[PDF] Microstructure evolution of hot-work tool steels during tempering and

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Microstructureev olutionofhot-worktoolsteelsduringtemperingand definitionofa kineticlaw basedonhardness measurements

Z.Zhang

a ,D.Delagnes b ,G.Bernhart b a InstituteofMaterials andTec hnology, DalianMaritimeUniver sity,Dalian116026,China b

ResearchCentreonTools, MaterialsandProcesses(CROMeP), EcoledesMines d'Albi-Carmaux,81013AlbiCTcedex 09,Fr ance

Abstract

Keywords:Martensiticsteel;Kinetic law oftempering;T emperingratio;Microstructure;Carbides;Hardness

1.Introduction

Hot-worktoolsteelsare widelyusedat varioustemper - ingstateswith different mechanicalproperties,according to theneedsof industrialapplicationswhere thesteelendures cyclicthermalandmechanicalloads. Furthermore,somein- vestigations[1-3]haveshownthatthetemperatureat the surfaceofthetoolmay exceed thetemperingtemperature. Inthatcase, thesteel maybesubjected toacontinuous evo- lutionofthe microstructureandassociated propertiesdur- ingservice.As aconsequence,it isimportantto understand theev olutionofthemicrostructureduringtemperingand duringservicein orderto controlthetool lifetime.There- fore,researchersha ve paidcloseattentiontotheevolution ofmicrostructureand propertieswith timeandtemperature overalongperiodoftime.F orexample, relationshipsbe- tweenthemicrostructure obtainedaftertempering andphys- icalpropertiesha vebeen widelystudied[4-6].Inthat field, Engelfinely investigated thesofteningrateofsteelwhen

Correspondingauthor. Tel.:86-411-4726897;

fax:86-411-4727184.

E-mailaddress: zzp@newmail.dlmu.edu.cn(Z.Zhang).

temperedfromdif ferentinitialstructures. Inotherrespects, relationsbetweentime, temperatureand compositionwith themicrostructureobtained aftertemperingha vealso been intensivelyinvestigated[7-15].

Withoutanydoubt,the famousworkofHollomon and

Jaffehasledtothe mostwellkno wnrelation.Hollomon

andJaffe [10]supposedthatthe samehardnesscould be reachedbydif ferenttemperinghistory ,i.e.bydifferent time-temperatureroutesassuming thatthehardness was afunctionof thetime andthetemperature: hardness te RT .Finally, HollomonandJaffehav eobtaineda relationbetweenthe hardnessanda temperingparameter

M:hardness[log].Thisrelation hasa

greatimportanceand hasbeenwidely usedtodetermine the differentpossibilitiesoftemperingconditions inindustry. But,asthis relationis notakinetic law(hardness isnot anexplicit functionoftimeandtemperature),this relation cannotdescribethe ev olutionofhardness duringatemper- ingandcannot beused topredictthe hardnessvariation ofsteelsin servicewith timeandtemperature. Moreover , someauthorsindicated thatthe Hollomon-Jafferelation isnotsuitable forallsteels [11-16].Inorder toextend theapplicationof theHollomonand Jaffew ork,somere- Fig.1.SEM micrographofthe as-quenchedmicrostructure. initiallaw .Forexample,withareferencestate temperedat 550

Cfor2 hand600

Cfor2 h,Jean[15]gavearelative

hardnessdecreasela wwithtime andtemperaturefor5% chromiumhot-work toolsteel. Inthisstudy ,ouraim istodevelopa kineticlaw describing thesofteningof thesteel duringtempering.At first,thesteel investigatedandexperimentalmethodsare presented.Then, afterthestudy ofmicrostructurale volutionsduring temper- ingatdif ferentscales(e x-austeniticgrains,martensiticlaths andcarbides),a kineticlaw oftemperingis proposed.This lawisbasedonmicrostructural investigations ofsecondary carbidesgrowth andtheassociatedhardnesse volution.In thelastpart, thiskinetic lawis discussedindetail usingdata comingfromthe literatureobtainedon differenttypes steels.

2.Materialsand experiments

2.1.55NiCrMoV7steel

Thegradein vestigated isthe55NiCrMoV7hot-work

toolsteelwith thefollowing chemicalcomposition(wt.%):

0.56C,1.7Ni,1.0Cr ,0.5Mo,0.1V ,0.2Siand0.7Mn.The

microstructureobtainedafter quenchingissho wninFig.1. Thequenchedmicrostructure containsmartensiticlaths andasmall quantityofprimary carbides.Inaddition, no retainedaustenitew asdetectedby X-raydiffraction(XRD).

Table2

Temperingconditionsforhardnesse volutionmeasurements andcarbidesanalysis

Temperingtemperature(

C)Part 1Part2

Temperingtime(h)Temperingtime(h)

3500.252665.000.0250.0830.5 175189

4600.25225.630.0250.0830.5 175100

5000.252*75.300.0250.0830.5 125100

5600.252*18.870.0250.0830.5 12575

6000.25*2*16.00*0.0250.083 0.5125 75

Table1

Heattreatmentconditions fortheanalysis ofex-austenitic grainsand temperedmartensiticlath sizes

SampleQuenchingTempering

R52Austenizingto 875

Cfor1 hoilquenching 510

C2h

R63Austenizingto 875

Cfor1 hoilquenching 605

C3.5h

2.2.Experiments

Heattreatmentconditions fortheanalysis ofex-austenitic grainsandmartensitic lathsarepresented inTable1.Heat treatmentconditionsfor hardnessmeasurementsand forin- vestigationsoftherelationshipbet weenha rdnessand sec- ondarycarbidesare shownin Table2.Thermocouplesfor temperaturecontrolwere weldedoneach sample.Vick ers hardnessmeasurementswere performedone verysample af- terthetempering. Consideringthe experimentalschedule,

15samplesof thepart1 wereusedto investigate theev o-

lutionofthe volumefraction ofcarbides,8 samplesindi- catedbybold typenumberswere selectedonthe basisof theHollomon-Jaffe relation20logconstantforthe steel,andfinally the6 samplesindicatedby numberswith anasteriskwere dev otedtothe determinationoftheevolu- tionofthe meancarbide sizeduringtempering. Themicrostructuree volutionof thesteelwasinvestigated byXRD,scanning electronmicroscopy (SEM)PHILIPS

XL30,transmissionelectron microscopy (TEM)JEOL

2010andPHILIPS CM12andquantitati veimage analy-

sis.Quantitativ eimageanalysiswascarriedoutwiththe softwareVISILOG .Inorder torev ealrespecti velath and priorausteniticgrain boundaries,sampleswere respectively etched3s withaNital2%solution(2% nitricacidethanol) and16h withthecorrosiveBeaujard-Bechet solution(4% picricacid1%Teepol aqueoussolution).Themorphology andthemean sizeofe x-austeniticgrainand martensitic lathwerein vestigatedby SEMobservationsandquantita- tiveimageanalysis.Thevolumefraction ofthecarbides wasestimatedbyenergy dispersiv eX-raydif fraction.The sizeandmorphology ofcarbides wereanalyzedby TEM observationsandquantitative imageanalysis.Pre viously, carbidesweree xtractedfromthe martensiticmatrixusing thewell-known replicatechnique[16].

Hardnessmeasurementswere performedusinga

Buehler

MICROMET

2001typemicrohardness tester.

Fig.2.SEM micrographofthe temperedmicrostructuresho wingsecondarycarbides. Fig.3.SEM micrographofthe priorausteniticgrains (a)andresult oftheimage analysis(b).

Thirtyindentationswere performedoneach samplewith

0.2kgload.Themean valuew asconsideredas thehardness

result.Thestandard deviationis approximately10V ickers (HV 02

3.Microstructur eevolutionduringtempering

Temperingisadiffusion typephasetransformation from aquenchedmartensite toatempered martensiticstructure containingferriteand carbidesas shownin Fig.2.Thepre- cipitationandgro wthofcarbides isstronglyrelatedtotem- peringtimeand temperature.Therefore,establishing arela- tionbetweentempering conditionsand microstructuresisof greatinterest.In ordertostudy evolution ofmicrostructural relevantparameters,sampleswereobservedat threediffer - entscales.

3.1.Ex-austeniticgr ains

Eachsample wasobserv edwithSEMand20photos with

amagnificationof 1000weretak en.Grainboundaries were drawnonatracingpaper .Then,the tracingpaperw astreated withtheVISILOG softwareasinitialphotos. Thesephotos weretreatedby medianfiltering,entrop ythresholding,la- belingandfinally theperimeter (P)andthe area( S)ofeach grainwerecalculated. Anexample ofex-austenitic grainob- servationandtheresultof suchimagetreatment aregiv en inFig.3. Asthegrains arenearly equiaxial,anequi valentdiame- ter4canbecalculated. Theresult ofthestatistics thesamedistrib utionofgrain equivalentdiameter.The mean 0 2 4 6 8 10 12 14

246 81012 1416 182022 242628

Grain equivalent diameter (µm)

Percent (%)

Fig.4.Distrib utionofprior austeniticgrainequivalentdiameters. Fig.5.SEM micrographofthe martensiticlaths(a) andresultof theimageanalysis (b).

Table3

Sizeandmorphology ofmartensiticlaths obtainedbyimage analysis

SampleNumberof

lathsanalysed

Meanlength

oflaths( m)

Meanwidth

oflaths( m)

Shapefactor

oflaths

R5225403.491.640.47

R6327533.201.550.48

equivalentdiameterforsamplesR52andR63 arerespec- tively10.08and10.63m,whichare nearlythesame values astheas-quenched sample.AnalyzingFig.4andthedata, wefoundout thatthetw opopulationscannot beseparated. Therefore,itcan beconcludedthat temperingdoesnot in- fluencethe priorausteniticgrain size.Thisconclusion isin agreementwithresults comingfromthe literaturemainly obtainedoncarbon steels[17].

3.2.Martensiticlaths

Eachsamplew asobserv edonSEMand10photos witha

withtheVISILOG softwarethroughmedianfiltering,en- tropythresholdingandlabeling.An exampleof martensitic lathsobservation andtheresultofsuchimage treatmentare giveninFig.5.Finally, parametersextractedfromtheim- ageanalysiswere thelength, thewidthand theshapef actor (width/length).Resultsof thestatistics (average values)are 0 10 20 30
40
50
60

123456

Lath width (µm)

Percent (%)

R52 R63 0 5 10 15 20 25
30

0.10.20.30.4 0.50.60.7 0.80.91.0

Lath shape

Percent (%)

R52 R63 (a)(b) Fig.6.Distrib utionofmartensitic lathwidthandshapefactor . Fig.7.Ev olutionofthe volumefractionofcarbideswith thetempering temperature. presentedinTable3.Distributions ofthewidthandshape factorofmartensiticlathsare shownin Fig.6.Thecon- clusionisthe sameas forprioraustenitic grainsize.The martensiticlathmorphology andsize arenotinfluenced by thetemperingtreatment.

3.3.Carbides

Thevolume fractionofcarbideswasestimated byXRD

withanener gydispersiv espectrometer(seeFig.7).More- Fig.8.TEM micrographofintra-laths carbidesextracted fromthematrix. toinv estigatetheevolutionofintra-lathscarbide sizewith temperingconditions.Using EDSanalysis, thechemical compositionofthe carbidesextracted fromthematrix (see carbides,M 3

Ctypewith anorthorhombicstructure.

Volumefractionsofcarbidesv aryfrom7.45 to8.67%and

themeanv alueisequal to8.2%,nearlycorrespondingtothe thatallthe carbonhas precipitated.Exceptfor 350

C,where

wecansee aslightinfluence ofthetempering timeinFig.7, timeandtemperature. Inconclusion,the volumefraction of carbidescanbe consideredasa constantforthese tempering conditions.Thisconclusion isinagreement withtheresults giveninliterature[18]. Sixsamples(see Table2)wereselected forthee valuation ofintra-lathscarbides size.Themean intra-lathscarbides size,thestandard deviationand the99%confidence inter- valonthemeansize arereportedin Table4.Confidence intervalswereestimatedusingthe t-distributionofStudent [19].Ata given temperature,themean sizeofintra-laths carbideincreasesrapidly duringtempering.When thetem- peringtimeincreases, thecoarsening rateofcarbide dimin- ishes.Inother respects,fora given temperingtime,the mean carbidesizeincreases withtemperingtemperature, signifi- Fig.9showsastrongcorrelationbetween thehardness and theav eragesizeofcarbides.

Table4

Evaluationofthemeansize ofintra-laths carbides

Tempering

temperature( C)

Tempering

time(h)

Hardness

(HV 02

Meansizeof intra-lath

carbides(nm)

Standard

deviation(nm)

99%confidenceinterv al

onthemean size(nm)

6000.2547936.3615.1534.10d38.62

600245350.0318.7946.92d53.14

6001640470.7827.7066.66d74.90

560248640.4217.0037.89d42.95

56018.8747244.1819.7941.63d46.73

500249331.1718.6028.51d33.83

Fig.9.Relation betweenhardnessand themeansize ofintra-lathscarbides.

4.Tempering kineticlaw

4.1.Definitionof atemperingr atio

Hardnessev olutionswithtemperingtimeatdifferenttem- peraturesaresho wninFig.10aand b.Asharp decreaseof hardnesstakes placeduringtheinitialstageof temperingat eachtemperature.Then, thisshortperiod isfollowed bya quasi-lineardecreaseof hardness,whichdepends ontem- peringtemperature(see Fig.11).

Temperingcanbeconsidered asaphase transformation

promotedbydif fusionfroman unstablestate(martensite) towardsaquasiequilibriumstate(ferrite globularcar- bides).Therefore,all kindsof hardnesscanbe usedtode- fineany temperingstatebetweenthesetw ostates.Ne verthe- less,hardnesscannot clearlyindicate thesofteningfrom a quenchedstateor thehardeningfrom anequilibriumstate. So,the followingdefinition ofatemperingratiocalled is introduced: 0 0 (2) whereH 0 isthehardness afterquenching,H thehardness intheannealed state,andH thehardnessof aninterme- diatestatebetween theas-quenched stateandthe annealed state.Accordingto thisdefinition,tempering ratiovalues fall between0(as-quenched state)and1 (annealedstate).F or the55NiCrMoV7steel, thevalues experimentallyobtained 200
300
400
500
600
700
800
900

0.00.51. 01.52.02.5 3.03.5 4.0

Tempering time (h)

Hardness (HV

0.2 200
300
400
500
600
700
800

0102 03040 5060708090 100

Tempering time (h)

Hardness (HV

0.2 Fig.10.Hardness evolutions duringtemperingfor differenttemperatures between100and 700 C. arerespectiv ely 0

776(HV

02 )and

210(HV

02

Evolutionsoftemperingratioare shownin Fig.12.Tem-

peringratioe xponentiallyincreaseswith thetemperingtime forvarious temperingtemperatures.Thehigherthetemper - ature,thegreater isthe temperingratiofor thesametem- peringtime. 200
300
400
500
600
700
800

0100200300400500600700800

Hardness (HV

0.2 Fig.11.Influence oftemperingtemperature onhardnessv alues. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

050001000015000

Tempering time (s)

Tempering ratio

v 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.E+001.E+052.E+05 3.E+05

Tempering time (s)

Tempering ratio

v Fig.12.Ev olutionoftempering ratiowithtemperingtimeandtemperature.quotesdbs_dbs33.pdfusesText_39

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