheballonthehammer.
AnotherinterestinganecdotealongthislinewastoldbyaprofessorofmetallurgyattheUniversityofCaliforniatotheeffectthatanearlyWrightdieselairplaneengineownedbyanAlaskanbushpilotdevelopedacrackinacriticalareaandthiscrackgraduallypropagateduntilitbecameamatterofextremeconcerntothepilot.
Ratherthanbeputoutofoperationwhilewaitingforanewpart,thepilottookachanceandheavilyballpeenedthemetalattheendofthecrack.
Thisenginewasusedforoverfiveyearsinthecrackedandpeenedstateandwastakenoutofserviceforgeneraldebilityandnotbecauseofthecrack.
Thisisacasewhereacrack(stressraiser)propagatingbyfatiguecouldnotpenetratethroughacompressivelystressedmetalarea.
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November1951665THESTORYOFSHOTPEENINGLEGHORN3.
Evensandblastedmetalpartsshowimprovementinfatiguelifeoveruncleanedparts.
Sandisaprettysharpedgedtypeofshot.
4.
Intheautomotiveindustry,shotblastingisusedforcleaningagoodmanypartsandthisblastingimprovesthemfatiguewise.
Shotblastingcanbeconsideredasshotpeeninginitsmostuncontrolledform.
5.
Withrespecttocoverage,ithasbeenshownthatacoverageofaslittleas30%willgive80%oftheeffectivenessinincreasedfatiguelifethatcanbeachievedbyfullcoverageandithasbeendemonstratedthattheunimpingedareasbetweentheshotindentationsareinastateofcompressivestressthoughnottothesamedegreeastheywouldbewithfullcoverage.
Itcanbeseenfromthisthataverysharpgrooveorfilletintowhichashotparticlewillnotquitepenetratewill,nonetheless,beimprovedfatiguewisebytheindentationssetupateithersideofitasshown:Inspiteofthefactthatimprovementwillresultinthiscase,thefilletatleastshouldbegoneoverwithablastoffinershottogiveaddedimprovement.
6.
Ifoneweretoexaminestrainpeeningtheoreticallyandwithoutknowingtheexcellentendresultsitachieves,wewouldbeveryapttocondemnsuchamethodonthebasisthatitwouldprobablyresultinover-peening.
Itcanbeseenfromtheforegoingthatshotpeeningbyoperatorjudgmentisavalid,thoughnotanoptimum,methodofapproachproviding,ofcourse,thattheoperatorhasreasonableskillandexperience.
UNINVESTIGATEDVARIABLESINSHOTPEENINGAnexampleofavariablewhich,tomyknowledge,hasnotbeeninvestigatedwouldbetheshotpeeningofapartatdifferenttemperatures.
Thereisnogoodreasontoassume666A.
S.
N.
E.
Journal,November1957thatapartreceivesoptimumbenefitsbybeingshotpeenedatroomtemperatureasisnowuniversallydone.
Itisentirelyprobablethatadifferentimprovement,orlackofit,wouldresultifapartwerepeenedwhilehot(asimmediatelyonremovalfromatemperingfurnace)orcold(asimmediatelyonremovalfromadeepfreezebox).
Thismightwellbeaninsignificantvariableandanyimprovementresultingfromsuchanoperationmightbetooinconsequentialtobeworththeaddedeffort;however,untilvalidexperimentshavebeencarriedoutinthisdirectionnoonecanassumethattheresultswouldnotbesignificantandworthwhile.
IhavemadesomeverycursoryexperimentsalongthislinebythesimpleexpedientoftakinganAlmen"A"strip,blockandholdingfixture(thestripbeingalreadyattachedtotheblockandfixture)toadesiredtemperatureandthenshotpeeningitimmediatelytoadefiniteroomtemperaturearcheight.
Thatis,theentireassemblywastakentothedesiredexperimentaltemperature,shotpeenedforadefinitetimeandintensity(.
015A2atroomtemperature)andimmediatelyremovedandmeasuredforarcheightwhilestillhot.
25°Fincrementsbetween-80°Fand+400°Fwerecheckedwithplottedresultsofarcheightagainsttemperatureshowinganincreaseof.
001A2archeightforaboutevery100°Fincreaseintemperature.
Archeightsvariedfrom.
013A2atthelowesttemperatureto.
019A2atthehighesttemperature.
Afterthestripshadreturnedtoroomtemperaturethearcheightsremainedwithin.
0005A2oftheoriginal(hotorcold)measurementwiththedistinctionthatthosemeasuredhotincreasedslightlybutverydefinitelywhenre-measuredatroomtemperaturewhilethosemeasuredcolddecreasedslightlybutverydefinitelyinarcheightwhenremeasuredatroomtemperature.
Thisdemonstratesthatthetemperatureofthepartbeingpeenedisabonafidevariable;however,agreatdealofresearchwillbenecessarytodeterminewhetherornotsuchvariationswillgreatlyimproveapartsotreated.
AnotherinterestingexperimentisthatdonebyRichardHarveywhereinhemarquenchedpartsandthenshotpeenedthemwhiletheywerestillatthemarquenchtemperatureand,hence,stillintheausteniticstate.
Theresultingbenefitsweresubstantialthoughthespecificapplicationsoftheprocesshavetobemuchmorethoroughlyinvestigated.
Thisisafairlylengthlytreatise;however,Ifeelthatitwillgivethereaderaninsightintotheprocessofshotpeeningwhichcannotbeattainedexceptbyexperienceandconsiderablereadingofotherliterature.
Inconcluding,Iwouldliketomentionthatthereareagreatmanyaspectsandusesofthisprocesswhichhavenotyetbeendeveloped.
Ifapartisfailing,itwon'thurttotryit.