J.Cent.SouthUniv. (2013) 20: 866870DOI: 10.1007/s117710131559y

EffectoftemperingtemperatureonmicrostructureandmechanicalpropertiesofAISI6150steel

LIHongying()1,HUJidong()1,LIJun()2,CHENGuang()1,SUNXiongjie()1

1.SchoolofMaterialsScienceandEngineering,CentralSouthUniversity,Changsha410083,China2.ResearchInstitute,BaoshanIron&SteelCo.Ltd,Shanghai201900,China

CentralSouthUniversityPressandSpringerVerlagBerlinHeidelberg2013

Abstract:EffectoftemperingtemperatureonthemicrostructureandmechanicalpropertiesofAISI6150steelwasinvestigated.Allsampleswereaustenitizedat870Cfor45minfollowedbyoilquenching,andthentemperedattemperaturesbetween200and600Cfor60min.Theresultsshowthatthemicrostructureoftemperedsampleat200Cmainlyconsistsoftemperedmartensite.Withincreasingthetemperedtemperature,themartensitetransformstotheferriteandcarbides.Theultimatetensilestrength,thehardnessandtheretainedaustenitedecreasewith increasingtemperedtemperature,and0.2%yieldstrengthincreaseswhenthe temperatureincreasesfrom200to300Candthendecreaseswithincreasingthetemperature,buttheelongationandimpactenergyincreasewithincreasingthetemperingtemperature.

Key words: temperingtemperatureAISI6150steel microstructure mechanicalproperty

Foundationitem:Project(2011BAE13B03)supportedbytheNationalKeyTechnologyR&DProgramofChinaReceiveddate:2012-09-24Accepteddate:2013-01-17Correspondingauthor:LIHongying,Professor,PhDTel/Fax:+86-731-88836328Email:lhying@csu.edu.cn

1Introduction

AISI 6150 steel is a fine grained, highlyabrasionresistantchromiumvanadiumalloysteel,whichhas high strength, high fatigue strength and largehardenability. In recent years,AISI 6150steelhas beencommonly used in many industries, particularly theautomotive industry [1-2]. However, some industriesdemandhighermechanicalpropertiesandlongerservicelifeofthematerials.InordertoimprovethemechanicalpropertiesofAISI6150steel,somemodificationstothesteel must be required. Considerable efforts have beendirected towardimproving themechanicalpropertiesbycompositionalmodifications and various heattreatmenttechniques. The small addition of Cr, V, Ni and Mnelementshas improved itsmechanical properties.Then,the technique of heattreatment should be employed toimprovethe mechanical propertiesofAISI6150steel.

Generally, quenching and tempering arewellestablishedmeanstoproducestrengtheninginsteel,while at the same time retaining or even increasing itsimpact toughness.This ismainly due to themartensiticstructure produced by quenching and the subsequentprecipitation of the fine carbides during the temperingprocess [3-4]. However, as large internal stressesassociatedwith themartensitic transformation cause thematerial to be lack of ductility, the martensite steel is

rarely used in nontempered condition. Tempering canincrease both the ductility and toughness, which areessential for enhancing impact energy absorption. Andtempered martensite lath structure also provides bestdynamic strength in steel [5]. In order to explore andunderstandtheeffectofheattreatmentprocesses on themicrostructure and mechanical properties of thematerials,manyresearchershavedonealotofworkoverthepastfewyears[6-11].SAYEDetal[6]havestudiedthe effect of the tempering temperature on themicrostructure andmechanical properties of dual phasesteels, and the results showed that tempering attemperatureslowerthan300Cfor1hwassuitableforattaining optimum strength and ductility. QIN et al [7]have researched thepropertiesof0Cr16Ni5Mostainlesssteels whichwere normalized at 1 000 C followed bytempering inthetemperaturerangeof525-625C.Theresults showed that the samples tempered at 550 and600 C for 2 h had an excellent combination of tensilestrength, elongation, impact energy, hardness andcorrosionresistance.SALEMIetal[8]haveexploredtheeffect of tempering temperature on the mechanicalproperties and fracture morphology of a NiCrMoV steel,anditwasfoundthatthetensilestrengthdecreasedandtheductility increased with the increasing of temperingtemperature.

However,verylimitedeffortshavebeendirectedtotheeffectsof theheat treatmentontheAISI6150steel.

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Therefore, in order to find out the effect of temperingtemperature on the microstructure and mechanicalproperties of the steel, in the present work, the tensileproperties, hardness, impact toughness andmicrostructures of tempered samples at differenttemperatureswerestudied.

2Experimental

The chemical composition ofmediumcarbonAISI6150steelused in thepresentwork isgiven inTable1.Before this work, we had conducted a series ofexperimentsatquenchingtemperaturesof800-1000Cforvariousholdingtimeof15-120min,and found that870 C, 45min is the best quenching condition for theAISI 6150 steel. In the present work, all the sampleswereaustenitizedat870C for45min, followedby oilquenchingtoproduce themartensite,andthentemperedat200-600 Cwiththeinternalof50C.Comparedwiththe previous works conducted, it was found thattemperingtimeof60minwasthe bestforthesteel.

Table 1 Chemical compositions of AISI 6150 steel (massfraction, %)

C Cr V Si Mn Ni P S Fe

0.53 1.02 0.15 0.23 0.83 0.03 0.01 0.01 Bal.

Before the microstructures of samples wereobserved by scanning electronmicroscopy (SEM), theywere mechanically polished and etched with 4% nitalsolution. The phase constituents in samples wereanalyzedbyaD/max2500Xraydiffractometry(XRD),whichwasoperatedat40kVand40mAwithscanning

speed of 0.5 ()/min. The volume fraction of retainedaustenite in the steel was determined by comparing theintegratedXraydiffraction intensitiesof the ferriteandaustenitephaseswiththetheoreticalintensities.

Themechanicalpropertiesoftheheattreatedsampleswereevaluatedbytensiletest,impacttoughnesstestandhardnesstest.Thetensilespecimensweremachinedfromthe heat treated samples in parallel to the rollingdirection. The tensile test was conducted on an InstronUniversal Testing Machine (Instron 5581) at roomtemperature according to ISO 68922009. The testspeed was 30mm/min, which corresponded to a strainrateof10 -2s -1.Atleast,fivespecimenswere testedandaverage valueswere calculated for each condition. Keyparameters obtained from stress-strain curves include0.2%yieldstrength(YS),ultimatetensilestrength(UTS),and percentage elongation. Standard CharpyVnotchedspecimenswiththesizeof55mm10mm10mmwasmachinedtofindouttheimpactproperties.Atleastfivesamples of eachheat treatmentprocesswere tested andtheir average value was taken as the impact toughnessvalue of plates under those conditions. The Vickershardness of all specimens was measured under 5 Napplied load for 10 s. The average hardness of onesample was reported from measurements over 10locations.

3Results

3.1MicrostructureThe microstructure of the steel in quenched

conditionisshowninFig.1(a),wherethemicrostructureconsists of the martensite (M). The microstructures of

Fig.1SEMmicrographsofheat treated samples: (a)Asquenched sample (b)Temperedat200C (c)Temperedat400 C(d)Temperedat600C

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samples temperedat200-600C for60minareshownin Figs. 1(b)-(d). Tempering is a diffusion type phasetransformation from the quenched martensite to thetempered martensite (M), ferrite (F) and carbides (C).The microstructure of the sample tempered at 200 C(Fig.1(b))mainlyconsistsof thelathmartensite,whichdiffers slightly from the quenched sample. Both thequenchedmartensiteandtemperedmartensiteareoflathshape.With the increase of the tempering temperature,the lath martensite transforms to ferrite and carbides.Oncethetemperingtemperatureincreasesto400C,themicrostructureofthetemperedsampleisclearlydifferentfromthatofthequenchedsample.Figure1(c)showsthatthemicrostructureofsampletemperedat400Cconsistsof lathmartensite, ferriteandrodshapecarbides.Whenthe tempering temperature increases to 600 C, themicrostructureofthetemperedsamplemainlyconsistsofferriteandcarbides,andthecarbideschangetheirshapefromrodtospheroidalshape.

3.2TensilepropertiesThe tensile properties of the AISI 6150 steel

samples depend on the tempering temperature. Thepropertiesofthequenchedsamplearealsoevaluatedforcomparison. The variations of ultimate tensile strength(UTS), 0.2% yield strength (YS) and elongation as afunctionoftemperingtemperatureareshowninFig.2.Itcanbeseen thatthevariationsof tensilepropertieswithtemperature consist of several stages: 1) With thetemperingtemperatureincreasingupto200C,theUTSand elongation slightly increase, and YS slightlydecreases2)From200to300C,theUTSdecreasesbyabout100MPa,whiletheYS risessharplyandreachesapeakvalueof1685MPa,andtheelongationincreasesbyabout1.2%3)From300 to600C,theUTSdecreasessharplyfrom1946to1115MPa,andtheYS decreasescontinuouslyfrom1685to1050MPa,whileelongationrisessharplyandcontinuouslyfrom7.5%to15.9%.

Fig.2Variationsoftensilepropertieswithtemperingtemperature

3.3ImpactenergyImpactenergygivesagoodindicationoftheenergy

required to initiateandpropagateacrack.ThevariationofimpactenergywithtemperingtemperatureisshowninFig.3.AscanbeseenfromFig.3,theimpactenergyofthe quenched sample is 10 J. With the temperingtemperature increasing up to 200C, the impact energyrises to 16 J. However, the impact energy changesslightly after tempering at 200-350 C. With furtherincreasing tempering temperature from 350 to 600 C,the impact energy increases continuously and reaches43Jatthetemperingtemperatureof600C.

Fig.3Variationofimpactenergywithtemperingtemperature

3.4HardnessFigure 4 exhibits the influence of tempering at

varioustemperaturesfor60minontheaveragevalueofVickershardness.ItcanbeseenthatthehardnessofAISI6150steelgraduallydecreases fromHV608toHV360withincreasingthe tempering temperatures in therangeof200-600C.Whencomparing theresultsof samplestempered in various temperature ranges, it is found thatthe rate of decrease of the Vickers hardness in lowertempering temperature range of 200-500 C is higherthanthatintemperaturerangeof550-600C.Moreover,

Fig.4Variationofhardnesswithtemperingtemperature

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thehardnessofthesteelwithouttemperingisthehighestandreachesHV689.

4Discussion

As can be seen from Figs. 2-4, the mechanicalproperties of AISI 6150 steel are quite sensitive to thetempering temperature. Compared with the samplestempered at different temperatures, the strength andhardness of quenched sample are the highest, but theductility is the lowest. This can be explained based onthe phase transformation of steel during the quenchingprocess, where the lattice structure of the steel changesimmediately from a facecentered cubic to abodycentered tetragonal phase.Martensite formation isaccompanied by a large amount of distortion, whichrapidly increases the strength and hardness of steel.However,theinternalstressgeneratedduringmartensiteformation causes significant reduction of the ductilityandtoughness.

Tempering process relieves the internal stressesacross the lath boundaries by permitting localrearrangement of atoms [11]. Below the temperingtemperature of300C,internalstressesgeneratedarenotfully released. With complete recovery of stresses attemperingtemperatureof300C,arearrangementofthedislocation structure takes place, which restricts theirmovement and leads to an increase in 0.2% yieldstrength.Withincreasingthetemperingtemperature,theconcentration of the temperedmartensite decreases andthepresenceoftheferriteandcarbidesincreases,mainlydue to thediffusionof carbonatoms intocementiteandthemovementofdislocationsbythermalassistance[12].Therefore, the ultimate tensile strength and 0.2% yieldstrengthdecreaseandtheelongationincreases.

AISI 6150 steels are susceptible to temperedmartensite embrittlement (TME) within a specifiedtemperature range. The phenomenon is usuallycharacterizedonaplotofimpactenergyasafunctionoftempering temperature. And it can be observed that amaximum in the ductile-brittle transition temperaturecorrespondingtotheminimumintheimpactenergy[13].It is seen from Fig. 3 that the impact energy increaseswithincreasingthetemperingtemperature.

Figure5depictstheXRDpatternsof the temperedsample at 200 C for 60min. The peaks represent thediffraction intensity of different crystal orientations ofthe ferrite. The volume fraction of retained austenite isachievedbytheresultofRietveldspectrumfittingofthetempered samples after XRD phase testing. Figure 6showsthevariationsofretainedaustenitewithtemperingtemperature. As can be seen in Fig. 6, the quenchedsample contains the maximum amount of retainedaustenite.With thetempering temperature increasingup

to 200 C, the volume fraction of retained austenitevaries from 3.60% to 2.93%. When the temperingtemperature increases from 200 to 300 C, the volumefractionofretainedaustenitesharplydecreasesto0.62%,mainlyduetothefactthatmostoftheretainedaustenitestransformtotemperedmartensiteaftertemperingatthesetemperatures. These lead to an increase in 0.2% yieldstrengthandelongation,andaslightdecreaseinultimatetensilestrength.Theretainedaustenitedecreasesslightlyin the tempering temperature range of 300-500 C. Attemperingtemperatureabove550C,thevolumefractionofretainedausteniteisclosetozero.

Fig.5 XRDpatternoftemperedsampleat200 C

Fig.6Variationofretainedaustenitewithtemperingtemperature

According to the HollomonJaffe relation of thehardness and a tempering parameter M: H=f(M)=f(T(t+lgt))[14],itisknownthatthehardnessisaffectedby the tempering temperatureif the tempering time isaconstant. Tempering can be considered as a phasetransformation promoted by diffusion from an unstablestate towards a quasi equilibrium state. Therefore, thehardness can be used to define any tempering state.Astheevolutionsofthetemperingtimeandtemperatureare

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also controlled by diffusion mechanisms (particularlycarbides precipitation and growth), a tempering kineticlawisproposedintheformoftheJohnsonMehlAvramitypeequation[15]:

v 0 0( ) exp( ( ) )mH H H H D t = + - - (1)

where H0 is the hardness after quenching, H is thehardness in the temperedstate,Hv is thehardnessofanintermediate state between the quenched state and thetempered state, t is the tempering time,m is the ageingexponent depending on the material and the previousheattreatment,andDdependsontemperingtemperatureandfollowstheArrheniusequation:

D=D0exp[-Q/(RT)] (2)

where D0 is the preexponential constant, Q is theactivation energy of the tempering transformation, R istheperfectgasconstant(equalto8.31J/(Kmol -1))andTisthetemperingtemperatureinK.AccordingtotheEq.(1) and Eq. (2), if the activation energy of temperingtransformation and tempering time remain constantduring the tempering process, it can be calculated thatthe hardness of the AISI 6150 steel decreaseswith theincrease of the tempering temperature.These calculatedresults closely agree with the measured experimentalvaluesofFig.4.

5Conclusions

1) At low tempering temperature of 200 C, thetempered microstructure mainly consists of lathmartensite. With increasing the tempered temperature,themartensitebecomeslessandtransformstoferriteandcarbides. The microstructures of the AISI 6150 steelconsist of ferrite and carbides at the high temperingtemperatureof600C.

2) Increasing the tempering temperature decreasesthe ultimate tensile strength and the hardness, butincreases the elongation and impact energy. However,0.2% yield strength increases when the temperatureincreases from 200 to 300 C and then decreases withincreasingthetemperature.

3) The retained austenite sharply decreases at thetemperedtemperaturesof200-300C,andthendecreasesslightly in the temperature range of 300-500 C.Whenthe tempering temperature is higher than 550 C, thecontentof retainedaustenitedecreasestozero.

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(Editedby YANGBing)