Strengtheningandtougheningofa2800-MPagrademaragingsteel
YiHe*,KeYang,WenshenQu,FanyaKong,GuoyueSu
InstituteofMetalResearch,ChineseAcademyofSciences,Shenyang110016,People’sRepublicofChina
Received9October2001;accepted23January2002
Abstract
AnewhighNi,TiandlowMomaragingsteelbasedon13Ni(400)maragingsteelhasbeendeveloped.Theoptimumheattreatmentwasfoundtobecryogenictreatment(CT)at200Kfollowedbyagingtreatmentat773Kfor4hinas-forgedorsolutioncondition.Opticalandtransmissionelectronmicroscopy(TEM)revealedthattheprecipitates,suchasNi3(Mo,Ti),werewelldevelopedinlathmartensite.Itachievedstrengthof2700MPaandfracturetoughnessofabove30MPam1/2.Thebalanceamongalloyelementsandtheremovalofundissolvedcoarseparticlesensuredthegoodcombinationofstrengthandtoughnessbyanormalthermomechanicaltreatmentcondition.D2002ElsevierScienceB.V.Allrightsreserved.
Keywords:Maragingsteel;Strengthen;Fracturetoughness;Precipitationhardening
1.Introduction
Maragingsteelsrepresentoneclassofultra-high
strengthsteelsthatattaintheirstrengthbyprecip-
itationhardening.Duringthepastfourdecades,
manykindsofmaragingsteelshavebeendeveloped.
18%Nimaragingsteelscontainingcobaltaretypical
category,whichresultinpracticalapplicationsof
variousgradesteelswithstrengthupto2400MPa,
designatedas18Ni(350).Inordertomeetthe
demandsofspecialproject,2800MPagrademarag-
ingsteel,ahigherstrengthmaragingsteelthan18Ni
type,suchas13%Ni–15Co–10Mo–0.2Tialloy
(designatedas13Ni(400)),whichwasfirstinventedbyMihalisinandBieber[1],hasbeenasubjectof
manyinvestigations[2–10].In13Ni(400)maraging
steel,asamainalloyingelement,nickelisobviously
lowerincontent,whereasbothcobaltandmolybde-
numcontentsaremarkedlyincreasedinordertoget
ahigherstrength,andthetitaniumisabandonedfor
maintainingthetoughness.However,inmostcases,
thestrengthof13Ni(400)hasdifficultyreachingthe
designedstrength,i.e.,2800MPa,becauseofundis-
solvedFe2Molavesphaseinthematrix,andits
ductilityandfracturetoughness(KIC)arelowerthan
whatwasexpectedintheforgedstate[2–10].In
ordertofurtherstrengthenandtoughenthe2800
MPagrademaragingsteel,someothertypesof
maragingsteels,suchas15Ni–25Co–7Mo[8],
16Ni–15Co–(5f7)Mo–2.5Ti[9],17Ni–15Co–
6Mo–1Ti[11],etc.,havebeendeveloped,butthey
stillhadbadtoughnessandductilitywithdesigned
0167-577X/02/$-seefrontmatterD2002ElsevierScienceB.V.Allrightsreserved.PII:S0167-577X(02)00610-
9*Correspondingauthor.Fax:+86-24-23891320.E-mailaddress:yihe@imr.ac.cn(Y.He).www.elsevier.com/locate/matletNovember2002
MaterialsLetters56(2002)763–
769strength.Althoughafteraspecialcomplicatedther-
momechanicaltreatmenttheductilityofthesealloys
couldhavegottensomeimprovementanditsKICwerecloseto30MPam1/2[7–11],itisnecessaryto
enhanceandstabilizethemechanicalpropertiesfor
practicalapplication.
Inthepresentwork,anattempttoimproveboth
ductilityandtoughnesswithoutdeteriorationof
strengthwasmadefora2800-MPagrademaraging
steelthroughaproperadjustmentofNi,MoandTi
contents,whichcanresultinanenhancementof
toughnessforthemartensitematrixandamodifica-
tionoftheprecipitatephase.
2.Experimentalprocedure
An18Nimaragingsteel,whosechemicalcompo-
sitionisgiveninTable1,wasmeltedina25-kg
vacuuminductionmeltingfurnaceandthenremelted
inavacuumarcmeltingfurnacefittedwitha100-mm
diametermould.Thesteelwashomogenizedat1473
Kfor24handthenforgedintoplateswith15mmin
thicknessandrodswith30mmindiameter,followed
byair-cooling.
Mechanicalbehaviorsweremeasuredonlongitu-
dinaldirectionfortheexperimentalsteelunder
differentsolutionandagingtreatments.Hardness
measurementsweremadetoinvestigatetheage-
hardeningbehaviorofthesteel.Thetransformation
temperaturesoftheheatwereestablishedbydila-
tometry.TensiletestingwereperformedonanAG-
5000AMTSmachineatadisplacementrateof0.2
mm/min.Theplanestrainfracturetoughness(KIC)
wasalsoevaluatedusingthree-pointbendingmethod
inaccordwithASTMspecification.Allthetests
wereconductedatroomtemperature.Microstructure
observationwasmadeonopticalmicroscopy,scan-
ningelectronmicroscopy(SEM)andtransmission
electronmicroscopy(TEM).QuantitativeX-raydif-
fractionmethodwasemployedtochecktheamount
ofretainedaustenite.3.Results
3.1.Hardness
Variationofhardnessasafunctionofagingtimeat
differenttemperaturefortheexperimentalsteelis
showninFig.1.Itcanbefoundthattheaging
responseisfairlyrapidat773and803K,andalmost
90%oftotalincreaseofhardnessisattainedwithinthe
first15minofaging.MaximumhardnessofHRC61.9
isobtainedat773Kwithinabout180minandslight
overagingoccursafterabout20h.However,at
temperatureof803K,thehardnessbeginstodecline
atabout120min,whichisrelativelyearlierforthe
steel.Agingat743Kgivesacontinuousincreaseof
hardnesswithagingtimeuntilabout16hatthepeak-
agingvalue.Itisalsoclearlyshownthattheaging
hardnessofthesteelsubjectedtoacryogenictreat-
ment(CT)at200Kismuchhigherthanthatofas-
forgedsampleswithoutCT.
3.2.Dilatometrycurve
Thetransformationtemperaturesfortheexperi-
mentalalloywereestimatedusingdilatometry.Fig.
2showsthedilatometrycurvetogetherwiththe
varioustransformationtemperaturesforthisalloy.
TheAs(873K)andAf(1058K)ofthereverse
transformationwerealmostthesameasthevalues
ofothercorrespondinggrademaragingsteel[9],while
theMsdeclinedtoabout323K,andtheMfwasbelow
theroomtemperature.Accordingtothemeasurement
ofretainedausteniteamountbyquantitativeX-ray
diffraction,itwasestimatedthattransformationof
austinitetomartensitefinishedatabout200K.
3.3.Microstructures
Fig.3showsopticalmicrographsoftheexperi-
mentalsteelundertwodifferentconditions,withand
withoutCTafterforging.Itwasfoundthatthetrans-
formationfromaustenitetomartensiteisnotcom-
Table1Chemicalcompositionofexperimental18Nimaragingsteel(wt.%)
NiCoMoTiAlCSPSiMnONFe
17.8814.756.691.100.080.00370.0020.0010.020.0070.00250.0015BalY.Heetal./MaterialsLetters56(2002)763–769764
