Shot Peening Technology Course No: T02-008

Credit: 2 PDH

Robert P. Tata, P.E.

Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980 P: (877) 322-5800 F: (877) 322-4774 info@cedengineering.com

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ShotPeeningTechnology

RobertTata,B.S.M.E.,P.E.

Copyright2014

AllRightsReserved

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INTRODUCTION

Shot peening is an exclusive mechanical process that is used extensively inapplicationswhere the performance of certain componentsmust be enhancedaboveconventionaldesignlimitations.

Theshotpeeningprocessderivesfromthesameprinciplethatearlyblacksmithsusedwhentheycontinuedtohammer(peen)ahotforgedironorsteelspecimenlongafteritcooledmakingitatougherandmoredurableproduct.

Thatarthasbeenrefined intoacloselycontrolledmanufacturingprocessthat iscapableofobtainingresults in improvingproductfatigue life inawaythat isnotattainable by any other similar production means. It is used successfully onmechanical power transmission and other components for the aeronautical,automotiveand industrial fieldsaswellas formanyotherapplicationswhetherconstructedofferrousornonferrousmaterials.

SHOTPEENINGPROCESS

Shot peening is a cold working process where certain stressed areas of aspecimenareblastedwithsmallsphericalelementscalledshot.Eachpieceofshotcreates a small crater in the area of the part being treated. The depth of thecratermaybeafewthousandthsofan inchdeepforsteelto.062ofan inchforsoftermaterials.

Eachpieceofshotimpactsandyieldsathinsurfacelayerofthecraterputtingitintension.Attheinstantthepieceofshotrebounds,thematerialbelowattemptstoforcethesurfacelayerbacktoitsoriginalconditionputtingthecompletesurfaceofthecraterinahighlycompressedcoldworkedstate.

Subsequentshotwillstrikeareaswheretherearenocratersorwherethis isanexistingcrater.Wherethereisanexistingcrater,thealreadycoldworkedsurfaceprevents any new formation.Where there are no craters, new craterswill beformed.Thisactioncreatesacontinuousformationofcoldworkedcraterswhichputthetotalareaintoalayerofcompressivestress.

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Most failures startat the surfaceofapart in tension. Since theentireworkingareaofashotpeenedpart is incompression,thepartwillexperience increasedlife.

Itmayseematfirstglancethatshotpeeningcratersonmetallicsurfaceswillhaveadestructiveeffecton the lifeofapartbyactingasstress risers;however, thecratersarevery shallowandhavea smooth spherical surface that is very largecomparedto itsdepth.Sincethestressrisingeffectofacrater increaseswith itsdepthanddecreaseswith its largearea,thosemade inpeeninghaveonlyslightstressconcentration. Inaddition, ithasbeenshownthatasolitarystressriser ismuch more dangerous than a number of closely spaced stress risers. Closelyspaced stress risers seem to share the intensified stress instead of the entireintensifiedstressbeingsupportedbyasinglestressriser.

A thin sheet of steelwhen shot peenedwill bow in bothmajor axeswith thepeenedsidebowedup(convexsideup).Thisisduetotheinternalforcesthataregenerated intheworkpiecebyshotpeening. Ifthethin layerofshotpeening isremoved,thematerialwillreturntoitsoriginalflatcondition.Thisdemonstratesthefactthatthe internalforces intheworkpiececausingthecurvaturewere inthethinsurfacelayersonly.

Oneoftheusesofthiseffectofshotpeening is intheformingofaircraftwings.Machineshavebeendevelopedwithcontrolsthatenablethemtoshotpeenformanumberofdifferentaircraftwingsandothertypesofsizesandshapes.

Manufactured parts have residual stresses that can be either compressive ortensile.Thesurfaceofaninductionheattreatedpartoftenwillhavebeneficialresidual compressive stress while the surface of the heat affected zone of awelded part will contain potentially destructive tensile stresses. This makesareasofweldedpartsgoodcandidatesforshotpeening.

Figure1belowhasatypicaltraceofthestresslevelsintheimpactareaofashotpeenedpart.Itcanbeseenontheplotthatthecompressivestressatthesurfacefollowstheknownprinciplethatthemaximumcompressivestressofapartaftershotpeeningisatleastequaltoonehalfthetensilestrengthofthematerialitself.

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Just below the surface layer, compressive stress increases slightly then rapidlydecreasestozeroandthenrevertstoslighttensilestress.Thissubsurfacetensilestresshastobalancethecompressivestressfortheparttoremaininequilibrium.The tensile stress cannot be so high as to create internal cracking, therebyweakening the part.An extreme example of this condition existswhere a thinspecimenisshotpeenedwithsuchgreatintensitythatthepartcanweakentothepointwherefracturingcanoccur.

Figure1

ShotPeeningStressProfile

Figure 2 below has a graph demonstrating the magnitude of the inducedcompressive stress in shot peened steel. It plots induced compressive stressversus tensile strengthof steel. Itdemonstrates theabovementionedprinciplethat the compressive stress induced by shot peening is at least onehalf thetensile strength of the material being treated. Variations in the shot peening

%TensileStrength

+100 +50 0 50 1000

2

4

6

8

10

12

DepthCompressiveStress

SurfaceStress

MaximumTensileStress

MaximumCompressive

Stress

%DepthBelowSurface

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processitselfhaveverylittleeffectaslongastheshotisashardasorharderthanthematerialbeingtreated.

AlmenTest:TheAlmentestisusedasameansofduplicatingapeeningintensitythathaspreviouslybeenestablishedasbeingoptimumonanexistingpart.

Figure2

MagnitudeofResidualStress

This isaccomplishedwithanAlmen test stripwhichmakesuseof theprinciplementionedabovewherebyathinstripofmetalwillbowwhensubjectedtoshotpeening and thatmore intensepeeningwill result inmorebowingof thepartbeingtreated.

Oneverysuccessfultreatmentthatisusedoccurswhenaproductionpartisshotpeened at anumberofdifferent intensities and fatigue tested.When the shotpeening intensity that resulted in the optimum fatigue test life has been

MPa

1000 1400 1900150,000

140,000

130,000

120,000

110,000

100,000100,000 200,000 300,000

965

877

690

TensileStrengthPSI

RockwellCScale

MaximumCompressiveStressPSI

17 34 43 50 57

MPa

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determined,anAlmenteststripisshotpeenedatthatsameintensityandusedasaqualitycontrolgauge tobeable tomaintain theshotpeening intensityof thepartatthecorrectlevelthroughoutproduction.

TheAlmentestusesthree,3.00 inchby0.75 inchteststrips;teststripN= .031inchesthick,A=.051inchesthick,andC=.094inchesthick.

The shot peened test strips bow in both the longitudinal and transversedirections.

Thefollowingtableservesasaguidefordeterminingteststriparcheight(bow)inrelationtothethicknessofthepartbeingshotpeened:

PartThickness ArcHeight1/16 .012N1/8 .008A1/4 .014A3/8 .018A .021A5/8 .007C .008C7/8(orgreater) .010C(orgreater)

Forotherthanflatparts,theabovevaluesaretobemodifieddependingontheexactshapetobeshotpeened.Forinstance,whenshotpeeningtheexteriorofatubularpart, thearcheightwillbemuchhigher than thearcheightof thewallsectionitself.

Figure3hasaplotofthedepthofthecompressivestresslayerversustheAlmentest strip deflection for three different materials. It shows that the depth ofcompressive stress is higher for lower hardness 31Rc steel than for 52Rc steelwithtitaniumfallingbetweenthetwo.

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Figure3

DepthofResidualStress

Forpartshavingathicknessof1/16inchorless,aslurryofverysmallglassbeadsandwater,propelledbypressurizedair,hasbeenfoundtocreatethebestresults.The results, similar to those using airpropelled dry metal shot, create smallcraters of coldworkedmaterial. Since the glass beads are smaller thanmetalshots, the craters are smaller, creating a very thin layer of compressed stressestablishedintheworkpiece.

Ingeneral,shotpeeningwillincreasethelifeofapartifitissubjecttoabendingortwistingstress;however,ithaslittleeffectonthelifeofapartthatissubjectto

31RcSteel

6AL4VTitanium

52RcSteel

.75

.50

.25

0.002 .004 .006 .008 .010

.035

.030

.025

.020

.015

.010

.005

0

DepthofCompression(Inches)

0 .005 .010 .015 .020 .025AlmenAScale

AlmenCScale

1.0

Millimeters

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axial(pushpull)stress,sincesuchstressesarereactedbytheentirecrosssectionofthepartratherthanprincipallyontheouterfibers.

Partssuchasboltsorothertypesofconventional fastenersthatareunderaxialstressonlymaynotbenefitfromshotpeening;however, ifexcessivebendingortwisting occurs during installation (torqueing), or if certain types of loads orvibrationsareencounteredthatputthepartinbendingortwisting,shotpeeningmaybeofvalue.Apartial listofpartsandproductswhere ithasbeenreportedthatshotpeeninghasbeensuccessfullyimplementedinproductionisasfollows:

rockerarms:1400%lifeincrease leafsprings:600%lifeincrease connectingrods:1000%lifeincrease coilsprings:1370%lifeincrease gears:1500%lifeincrease steeringknuckles:475%lifeincrease rockerarms:1400%lifeincrease

Ithasbeenfoundthatwith increase infatigue life,costscanbereducedasshotpeenedpartscanbemadesmallerandlighter,andinsomecases,bemadeoflessexpensivematerial.

Shotpeeninghasbeen found to replaceproductionprocessessuchaspolishingandhoning. Ithasalsobeenfoundto improvetheresistancetostresscorrosionofamagnesiumalloy,eliminateporosity inaluminumdiecastings,and improvethelubricationofcrankshaftsbecauseofoilcollectinginthecraters.

Shotpeeningspringswhilebeingstaticallyloadedunderbending(leafsprings)ortorsion(coilspringsortorsionbars),calledstresspeening,producesevengreaterlifeexpectanciesthanshotpeeningunloadedsprings.

An interestingapplicationof shotpeening ison the seal lip contact surfacesofrotating shafts. With increasing speeds and higher pressures, seal leakagebecomes a challenge to the mechanical designer. Many times the sealing lipsurfaceof rotating shaftshasmicroscopic random sharpedges thatwearawaymuch softer seal lips, usually constructed of a form of rubber. Sometimes the

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sharp edges are not random but occur in parallel forming a hydrodynamicpumping action causing leakage. These microscopic surface deformities aredifficult toeliminateorcontrol. Ithasbeen found that themuchsmootherandrandomsurfaceproducedbyshotpeeningeliminatesbothproblemsresulting inimprovedsealing.

Figure4depictsagraphofseal leakageversusoperatingtimeforbothstandardgroundandshotpeenedshaftsealingsurfaces. Itcanbeseenthere isamarkedimprovementwithpeenedshaftsovergroundshafts.

Figure4

Groundvs.ShotPeenedSealingSurface

Hours

Shot peening isnt the onlymethod bywhich the fatigue life of a part can beincreased. Others include cold working by rolling, stretching, compressing,twisting; heat treating including inductionhardening and flame treatment; andalteringthesurfacecompositionbycarburizing,nitriding,etc.However,shotpeeningpossessesadvantagesoveralloftheabovesuchas improved flexibility,control,safetyandcost.

SealLeakageCubicCentimeters StandardGround

Peened

0 100 200 300 400 500

20

40

60

80

100

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Peening Shot: Shotpeening shot consistsof sphericalparticlesobtainedby thedispersionofamolten streamofmetal immersed inwater,airorothermedia.Theearliestshotusedforcommercialpurposeswasmadefromchilledcastiron.Highcarboncontentcastiron,meltedinacupola(smeltingfurnace),isimmersedinwaterwherethemetalisquenchedandbrokenupintosmallsphericalparticlesofhardnessrangingfrom55to66RockwellC.

Anotherformofcastironshotmaterialismalleablecastiron.Malleablecastironshot is formed by heat treating (annealing) chilled cast iron shot down to ahardnessof22to40RockwellC.Thismakesitnotashardaschilledcastironshotandnotrecommended forhigherhardnessmaterials,but lessbrittleand longerlastingforlowerhardnessmaterials.

Anadvancementinpeeningapplicationsissteelshot;eithercaststeelwhichhasbeen heat treated to the proper hardness, or cut wire which has been coldworked by a drawing operation. Steel shot has a hardness range of 40 to 65RockwellCwhichgivesitgoodpeeningactionandthetoughnesstoresistfracturebetter thananyof thecommon ferrousshotmaterialsused. IthasbeenshownthatsteelshotofRockwell42to50ChardnesswilleffectivelypeenRockwell60ChardnessworkcomparabletochilledironshotofRockwell55to60Chardness.

Testshaveshownsteelshottobemoreeffectivethan ironshot inperformanceanddurabilityofuse,andhavebecomethestandard.Testshavealsoshownsteelshot to be more effective in increasing the fatigue life of Rockwell 60C workbetter than any other ferrous material. Besides having great resistance tofracture,steelshothasbeenfoundtocreatelesswearonpeeningequipmentandgreater economics of operation making it an important achievement in shotpeeningadvancement.

Shot isclassified in sizesnormally ranging from .016 to .094 inches indiameteralthough standards have been established for shot .005 to .111 inches indiameter.Shotmustbeofuniform size, shapeandhardnessandbedurable inorderforpeeningtobeeffective.Broken,outofround,orundersizedshotmustbeautomaticallyremovedfromthesystemtoensureproperpeeningaction.Shothardnessdeterminestheamountanddepthofcompressivestress.Inmostcases,

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shothardness shouldbeequal toorharder than thepartbeing treatedunlesssurfacefinishisaconcern.

Peening intensity depends on the mass of individual shot; therefore, it isimportant thatshotusedbeofuniformsize inorder toobtainuniformpeeningaction. Shot hardnessmust be uniform and shot shapemust be spherical anduniformandwithoutsurfacedefects.Theuseofdurableshotusuallyovercomesitsextracostresultingingreateroverallsavings.

Shot size has a different effect on the depth of the compressive layer onaluminum alloys than it does on any othermetals. In steel and titanium, thedepthofcompressionremainsfairlyconstantwithpeeningintensityregardlessofshot size. An aluminum part, peened to a given Almen intensity, will have adeeper layer of compression stress when peened with larger shot than whenpeenedwithsmallershotatthesameintensity.

ShotPeeningMachines:Shotpeeningisgenerallycarriedoutinacabinetinorderto confine the shot and thedust thatoccurs as a resultof theprocess and tofacilitatethecollectionandreuseoftheshot. Infullmassproductionmode,thework isautomatically carried inandoutof theenclosurebymechanicalmeanswiththeareatobepeenedpositionedinaneasilyaccessiblelocation.

Shotmaybepropelledbyair,water,orbyawheelwithvelocitiesintheorderof200 fps.Airorwatersystemspropel theshot throughanozzlewhile thewheelmethodslingstheshotusingarotatingvaneddevice.

Theareaimpactedbytheshotstream iscalledtheshotpattern.Inthecaseofapneumaticdeliverymachine,shotissprayedbyanozzlecoveringacirculartargetareaofabout2or3inchesindiameterdependingonnozzlesizeanddistancetothework.Theshotpatternfromawheelisfanshapedwiththeincludedangleofabout40degrees,andwidthsomewhatgreaterthanthewheelwidth.Thelengthandwidthofthepatternisgovernedbythedistanceoftheworktothewheel.

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Atypicalpeeningmachineismadeupofthefollowingmajorsystems:

A cabinet to contain the work piece including various shot delivery andrecoveryaswellasdustanddebrisremovablesystems

Conveyersystemtotransportthework inandoutofthecabinetand/oramountingdevicetopositiontheworktobeshotpeened

A shothandling system fordelivering shotandaccuratelypropelling it tothetarget

Adevicetoreturntheshotthathasbeenusedinthepeeningprocesstoberecycled

Separatortoremovebrokenorundersizedrecycledshot Devicetoreplacebrokenorundersizedshotwithnewmaterial Systemtocollectandremovedustanddebrisfromthecabinet

Shotpeeningmachinesmaybeclassifiedintotwomajorcategoriesdependingonthesystemusedtodeliverandpropeltheblasttothetargetarea.Theyare:

AirBlastMachines CentrifugalBlastMachines

Air Blast Machines: There are three different types of air blast machinesdependingonthemethodusedtointroducetheshotintotheairstream:

1. SuctionInductionAirBlastMachine:Inthistypeofmachine,agun isusedwhichhas two inletportsandoneoutletnozzle.One inletport receivesand sends compressed air out the nozzle. At the same time, thepressurized air draws shot through the second inlet port from a lowerstoragebinandpropels it to the target.This is the simplest typeand isused to peen small parts or small quantities, or when the requiredintensity is low. It is used for laboratorywork or for other applicationswheretheshotsizeischangedfrequently.

2. Gravity Induction Air Blast Machine: In this type of machine, the samenozzle isused;however, the shot isdelivered to thenozzlebymeansof

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gravityfromanupperstoragebin.Thisresultsinbettercontroloftheshotvelocityandflowrate.Thesemachinesareusedwhenthevacuumthroughthenozzlecreatedbytheflowofpressurizedairisnotgreatenoughtolifttheshotfromalowerstoragebin.

3. DirectPressureAirBlastMachine:Inthistypeofmachine,theshotisstoredinavesselatthesamepressurethatairissenttothenozzleintheaboveinductionmachines.Theairshotmixture isdeliveredtothenozzlewhichdirects ittothetarget.Thissystemallowsgreaterfreedomofmovementof thenozzle and isused topeen smaller areas such as fillets athigherintensities. The nozzle in the gun used in all three air blast machinesexperiences wear from the abrasive action of the shot and has to bereplacedperiodically.Airblastgunshavebeendevelopedwith improvednozzlesthatprovideauniformshotstreamforlongerperiodsoftimethanpreviouslyusednozzles(SeeFigure5).

Figure5

ShotPeeningGun

PressurizedAirInducedShot

ShotPeeningStream

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Centrifugal Blast Machines: In this class of machine, the shot is propelled bycentrifugalforce.Theshotisgravityfedtothehubofarotatingwheelwhichhasradial vanes or blades where it is propelled to the target. The spray pattern,insteadofbeingcircularlikethatofairblastnozzles,issomewhatrectangularforcoveringdifferentshapedtargets(SeeFigure6).

High Frequency Impact Treatment (HiFIT):HiFIT involves using an airoperatedhandgunwitha3millimeter (0.118 inch)hardenedballattachedat theend toimpacttheworkpieceatanadjustablefrequencyrateof180to300Hertz(cps).Manytimesthedurabilityandlifeofstructuresaredeterminedbythestrengthofthewelds.HiFIThas foundexceptionaluseonweldedstructures,bothnewandexisting,by strengthening the critically stressedextremeedges (toe)where theweld thins out and meets the parent material. HiFIT creates a track of localdeformations plastically deforming and bonding the toe to the parent metalcreating an area of induced compressive stress. At institutional testing, itwasshownthatHiFITincreasesthefatiguestrengthofweldsby80to100percentandan increase inweld life of 5 to 15 fold. The advantages ofHiFIT are that it issimple,portable,effective,reliableandeconomical.

Figure6

ShotPeeningWheel

Shotin

ShotFeedRotation

Vanes

ShotStream

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LaserShockPeening:TheLaserShockPeeningsystemproducescompressionwithminimumcoldworkingusingshockwavestoyieldthematerial.Highspeed,highpowered lasers are used to focus a short duration energy pulse on a coatingapplied to theworkpiece,usuallyblack tape, toabsorb theenergyof the laserbeam.Atransparent layer,usuallywater flowingoverthetape,allowsthe laserbeamtopassthroughbutactsasabarriertotheresultantshockwave.Whenthelaserisfired,thebeampassesthroughthewaterexplodingthetapeandcreatinga shockwave that is confinedby thewaterbarrier anddirected into theworkpiece,therebycoldworkinganindentationofapproximatelyonemillimeter(.039inches).Theprocedureiscontinuedacrossthesurfaceoftheworkpiececreatinga serious of computer controlled slight depressions resulting in a region ofsubsurfacecompressivestress.Lasershockpeeninghasbeenappliedtoavarietyof alloys used in aircraft engines and airframes as well as other engineeringapplications.Althoughlimitedbycost,qualitycontroland logistics, laserpeeninghas been applied successfully to improve the damage tolerance to compressorbladeleadingedges.

Oneadvantageof laserpeening isthatthedepthofcompressivestressexceedsthat of shot peening with less cold working (i.e., shallower indents). Also thecompressive stress tends to bemaximum at the surface and diminishes nearlylinearwithdepth.Theminimumofcoldworkingprovidesthermalandmechanicalstability in high temperature applications or where there may be momentaryoverloadduetoimpact(birdsdrawnintoaircraftturbineengines).

Disadvantageswith laserpeening includea clean roomenvironmentandaonemilliondollarequipmentinstallation.Laserpeeningrequiresrepeatedoperationswith newly applied tape to obtain the correct compressive stress depth. Laserpeeningcancostmultipletimesthecostofshotpeening.

LowPlasticityBurnishing (LPB): LPBencompasses applyingpressure toa rollingball (or roller) over the surface to be treated with sufficient normal force todeformthesurface layers.ThebasicLPBtool isaballsupported inanoverheadhemispherical hydrostatic bearing. The tool can be mounted in a lathe or aComputerNumericalControl(CNC)machinewhereamachinetoolcoolantisused

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topressurizethehydrostaticbearing,therebypreventingtheballfromcontactingthemetal bearing seat (See Figure 7). The ball is loaded in a normal directiondownon thesurfaceof theworkpiecewithahydrauliccylinderwhich is in thebodyofthetool.Theballrollsacrossthesurfaceoftheworkpiece inapatternestablishedbytheCNCoranyothermachinebeingused.Sincethe lateralforcebeingappliedtotheballisthroughathinlayerofhydrostaticfluid,theballisfreeto roll in anydirection.As theball rollsover theworkpiece, thenormal forceappliedbytheballcausesplasticdeformationtooccurinthesurfaceoftheworkpieceunderneath.Sincethesurroundingmaterialintheworkpiecetendtoreturntheplasticallydeformedmaterialbacktoitsoriginalconfiguration,thedeformedarea isput inastateofcompressivestress.Thepatternoftheresidualstress isdesigned to increase the performance of a part against the effects of fatiguefailureand stress corrosion. LPB removesnomaterial, smoothsasperities, andleavestheworkpiecewithanalmostmirrorlikefinish.

Figure7

LowPlasticityBurnishingTool

NormalForce

HydrostaticPressure

HemisphericalHydrostaticBearing

BurnishingBall

WorkPiece

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The coldworkingproducedby LPB isminimal compared to shotpeening, laserpeeningordeep rolling.Aminimalamountofcoldworking results in improvedperformanceatelevatedtemperaturesandmechanicaloverloadconditions,bothwhichtendtorelievecompressivestressthatiscoldworkedintocomponentsbythe other abovementioned surface treatments. LPB has been used on turbineengines, piston engines, propellers, landing gear, biomedical implants (kneereplacements)andwelded joints.Theapplications involve titanium, iron,nickelandsteelbasedcomponents.