Microstructure evolution during high-temperature partitioning of a Medium-Mn Quenching and Partitioning steel

Delft University of Technology

Microstructure evolution during high-temperature partitioning of a Medium-Mn Quenching

and Partitioning steel

Ayenampudi, S.; Celada-Casero, C.; Sietsma, J.; Santofimia, M. J.

DOI

10.1016/j.mtla.2019.100492

Publication date

2019

Document Version

Final published version

Published in

Materialia

Citation (APA)

Ayenampudi, S., Celada-Casero, C., Sietsma, J., & Santofimia, M. J. (2019). Microstructure evolution during

high-temperature partitioning of a Medium-Mn Quenching and Partitioning steel. Materialia, 8, [100492].

https://doi.org/10.1016/j.mtla.2019.100492

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ContentslistsavailableatScienceDirect

Materialia

journalhomepage:www.elsevier.com/locate/mtla

Full

Length

Article

Microstructure

evolution

during

high-temperature

partitioning

of

a

medium-Mn

quenching

and

partitioning

steel

S.

Ayenampudi

_,

_C.

_{Celada-Casero}

_,

_J.

_Sietsma

_,

_M.J.

_Santofimia

a

r

t

i

c

l

e

i

n

f

o

Quenching and partitioning Medium manganese steel High-temperature partitioning Carbon partitioning Austenite stability

a

b

s

t

r

a

c

t

Medium-Mn Quenching & Partitioning (Q&P) steels have been recently considered as potential candidates for the 3rd generation advanced high-strength steels. The processing of these steels aims to induce the partitioning of substitutional alloying elements from martensite to austenite during an isothermal treatment at high tempera- ture, where the diffusivity of substitutional alloying elements is sufficiently high. In this way, austenite increases its concentration of austenite-stabilising elements and thus its thermal stability. The present study aims to in- vestigate the microstructural evolution during high temperature partitioning treatments in a medium-Mn steel and the possible occurrence of additional phase transformations that may compete with the process of atomic partitioning between martensite and austenite. Q&P routes in which the partitioning steps take place in the range of 400 °C–600 °C for times up to 3600 s were investigated. The final microstructures display an increased fraction of retained austenite with increasing holding times during partitioning at 400 °C, while at higher partitioning temperatures, 450 °C–600 °C, leads to cementite precipitation in austenite films and pearlite formation in blocky austenite, resulting in a decrease of the fraction of retained austenite with the holding time. This observation is supported with theoretical calculations of the volume change, suggesting that for maximising the fraction of retained austenite, short holding times are preferred during partitioning at high temperatures. Observations from the current study reveal that the successful application of high-temperature partitioning treatments in medium- Mn steels requires microstructure design strategies to minimize or suppress competitive reactions.

1. Introduction

Thequenchingandpartitioning(Q&P)process,proposedbySpeer andco-workers[1],hasbeenconsideredasoneofthemostpromising heattreatmentsfortheproductionofthirdgenerationadvancedhigh strengthsteels(AHSSs)withexceptionalcombinationofstrengthand ductility.The typicalQ&P processinvolvesan initial austenitization (partialorfull)followedbyquenchingtoatemperaturebetweenthe martensitestarttemperature(Ms) androomtemperaturetocreate a

controlledfraction ofprimary martensite(M1).Thesteel is then re-heatedtoahighertemperaturetoallowthecarbondiffusionfromthe supersaturatedmartensiteintotheaustenite,whichisreferredtoas par-titioningstage.Thecarbonenrichmentoftheausteniteleadstoits sta-bilizationatroomtemperature.Ifpartoftheausteniteisinsufficiently enrichedwithcarbon,this maytransformintofreshmartensite(M2) duringthefinalquenchtoroomtemperature[2–4].

Speeretal.[4]proposedtheconstrainedcarbonequilibrium(CCE) modeltodescribethethermodynamicsofthecarbonpartitioning

pro-∗_{Corresponding author.}

E-mail addresses: s.ayenampudi@tudelft.nl (S. Ayenampudi), c.celadacasero@tudelft.nl (C. Celada-Casero), J.Sietsma@tudelft.nl (J. Sietsma),

M.J.SantofimiaNavarro@tudelft.nl (M.J. Santofimia).

cess.TheCCEmodelischaracterizedbytwoassumptions:a)thecarbon partitioningfrommartensitetoausteniteisfinalizedwhenthe chem-ical potential of carbon in both phases is equal andb) the austen-ite/martensiteinterfaceisimmobileduringthepartitioningstepasthe numberofironatomsineachphaseareconserved.Typicalpartitioning temperaturesintheQ&Pprocess(350°C–450°C)arerelativelylowand thediffusivitiesofsubstitutionalalloyingelementsduringthe partition-ingstepcanbeignoredatthetimerangesthatarenormallyconsidered. Therefore,moststudiesareconcentratedonstudyingthestabilization ofaustenitebycarbon[2,5–8].

Recently, the idea of stabilizingthe austenite through Q&Pheat treatmentsinwhichthepartitioningstagetakesplaceattemperatures highenoughtostimulatethepartitioningofsubstitutionalalloying ele-mentshasbeenproposedbysomeauthors[9–13].Thispossibilitystems fromtheobservationofanapparentpartitioningofsubstitutional alloy-ingelementsatrelativelylow temperatures.Forexample,Santofimia etal.[9]reportedthepartitioningofmanganeseattypical partition-ingconditions(400°Cfor50s)atsomemartensite/austeniteinterfaces.

https://doi.org/10.1016/j.mtla.2019.100492

Received 29 July 2019; Accepted 27 September 2019 Available online 1 October 2019

2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY license. ( http://creativecommons.org/licenses/by/4.0/ )

Later,severalauthors[10–13]observedthattherangeofmanganese partitioningisincreasedwiththeincreaseinpartitioningtemperature from400°Cto450°C.Someoftherecentworks[14,15]alsoaimedat in-vestigatingaustenitestabilityintheintercriticalrangeoftemperatures bypromoting austenitereversetransformation.However,manganese tendstopartitionintoausteniteonlyafewnanometres(typicallyless than10nm)afterisothermalholdingtimesofupto1hatthese temper-atures.Therefore,thepartitioningrangesofmanganesereportedearlier maynotbesufficienttostabilizetheentirefilmsofretainedaustenite, whichtypicallyhaveathickness5–20nm.

Thediffusivityofmanganeseinaustenite,attypicalQ&P temper-atures (400°C–450°C), is in the order of magnitude of 10−26 _m2_/s

[16],whichisverylowcomparedtothatofcarbon,whichisaround 10−16 _m2_{/s[17]. Inorder to promote manganesepartitioning from}

martensiteintoausteniteitis importanttoapply higherpartitioning temperatures.However,higherisothermalholding temperaturesmay increase the probability of occurrence of competitive reactions like austenitedecompositionintobainiteorpearlite,orcarbideformation [18]oraustenitereversetransformation[14–15].Mostoftheearlier worksreportedtheoccurrenceofbainiteformation[3]andcarbide pre-cipitationinsideprimarymartensite[6,8]duringisothermalholdingat lowerpartitioningtemperatures(400°C–450°C).However,thereareno researchworksfocusedonthemicrostructuraldevelopmentthattakes placeduringpartitioningstepsathighertemperatures(500°C–600°C) belowtheintercriticalregion.Hence,thecurrentresearchaimstogain insightintothemicrostructuralevolutionandcompetitivereactions oc-curringin amedium-Mn steelduringpartitioningattemperaturesof 400°C–600°Candtimesrangingfrom180sto3600s.Theresultsofthis studyopenupthepossibilitytonewQ&Pmicrostructuraldesign strate-giesinmedium-Mnsteelsthatminimizeorsuppresstheoccurrenceof competitivereactionsduringhigh-temperaturepartitioningtreatments.

2. Materialsandexperimentalmethods

Thechemicalcompositionofthemedium-Mnsteelusedinthisstudy isshowninTable1.Thecontentofmanganeseisexpectedtocontribute tothestabilizationoftheausteniteandtodelaysignificantlythe for-mationofstructuressuchasferrite,bainiteandWidmanstättenferrite duringcoolingtothequenchingtemperature.Siliconisnormallyused todelayanycementiteprecipitationduringthepartitioningstep.

Thesteelwasproducedintheformof aforgedbillet.Cylindrical specimensof10mminlength and4mmin diameterweremachined fromtheforgedbillet.ThesespecimenswereheattreatedinaBähr805 DILA/Ddilatometer.AtypeSthermocouplespot-weldedonthe sur-facewasusedtomonitorandcontroltemperature.Lowpressureonthe orderof10−4_{mbarwasusedduringheatingorisothermalsegments,}

andheliumwasusedasthecoolant.Theerrorinchangeinlengthfrom dilatometryexperimentswasestimatedas±0.01%.

TheappliedthermalroutesareshowninFig.1.Thesethermalroutes includeafullaustenitizationat950°Cduring120s,quenchingat30°C/s to190°Candpartitioningattemperatures(TP)rangingfrom400°Cto

600°Cforpartitioningtimes(tP)ofupto3600s.Inthefollowing

sec-tions,conditionswillbeindicatedasQPT_P-t_Pforconciseidentification ofspecimens.Aheattreatmentinvolvingadirectquenchfrom austeni-tizationconditionswasalsoincludedintheinvestigations.

Aftertheapplicationoftheheattreatments,thespecimenswerecut intohalfandthesurfacewaspreparedbygrindingwithP800,P1000, andP1200abrasivepapersandpolishingwith6,3and1μmdiamond paste.Thepolishedspecimenswereetchedwitha2%Nitalsolutionfor

Table1

Chemical composition (wt. %) of the steel investigated.

C Mn Si Mo Al Cr Fe

0.31 4.58 1.52 < 0.005 0.01 0.02 Balance

Fig.1. Schematic drawing of the Quenching and Partitioning heat treatments.

thesubsequentobservationusingscanningelectronmicroscopy(SEM), forwhichaJEOLJSM-6500Ffieldemissiongunscanningelectron mi-croscope(FEG-SEM),operatingat15kV,wasused.

Thevolume fractions(fRA) andlatticeparameter(a𝛾) of retained

austeniteweredeterminedbymeansofX-raydiffraction(XRD) analy-sisusingaBrukertypeD8-Advancediffractometer,ina2𝜃 rangefrom 40° to130°,withCoK𝛼 radiation(wavelength0.1789nm),usingastep sizeof0.0420_{2𝜃,withacountingperstepof3s.This2𝜃 rangecovers}

the(110),(200),(211),(220)ferritepeaksandthe(111),(200),(220), (311)austenitepeaks.Thevolumefractionofretainedausteniteandthe errorsindeterminingtheretainedaustenitefractionwerecalculatedby thedirectcomparisonmethodofausteniteandmartensitepeaksusing theproceduredescribedbyJatczak[19].Inthepresentwork,volume fractionsofretainedaustenitebelow0.03areneglectedasthisisthe detectionlimitoftheX-raydiffractionmeasurements.Thecarbon con-centration withintheretainedausteniteis calculatedfrom its lattice parameter,a_𝛾(inÅ)usingthemethoddescribedin[20]:

𝑎𝛾=3.556+0.0453⋅ 𝑥C+0.00095⋅ 𝑥Mn+0.00157⋅ 𝑥Si (1)

wherexi,representstheconcentrationofthealloyingelementiinwt.

%.

Magnetizationmeasurementswereperformedatroomtemperature oncubicspecimensof2.0mminsidelengththatweremachinedfrom thecentreofheat-treateddilatometryspecimens.A7307vibrating sam-plemagnetometer,calibratedwithaNationalInstituteofStandardsand Technologynickelspecimen,wasused.Withthisequipment, magneti-zationcurvesatroomtemperatureweremeasuredbyastepwisechange intheappliedmagneticfieldfrom+1.6to−1.6T.Thesaturation mag-netizationvalueswereobtainedbyfittingtheapproachtothe satura-tionoftheexperimentallyobtainedmagnetizationcurve,accordingto Ref.[21].Thevolumefractionofmartensite(fM)inthequenched

spec-imenisdeterminedbycomparingthesaturationmagnetizationvalues obtainedbothonthespecimenwithmartensitetobemeasuredandon pureFe-BCCspecimensaccordingtothemethodindicatedinRef.[21].

3. Results

Inthissection,themicrostructuralevolutionduringthedifferent ap-pliedQ&Pheattreatmentsisevaluatedbasedonthedilatometry mea-surements,X-raydiffractionanalysis,magnetisationmeasurementsand microstructuralobservations.

The volumefraction of martensiteformedat thequenching tem-peraturewasdeterminedbyanalysingthedilatometryresponseofthe as-quenched specimen austenitized at 950°C for 120s and directly quenchedtoroomtemperature,asFig.2ashows.Thelinearexpansion behaviourof theBCCandFCClatticesinthedilatometrycurvewere used tofitthethermalexpansionof bothphases.Inordertoextract

Fig.2. (a) Relative change in length versus temperature of an as-quenched (solid blue line) and QP400-3600 (dashed red line) specimens. f Mis the volume fraction of martensite formed after an as-quench heat treatment; f M1and f M2are the volume fractions of primary and fresh martensite formed during QP400-3600 heat treatment, respectively (b) Volume fraction of martensite as a function of temperature obtained applying the lever rule to the as-quench dilatation curve.

informationregardingthephasefractionsformedduringthequench, thefinalchangeinlengthisassociatedwiththeformationofavolume fractionofmartensiteequalto0.94±0.01,asexperimentallymeasured usingthemagnetometer.Byapplyingtheleverrule,thevolumefraction ofmartensiteisdeterminedasafunctionofthetemperatureduringthe quenchingtreatment,asindicatedbyasolidbluelineinFig.2b. Ac-cordingtothisdata,theselectedquenchingtemperatureof190°C cor-respondswiththeformationofavolumefractionofprimarymartensite (M1)equalto0.60±0.01,leavingavolumefractionofuntransformed austeniteof0.40±0.01.Thesevolumefractionsofprimarymartensite and,consequently,austenitewerechosenwiththeaimtostabilizea sig-nificantvolumefractionofausteniteinthefinalmicrostructures,asthe steelhasrelativelyhighcarboncontent.

Fig.2aalsoshowsthedilatationcurveofQP400-3600(dashedline) specimens,asanexample.ThisdilatometrycurveofQP400-3600 spec-imen is used toexplainthemicrostructural development duringthe Q&Pprocessing routes.Initially,alinearcontractionisdetected cor-respondingtothecoolingfromtheaustenitizationtemperature.When thetemperaturedecreasesbelowtheMsanduntilthequench

temper-ature(TQ),adilatationcorrespondingtotheformationof0.60volume

fractionofathermalmartensiteisobserved.Then,thespecimenis re-heatedto400°C,duringwhichanexpansionisobserved,indicatingno phasetransformations.Thesmall positivechange in lengthobserved duringtheisothermalholdingat400°Ciscausedbytheformationof carbidesand/orpearliteandtothecarbonpartitioningfrommartensite toaustenite,anditwillbediscussedindetailinthefollowingsections. Duringthepartitioningstep,partoftheremainingausteniteenriches sufficientlyincarbontobethermallystabilizedatroomtemperature.A smalldeviationfromlinearityofthedilatometriccurvesduringthefinal quenchtoroomtemperatureindicatestheformationofasmallvolume fractionoffreshmartensite(M2)fromthelessstableaustenite.The vol-umefractionsoffreshmartensite(fM2)weredeterminedbycomparing

themeasuredchangeinlengthwiththechangeinlengthobservedin thedirect-quenchedspecimenasexplainedintheref.[22,23].The re-tainedaustenitevolumefractions(fRA)inthefinalQ&Pmicrostructures

weremeasuredusingX-raydiffractometryasexplainedinthe experi-mentalprocedure.Theremainingconstituentsinthefinal microstruc-tureswillbecarbidesand/orpearlite.Thetotalvolumefractionofthese constituents,fc+p,wascalculatedbybalanceofthephasefractions:

𝑓𝑀1−𝑓_𝑀2−𝑓_𝑅𝐴−𝑓_𝑐+𝑝=1 (2) Thesamemethodwasappliedtodeterminethevolumefractionof phasespresentallfinalQ&Pmicrostructures.Theresultsaredisplayed

on theleft-handsideof Fig.3. Also,on theleft-handsideofFig.3, thedilatometrycurvesagainsttimeregisteredduringthepartitioning stepsfor3600sarerepresented.Inthefollowing,thisinformationwill beemployedtogetherwiththemicrostructuralobservationsinorderto understandthemicrostructuralevolutiontakingplaceduringthe parti-tioningstepatthedifferentstudiedtemperatures.

3.1. Partitioningat400°C

Fig.3ashowsthechangeinlengthobservedinthedilatometry spec-imensduringtheisothermalholdingat400°Cfor3600s.The dilatome-trycurveshowstwostages.Thefirststageisanexpansion,whichis ob-servedduringapproximatelythefirst1800s.Thisexpansionisrelated totheprocessof carbonpartitioningfromthecarbon-supersaturated martensite(M1) intotheaustenite[24].Thesecond stageisaslight contraction,whichislikelyduetotheprecipitationofcarbidesin pri-marymartensite,aspreviouslyobservedbyTojietal.[13].

Thefinalmicrostructuresshow anincreaseinthevolumefraction of retainedaustenite andadecrease in thevolume fractionof fresh martensitewithincreasingholdingtimes.Thisevolutionresultsfrom theprocessofcarbonpartitioningfrommartensitetoaustenite,which progressivelystabilizestheausteniteduringthepartitioningstep.

Fig.3bshowsthemicrostructureofthespecimenafterpartitioning at400°Cfor3600s.Theprimarymartensiteischaracterisedbythe pres-enceofcarbides.Themicrostructurealsoshowblockyislandsoffresh martensite/retainedaustenite(MAislands)withathicknessoffew mi-crometres.Nanometricretainedaustenitefilmsarealsoobservedin be-tweenthemartensitelaths.

3.2. Partitioningat450°C

The dilatometry curve during the partitioningstep at 450°C for 3600sisshowninFig.3c.Asmallexpansionisobservedwithinthefirst 40s(zoomed-intheinset),whichisrelatedtocarbonpartitioning.After 40s,acontinuousdecreaseinlengthisobserved,whichmayberelated withamorepronouncedprecipitationprocessthanthatobserved dur-ingpartitioningat400°C.Kannanetal.[25]andOninketal.[26] ob-served thataustenitefilmssaturatedwithcarbontend todecompose intocarbon-depletedausteniteandcementite,andthisphenomenonis accompaniedbycontraction.

Afterpartitioningat450°C,thefinalvolumefractionsof microstruc-turalconstituentsindicateasimultaneousincreaseinthevolume frac-tionoffreshmartensiteandadecreaseinthevolumefractionofretained

Fig.3. For every partitioning temperature, the figures on the left show the change in length and the volume fraction of phases present at the end of the different Q&P heat treatments as a function of the partitioning time. Figures on the right side show microstructures of the specimens observed under the SEM after partitioning for 3600 s at partitioning temperatures of 400 °C–600 °C.

austenite.Afterpartitioningfor3600sat450°C,thevolumefraction ofretainedausteniteisessentiallythesameasthatpresentinthe as-quenchedstate.

Fig.3d shows aSEM micrograph of thespecimen partitioned at 450°C for 3600s. In this case, the precipitation of carbides inside

primary martensite is not very evident. Arrays of parallel carbides alignedinthedirectionofthemartensitelathscanbeclearlyobservedin theprimarymartensite,asindicatedinFig.3dwithdashedlines.These arraysofcarbidesappeartooccupythelocationswhereaustenitefilms wereobservedatshorterpartitioningtimes.

Fig.4. The figures on the left and right show microstructures of the specimens observed under the SEM after partitioning at temperatures of 500 °C, 550 °C and 600 °C for 180 s and 900 s, respectively. Evolution of microstructural features with parti- tioning time and temperature are shown in this figure.

3.3. Partitioningat500°C

Thedilatometrycurvecorrespondingtopartitioningat500°Cshows adecreasein changeinlength forthefirst600s(Fig.3e).This con-tractionisofthesameorderofmagnitudeastheoneobservedduring partitioningat450°C(Fig.3c)althoughinthepresentcaseitoccursin ashortertime.Thiscontractionisfollowedbyacontinuousdilatation untiltheendofthepartitioningstage.

Fig.3eshowsthat,withtheincreaseinholdingtime,thevolume fractionofretainedaustenitedecreasescontinuouslyuntilitisnot de-tectedbyX-raydiffractionafterpartitioningfor3600s.Thisdecrease intheretainedaustenitevolumefractioncoincideswiththeincreasein thevolumefractionoffreshmartensite,carbidesandpearlite.

Themicrostructureofthespecimenafterpartitioningat500°Cfor 3600s(Fig.3f)showsthepresenceofpearliteinthemicrostructure.This suggeststhattheincreaseofchangeinlengthobservedinthe dilatome-trycurveisrelatedtopearliteformation,whichbecomesthedominant processafter900sofisothermalholding,ascanbeseenfromtheSEM micrographs,Fig.4aand4b.Precipitationofcarbidesisobservedatthe phaseboundariesoffreshmartensite/retainedausteniteislandswiththe surroundingprimarymartensite.

3.4. Partitioningat550°C

Thedilatometrycurveduringpartitioningat550°C(Fig.3g)shows averysimilarbehaviourastheoneat500°C.However,inthepresent case,thetransitionfromcontractiontoexpansionoccursatashorter holdingtime(200s)andthemagnitude ofthecontractionissmaller.

Moreover, the final expansion observed in the dilatometry curve is higherthaninthecaseofpartitioningat500°C.

Theevolutionofphasefractionsinthefinalmicrostructurepresented in Fig.3g showsthatno retainedausteniteisdetectedbyXRDafter 900sofpartitioning,whilethevolumefractionofcarbidesandpearlite significantlyincreaseswithincreasingtheholdingtimewhichisalso evidentfromtheSEMmicrographs,Fig.4cand4d.

The SEM micrograph of the specimen partitioned at 550°C for 3600s (Fig. 3h) shows a dense distribution of pearlite in the final microstructure,whereasthefreshmartensite/retainedausteniteislands (MAislands)arelessevidentinthepresentcasethanafterpartitioning at500°C.

3.5. Partitioningat600°C

Thedilatometrycurveregisteredduringpartitioningat600°Cfor 3600s,Fig.3i,isverysimilartotheoneobservedduringpartitioning at550°C.However,thespecimenpartitionedat600°Cexperiencesa smallerexpansionthaninthecaseofpartitioningat550°C,indicating theformationofalowervolumefractionofpearlite.

Thefinalvolumefractionsalsoshowsimilartrendswiththe parti-tioningtimeastheonesobservedduringpartitioningat550°C.Thatis, alowvolumefractionofretainedaustenitethatbecomesundetectable byXRDafterpartitioningfor900sandasimultaneousincreaseofthe volumefractionofcarbidesandpearlitewithpartitioningtime.

Pearliteis alsoobserved in theSEMmicrographof thespecimen partitionedat600°Cfor3600s(Fig.3j).SEMmicrographs,Fig.4b -4f,indicate thatpearlite formationduringpartitioningstage ismore

significant at 550°C than at 600°C. Moreover, carbides in primary martensiteobservedat600°Cseemtobecoarserthanafterpartitioning at550°C,Fig.3j.Besidespearlite,lamellarcarbidesareobservedinthe freshmartensite/retainedausteniteislandsafterpartitioningat600°C.

4. Discussion

Theprevioussectionhaspresentedaqualitativedescriptionofthe microstructuralevolutiontakingplaceduringthepartitioningstepatthe differentstudiedtemperatures.Inthissection,aquantitativeassessment isperformedinordertoevaluatetheextenttowhichmicrostructural processeshinderorinhibitthepartitioningofcarbonandsubstitutional alloyingelementsfromthemartensiteintotheausteniteand,therefore, promote anadequatestabilizationoftheausteniteat room tempera-ture.Forthispurpose,firstly,theredistributionofcarbonamongphases andduetophasetransformationsduringthepartitioningstageis anal-ysedbasedonthecarbonbalanceatdifferentpartitioningtemperatures. Then,thesequenceofmicrostructuralmechanismsoccurringat differ-entpartitioningtemperaturesisvalidatedthroughtheoretical calcula-tionsofthelength change.Finally,itisdiscussedhowsimultaneous microstructuralphenomenaduringpartitioninginfluencethe stabilisa-tionoftheausteniteatdifferenttemperaturesandthemostpromising routesareidentify.

4.1. Carbonbalanceatdifferentpartitioningtemperatures

Itiscrucialtounderstandhowcarbonredistributesinthe microstruc-tureduringthepartitioningstagetounderstandthestabilisationprocess oftheaustenite.Therefore,in thissection,thecarbon distributionis quantifiedbytheanalysisofthecarboncontentofallmicrostructural constituentspresentinthemicrostructuresafterQ&Pheattreatmentsin whichthepartitioningtimelastedfor3600s.Thisevaluationprovides informationontheeffectivenessofthecarbonpartitioningfrom marten-sitetoausteniteinordertostabilizeausteniteatroomtemperature.

Thecarboncontentinthephasesattheverybeginningof partition-ingstage(tp=0s)canberepresentedby:

̄𝑥= 𝑓_𝛾(_𝑡

𝑝=0)⋅ 𝑥𝛾(𝑡𝑝=0)+ 𝑓𝑀1(𝑡𝑝=0)⋅ 𝑥𝑀1(𝑡𝑝=0) (3)

where ̄𝑥isthetotalcarboncontentpresentinthealloy(0.31wt.%), f_𝛾andfM1arethevolumefractionsofausteniteandmartensitepresent

atthebeginningofthepartitioningstage(0.40and0.60respectively), andx_𝛾andxM1arethecarbonconcentrationspresentinausteniteand

martensiteatthebeginningofthepartitioningstage.Sincethe marten-sitictransformation is diffusionlessand considering thatthere is no changeincarbonconcentrationduringthereheatingstage,martensite andausteniteareassumedtohavethesamecarboncontent(0.31wt. %)attheonsetofthepartitioningstage.Theseinitialconditionsatthe beginningofthepartitioningstageareconsideredequalforallstudied Q&Pheattreatments.

Section 3 has shown that, during the partitioning step, several competitive phenomena occur at different stages of the isothermal holdingdependingonthepartitioningtemperature.Thesephenomena arecarbideprecipitationinmartensite,pearliteformation,carbide pre-cipitationinausteniteandcarbonenrichmentoftheaustenite.Allthese phenomenacompete for thecarbon available in the microstructure. Therefore,after3600sof partitioning, thefollowing carbonbalance canbeapplied:

̄𝑥= 𝑓_𝛾(_𝑡

𝑝=1ℎ)⋅ 𝑥𝛾_𝑐(_𝑡𝑝=1ℎ)+𝑓𝑀1(𝑡𝑝=1ℎ)⋅ 𝑥𝑀1𝑐(𝑡𝑝=1ℎ)+𝑓𝑝(𝑡𝑝=1ℎ)⋅ 𝑥𝑝_𝑐(_𝑡𝑝=1ℎ)+𝑋𝑐

(4) wherē𝑥isthetotalfractionofcarbonpresentinthealloy(0.31wt.%), f_iand𝑥𝑖

𝑐representthevolumefractionandcarboncontentofphasei

(i=𝛾,M1andP)after3600sofisothermalholdingandbeforethefinal quenchtoroomtemperatureandXcisthetotalfractionofcarbonthat

isprecipitatedincarbides.

Thevolumefractionofausteniteattheendofthepartitioningstep andbeforethefinalquench,f_𝛾,canbecalculatedasthesumofvolume fractionsofretainedausteniteandfreshmartensiteobservedinthe fi-nalmicrostructures.Thecorrespondingcarboncontent(𝑥γ

c)iscalculated

consideringthecarboncontentinretainedaustenite(𝑥RA

c )measuredby

X-raydiffractometerandthecarboncontentinfreshmartensite(𝑥M2 c ).

Thisbalancecanbeformulatedas: 𝑓𝛾(𝑡𝑝=1ℎ)⋅ 𝑥𝛾_𝑐(_𝑡

𝑝=1ℎ)=𝑓𝑅𝐴⋅ 𝑥 𝑅𝐴

𝐶 +𝑓𝑀2⋅ 𝑥𝑀2𝐶 (5)

wherefRAandfM2arethevolumefractionsofretainedausteniteand

freshmartensite,respectively.

Thecarboncontentinfreshmartensite(𝑥M2

c )isdeterminedbasedon

themartensitestarttemperatureduringthefinalquench(experimental curve)andapplyingtheRowlandandLyleequation[27]thatrelates themartensitestarttemperature(in°C)withthechemicalcomposition ofthealloy.Inthepresentstudytheequationhasbeenadaptedtothe chemicalcompositionofthesteelas

𝑀𝑠=499−324⋅ 𝑥M2c −32.4⋅𝑥Mn−27⋅𝑥Cr−10.8⋅𝑥Si−10.8⋅ 𝑥Mo (6)

wherex_irepresentstheconcentrationofelementi(i=C,Mn,Cr,Siand Mo)inthealloyinwt.%.

Thecarboncontentinsolidsolutioninprimarymartensite,𝑥M1 c ,is

assumedtobezeroafter3600sofpartitioningtimeatallstudied parti-tioningtemperaturesduetotheformationofcarbidesinthematrixand thecarbonpartitioningtoaustenite.Thecarboncontentinpearliteis assumedtobetheeutectoidcarboncontent.

Undertheseassumptions,thecombinationofEqs.(4)–(6)provides informationregardingthecarbonpresentineverymicrostructural con-stituentandincarbidesafter3600sofpartitioningatallstudied tem-peratures.Theresultsanddetailsofthenumericalvaluesusedinthe calculationsarepresentedinTable2andareexplainedbypartitioning temperaturehereafter.

4.1.1.Partitioningat400°C

Duringtheisothermalholdingat400°Cfor3600s,carbon partition-ingfrommartensitetoausteniteandcarbideprecipitationinprimary martensite occurin themicrostructure.Pearliteformation is not ob-served.Withthisinformation,theapplication ofEq.(4)revealsthat thefractionofcarbonthatprecipitatesintheformofcarbides(XC)in

primarymartensiteisaround0.01wt%C.

4.1.2.Partitioningat450°C

Duringpartitioningat450°C,carbideprecipitationinaustenitefilms takesplacealongwithcarbonpartitioningfrommartensitetoaustenite andcarbideprecipitationinprimarymartensite.Inthiscase,thebalance ofcarbonshowsthatthefractionofcarbonthatprecipitatesincarbides, X_C, isaround0.08wt%C,whichishigherthanthatobservedin the case of partitioningat 400°C andcoincides withthemicrostructural observations.

4.1.3.Partitioningat500°C,550°Cand600°C

Partitioningat500°C,550°Cand600°Cpromotespearliteformation alongwithcarbonpartitioningfrommartensitetoaustenite,carbide pre-cipitationinprimarymartensiteandcarbideprecipitationinaustenite. Thecarbonbalanceshowsthatthefractionofcarbonthatprecipitates intheformofcarbides,XC,isaround0.11wt.%inthecaseof

partition-ingat500°Cand550°C,andof0.12wt%inthecaseofpartitioningat 600°C.

Themaximumvolumefractionofpearliteisobservedafter partition-ingat550°C(Fig.3).Thiscoincideswiththenoseofthepearlite forma-tioninthetheoreticallycalculatedTemperature-Time-Transformation diagram,usingthefreeprogramMUCG83[28].

Table2

Volume fraction and carbon content of phases present at the end of the partitioning step for different partitioning temperatures. These phases are carbon enriched austenite ( 𝛾), primary martensite (M1), pearlite (p) and carbides ( Xc). The Table also shows the volume fractions and carbon contents of fresh martensite (M2) and retained austenite (RA) used for the estimation of the volume fraction ( f𝛾) and carbon content 𝑥𝜸𝐜 of carbon enriched austenite ( 𝛾)

present after 3600 s of partitioning time.

( T P∘C) C enriched austenite C depleted martensite Pearlite Carbides

f𝛾 𝑥 γc (wt. %) fM1 𝑥 M1c (wt. %) f p 𝑥 p c (wt. %) X c (wt. % C) M2 RA 𝑥 𝑀2 𝑐 f M2 f RA 𝑥 RAc 400 0.39 0.68 0.16 0.23 0.80 0.6 0 0 0 0.01 450 0.39 0.58 0.32 0.07 0.60 0 0 0.08 500 0.31 0.45 0.31 – – 0.08 0.73 0.11 550 0.22 0.32 0.22 – – 0.17 0.73 0.11 600 0.27 0.35 0.27 – – 0.12 0.73 0.12

4.2. Lengthchangesassociatedtothereactionsduringthepartitioningstage Thefocusofthissectionisontheevaluationandvalidationofthe influenceofeachmicrostructuralmechanismontheoverallchangein lengthobserved attheendofthepartitioningprocess.This provides insight intothe sequence of the microstructuralprocesses occurring at different partitioningtemperatures. Accordingto themechanisms proposedintheprevioussectionbasedonthedilatometryresultsand microstructuralobservationsandusingthephasevolumefractionsand carboncontentscalculatedinTable2,thetheoreticalchangeinlength associatedtoeachmicrostructuralprocessduringthepartitioningstage arecalculatedandcomparedwiththeexperimentalvalues.

Therelationbetweentherelativechangeinlengthrecordedduringa dilatometryexperimentandtheactualchangeinvolumethatdevelops inthematerialcanbeexpressedas:

Δ𝑳 𝑳𝒊 = 1 3⋅ 𝑽 𝒇_−𝑽𝒊 𝑽𝒊 (7)

whereΔ𝐿=𝐿_𝑓−𝐿_𝑖isthedifferencebetweenthefinal(Lf)andinitial

(Li)lengthofthematerialafterandbefore(thepartitioningstage.Vfand

Vi _{arethetotalspecificvolumesofthematerialafterandbeforethe}

partitioningstage,respectively.Inthepresentanalysis,theinitialstate, i,representsthestartingpointofthepartitioningstep(t_p=0s)andthe finalstage,f,representstheendofthepartitioningstageafter3600s (tp=3600s).

Thetotal specificvolume,V, of thematerial at any stageof the isothermalholdingcanbeexpressedas:

𝑉 =∑

𝑗 𝑣𝑗⋅ 𝑓𝑗

(8)

wherev_j andf_j arethespecificvolumeandvolumefractionofevery microstructuralconstituent,j.Inthiscontext,thephasespresentinthe microstructureatthebeginningofpartitioningstage(tp=0s)are

pri-marymartensiteanduntransformedaustenite,whereasthe microstruc-turalconstituentsthatarepresentattheend ofthepartitioningstep (tp=3600s)dependonthepartitioningtemperature(seeTable2).

Eq.(7)canberewrittenincludingthespecificvolumes,asexpressed inEq.(8),ofallpossibleindividualphasesatthebeginningandatthe endofthepartitioningstepas:

Δ𝐿 𝐿𝑜 = 1 3⋅ {(𝜗f γ⋅ 𝑓γf+ 𝜗 f M1⋅ 𝑓 f M1 +𝜗 f p⋅ 𝑓pf+𝜗 f carbides⋅ 𝑓 f carbides)− (𝜗 i γ⋅ 𝑓γi+ 𝜗 i M1⋅ 𝑓 i M1)} (𝜗i γ⋅ 𝑓γi+ 𝜗iM1⋅ 𝑓 i M1) (9) where 𝜗i γ, 𝜗iM1 and 𝑓 i

γ, 𝑓M1i are the specific volumes and volume

fractions of austenite and martensite before partitioning stage. 𝜗f γ, 𝜗 f M1, 𝜗 f p, 𝜗 f carbidesand 𝑓 f γ, 𝑓 f M, 𝑓 f p, 𝑓 f

carbides stands for the specific

volume and volume fraction of carbon enriched austenite, primary martensite, pearlite and carbides at the end of partitioning stage, respectively.Intheabove equation,volumefractionofausteniteand primarymartensitearequantifiedfromexperimentaltechniques.While,

volume fraction of pearlite and carbidesare estimated from carbon balancing(Eq.(4))asexplainedinSection4.1.

Thespecificvolumesofthecrystalstructuresarecalculatedfrom thecorrespondinglatticeparametersandthermalexpansioncoefficients accordingtotheformulaepresentedinTable3.Thelatticeparameters ofaparticularcrystalstructureatagivenpartitioningtemperature,T, canbecalculatedusingthefollowingequation:

𝑎𝑙𝑎𝑡𝑡𝑖𝑐𝑒,𝑇 =𝑎_{𝑙𝑎𝑡𝑡𝑖𝑐𝑒,𝑅𝑇}⋅ (1+𝛽 ⋅ (𝑇−300𝐾)) (10)

where𝛽 is thethermalexpansion coefficient,a_lattice,T anda_lattice,RT are the lattice parameters at the partitioning temperature and room temperature (300K), respectively. Lattice parameters at room temperaturea_lattice,RT foraustenite(𝛾),martensite(𝛼│_{)andcementite}

(𝜃)arecalculatedasafunctionofchemicalcomposition(inat.%)and areshowninTable3.

Dependingonthephenomenaobservedateachpartitioning temper-ature,Eq.(9)ismodifiedaccordinglytocalculatetheoreticalchanges inlength.

Forthetheoreticalcalculationsofchangeinlength,carbide precip-itationin primary martensiteisneglectedat allpartitioning temper-atures,as thevolumefraction of carbidesformedat 400°C islower than0.01andevenlowerathigherpartitioningtemperatures(450°C – 600°C).

Duringpartitioningathightemperatures(450°C–600°C), precipita-tionofcarbidesinsideausteniteisobserved.ThroughEBSDphasemaps, Kannanetal.[25],observedthatthenatureofcarbideprecipitated in-sideausteniteduringisothermalholdingat500°Ciscementite(6.67wt. %).BymeansofThermoCalccalculationsandDictrasimulationsofthe carbon redistributionbetween martensiteandausteniteduring parti-tioningat500°C,ithasbeenrecentlyshownthatthecarboncontent ofaustenite-filmsinbetweenmartensitelathscanreachvaluesabove 1.50wt.% C in less than1s[35]. This carboncontent in austenite iswellabove thatatwhichthemolarGibb’sfreeenergyof austenite andferriteareequalat500°C(0.48wt.%C).Thismeansthat,within therangeoftemperaturesof450°C–600°C,theaustenitemightbe suf-ficiently supersaturated in carbon so thatcementite can form, caus-ingcarbonimpoverishmentinthesurroundedaustenite.Fig.3showed that,at450°C,thisprocesscausesacontinuouscontractionover1hof

partitioningtime,whereas,at500°C,asimilarcontractionin magni-tudeoccurspredominantlyduringthefirst600s.Therefore,underthis assumption,calculationsshowthatthevolumefractionofcementite pre-cipitated inausteniteatallpartitioningtemperaturesisaround0.01. Forthetheoreticalchangeinlengthcalculationsatpartitioning temper-aturesof500°C– 600°C,pearlite(𝛾 → 𝛼 +𝜃)isalsoincludedasitwas alsoobservedduringthepartitioningstage.

Table3

Equations used to calculate the specific volume and lattice parameter of martensite ( 𝛼|_{), austenite ( 𝛾) and cementite ( 𝜃). Carbon} concentrations are in at.% and temperatures are in Kelvin.

Specific Volume Lattice parameter ( ˚A) Ref. Linear thermal expansion

coefficient ( 𝛽, K −1 ) Ref. 𝜸 𝑣 γ= 1 ∕ 4 ⋅ 𝑎 3γ a𝛾= 3.556 + 0.0453· x c + 0.00095· x Mn [29] 1 . 244 ⋅ 10 −5 [30] 𝛼| _𝑣 α′= 1 ∕ 2 ⋅ 𝑐 _α′⋅ 𝑎 _α′2 𝑎 _α′= 2 . 86640 . 0028 ⋅ 𝑥 _𝑐 [29] 2 . 065 ⋅ 10 −5 [30] 𝑐 α′= 2 . 8664 + 0 . 0256 ⋅ 𝑥 _𝑐 𝜃 𝑣 θ= 1 ∕ 12 ⋅ 𝑎 θ⋅ 𝑏 θ⋅ 𝑐 θ 𝑎 θ= 5 . 0895 , = 6 . 7449 , = 4 . 5250 [31] 5 . 586 ⋅ 10 −6 [31] Table4

Experimental and calculated changes in length at the different partitioning temperatures ( Tp) after 3600 s of partitioning time ( tp) in relation with the dominant phenomena occurring during the partitioning stage.

T p , °C Major phenomena Experimental change in

length (% ± 0.01%)

Theoretical change in length (%) at t p = 3600 s 400 C-partitioning 0.029 0.032 450 C-partitioning; − 0.023 − 0.020 Carbide precipitation in 𝛾 500 C-partitioning; − 0.008 − 0.017 Carbide precipitation in 𝛾; Pearlite formation 550 C-partitioning; 0.015 0.010 Carbide precipitation in 𝛾; Pearlite formation 600 C-partitioning; − 0.001 − 0.002 Carbide precipitation in 𝛾; Pearlite formation

Table4summarizesthemajorphenomenaoccurringateach parti-tioningtemperatureaswellastheexperimentalandtheoreticalchanges inlengthattheendofthepartitioningstep(tp=3600s).The

theoreti-calchangesinlengtharecalculatedusingequations7-10dependingon thephasespresentattherespectivepartitioningtemperatureandusing thedatafromTable2.Thereexistsagoodagreementbetweenthe ex-perimentalandtheoreticalchangeinlengths,whichindicatesthatthe above-mentionedconsiderations,i.e.cementiteasthecarbidethat pre-cipitatesinsideprimary martensiteandfromaustenite,andcomplete carbon-depletioninprimarymartensite,arevalid.

4.3. Analysisofsimultaneousphenomenaduringhigh-temperature partitioningstages

ItiswellknownthatQ&Pheattreatmentsaimtoproducesteelswith goodcombinationsofductilityandstrength,whichisachievedmainly fromretainedausteniteandprimarymartensite,respectively[32–34]. Inorder tostabilizeasignificantfractionofretainedausteniteinthe finalmicrostructureitisimportanttoavoidotherreactionsduringthe partitioningstagethatmightcompetefortheavailablecarbon.

Themicrostructuralevolutionobservedduringpartitioningat400°C confirmsthatthemajorphenomenaoccurringisthecarbonpartitioning frommartensitetoaustenite,responsiblefortheretentionofavolume fractionofaustenitebetween 0.19and0.24at roomtemperature af-terpartitioningfor180sand3600s,respectively.Onthecontrary,at 450°C,anincreaseinpartitioningtimeleadstoareductioninthe vol-umefractionofretainedaustenite,whilethefractionoffresh marten-site,consequently,increases.Thismightbeattributedtothe precipita-tionofcarbidesinausteniteduringthepartitioningstage,whichreduces thetotalfractionofcarbonavailabletostabilizetheausteniteatroom temperature[35].Consideringthepartitioningtemperatureof500°C, themaximumvolumefractionofretainedausteniteisobservedafter partitioningfor180sandtheformation of pearliteis observedafter 900s.Whereas,athigherpartitioningtemperatures,550°Cand600°C, pearliteisobservedafter180sofisothermaltreatmentandthevolume fractionofretainedausteniteislowerthan0.05.Thefurtherincrease intheisothermalholdingtimedoesnotrisethevolumefractionof

re-tainedaustenite;however,thepearlitevolumefractionisobservedto increase.Itisevidentthatthemicrostructuresshowatendencytoform pearliteathighpartitioningtemperatures(500°C-600°C).The forma-tionofpearlitefromtheaustenitegrainsduringpartitioningconsumes partofthevolumefractionofausteniteandpartofthecarbonavailable foraustenitestabilisation.Thus,theretainedaustenitefractioninthe finalQ&Pmicrostructureisreduced.

Asdiscussedearlier,thedilatometryanalysisatthepartitioning tem-peraturesof500°Cto600°Cindicatethatcarbideprecipitationinside austeniteandpearliteformationoccursimultaneously.Thechangein slopesofdilatometrycurvesduringisothermalholdingindicatea tran-sitionfromadominantprocessofcarbideprecipitationinsideaustenite (causingcontraction)toadominantprocessofpearliteformation (caus-ingexpansion).Fig.5a,bandcshowthedilatometrycurve(redline), extrapolationofcontraction(blueline)andexpansion(greenline) be-haviourof theactual dilatometrycurve at500°C,550°Cand600°C respectively. Byrelatingthefinalvolumefractionofpearliteandthe extrapolationof thedilatometrycurve corresponding topearlite for-mation(Fig.5),theevolutionofthevolumefractionofpearlite with isothermalholdingtimeatpartitioningtemperaturesof500°C,550°C and600°C iscalculatedandshown inFig.6a.FromFigs. 5and6a, thetransitioninthepre-dominantbehaviourofcarbideprecipitation inaustenitetopearliteformationduringthepartitioningstageis iden-tified tobe when thevolume fraction of pearlite is in the rangeof 0.01– 0.03.

Fortheinvestigatedsteel,theprecipitationofcarbidesinside austen-iteseemstobeunavoidable,asitoccursattheveryearlystageofthe partitioningandasaresultoftherapidcarbonenrichmentinaustenite promptedathighpartitioningtemperatures.However,pearlite forma-tioncanbeminimizedto0.01volumefractionbyrestrictingthe isother-malholding toshorttimes.UsinginformationfromFig.6a,theTTT diagramshowninFig.6bisconstructed,wherethepartitioningtimes corresponding topearlitevolumefractionsof0.01and0.05are indi-cated.ItcanbeseenfromFig.6bthat,similartowhatispredictedfrom thetheoreticalcalculationsusingtheMUCG83program,thekineticsof pearliteformationisfasteratpartitioningtemperaturesaround550°C (closetothenoseofpearliteformation)thanaboveorbelow.

Fig.5. Representation of change in length (red line) during isothermal holding at (a) 500 °C, (b) 550 °C and (c) 600 °C for 3600 s. The blue and green lines represent a polynomial fit for the observed contraction and expansion in the dilatometry curve, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig.6. (a) Volume fraction of pearlite formed during the isothermal holding at partitioning temperatures of 500 °C, 550 °C and 600 °C. (b) TTT diagram showing partitioning times corresponding to 0.01 and 0.05 volume fraction of pearlite formation during partitioning at 500 °C, 550 °C and 600 °C.

Themorphologyandlocationoftheausteniteaftertheinitialquench seems toplayarolein thedegree of carbonenrichment duringthe partitioningstep and,thus,intheprecipitationofcarbidesor forma-tionofpearlitewithinaustenite.BasedontheSEMobservations,itis roughlyestimatedthatarounda0.20ofthetotalvolumefractionof thefilm-typeof austeniteisoccupied withcementite.Assuming that theaustenitesurrounding thecementiteparticlesisretainedatroom temperatureand,hence,ithasatleasttheminimumcarboncontent re-quiredforaustenitestabilisationatroomtemperature(0.68wt.%Cas measuredbyXRD),carbonbalancecalculationsyieldthatacarbon con-tentofabout1.88wt.%Cisrequiredsothata0.20volumefraction ofcementiteformswithinafilm ofaustenite.This carboncontentis ingoodagreementwiththeresultsofcarbon-redistributionsimulations performedbyDictraat500°CbyHidalgoetal.[35].

Inthecase ofpearlite, itisobserved tonucleatealong theprior austenitegrainboundaries,wherethediffusionofcarbonisenhanced comparedtothatinbulkand,thus,arapidcarbonenrichmentofthe austenitecanbeexpectedattheselocations.Yangetal.[18]reported thatlargeaustenitegrainsarehighlyfavourableregionstoformpearlite. Hidalgoetal.[35]showedbyDictrasimulationsthat,ata partition-ingtemperatureof500°C,austeniteblocksof0.3-0.5μminthickness canreachhomogeneouscarbonconcentrationsclosetotheeutectoid composition(0.80wt%C)after50sofisothermalholding.Thismakes thermodynamicallyandkineticallypossiblethetransformationofsuch

austeniteintopearlite.Athigherpartitioningtemperatures(550°Cand 600°C),carbondiffusesevenfasterandtendstohomogenizeacrossthe austenitegraininashortertime,whichwouldallowanearlierpearlite formation.Duetopearliteformation,thevolumefractionofaustenite availableforstabilizationthroughmanganesepartitioningisconsumed. Inthecaseofpartitioningat500°C–600°C,manganesemighthave partitionedfrommartensiteintoausteniteattheinterfaceswhereno pearliteformationorcarbideprecipitationinaustenitewereobserved. However,theausteniteisnotstabilizedatroomtemperature(Table2). Thisisbecauseofinsufficientcarbonenrichmentintheausteniteduring thepartitioningstageduetopearliteformationorcarbidesprecipitation inaustenite.Thisindicatesthat,inthepresentcase,thoughmanganese ispartitioned,itdidnotplayaroleintheaustenitestabilization.

Basedontheaforementionedanalysis,theaustenitestabilisation pro-cessmightbeenhancedduringhigh-temperaturepartitioningprovided theminimisation/suppressionofcompetitivereactions.Theformation ofpearliteispointedasthemainprocesscompetingforthecarbon avail-ableinthemicrostructureduringthepartitioningstageduetothelarge carboncontentsandvolumefractionsofaustenitethatitconsumes.To increasethefractionofretainedausteniteafterthefinalquench,itis recommendedtoselectrelativelylowquenchingtemperaturestocreate microstructureswithalowvolumefractionofuntransformedaustenite andsmallgrainsizewhichcan1)stabilizeahighvolumefractionof austeniteduringthefinalquenchduetosufficientcarbonenrichment

[36],2)avoidpearliteformationwhenisothermalholdingisrestricted toshorttimes.Carbideprecipitationinsideausteniteisoccurringinthe initialstageofpartitioningandseemstobeunavoidable.However,an alloywithlowcarbon content(lower thanthatinthecurrent work) mightaidindelayingorsuppressingcarbideprecipitationinside austen-itebyavoidingcarbonsupersaturationinaustenitefilms.

5. Conclusions

Thepresentstudyinvestigatestheevolutionof themicrostructure duringQuenching&Partitioningprocessing inamediummanganese steelatpartitioningtemperaturesbetween400°C–600°Cand partition-ingtimesupto3600s.Themainconclusionsextractedare:

• Partitioningat400°Cleadstoaustenitestabilizationthroughcarbon partitioning,whilepartitioningat450°Cleadstocarbide precipita-tioninsideaustenitegrains.Atevenhigherpartitioningtemperatures (500°C–600°C)carbonpartitioningalsostimulatespearlite forma-tion.

• Carbon balancing at partitioning temperatures of 400°C–600°C showsthatalmostnocarbonisavailableintheprimarymartensite bytheendof3600sofisothermalholding.

• High-partitioningtemperatures(above450°C)resultinamorerapid carbonpartitioningkineticsthantheusual400°Candprovide suf-ficientdrivingforcefortheoccurrenceofadditionalreactionsother thancarbon partitioning. This results in carbide precipitationin austenitefilmsandpearliteformationin austeniteblocks depend-ingonthemorphologyofthegrainandpartitioningconditions. • Competitivephenomena,likecarbideprecipitationinausteniteand

pearliteformation,influencenegativelythestabilisationof austen-ite,astheyconsumesignificantfractionsofausteniteandthecarbon availableforpartitioningduringthepartitioningstage.

• ForthecurrentmediumMnsteel,carbideprecipitationinaustenite andpearlite formationoccursimultaneouslyduringthe partition-ingstage.Itisdeducedthatcarbideprecipitationinausteniteand pearliteformationaredominantintheearlyandlaterstagesofthe partitioningstage,respectively.

The results of the current studyprovide better understandingof the microstructuralchanges that occur during Quenching and high-temperaturepartitioningprocessing(450°C–600°C)inmediumMnQ&P steels.Itisunderstoodthatthesuppressionofthecompetitivereactions athighpartitioningtemperatureswillhelpinoptimizingtheaustenite stabilizingeffectofcarbonandmanganese.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingfinancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinfluence theworkreportedinthispaper.

Acknowledgments

Theauthorsgratefullyacknowledgethefinancialsupportfromthe ResearchFundforCoalandSteel(RFCS)projectHighQP(proposal num-ber:709855).

Dataavailability

Therawandprocesseddatarequiredtoreproducethesefindingsare availabletodownloadfromdoi:10.1016/j.mtla.2019.100492.

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