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Journal of Materials Processing Technology
j ou rn a l h o m epa ge :w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c
Ultrasonic characterization of heat-treatment effects on SAE-1040 and -4340 steels
Magdy M. El Rayes
∗,1, Ehab A. El-Danaf
2, Abdulhakim A. Almajid
MechanicalEngineeringDepartment,CollegeofEngineering,KingSaudUniversity,P.O.Box800,11421,Riyadh,SaudiArabia
a r t i c l e i n f o
Articlehistory:
Received13May2014
Receivedinrevisedform4September2014 Accepted5September2014
Availableonline16September2014
Keywords:
Ultrasoniccharacterization Soundvelocity
Attenuation
Heattreatmentofsteel Microstructuralphases Mechanicaltesting
a b s t r a c t
Inthisworkmicrostructuralcharacterizationandmechanicaltestingresultswerecorrelatedwithultra- sonicvelocityandsoundattenuationofsteelsSAE-1040andSAE-4340.Bothtypesweresubjectedto threetypesofheattreatment;thefirstwasannealingat850◦C,thesecondwasaustenitizingat1000◦C followedbyoilquenchingandthethirdwassimilaraustenitizingthenwaterquenching.Treatmentsof SAE-1040steelresultedinmicrostructurescontainingdifferentferriteandpearlitecontents,different inter-lamellarspacingandalsodifferentgrainsize.Similarferriteandpearlitecontentwasobtained whenannealingSAE-4340whereas,oilandwaterquenchingresultedintomartensite.WithSAE-1040, thesoundvelocitywasreducedintheorderofannealing–oil–waterquenchingduetothereductionof ferriteontheexpenseofpearlite.Thesameorderinsoundvelocityreductionwasalsoobtainedwith SAE-4340duetothechangeinmicrostructuralphasesfrompearlitetomartensite.Incomparisonto pearlite,themartensitepossessedhighercrystallatticedistortion,higherdislocationdensityandlower elasticmodulusallofwhichcontributeinreducingsoundvelocity.AttenuationofSAE-1040increased intheorderofannealing–oil–waterquenchingbecauseofhigherpearlitecontentandthereductionin inter-lamellarspacing.AttenuationofSAE-4340gaveanoppositeorderduetothereductionoftheextent ofmicrostructuralanisotropy.Themechanicalpropertiesandhardnesswerepredominantlyaffectedby themicrostructuralphasesleadingtothelogicalcorrelationwithultrasonicparameters.
©2014ElsevierB.V.Allrightsreserved.
1. Introduction
Oneoftheprimeobjectivesofnon-destructivetesting(NDT)is tocertifythatthecomponentbeingexaminedisfitfortheintended service.Themostcommonwayofdoingsoisbyexaminingthe componentwithNDTtodetectflaws ordiscontinuitiessuchas voids,inclusions,cracks,inmaterialsorstructures.Anotherparam- eterwhichisequallyimportanttoflawdetectionistoassessthe materialproperties.AmongvariousNDTmethods,ultrasonictest- ing(UT)wasappliedratherextensivelyinavarietyofpublications relatedtothecorrelationofultrasonicmeasurementswithmicro- structuralphasesofsteelasconductedbyGürandTuncer(2004), duplexstainlesssteelbydeAlbuquerqueetal.(2010a,b),Ni-base superalloybydeAlbuquerqueetal.(2012),thermallyagedNi- basealloybyNunesetal.(2013)andgrainsizeasbyBoudaetal.
∗Correspondingauthor.Tel.:+966114679906/500991132;fax:+9664676652.
E-mailaddresses:[email protected],[email protected](M.M.ElRayes).
1 OnleavefromProductionEngineeringDepartment-FacultyofEngineering- AlexandriaUniversity.
2 On leave from Mechanical Design and Production Department-Faculty of Engineering-CairoUniversity.
(2003).UTresultswerealsocorrelatedwithmechanicalproperties asinvestigatedbyVijayalakshmietal.(2011)anddeAlbuquerque etal.(2010a,b),aswellasresidualstressesasbyChakiandBourse (2009).Therefore,theutilizationofultrasonictechniquestoindi- rectlydeterminethemicrostructuralfeaturesandthemechanical propertiescanbeusefulfornumerousindustrialapplications.
Plain carbon steels SAE-1020 and -1050 were heat treated byvaryingtheaustenitizationtemperaturebetween860◦C and 1060◦C,aswellasthecoolingratewasvariedbetweenfurnace cooling,aircoolingandoilquenching.Thesematerialswerechar- acterizedbyultrasonicvelocityandattenuationmeasurementsas reportedbyGürandKeles(2003).Metallographicstudiesrevealed thattheamountofproeutectoidferrite,thesofterphase,inSAE- 1020washigherthanthatinSAE-1050.Ultrasonicresultsshowed thatsoundvelocitiesvarydependingontheseverityofcooling.The lowestsoundwavevelocitywasfoundwiththeoilquenchedSAE- 1050consistingofmartensite,whichpossessedhighdislocation density,distortion ofcrystalline latticeand maximum hardness among otherspecimens. On theother hand,thehighest sound velocitywasobtainedwithfurnacecooledSAE-1020whichhad thesofteststructure.Itwasalsofoundthatprioraustenitegrain size;andnotthetransformationproductswithinit,asmanifested http://dx.doi.org/10.1016/j.jmatprotec.2014.09.005
0924-0136/©2014ElsevierB.V.Allrightsreserved.
Table1
ChemicalcompositionofsteelsSAE-1040and-4340inwt.%.
SteelgradeSAE C Mn P S Si Cr Mo Ni
1040 0.41 0.6 0.04 0.05 0.3 – – –
4340 0.36 0.25 0.04 0.04 0.25 1.4 0.2 1.4
bytheauthor,hadthepredominanteffectontheattenuationval- ues.Thelargertheprioraustenitegrainsizethehigherthesound attenuation.Slowercoolingratesledtolargerprioraustenitegrain size,largeramountsofsoftferriteandwiderinterlamellarspacing betweencementite[Fe3C]platesofpearlite.
PrasadandKumar(1994)studiedtheinfluenceofvaryingthe grain size of cast steel on ultrasonic velocity and attenuation.
Variousgrainsizeswereachievedviahotupsettingatdifferent percentagesofheightreduction.Thesesampleswerefurtherheat treatedthrough annealing,normalizingand hardeningusing oil quenchfollowedbytempering.Theyreportedthatincreasingthe degreeofdeformationdecreasestheultrasonicvelocity.However, numerousresearchworkconcludedthefactthatthelongitudinal ultrasonicvelocityvariedfromgraintograinbecauseofmisorien- tationofgrains,whichwasrelatedtothevariationintheelastic constantaswell.
The objectivesof thepresent workwere to correlate ultra- sonicmeasurementsnamely;ultrasoniclongitudinalsoundwave velocityandultrasonicattenuationwiththemicrostructuraland mechanicalpropertiesoftwotypesofsteelwhichweresubjectedto differentheattreatments.Thesetreatmentswereselectedtoobtain differentmicrostructuralphasesaswellasdifferentgrainsize.
2. Materialsandmethods
Twotypesofsteels,namelySAE-1040(C-Mnsteel)andSAE- 4340(Cr-Ni-Mosteel),suppliedintheformof50mmdiameters rods,fromwhichbothmaterialswerecutintothreeequalpieces 100mm long usinghacksaw withlubricanttoavoid excessive heating. The selection of these steel types was based on their importancein various fieldsof industrial applicationsin which theyaremainlycharacterizedbydurability.Steel1040isnormally usedinaxels,crankshaftsandgears,whereas,steel4340isused in aircraft landinggears, powertransmission gears andshafts.
Inaddition,thetypesofheattreatmentchosenwereexpectedto resultintodifferentphasesthatwereoughttosignificantlyaffect the microstructural, mechanical and ultrasonic characteristics.
Table1presentstheaveragechemicalcompositionofthesesteels inwt.%afterthreerunsforeachtypeusingspectroscopicchemical analyzer.
Bothmaterialsweresubjectedsimultaneouslytothesameheat treatmenttypeinanelectricresistancefurnace.Thefirsttreatment wasaustenitizingat850◦Cfor2hfollowedbyfurnacecooling(full annealing).Thesecondandthirdwereaustenitizingat1000◦Cfor 3hfollowedbyoilandwaterquenching, respectively.Theheat treatedsamplesweresectionedintodisks usinghacksawwith coolanttoextractultrasonicmeasurementssamples[50mmlong], microstructural,hardnessandtensiletestsamples.Themicrostruc- turewasexaminedbysecondaryelectronimagingusingscanning electronmicroscopy(SEM),andelectronbackscattereddiffraction (EBSD).
For SEM the samples were prepared according to standard metallographicsamplepreparationwhichincludesgrindingusing SiCsandpaper,polishingusingdiamondpasteof1.0and0.05m, andetchedwith5%Nitaltorevealthesamples’microstructure.
ThemicrostructureforbothmaterialswasalsostudiedbyEBSD usingOxfordHKLsystemincorporatedonafieldemissionscanning electronmicroscope(FESEM)7600JEOL.Thesesampleswerepol- ishedwithcolloidalsilicaasafinalsteppriortoimaging.
The ultrasonic measurement samples were machined using endmillingthensurfacegroundonbothsidesinordertoensure completeparallelismbetweenfaces.Inordertoeliminaterough- ness,visibleirregularitiesandanyoxideswhichmightaffectthe ultrasonic measurements, the samples were further processed similar to the metallographic preparation steps. For ultrasonic measurements, the pulse-echo technique and direct contact methodwereappliedtoobtainultrasonicvelocitiesandattenu- ationcoefficientsusingultrasonicpulse-receiverequipment-Karl Deutsch(Model:Echograph-1085).Ultrasonicmeasurementsfor allsampleswereobtainedbyusingcommercialNDTultrasonic- longitudinalwavetransducersof4MHz.ThecouplingmaterialKarl Deutsch-Ecotrace gel wasused for thelongitudinal wave mea- surements.Duringmeasurementsaconstantloadwasappliedto theprobeagainstthespecimensurfacesoastohaveaconstant thicknessofcouplantlayerattheinterfacebetweenthespecimen surfaceandtheprobe.Ultrasonicvelocitywasdeterminedbydivid- ingtwicethespecimenthicknessbytimeofflight(TOF)obtained betweenzerocrossingofthefirstand secondback-wallechoes usingEq.(1)asappliedbyVijayalakshmietal.(2011).Inorderto checkresultsrepeatability,sevenultrasonicreadingsofeachspec- imenwereaveragedtorepresentthedataobtained,whichgavean errorofaround±0.5%withbothtypesofsteel.
Velocity (m/s)=2×thickness (m)
time (s) (1)
Duringthesamemeasurement,ultrasonicvelocitywasmea- suredagainusingafunctionintheultrasonicequipmentwhich automaticallycalculatestheultrasonicvelocitywhenlocatingtwo correspondingpointsontwosuccessiveechoes(firstandsecond echoes)attheextremesidesofthescreenwithanerroroflessthan theabovementionedone.
Theultrasonicattenuationvalueswerecalculatedaccordingto Eq.(2)whichisbasedonthereductionoftheamplitudeofanultra- soundpulse,measuredindecibelspermillimeter(dB/mm).This equationappearedinseveralpublicationssuchthatofStellaetal.
(2009),Vijayalakshmietal.(2011)andFreitas etal.(2011)and giveninas:
˛=20 2x logA0
A1
(2) where˛istheattenuationcoefficient[dB/mm],xisthethickness ofthesamplemeasuredinthetest[mm],A0istheamplitudeof thefirstechoindBandA1 istheamplitudeofthesecondecho.
Theconstant2isbecausethepulse-echotechniqueisused.For eachsample,themeasurementswererepeatedseventimeswith maximumerrorof±0.1%withbothsteels.
Vickersmacro-hardnesstestswereconductedusing10kgforce andwereperformedfivetimesforeachheattreatedspecimenand theaveragewastaken.Inordertoevaluatethemechanicalbehav- iorofheattreatedsteels,tensilespecimenswereextractedfrom thecenter of thediskalong itsaxisand werecutaccording to ASTME-08.Tensiletestswereconductedatroomtemperatureand across-headspeedvelocityof2mm/minusingInstronmachine model3385H.Themachinewasequippedwithacomputerhaving softwarethroughwhichtheload-elongationdatawererecorded.
Thetensiletestforeachtypeofsteelandtreatmentwasrepeated threetimesandtheaveragevaluewastakenandpresentedhere- after.
3. Resultsanddiscussion
3.1. Microstructure
Fig.1a–cshows theSEM microstructures obtainedwith the threedifferenttreatmentsappliedonSAE-1040steel.Fig.1ashows
Fig.1.MicrostructuresobtainedbySEMofsteelSAE-1040in(a)annealed,(b)oil quenchedand(c)waterquenchedconditions.
the annealed microstructure which contained pearlite colonies (light)inamatrixofferrite(dark).Thepearlitecontainedalternate lamellasofeutectoidferrite(Fe)andcementite(Fe3C)withrandom orientation.Thepearliteconstituted46%volume-fraction,whereas ferritewas54%.Theaveragegrainsizeofthisstructurewasaround 18m.Theoilquenchedmicrostructure,showninFig.1b,wasalso composedofpearliteandferritewhichtransformedfromaustenite uponquenching.Thismicrostructureconformstothecontinuous coolingtransformation(CCT)diagramofSAE-1040steel.Measure- mentsofphasesyieldedabout80%pearliteand20%ferriteand meangrainsizeof31.1m.ItcanbealsonotedfromFig.1bthat thepearliteincreasedontheexpenseofferriteandalsotheinter- lamellarspacingweremuchreducedduetothefastcoolingrate comparedtotheannealedtreatment.
The water-quenched microstructure was similar to that obtainedwithoil-quenchwithrespecttoitspearliteandferrite constituents.However,thevolumefractionofpearliteincreased to92%stillontheferriteexpense8%aswellasmoredenseinter- lamellarspacingbetweenFe3CandFetookplace.Themeangrain sizecorrespondingtothistreatmentwasaround54.5m.Hence,it canbestatedthatthemaindifferenceinthemicrostructureresult- ingfromthethreetreatmentswasthecontentandsizeofpearlite andferritephasesaswellastheinterlamellarspacing.
Fig.2a–cshows,athighermagnifications,themicrostructure correspondingtoannealed,oilandwaterquenchedrespectively, withemphasis onpearlite structure. In the annealed condition theinterlamellarspacing was relativelywider than that occur- ringwithoilandwaterquenchedstructurewherethesespacing became much less,i.e. denserlamellas.Measurements of spac- ingintheannealedsample gavean averagevalueof 0.362m, whereas withthewater quenched samplewas much narrower around0.0178m.Fig.3showsanexampleofthesemeasurements withSAE-1040intheannealedconditionusingasoftwareavailable inSEM.
Fig.4a–cshowstheorientationimaginggrainboundarymaps representing the extent of sub-grain boundaries in red color, definedwithmisorientationlessthan2◦forSAE-1040steelinthe annealed,oiland waterquenched conditions,respectively. Also showninthesamefigure,thetruegrainboundaries,withmisori- entationlargerthan15◦acrossthem,displayedinthickblackcolor.
Besideeachgrainboundarymaptherespectivehistogramforthe misorientationangledistributionispresented.Itisevidentthatthe annealedstructurehadarelativelylargeaveragemisorientation angleofabout35◦ witharelativelylowpercentageoflowangle grainboundaries(LAGBs)of12%,whichisatypicalfindingforwell recrystallizedannealedmicrostructures.Theoilquenchedsample revealedarelativelygoodamountofLAGBswithapercentageof about67%,andanaveragemisorientationangleof12◦.Thewater quenchedsamplerevealed,evenmoreLAGBswithapercentageof about73%andanaveragemisorientationangleof16◦.
Fig. 5a–c shows the annealed, oil and water quenched microstructures respectively of SAE-4340 steel taken by SEM, respectively.Theannealed structurewascomposedof grainsof pearlite(light)inamatrixofferrite(dark).Measurementsshowed thatthismicrostructureiscomposedof59%pearliteand41%fer- rite.Thegrainsizemeasuredisaround11.6m.Theoilquenched structure,Fig.5b,iscomposedoflongmartensitelaths(light)coex- istingwithasubstructureoffewplatemartensite(regionsmarked P-dark),whichnormallyconsistsoffineinternaltwinsasreferredin ASMMetalsHandbook.Thewaterquenchedmicrostructureiscom- posedof100%martensitewithrelativelyshortlathsasinFig.5c.It shouldbenotedthatduetothepresenceofmartensitelaths,itwas notpossibletoseetheprior-austenitegrainboundariesclearlyin thequenchedsamples.
DuetodifficultiesinindexingtheKikuchipatternsofmartensite obtainedintheoilandwaterquenched4340,theEBSDstudywas
Fig.2.VariationofinterlamellarspacingwithheattreatmentofSAE-1040 (a) annealed,(b)oilquenchedand(c)waterquenched.
confinedtotheannealed4340.ThestructureispresentedinFig.6.
Theaveragemisorientationangleexhibitedavalueofabout33◦ witharelativelowpercentageofLAGBsof25%.Worthnoting,that bothSAE-1040and-4340steelsinannealedconditiondisplayed almostsimilaraveragemisorientationangle;howevertheannealed
Fig.3.MeasurementsofpearliteinterlamellarspacinginSAE-1040intheannealed condition.
4340displayedarelativelyhigheramountofLAGBsalmostdouble thatdisplayedintheannealed1040.
3.2. Hardnesstesting
Vickershardnessmeasurementswereconductedtoindicatethe hardnessofphasesobtainedfromdifferentheattreatments.The resultsofthemetallographicexaminationsareverifiedbyhard- nessmeasurements.Fig.7aandbshowsthathardnessincreased whentheheattreatmentofbothsteeltypeschangedfromanneal- ingtooilandtowater quenching.WithSAE-1040steel,Fig.7a, hardnessincreasedduetotheincreaseinpearlitecontent(hard phase)withrespecttotheferrite(softphase),asshownearlierin microstructuralresults.Similarly,withsteelSAE-4340,thehard- nessincreasedduetothechangeinmicrostructuralphasesbeing pearlite+ferriteobtainedfromannealing,martensitetransformed atslowandfastcoolingratesobtainedfromoilandwaterquench, respectively,asdepictedinFig.7b.Thesoftnessofoil-quenched martensitecomparedtothewater-quenchedoneisduetothatoil quenchingleadstolessextentoflatticedistortion,residualstresses anddislocationdensityasreportedbyGürandTuncer(2004).
3.3. Tensiletesting
Table2summarizestheresultsobtainedfromtensiletestscon- ducted onboth types of steelssubjected tothe different heat treatments.Theresultsoftensiletestsareinlinewithhardness results, which in turn can be related to the phases present in themicrostructure.Withbothtypesofsteel,theultimatetensile strength(UTS)andyieldstrength(YS)increasedwhenthetreat- ment waschangedfromannealing tooilandwater quenching, whereastheelongationwasreducedinthesameorder.Thephases ofsteelobtained,asexpected,wereresponsiblefortheincrease inUTS andYS andthereductionin elongationwhichis similar tothatfoundbyGürandTuncer(2005).InSAE-1040,increasing Table2
MechanicalpropertiesofSAE-1040and-4340steels.
Sample UTS,MPa YS,MPa Elongation%
1040-annealed 625 326 37.4
1040-oilquench 798 464 26.6
1040-waterquench 854 690 22.4
4340-annealed 693 406 36.6
4340-oilquench 1721 1425 23.1
4340-waterquench 1947 1633 21.6
Fig.4.Orientationimaginggrainboundarymapping(left)andmisorientationangledistributionhistogram(right)ofSAE-1040inthe(a)annealed,(b)oilquenchedand(c) waterquenchedstructures.
thepearlitecontentontheexpenseofferriteincreasedtheUTS andYSandalsoreducedelongationasseeninTable2.Asimilar trendwasnotedwithSAE-4340wheretheUTSandYSincreased with simultaneous reduction in elongation accompanying the treatment.
TheresultsinTable2indicatedthatforbothtypesofsteelthe mechanicalpropertiesarepredominantlyaffectedbythemicro- structuralphasesresultingfromdifferentheattreatments,which inturnaffectstheultrasonicvelocityand attenuationaswillbe showninthefollowingsections.
3.4. Ultrasonictesting 3.4.1. Soundvelocity
Theultrasonic velocity measurements of longitudinalwaves weresensitivetomicrostructuralvariationsresultingfromdiffer- entheattreatmentsappliedonSAE-1040steel.Thiscanbenoted whenplottingsoundvelocityversusdifferentfractionpercentof pearliteandferritephasesasinFig.8.Thereasontothiswasrelated tothepercentageofpearliteandferritephaseswithinthewhole structureaswellastheinterlamellarspacingbetweenferriteand Fe3Cphaseswithinthepearlitegrain.
Gürand Tuncer(2004),Freitaset al.(2010)andothers have agreedupon that sound velocitybecomes lower when thefer- ritecontentisreducedaswellaswhentheinterlamellarspacing isnarrowercomparedtocoarsepearlite.Thisis duetothefact thattheferritephasehastheleastresistancetoultrasonicwaves henceallowingthehighestvelocity.Simultaneously,pearlitegrains withdense/narrowinterlamellarspacingpossesshighresistanceto ultrasonicwavesandconsequently,havelowsoundvelocityasdis- cussedbyFreitasetal.(2010).Thevariationofinterlamellarspacing withdifferenttreatmentswasnotedinFig.2.Therefore,itcanbe statedthataninverserelationexistsbetweenpearlitecontentand soundvelocity.
The evolution of sub-grains and sub-grain boundaries con- tributeinthereductionofsoundvelocity.GürandKeles(2003)
reported that the structures consisting of sub-grains and their boundarieshaveaneffectonlatticestrainingandinterruptsthe matrix continuity thus lowers the elastic modulus and conse- quently,lowersthepropagationrateofsoundvelocity.Thiscan beinferred fromFig. 4, which shows a tremendous amountof sub-grainboundariesaccompanyingthewaterquenchtreatment (LAGBs=73%)whichismuchhigherthanthatoccurringwiththe annealingtreatment(LAGBs=12%).
Furthermore,thevariationoftheelasticconstantfromgrainto graininthedirectionofsoundwavepropagationmaybeanother reason for the reduction in sound velocity as foundby Prasad andKumar(1994).Papadakis(1964)reportedthatthereisalin- earproportionbetweensoundvelocityandtheelasticmodulusof thestructurewithinwhichthesoundwavepropagates.Theelas- ticmodulidecreasesintheorder offerrite–pearlite–martensite.
Hence,lowerpercentageofferritewithrespecttopearlite con- tentcorrespondstolowerelasticmodulusandconsequently,lower soundvelocity.
AsimilartrendisalsofoundwithsoundvelocityforSAE-4340 whenchangingtheheattreatmentfromannealingtooilandwater quenchingcausingareductioninsoundvelocityasseeninFig.9.
Thereasontothisreductionisduetothechangeinmicrostructure frompearlitetomartensitesimilartothatconcludedbyPrasadand Kumar(1994).Whenquenching,thesteeliscooledrapidlyfromthe austenitizingrangetoroomtemperature,inwhicheachaustenite grain(FCC)suddenlytransformsintolathsofmartensite(BCT)by diffusionlesslatticeshear asconfirmedbyGür andC¸am(2006).
Thistransformation leadstohighcrystallatticedistortions due totheincreaseinvolumeduringtheaustenite–martensitetrans- formationresultingintogreatamountofinternaltension/residual stresses.Thereforeit canbestatedthat martensiteisthephase withveryhighdislocationdensityandmaximumrandomnessas reportedbyPapadakis(1964)andGürandC¸am(2007).
Inthesamecontext,GürandTuncer(2005)reportedthatthe ultrasonicvelocityinmartensiteisessentiallyaffectedbychanges in the modulus of elasticityof individualgrains, in the crystal
Fig.5.MicrostructuresobtainedbySEMofsteelSAE-4340in(a)annealed,(b)oil quenchedand(c)waterquenchedconditions.
Fig.6.Orientationimaginggrainboundarymapping(left)andmisorientationangle distributionhistogram(right)ofSAE-4340intheannealedcondition.
latticedistortion levelandintheorientationofprimaryausten- itegrains.The increasein latticedistortionand thesubsequent increaseindislocationdensityreducesoundvelocityasreported byGürandC¸am(2007).ThiswasconfirmedearlierbyPapadakis (1964)whoreportedthattheelasticmodulidecreaseintheorder ofpearlite–bainite–martensiteandbasedontheproportionalrela- tion betweentheelasticmodulus and soundvelocity it can be statedthatmartensitehaslowersoundvelocitywhencomparedto pearlite.Inaddition,martensitepresentshighresistancetoultra- sound waves,because ofits compact and finestructure, hence possessinglowsoundwavepropagationvelocityasreportedby Freitasetal.(2010).Theseresultsareinlinewiththoseobtainedby Papadakis(1964),GürandC¸am(2006)andGürandTuncer(2005).
3.4.2. Attenuation
Theattenuationofultrasonicwavesinapolycrystallinemate- rial is mainly due to either scattering within the grains and their structural boundaries or absorption due to dislocation damping, thermoelastic losses, and magnetic hysteresisloss or bothasreportedinvariouspublicationssuchasPapadakis(1964) andKumaretal.(2002).Whencalculatingattenuationcoefficient (˛),mostliteraturesuchthatreportedbyLiuetal.(2007)have
Fig.7.Hardnessvariationwithdifferentheattreatmentswith:(a)SAE-1040and (b)SAE-4340.
appliedEq.(3),whichindicatesthat (˛)isequal tothesumof theabsorptioncoefficient(˛a)andthescatteringcoefficient(˛s) asfollows:
˛=˛a+˛s (3)
Scattering is usually divided into three different regimes dependingontheratioofgrainsize(d)toultrasonicwavelength ()assummarizedbyKhafrietal.(2012).Theseregimesarethe Rayleighregionwhered,thestochasticregionwhere≈dand thegeometricregionwhere<d.InthepresentworktheRayleigh regimeisapplicablebecauserangesbetween1.353and1.366mm whereasthemaximumgrainsizeisabout54m(dependingonthe soundvelocitiesmeasuredforeachspecificsample).Inpolycrys- tallinematerials,itisthescatteringofultrasoundfromgrainsand interfacesthatisthemaincauseofattenuation.Inthesematerials,
grainscatteringlossesarelargecomparedtoabsorptionlossesas agreedbyPapadakis(1964)andKhafrietal.(2012).
Fig.10showsthattheattenuationcoefficientforSAE-1040was lowestwiththeannealedmicrostructureandgraduallyincreases withoilthenwaterquenchedones.Thisisduetotheincreasein pearlitecontent(i.e.decreaseinproeutectoidferrite),asinferred fromFig.1,andthedecreaseininterlamellarspacinginpearlite as shown in Fig. 2, all of which contribute in increasing scat- tering, hence increasing attenuation.The reason tothis is that scatteringincreaseswithsampleshavinglargerpearlitecontent and consequently,larger areas of interface [phase boundaries]
betweenhardFe3Candsoftferrite,asreportedbyGürandTuncer (2004)and Gürand Keles(2003).In thesamesense,ultrasonic wave attenuation due to scattering in pearlite microstructure is greater than that in ferrite because pearlite is more elasti- cally anisotropic than ferrite. This result is confirmed by Ahn and Lee (2000). Since the pearlite content increasedwhen the treatment changed from annealing to oil then water quench- ing, therefore the interface areaincreased as well. It wasalso reportedbyGürandKeles(2003)thatinpearlitecontainingsteels, theattenuationof ultrasonic beamspassing through grainsare affectedbypearlitedispersion,whichconformstothatshownin Fig.2.
Fig. 11 for SAE-4340 shows that attenuation decreased as the steel phases changed from ferrite+pearlite, oil quenched- martensite and water quenched-martensite. This result is in agreementwiththatobtainedinvariousearlierworkssuchasthat ofKumaretal.(2002,2003),whichwasappliedoncarbonandalloy steels.Theyreportedthattheattenuationofpearliteishigherthan thatofmartensite.
GürandC¸am(2007)indicatedthatuponquenching,thesud- dentransformationofausteniteintomartensiteresultedintohigh amountoflatticedistortion,highdislocationdensityandmaximum randomnessofthestructure.Fromthepoint ofviewofscatter- ingtheory,therandomnesscausesanincreaseinelasticisotropy ofthegrainvolume,thusdecreasesscatteringpowerasreported by Papadakis (1964) and Kumar et al. (2002).In addition,fine lathsofmartensitemadethestructuremoreisotropicthanpearlite andferritehencedecreasingitsscatteringpower(GürandKeles, 2003).
3.5. Correlationbetweenultrasonicandmechanicalproperties Fig.12aandbshowstherelationbetweenhardnessandsound velocityandattenuationcoefficientofSAE-1040steelatdifferent pearlite–ferritecontents,respectively.Generally,itcanbestated
Fig.8. Theinfluenceofdifferentpearlite(P)andferrite(F)contentsonsoundvelocityinSAE-1040.
Fig.9.TheinfluenceofdifferentphasesinSAE-4340onsoundvelocity.
Fig.10.Theinfluenceofdifferentpearlite(P)andferrite(F)contentsonattenuationcoefficientinSAE-1040.
Fig.11.TheinfluenceofdifferentphasesinSAE-4340onattenuationcoefficient.
Fig.12.RelationshipbetweenhardnessVickersand(a)soundvelocityand(b)atten- uationcoefficientatdifferentpearlite–ferritecontentsinSAE-1040.
thataninverserelationshipexistsbetweenultrasonicvelocityand hardness,whichisinagreementwiththatfoundbyGürandC¸am (2007).Sincethehardnessofthephasesincreasedintheorder ofincreasingpearlitecontent, acorrespondingreduction inthe soundvelocitywasnoted.Theattenuationcoefficientcanalsobe correlatedwithhardnesswhichinturnisrelatedtothevolume percentof pearliteand ferrite and thecorrespondingextent of elasticanisotropy.Inaddition,PrasadandKumar(1994)indicated thathardeningprocessintroducesinternalstressesintothelattice resultingintolatticedistortion/deformation;which isaccompa- nied by an increase in dislocation density thus increasing the attenuationcoefficient.
The same inverse relation between hardness and ultra- sonic velocity was obtained with SAE-4340 steel as shown in Fig.13a. Thisis attributed to thedifference in microstructures where martensite possesses higher resistance to sound waves as well as higher lattice distortion and dislocation density all of which reduce the sound velocity more than pearlite and ferrite structure does. On the other hand, Fig. 13b shows that increasing hardness markedly reduced the attenuation coefficient.ThisresultisconfirmedwiththatobtainedbySeokand Kim(2005).Again,thedifferenceinmicrostructurewasresponsi- bletothisreduction.Thisisduetothefactthatlathsofmartensite
Fig.13.RelationshipbetweenhardnessVickersanda)soundvelocityandb)atten- uationcoefficientatdifferentpearlite-ferritecontentsinSAE-4340.
makethestructuremoreisotropicthanpearlite+ferritestructure does,hencedecreasingitsscatteringpower.Thisresultisinline withthatreportedbyGürandKeles(2003)andPapadakis(1964) which statedthat attenuationin pearliteis higher thanthat in martensite,whichpossessesleastattenuationevenamongother steelphasessuchasferriteandbainiteasreportedbyKumaretal.
(2002).
Fig. 14a shows that the increase of UTS and YS of SAE- 1040steel, wasaccompaniedby a reduction in soundvelocity.
A similarinverse relation,as that of hardness,is obtained due totheproportionalrelationbetweenpearlite contentand hard- ness onone hand and the mechanical properties on theother hand. Similarly, Fig. 14b plots the strength versus the atten- uation coefficient. The strength is related to the increase in pearlitecontent whichin turncausesthestructure tobemore anisotropicthusincreases theattenuation[scattering power]of thisstructurecomparedtothelessanisotropicone[lowerpearlite content].
WithSAE-4340steel,increasingstrengthwasaccompaniedbya reductioninultrasonicvelocityasinFig.15a,whichisatrendsim- ilarthatobtainedwithSAE-1040steel.Thereasontothisreduction
Table3
ComparisonbetweenSAE-1040and-4340inannealedcondition.
Sample Soundvelocity,m/s Attenuationcoefficient,dB/mm Pearlie%,ferrite% LAGBs%
1040-annealed 5913 0.0072 46%P,54%F 12
4340-annealed 5906 0.0074 59%P,41%F 25
Fig.14.RelationbetweenUTSandYSwithultrasonicparametersofSAE-1040steel:
(a)soundvelocityand(b)attenuationcoefficient.
isrelatedtothetypeofmicrostructure,wherepearliteoffersless resistancetoultrasonicwavesthusallowingfastersoundvelocity totakeplacecomparedtomartensitewhichpossesshigherresis- tanceandconsequentlylowestspeedasdiscussedbyFreitasetal.
(2010).Therelationbetweenstrengthandattenuationisplotted inFig.15b.Increasingstrengthwasaccompaniedbyareduction inattenuation.Thestrengthincreasedbecauseofthechangein microstructurefrompearlite+ferritetomartensitewhichdirectly influencedtheattenuationbasedonthefactthatmartensitestruc- ture is more isotropic thus possessing lower scattering power thanpearlitedoes.Thisresultagreeswellwiththatreportedby Papadakis(1964),andKumaretal.(2002).
Althoughthepresentworkwasnotintendedtocomparethe two types of steel, however, there were two reasons behind their comparison. First, was that both types of steel were annealed at the same temperature and second was that simi- lar microstructural components were obtained. In light to the earlier discussion, comparison was done as a trial to further verifytheinfluenceof phase percentandthesub-grainbound- aries on sound velocity and attenuation when steel type is changed. Table 3 summarizes the microstructural phases and grain size and their corresponding ultrasonic measurements conducted with both types of steels annealed at the same conditions.
It canbenoted thatsound wavespropagated faster inSAE- 1040steelthaninSAE-4340.Thisisduetothattheformersteel possessedhigherpercentofferrite(54%;leastresistanttoultra- sonicwaves)thanthelatter(41%).AttenuationofSAE-1040,on theotherhand,waslowerthanthatofSAE-4340asinTable3.
This wasagain due tothe lower percentage of pearlite in the former(46%)thaninthelatter(59%).Higherpearlitecontentmade thestructuremoreanisotropicthusleadingtohigherattenuation capabilitythanlowerpearlitecontent.Also,thehigherpercentage
Fig.15.RelationbetweenUTSandYSwithultrasonicparametersofSAE-4340steel:
(a)soundvelocityand(b)attenuationcoefficient.
of sub-grainboundariesoccurringwithSAE-4340(LAGBs=25%) compared tothat withSAE-1040(LAGBs=12%)maybeanother reason why the former possesses higher attenuation than the latter.
4. Conclusions
•InSAE-1040steel,ultrasonicvelocityisreducedintheorderof annealing–oil–waterquenching.
•In SAE-4340, ultrasonic velocity is also reduced in the order of annealing–oil–water quenchingas wellasattenuation also decreasesinthesameorder.
•Themainmicrostructuralparametersaffectingthenondestruc- tive measurements are the volume fraction of phases and percentageofsub-grainboundaries.
•Theattenuationandultrasonicvelocitycanbewellcorrelated to evaluate the microstructural phases and the mechanical properties such as ultimate and yield strength as well as hardness.
Acknowledgment
ThisworkwassupportedbyNationalScience,Technologyand InnovationPlan(NSTIP)strategictechnologiesprogram,withinthe projectnumber(08-ADV-209-02)intheKingdomofSaudiArabia.
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