Lecture 38

NPTELPhaseII:IITKharagpur:Prof.R.N.Ghosh,DeptofMetallurgicalandMaterialsEngineering||||

1

Module 38

Structural steel I

Lecture 38

Structural steel I


2

Keywords:Killed,semikilled&rimmingsteel,propertiesofstructuralsteel,steelspecification,strongstructuralsteel,mechanismofstrengthening,strainhardening,grainrefinement,solidsolution

strengthening,precipitationhardening,cold&hotworkingIntroductionBy now you have a fairly good idea on the basic concepts of physicalmetallurgy.Next fewmoduleswillbedevotedtotheapplicationsofphysicalmetallurgy.Themain focuswillbetoillustratewithexampleshowthebasicprinciplesthatyouhavelearntsofarhavebeenusedtoimprovetheperformanceofavarietyofengineeringmaterials.Letusbeginthismodulewithstructuralsteel,themostwidelyusedandabundantlyavailablematerialofconstruction.Bulkofsteelproducedallover theworldcomesunder thiscategory.Theseareusedas loadbearingcomponents.Theseareavailableinvariousformsandshapeslikeplates,rods,sheet,Isection,T section, channels, angles, rods, wires, wire ropes, bars, pipes, and tubes. It is the mainmaterial of construction of buildings, sheds, ware house, hangers, bridges, towers, powerplants, refineries,automobiles, commercial vehicles, ships, railway carriages, storage vessels,pipelines,andvarietiesofotherapplications.Themainfeaturesorcharacteristicsofstructuralsteelareasfollows:

Cheapestmaterialofconstruction(nexttocementandconcrete) Accountsfor9095%ofworldsteelproduction %C


3

(Blast Furnace), BOF (Basic Oxygen Furnace) and CC (Continuous Casting) route or the DRI(Direct reduced Iron), EAF (Electric Arc Furnace), and CC route. Amajor trend today is theincreaseduseofrecycledsteelscrap.Thisisoftenasourceofseveralharmfulelements(Sn,Cu,etc)whichmustberemovedduringsteelmaking.Introductionoflargescalevacuumdegassing,streamdegassing, ladle treatmentandvacuumarcremeltinghasalsoplayedamajorrole inthe improvement in the quality of steel being produced today. Oxygen is used to removecarbon frommolten iron.Apartof itremains insolutionwhentheamountofcarbon in irongoesdown.Unless it is removed itwouldbe impossible toproducedefect freecastingor tomakealloyaddition.Letusfirstlookatthebasicprincipleinvolvedinthecontroloftheamountofoxygenpresentinsteelwithrespecttoingotrouteofsteelmakingwhichisstillpracticedforcertaingradesofsteel.Killed,semikilled&rimmingsteel:Oxygen is used to remove the impurities (unwanted solutes) present in steel. The list ofimpurities includes C, S, P, etc.Once these are removed to the desired level, a part of theoxygenremainswithinthemolteniron.Atagiventemperaturethereisaninverserelationshipbetweentheconcentrationofdissolvedoxygen [O]andcarbon [C] inthemolten iron.This isshownwiththehelpofasetof%Cversus%Oplotsattwodifferenttemperaturesinadiagraminslide1.Thesolubilityproductdefinedas([C][O])isafunctionoftemperature.Itdecreasesasthetemperatureofthemoltenirondropsasitiscoolsinaningotmold.Thedissolvedoxygeninsteelmakesitbrittle.Itisremovedeffectivelybyaddingelementshavingverystrongaffinityforoxygen in the formof FerroAlloys. These are known asdeoxidizers. Elements like SiorAlreactswithdissolvedoxygentoformsolidparticlesofoxides.Someofthesemayfloatupandgetremovedwhilethesteelisstillmoltenbutasubstantialpartmayremainentrappedwithinthedendritic channelsduring solidification.These are retained as inclusionswithin the steelevenafterithassolidified.Fullydeoxidizedsteelisknownaskilledsteel.Itsolidifieswithinthemoldquietlywithoutmuchagitation.Solidificationofsteelisaccompaniedbyshrinkage.Everyattempt ismade to impose the conditionofdirectional solidification so that itbeginsat thebottom and the top is the last to solidify. It is often necessary to use hot top or to addexothermic mixture at the top of the solidifying steel. Slide 1 also shows the solidificationbehaviorofmoltensteelwithasetofingotsunderthreedifferentconditions.Killed steel: It is characterizedbyapronounced shrinkage cavity.This isoften referred toaspipe.Apartcontaining thepipehas tobecroppedandscrapped.Therefore theyield is low.This iswhy ingot route forkilledsteelhasnowbeen replacedbycontinuouscasting forbulksteelproduction.Killedsteelhaslittledissolvedoxygen.ItisductileithaslowDBTT.Howeveritisnotsoclean.Ithasinclusions.


4

Rimming steel:Deoxidation canalsobecarriedoutbyallowing the residualcarbon to reactwiththedissolvedoxygen inmoltensteelas itcoolswithinan ingotmold.TheexcesscarbonandoxygenreactstoformbubblesofCO.Thebubblesfloatupduetothedifferenceindensitycausingsevereagitationwithinthemoltenmetal.Thisiscalledrimmingaction.Theproductisknown as rimming steel. Themain characteristicof thisprocess is the absenceof shrinkagecavityasitisoffsetbythepresenceofalargenumberoftinyCOgasbubbles.Theporesintheingotgetweldedduringsubsequentrolling/hotworking.Thesteelisrelativelycleanasthereare no solid oxide inclusions. The yield too is high. The rim of the ingot is extremely pure.However rimming steel does have some amount of dissolved oxygen. Therefore DBTT ofrimmingsteelishigherthanthatofthekilledsteel.

Killed, semi killed & rimming steelBulk steel production: impurities in hot metal are

removed by oxidation. It picks up oxygen. The above classification is based on how it is removed.

%C

%O

T

C + O = CO

[C] [O] = k : f(T)As T decreases this rimming

reaction (CO bubbles) occurs.

killed SK rimmingIngots

Semikilledsteel:This iswheredeoxidation ispartlyduetoadditionof ferroalloysandpartlydue to the rimming action. As a result the size of the pipe that forms due to shrinkage isrelativelysmall.Theyieldisthereforealittlebetterthanthatofthekilledsteel.Continuouscasting(CC):BulkofsteeltodaycomesfromeitherBOF/CCorElectricArcFurnace/CCroute.Moltensteelfrom eitherBOFor EAF comes to the castingunit in ladle.Anumberofoperations likedeoxidationandalloyadditionsbeforecastingaredonewhile it is in the ladle.Figure1givesa

Slide1


5

sketch showing different parts of a continuous casting unit. Liquid steel from the ladle istransferred into a tundish where there is a protective layer of slag. It prevents pickup ofimpurities from the surrounding. Liquid steel from the tundish enters into a water cooledcopper mold through a submerged entry nozzle. A layer of molten and solid flux cover ismaintainedovertheliquidwithinthemold.Assolidificationbeginsfromthemoldwall,excesssolute present in steel gets rejected into the liquidwhere some thesemight react to forminclusions that float up and get captured by the flux. This is how a relativelyhomogeneouscontinuouslycastbilletsorslabsareproduced.Althoughthesetupisexpensivethecostofsteelproduced by this route is significantly less because of higher yield, high productivity, lowerrejectionandfewerdownstreamprocessingsteps.Aboveallthequalityofsteelisfarsuperior.Howeveritneedsmoreprecisecontroloftemperatureandcompositionofliquidsteel.Someoftheseareachievedthroughvacuumdegassingandladlerefinement.Duringthisstagesomeoftheharmfulelementsareremovedandsomeusefulelementsareadded.Thisisoftenreferredto as secondary steel making. It has been covered well by Ahindra Ghosh in his book onSecondarySteelmaking:Principles&Application,CRCPress(2000).

Propertiesofstructuralsteel:Microstructureofstructuralsteelconsistsofferriteandpearlite.Ferriteissoftwhereaspearliteis hard. The amount of pearlite increases as %C increases. The strength of structural steel

Tundish

Coppermold

Billet

Liquidsteel

Protectiveslaglayer

SubmergedentrynozzleMoldflux

Impuritiesmoveintotheliquid

Fig 1: Shows a schematic representation of different parts of a continuous casting (CC) unit.Solubilityof impurities in solid is less. Therefore atomsof such elementsmove into the liquidduringsolidification.Thishelpsminimizethelevelofsegregationinthecastbilletorslab.


6

depends on the amount of pearlite and its lamellar spacing. Finer the pearlite higher is thestrength.Usually structural steel isused innormalized condition (air cooled). Thedegreeoffinenessisthereforedependentonthesectionsizeofthecomponent.Slide2showstheeffect%Conthemechanicalpropertiesofsteel.Itfollowstheruleofmixture.Notethattheincreaseinstrength isaccompaniedby lossofductility.Theslidealso includesatablegivingthe lowerandtheupper limitsofstrengthandductility.NotethatPstandsforpearliteandCRdenotescoolingrateinthisslide.

Effect of % C on properties of + P steel

YS

UTS

BHN

0 0.8% C

%RA

%El

%C 0 0.8BHN 50 200UTS 250 750

YS 100 400%El 40 10

%RA 70 25

Strengthening follows rule of mixture. It increases with increasing % P. CR depends on section size.

Steelspecification:

The properties of steel depend primarily on its composition and processing. The bulk ofstructuralsteelarehotrolledandaircooled.BesidesCitalsohascertainamountsofimpurities(unwantedelementslikeS&P)andsomeamountofintentionallyaddedSi(asdeoxidizer)andMn(addedprimarilytoreducetheharmfuleffectsofSonthepropertiesofsteel).AsaresultthephasediagramandtheCCT(ortheTTT)diagramofsuchsteelarelikelytobedifferentfromthoseoftheplaincarbonsteel.Withincreasingconcentrationofalloyingelement%CinPearlitedecreases.ItmeansthattheeffectiveconcentrationofCinsteelincreasesastheconcentrationofalloyingelementsincreases.Oftentointerpretthestructureofsteelthecontributionofeachalloying element is expressed in terms of its carbon equivalent. The expressions for carbonequivalents are derived empirically. These are extremely useful in predicting the structure,hardenabilty and HAZ (Heat Affect Zone) of steel. One of the earliest and still popularexpressions is the one introduced by the International Institute ofWelding (IIW). This is asfollows:

(1)

Slide2


7

Itwas found souseful that itwas incorporatedeitheras it isorwitha littlemodification inseveral material standards. For example AWS (American Welding Society) introduced anadditionalfactor,Si/6toincludetheeffectofSiwhichisinvariablypresentinalmostallgradesofsteel.

Steel specificationISS C07: %C ~ 0.07; C15: %C ~ 0.15; 15Mn1:

0.15C 1Mn; 25Mn1S14 (?)

AISI-SAE

xx25: %C~0.25; xx=10 carbon steel, 11: resulfurized; 13: Mn steel; 40: Mo steel; 41: CrMo; 43:NiCrMo; 51:0.8Cr; 52:1.45Cr; 61: CrV, 92: SiMn: Example: 1025 0.5 C steel; 52100: ball bearing steel 1.45Cr1C

BS oldBS 970

En 24 ~ SAE4340: 1.8Ni0.8Cr0.25Mo0.4C xxx (A/M/H) xx: A: analysis; M: mechanical property; H: hardenability; last digits: C

UNS Unified numbering system: G: carbon steel; S: stainless; T: tool steel; UNS G 10200 (?)

Althoughitispossibletomakesteelofvariouscompositionsbyavarietyofprocessingroutesitis necessary to classify these into different groups and grades for the benefit of the steelproducerandtheuser.Theneedwasfelteversincethetimesteelbecameavailableinbulkfortheconstructionofvarietyproductsandstructures.Oneoftheearliestandwelldocumentedsystemsofdescribing the typesofsteelwas thatdevelopedbyAISI (American IronandSteelInstitute)andSAE(SocietyofAutomobileEngineers).Anumericalindexsystemconsistingof4digitswas introducedtorepresentaspecificgradeofwroughtsteel.Thenumbersareused insuchamannerthatitgivesatleastapartialdescriptionofthecompositionofsteel.AlphabetsarealsousedtodenotethepresenceelementslikeB(Boron)orL(Lead).

Table 1:NumericalIndexSystemusedbyAISI/SAEforthedesignationofwroughtsteelDesignation Typeofsteel Remarks10xx Plaincarbonsteel(S&P


8

43xx 1.8%Ni,0.500.65Cr,0.25%Mosteel 4340:%C=0.4051xxx 0.81.05%Cr 51110means%Cis1.161xx 0.80.95Cr,0.10.15V81xx 0.3Ni0.4Cr0.12Mo92xx 0.85Ni2.0Si93xx 3.25Ni1.25Cr0.12Mo

Similardesignationshaveevolvedever sinceby severalcountriesandorganizations.A setofalphanumericdigitsisusedtorepresentdifferentgradesofsteel.Thenumberingsystemoftenprovides some idea about the composition, processing or the yield strength of steel. Forexample ASTM A27U6030 represents casting grade of plain carbon steel. The number 60denotes itsUTS in ksi and30= YS. Thenumbering systemdoesnot give any idea about itscomposition.Youneed to lookat the full specification toget thedetails.However from themagnitudeofYSyougetanestimateofits%C.Itshouldbearound0.25ifthesamelevelofYSistobeobtainedwithoutanyexpensiveheattreatmentprocess.Byandlargethedesignationofsteelhasevolvedover theyearsby themanufacturers,buyers, societyorcountriesmorebyconvention rather than any scientific basis. Some of the more popular and widely useddesignationsystemsforsteelandotherengineeringmaterialsareasfollows:

ASTM(AmericanSocietyofTestingMaterials) BS(BritishSpecification:OLD/NEW) UN(ISO) UNS(UnifiedNumberingSystem:USA) DIN(Germany) AFNOR(France) UNI(Italy) JIS(Japan) GOST(Russia) IS(India)

Thedesignationsystemsthatarefollowedbyusers/buyersandmanufacturersarenumerous.Thereforeitisnecessarytohaveconversiontablesfromonesystemtoanother.Anexampleofthesameisgivenintable2.

Table 2:AnexampleofequivalentgradesofplaincarbonsteelunderdifferentdesignationsystemASTM BS UNS DIN AFNOR UNI JIS GOST

LC 1015 040A15 G10150 Ck15 XC15 C15 S15C 1020 040A20 G10200 C22 C20 C21 S20C 1050.2

MC 1030 080A30 G10300 C30 XC32 C30 S30C 1050.3 1050 080M50 G10500 C50 XC50 C50 S50C 1050.5

HC 1080 060A78 G10800 Ck75 XC75 C75 1050.8


9

Table3givesan ideaaboutthe IndianStandard(IS)specificationsusedforthedesignationofdifferenttypesofsteel.Thisisprimarilybasedonthecompositionofsteel.Howeverapartfromcompositionthepropertiesofsteeldependonitssectionsize/shapeandtheprocessingroute.Thedetailsaregiveninthespecification.

Table 3:AnexampleofthedesignationsystemadoptedbytheIndianStandard(IS)Designation Composition TypeC07 0.07C Plaincarbonsteel55Mn1 0.55C1Mn Mediumcarbonsteel05Mn75 0.05C0.75Mn Lowcarbonsteel25Mn1S14 0.25C1Mn0.14S Freecuttingsteel(goodmachinability)T118 1.18C ToolsteelT105Cr1Mn60 1.05C1Cr0.6Mn LowalloytoolsteelT75W18Cr4V1 0.75C18W4Cr1V HighspeedtoolsteelStrongstructuralsteel:

The strength of steel depends on its composition and the way it has been processed. Thesimplestwayto improve itsstrength isto increase its%C.This iscertainlythecheapestofallalloyingelementsusedtoimproveitsstrength.Thereareseveralbenefitsthatmayaccruefromthe use of high strength structural steel. It makes structures lighter resulting in significantsavings in the transportationand thematerialhandling costsduring itsentire life cycle. If itformsapartofautomobile,aircraft,orrailwaycarriagetheimpactismuchmore.Itsignificantlybringsdowntherunningcostbymakingthesemorefuelefficient.Eversincethefuelcrisisofthe1970stherehavebeenconcertedeffortsfromvarioussectorsto improvethestrengthofsteelwithout adversely affectingotherusefulpropertiesof steel. This iswhere the growingknowledgeofphysicalmetallurgyprovedtobeuseful.Thefourbasicmethodsofimprovingthestrengthofmetalsareasfollows:

Work(Strain)hardening Grainrefinement Solidsolutionstrengthening Precipitationhardening

Themetalsareductileeven though theirmeltingpointsandelasticmodulusarehigh.This isprimarily because of the presence of crystal defects known as dislocations. It is extremelydifficulttoproducedislocationfreemetalcrystalsbyconventionalprocessingroutes.Extremelythin filaments or whiskers of Fe are known to have been produced by vapor depositiontechnique.Thewidthsofsuchwhiskersaretoosmalltocontainadislocation.Thestrengthofsuchwhiskersisoftheorderof10GPa.Thisisquiteclosetothetheoreticalstrengthofdefectfreecrystal.Howeverthiscannotbeafeasibleroutefortheproductionofultrahighstrengthsteelintonnagescale.Neverthelessbysuitablecombinationofalloyingandthermomechanicalprocessingsteelshavingnearlythesameordersofstrengthareroutinelybeingmanufactured.Afewofthesearelistedbelow:


10

Colddrawn1%Csteel~3GPa Bainiticsteel0.8C~1.5GPa LowalloyMartensiticsteel:~2GPa

Work (Strain) hardening: Metalsweusearemostlypolycrystalline.Plasticdeformationalterstheshapeofthegrainsortheindividualcrystalsofwhichthemetalismade.Thedeformationofindividualgrainsoccursbydislocationglide.Thedistance throughwhich it canmove isgovernedby itsgrain size. Itcannotcrossagrainboundarytomoveovertoaneighboringgrain.Everynewdislocationthatis generated from thedislocation sourceswould remainwithin the grain. This results in anincreaseindislocationdensity.Theincreaseinshearstrengthisproportionaltothesquarerootofdislocationdensity.Slide4illustrateswiththehelpofasetofdiagramsthenatureofstressstrainplotsofsingleandpolycrystallinemetal.

Strain hardening

III III

I: Easy glide: interaction with forest dislocation

II: Linear hardening: Duplex slip :Cottrell -Lomer locks & jogs

III: Softening: cross slip

SC PC

0 Gb Dislocation density

increases with cold work

Thebasicprincipleswerealreadyexplainedinmodule9&13.ThedeformationbehaviorofSC(asinglecrystal)dependingon itsorientationmayconsistofthreedistinctstages.Thetypeofdislocation interactionsthattakesplaceare indicated intheaboveslide.Recallthatdenotesresolvedshearstressonaslipplanealongaspecificslipdirectionand isthecorrespondingresolved shear strain. During stage I slip occurs only on one slip system. Therefore themagnitudeofstrainhardeningisnegligible.SignificantstainhardeningoccursduringthestageII. This isbecauseofdislocationglideoccurringonmultiple slipplanesand the formationofCottrell Lomer locksanddislocationpileups.This imposessevere restrictionon theaverageglidedistanceofadislocationleadingtosignificantstrainhardening.ThestageIIIsetsinwhenthedislocationsgetstuckupatobstacles(GrainboundaryorCottrellLomerlock)moveover

Slide4


11

tothecrossslipplane.Asaresultthere issomeamountofstrainsoftening.Theshapeofthestressstrainplotisastrongfunctionoftheorientationofthecrystalwithrespecttoitstensileaxis.ForexamplethestageImaybetotallyabsent.

Thedeformationbehaviorofapolycrystalline(PC)metalwoulddependontheorientationandthenumberofindividualgrainsinaunitvolume.Atypicaltensilestress()versusstrain()plotisshown inslide4.Unlikemultiplestages inthecaseofSC ithasonlytwostages:elasticandplastic. This suggests that because of the constraints imposed by the neighboring grainsdeformationoccursinPConlywhenthereissliptakingplaceonmultipleslipsystemsineverygrain.NotethatthestrengthofPC increaseswith increasingstrainbeyond itsyieldpoint.Theyieldstrengthintensionisapproximatelytwicetheyieldstrengthinshear.Inshorttheincreaseinstrengthduetoworkhardeningmaybegivenby: (2)The dislocation density in annealedmetalmay be in the range 105 106 permm2. Itmayincreasetoaround1091010permm2asaresultofcoldwork.Itshortthedislocationdensitycan increasebyfourordersofmagnitudeduetocoldwork.Thissuggeststhatthe increase instrengthdue tostrainhardeningcanbe100 folds.Therefore inprinciple thestrengthof lowcarbon steel could be increased from 100MPa to around 10GPa. However this may be oftheoreticalinterest.Coldworkmakessteelbrittle.Suchamaterialmaynotbeofanyuse.

Grain refinement:

Grainboundariesactasdislocationbarriers.Thetotalplasticdeformation()isproportionaltodislocationdensity ().Recall that ,where b isBurgers vector and x is the averagedistanceadislocationcouldmovebyglide.Withincreasingstrainthenumberofdislocationsinthe pileups at the grain boundaries increases. When it exceeds a certain number, thedislocationatthetipofthepileupovercomesthebarriertoallowfurtherdeformationtotakeplace.Using thisconcept the following relationbetween theyieldstrengthandgrainsize (d)canbeobtained.ThisisknownastheHallPetchequation(seemodule13).

(3)

Thustheincreaseinyieldstrengthduetograinrefinementisgivenby:

/ (4)

The effectof grain refinement is illustratedwith the help of a set of diagrams in slide 5.Adecrease ingrainsize from1mmto1mcanresult inasix fold increase in itsyieldstrength.Thisistheonlystrengtheningmechanismthathasnoadverseeffectonductilityandtoughness.


12

Grain refinement

Because of back stress pile up can

have a limited no. of dislocations

Hall Petch relation

0kd

YS

1/d100

6001mm 1m

Solidsolutionstrengthening:Whenadislocationmovesinapuremetalithastoovercomeonlytheresistanceofferedbyaperiodicarrayof identicalatoms.This isknownas the friction stress (0). Ina solid solutionhoweverapartfromthefrictionstressithastoovercometheresistanceduetothestressfieldssurrounding the solute atoms present in the matrix. The solid solution strengthening isthereforeprimarilydue to the interactionbetween the stress fieldsof a solute atom in thelatticewiththatofanearbydislocation.Themagnitudeofthestressfield isafunctionofthedifferenceintheatomicdiameterofthematrixandthatofthesolute.Ifthesoluteatomsaresmallerthanthatofthesolvent,thenatureofthestressfieldarounditistensile.Thereforethepreferredsitesforsmalleratomsaretheupperhalfoftheslipplaneofanedgedislocation.Thisisbecausethestressfieldwithinthisregioniscompressive.Thesamelogiccanbeextendedtothe case of larger atoms. These are surrounded by compressive stress field. The preferredlocationsofsuchatomsarebeneaththeslipplane.Thuseverydislocationinasolidsolutionislikelytohaveanatmosphereofsoluteatoms.ThisisoftenreferredtoasCottrellatmosphere.Slide6givesaschematicrepresentationofadislocationlyingonaslipplanewherethereareafew solute atoms. The solute atoms can be visualized as pointswhere the dislocations arepinnedoranchored.LetFbetheforceneededtounpinthedislocationfromananchoranddbe the averagedistancebetween two anchorsor solute atoms.Higher the concentrationof

Slide5


13

soluteatomsshorter isthedistancebetweentheanchors. Ifxfdenotesthenumberofsoluteatoms/unitareaof theslipplane thend isgivenby1/.Therefore the forceneeded tounpinadislocationfromananchor(orasoluteatom)isequaltoF/d=.Ifdenotestheappliedresolvedshearstress,theforceperunit lengthofthedislocation isequaltob.Thus: .Theincreaseinstrengthduetotheadditionofanalloyingelementisgivenby (5)

Solid solution strengthening

Solute atoms having different size exerts a force / unit length

F/d = b also d 1/xf2 0.250.83 f f

F x G xb

Substitutional atoms

Interstitial atoms produces symmetric distortion in fcc

but asymmetric in bcc. Offers higher effect as it

blocks both edge & screw dislocations

1 27.5 f

Gx

Theproportionalityconstantk3inequation5dependsonthedifferencebetweenthediametersof the two atoms. The higher the difference higher is the strengthening. However a largedifference indiameterputsarestrictionontheamountofsolutethatcanbeaccommodatedwithin the lattice. The constant k3 also depends on the typeof the solute element and thecrystal latticeof theparentmetal. Interstitialatoms likeC&N in ironaremoreeffective inpinning dislocations. The effect ismore pronounced in ferrite because it has BCC structurewherethe interstitialsitesareasymmetric.Thestress fieldsurroundingsuchsites inacrystalhashydrostaticaswellasshearstressfield.Thereforeitcaneffectivelypinbothedgeandscrewdislocations.

Slide6


14

Precipitationstrengthening:

Particle looping vs cutting

Smaller precipitate: cutting more likely

Initial stage: small coherent particles form & f keeps increasing till its limiting

value. Strength increases due to both f & r

Once f reaches its limit particles coarsen & become large enough to allow looping

r

Looping f1/2/r

cutting f1/2 r1/2

CRSS

f

The strength of steel can also be significantly improved by precipitation hardening(strengthening).Precipitatesactasobstacles to themovementofdislocations.Thereare twowaysadislocation canmovebeyondaprecipitate. It couldmove through theprecipitatebycuttingitintotwohalves.Thismodeofmovementiscalledcutting.Alternativelyitmaymovebeyondtheprecipitatebyleavingdislocationloopssurroundingtheprecipitates.Thisisknownas looping. Themechanism of hardeningwas explained in detail inmodule 30. The factorsdeterminingthemagnitudeofstrengtheningarethevolumefraction(f)oftheprecipitateandits size (radius r). Slide 7 shows the effect of f and r on the shear strength of steel in thepresenceofprecipitates.Thefigureinthisslidesuggeststhatforagivenfthestressneededtocuttheprecipitate increaseswithr. Itreachesapeakwhen it intersectsthestressatwhichaloop can form around the precipitate. If r increases beyond this, the strength decreases. Inorder to get thehighest strength f shouldbe ashigh aspossible and r shouldbe just largeenoughtoresistcuttingandsmallenoughtoallowlooping.Thustheincreaseinstrengthduetoprecipitationstrengthening(ps)isgivenby: (6)Theresultantstrengthofsteelduetoallthefourmechanismsisthusgivenby: / /(5)You may wonder why martensitic transformation has not been included as a separatemechanismofstrengthening.Thisisbecausemartensiteowesitsstrengthduetoacombinationof several factors.Theseare solid solution strengthening, finegrain sizeandhighdislocationdensity.

Slide7


15

Strengthversusductility(ortoughness):

Strength vs. ductility

Brittleductile

Hardness

YS

TS%El

DBTT

% C, %, % PearliteGrain refinement is the only mechanism that

improves YS , ductility, toughness & lowers DBTT

Highstrengthisnottheonlycriterionfortheselectionofmaterialsforstructuralapplications.Often there is an inverse relationship between strength and ductility. You may be able toachieve significant strengthening in steel by increasing its carbon content, by coldwork, bygiving special heat treatment, by precipitation hardening or by grain refinement. Howeverstrengtheningisoftenassociatedwithlossofductility.Harddrawnsteelwiresarebrittle.Trytobend it. Itwouldcrack.Slide8showsthenatureof plotsofductileandbrittle formsofsteel.Elasticmodulusofsteelisinsensitivetoprocessing.Theinitialpartsofthetwoplotsareidentical. Ductile material is characterized by high ductility (for example annealed steel)whereasbrittlematerialmay failmuchbeforeplasticdeformation sets in (forexampleharddrawn steel wire). Structural steels normally are not used in hardened and temperedconditions.Themostcommonwayof increasing itsstrength iseitherbyaddingcarbonorbycoldwork.Slide8showstheeffectoftheseonthestrength(TS&YS),ductility(%El)andductiletobrittletransitiontemperature(DBTT).Asyouincrease%Cinsteeltheamountofpearlite inthemicrostructure increases.Pearlitebeinga finemixtureof soft ferriteandhardcementitehasmuchhigherstrengththanthatofferrite.Thisiswhywithincreasing%C,thestrength(TS&YS)ofstructuralsteelincreases.However%EldecreasesasYSorTScontinuestoincrease.Withincreasing strength theDBTT too increases.HighDBTTmeans thematerial is susceptible tobrittlefailure.Itiscertainlyundesirable.FormoststructuralapplicationsDBTTshouldbebelow0C. Asagainstthese,grainrefinementcan improvethestrengthofsteelwithoutany lossofductility or an increase in its DBTT. This is the current trend in improving the strength ofstructuralsteel.Thiscanbeachievedbycontrolledthermomechanicalprocessing.

Slide8


16

Coldworking:

Cold working & annealing of steelBH

N

Temp0 700

250

Recovery

Re-crystallization

Grain growth

BW CW Recryst FG

Cold working increases S/V. It means more nucleation sites

LC steel mostly

%C

910

Annealing temp

Pearlitic area remains elongated

Slide9illustrateswiththehelpofasetofsketcheshowextremelyfinegrainscanbeproducedin lowcarbonsteelbycoldworkandannealing.Lowcarbonsteel ismadeofmostlygrainsofferrite. During cold work the grains get elongated. The surface area to volume ratio (S/V)increases.Thedislocationdensitywithin thegrains ismuchhigher.Therefore thenet storedenergyperunitvolume(Gv)tooismuchhigher.Thestateofcoldworkisthermodynamicallyunstablealthough itcould remain in thatstate indefinitelyat room temperature. Inprinciplecoldworkedsteelcouldbeusedasitisunlessductilityortoughnessisanimportantcriterionofacceptance. The coldworked steel can be annealed at a temperature below A1 (the lowercritical temperature). Normal annealing time is 1 hour. Slide 9 shows how the hardness isexpectedtochangewithincreasingannealingtemperature.Theremaybeaninitialincreaseinhardness.This isdue to strainageing.Duringcoldworkingdislocationsmoveaway from thesolute atmosphere. With a little thermal activation during low temperature annealing thesoluteatomsmovetowardsdislocationsandoccupypositionsoflowestenergy.Thismakesthedislocations immobile. Therefore the strength increases. Simultaneously the dislocations toorearrangethemselvesintoalowenergyconfiguration.Onthewholeduringtherecoverystagetheremaynotbeanysignificantlossofhardness.Howeverforgrainrefinementthenextstageofannealingcalledrecrystallization ismore important.This isthestagewheretheelongatedgrainsarereplacedbyanewsetofstrainfreegrains.The largeS/Vratioandhighdislocationdensityprovidesmorenumbersoffavorablesitesforthenucleationofsuchgrains.Recalltheexpressionsforthecriticalnucleussize,activationbarrier,degreeofcoldwork(storedenergy)andnucleationrate:

Slide9


17

:Ascoldwork(Gv)increasesr*decreases(6) :Ascoldworkincreasestheactivationbarrierdecreases(7) :Ascoldworkincreasesnucleationrateincreases(8)

The key to get fine grain structure is to have high prior cold work, high S/V ratio, highnucleation rate,anda relatively lowerannealing temperature.Ahighannealing temperaturemay lead to excessive grain growth. Therefore it should be avoided. The annealing at atemperaturebelowA1doesnothaveanyeffectonthepearliticareas.Thesetoogetelongatedgetelongatedduringcoldwork.Thisshapeisretainedevenafterannealing.Thisisbecausethemorphologyofcementitecannotundergoanychangeatsucha lowtemperatureandashortannealing time. Themicrostructure of low carbon steel after coldwork and annealingmayconsistofelongatedpearliticregions inamatrixofferrite.Suchastructure is likelytoexhibitanisotropic behavior. The only way to avoid this is to anneal above A1 or above A3temperatures.Theferritegrainsizeaftersuchtreatmentmaynotbethatfine.Hotworking:Deformation processing of metals above its recrystallization temperature is known as hotworking.Thetemperatureofrecrystallizationdependson themicrostructure,theamountofcoldwok, thestrain rateand thedurationof thermalexposure.Byand large it isaround0.5timesitsmeltingpointinK.Thestrainrateduringhotworkingisquitehighandthetimegapbetweendifferentstagesofdeformationmaybeshort.Thereforehotworkingofmostmetalsisdone at 0.70.9Tm. The temperature at which the last stage of deformation is given is ofconsiderable importance in all thermo mechanical processing. It determines the finalmicrostructure and the properties of the product. During hot working recovery, recrystallizationandgraingrowthmayoccur.Thesearethermallyactivatedprocesses.Athighertemperatures the recrystallization time is short.Thematerial softensquickly.Therefore theloadonthehotrollingmill isnevertoohigh.However inordertogetafinegrainstructure itmay be necessary to increase the amount reduction per pass, lower the finish rollingtemperature,andadoptarelativelyfastercoolingsothatthegraingrowthcanbesuppressed.Theprioraustenitegrainsizealsoaffectsthepropertiesofsteelafterhotworking.Iftheinitialgrainsizeislessthefinalgrainsizeisgoingtobeevenfiner.Iftheinitialgrainsizeiscoarsethefinalgrainsizeisunlikelytobefine.Thisisillustratedwiththehelpofasetofdiagramsinslide10. Fine austenite grains providemore nucleation sites. Therefore the final structure is stillfiner.


18

Effect of austenite GS on hot worked structure

Coarse grain

Fine grain

CCT diagram

T

time Finer initial GS finer is the final structure

Coarse

Fine

Fine

Finer

ThegrainsizeofaustenitealsoaffectstheCCTdiagram.Thediffusioncontrolledprocessesliketheformationferriteandpearlitegetsuppressed.Thereforecoarsegrainaustenite is likelytohaverelativelymoreamountofpearlite.SummaryInthismodulewelearntaboutstructuralsteelandhowthesearedesignated.Thepropertiesofsuchsteelaredeterminedby itscompositionandprocessing.Steel isprimarilyan ironcarbonalloy.%Cinbulkofthestructuralsteelislessthan0.25.ApartfromcarbonithasSiandMn.TheformerisaddedasadeoxidizerwhereasthelatterisaddedtofixSwhichisinvariablypresentinsteel.The totalamountofSi&Mnmaynotexceed2%.P is theotherunwantedelementpresentinsteel.Itmakessteelbrittle.S&Pshouldbewithin0.05%.Wealsolookedatthewaysthe strength of steel can be improved. Steel is amenable to all the four mechanisms ofstrengthening.However, strengtheningdue to increased carbonorotheralloyaddition, coldworkorprecipitationhardening isaccompaniedby lossofductilityandtoughness.SuchsteelmayalsobecomepronetobrittlefailurebecauseallthethreestrengtheningprocessesincreaseDBTT.Grain refinement is the onlymechanism that improves strengthwithout any adverseeffectson itsductilityand toughness.Wealsosawhowbydeformationprocessinggrainsizecanbecontrolled.

Slide10


19

Exercise:

1. Microstructure of a cold rolled annealed mild steel sheet shows elongated pearliticregion inequiaxedferriticmatrix. Commentontheprocessingroutethatwillgivethistypeofmicrostructure.

2. Name three common grades of steel produced by ingot route. Which of these hashigheryield?

3. Why does compressive stress develops at the surface in a case carburized andsubsequentlycasehardenedsteel?

4. Whylowcarboncontentispreferredinmoststructuralapplication?Ishighstrengthnotamajorcriterion?

5. Whatisthecompositionofthesteeldesignatedas25Mn1S14?Whatistheroleofsulfurinthissteel?

Answer:1. Elongated pearlite is a sign of cold work whereas equiaxed ferrite means annealed

structure. It looks that the steelwas coldworkedwhenbothpearlite and ferrite gotelongated. Later on annealing just below eutectoid temperature ferrite has recrystallizedbut pearlite remains untransformed. Tomodify pearlite shape itmust beheatedabovetheeutectoidtemperature.

2. Steelproducedbyingotrouteisclassifiedaskilled,semikilledandrimmingsteel.Thisisbasedonthewaythedissolvedoxygen inmoltensteel isremoved.Rimmingsteelhasthehighestyield.Theoxygengetsremovedbythereactionbetweendissolvedcarbonand oxygen with evolution of CO bubbles as the steel cools inmold. This gives therimmingaction.Formationofsolidcrustatthetopoftheingotdoesnotallowthegastogo out. It remains entrapped as porosity and balance the shrinkage associatedwithsolidification.There isnoshrinkagepipe. Thereforetheentire ingotwithoutcroppingcangoforhotrolling.Theporesgetweldedduringhotworking.

3. This is illustratedwith thehelpof followingdiagram.Herewehavehighcarbon steelwithlowerMs&Mftemperatureatthesurfacewhereascoreisalowcarbonsteelwithhighertransformationtemperatures(Ms&Mf).Thereforetransformationoccursfirstincorewithexpectedexpansionwhen surface is still softausteniteand therefore itcanallow the core to expand (due to transformation) by plastic deformation. Whenmartensite starts forming at the surface accompaniedby volume expansion the corewhichisnowstrongwillinhibitthis.Thisiswhythereisaresidualcompressivestressatthesurface.


20

4. Often structural steel is supposed to have additional properties like weldability &

formability apart from high strength. Low carbon content gives better formability&weldability. Loss in strength due to lower carbon is made up by grain refinementthroughcontrolledthermomechanicalprocessingwithadditionofmicroalloyelementslikeNb,V,&Ti.Theypreventgraingrowthduringhotrolling.

5. Thisisafreecuttingsteelhaving0.25%carbon,1%Mnand0.14%S.InpresenceofMn,SispresentasMnSinclusion.Thisimprovesmachinability.Itismostlyusedformakingnuts&bolts.IfMnisnotpresentthenSformsalowmeltingeutecticbetweenFe&FeS.This segregates along austenite grain boundary leading to hot shortness. This isundesirable.ThisiswhyinmoststeelSislessthan0.05%.

Ms1

Ms2Mf2

Mf1surface

core

CS

T

tc ts

Note that transformationwould startat tcwithin core although it is at a highertemperaturewhereaswhentransformationstartsatsurfaceattstransformationatcoreiscomplete.

KeywordsIntroductionKilled, semi killed & rimming steel:Continuous casting (CC):Properties of structural steel:Steel specification:Table 1:Table 2:Table 3:Work (Strain) hardening:Grain refinement:Solid solution strengthening:Precipitation strengthening:Strength versus ductility (or toughness):Cold working:Hot working:SummaryExercise:Answer:

Lecture 38

Documents

Transcript of Lecture 38