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CASTINGMETALLURGY

3.2 MaximumFluidity(TheScienceofUnrestrictedFlow)..................................................95 FluidityDefinition..............................................................................................................96

3.2.1ModeofSolidification..............................................................................................97

3.2.2EffectofVelocity...................................................................................................110

3.2.3EffectofViscosity(IncludingEffectofEntrainedBifilms).................................110

3.2.4EffectofSolidificationTime tf ..............................................................................112

3.2.5EffectofSurfaceTension.......................................................................................121

3.2.6EffectofanUnstableSubstrate.............................................................................124

3.2.7ComparisonofFluidityTests.................................................................................125

3.2.8EffectofVibration..................................................................................................128

3.3 ExtendedFluidity.............................................................................................................128

3.4 ContinuousFluidity.........................................................................................................131

CHAPTER4MouldsandCores .....................................................................................

4.1 Moulds:InertorReactive................................................................................................135

4.2 TransformationZones......................................................................................................136

4.3 EvaporationandCondensationZones.............................................................................139

4.4 MouldAtmosphere..........................................................................................................144

4.4.1Composition............................................................................................................144

4.4.2MouldGasExplosions...........................................................................................145

4.5 MouldSurfaceReactions.................................................................................................147

4.5.1Pyrolysis.................................................................................................................147

4.5.2LustrousCarbonFilm............................................................................................148

4.5.3SandReactions.......................................................................................................149

4.5.4MouldContamination.............................................................................................149

4.5.5MouldPenetration..................................................................................................151

4.6 MetalSurfaceReactions..................................................................................................155

4.6.1Oxidation................................................................................................................155

4.6.2Carbon(PickupandLoss)......................................................................................155

4.6.3Nitrogen..................................................................................................................156

4.6.4Sulphur....................................................................................................................157

4.6.5Phosphorus..............................................................................................................157

4.6.6SurfaceAlloying.....................................................................................................158

4.6.7GrainRefinement...................................................................................................158

4.6.8Miscellaneous.........................................................................................................159

4.7 MouldCoatings................................................................................................................159

4.7.1AggregateMoulds..................................................................................................159

4.7.2PermanentMouldsandMetalChills.....................................................................160

4.7.3DryCoatings...........................................................................................................161

5.1

5.1.1ResistancestoHeatTransfer..................................................................................163

5.1.2IncreasedHeatTransfer.........................................................................................178

5.1.3Convection..............................................................................................................193

5.1.4Remelting................................................................................................................193

5.1.5FlowChannelStructure..........................................................................................194

5.2 DevelopmentofMatrixStructure....................................................................................197

5.2.1General....................................................................................................................197

5.2.2NucleationoftheSolid..........................................................................................198

5.2.3GrowthoftheSolid................................................................................................201

5.2.4DisintegrationoftheSolid(GrainMultiplication)................................................209

5.3.1PlanarFrontSegregation........................................................................................213

5.3.2Microsegregation....................................................................................................216

5.3.3DendriticSegregation.............................................................................................218

5.3.4GravitySegregation................................................................................................219

6.1

6.2.1FilmsonLiquidMgAlloys

6.2.2StrengtheningMgAlloys.......................................................................................230

6.2.3Microstructure........................................................................................................234

6.2.4Inclusions................................................................................................................235

6.3 Aluminium.......................................................................................................................236

6.3.1OxideFilmsonAlAlloys......................................................................................237

6.3.2EntrainedInclusions...............................................................................................239

6.3.3GrainRefinement(NucleationandGrowthoftheSolid).....................................239

6.3.4DendriteArmSpacing(DAS)andGrainSize......................................................244

6.3.5ModificationofEutecticSiinAl-SiAlloys..........................................................244

6.3.6Iron-RichIntermetallics.........................................................................................259

6.3.7OtherIntermetallics................................................................................................262

6.3.8ThermalAnalysisofAlAlloys..............................................................................264

6.3.9HydrogeninAlAlloys...........................................................................................267

6.4 CopperAlloys..................................................................................................................269

6.4.1SurfaceFilms..........................................................................................................270

6.4.2GasesinCopper-BasedAlloys..............................................................................270

6.4.3GrainRefinement...................................................................................................274

6.5 CastIron...........................................................................................................................275

6.5.1ReactionswithGases...........................................................................................275

6.5.2SurfaceFilmsonLiquidCastIrons.....................................................................277

6.5.3CastIronMicrostructures.....................................................................................288

6.5.4FlakeGraphiteIronandInoculation....................................................................290

6.5.5NucleationandGrowthoftheAusteniteMatrix.................................................299

6.5.6CoupledEutecticGrowthofGraphiteandAustenite..........................................300

6.5.7SpheroidalGraphiteIron(DuctileIron)..............................................................302

6.5.8CompactedGraphiteIron.....................................................................................308

6.5.9ChunkyGraphite..................................................................................................310

6.5.10WhiteIron(IronCarbide)....................................................................................312

6.5.11General..................................................................................................................313

6.5.12SummaryofStructureHypothesis.......................................................................314

6.6 Steels................................................................................................................................314

6.6.1CarbonSteels..........................................................................................................315

6.6.2StainlessSteels.......................................................................................................316

6.6.3InclusionsinCarbonandLow-AlloySteels:GeneralBackground......................317

6.6.4EntrainedInclusions...............................................................................................319

6.6.5PrimaryInclusions..................................................................................................322

6.6.6SecondaryInclusionsandSecondPhases..............................................................324

6.6.7NucleationandGrowthoftheSolid......................................................................326

6.6.8StructureDevelopmentintheSolid.......................................................................329

6.7 Nickel-BaseAlloys..........................................................................................................330

6.7.1AirMeltingandCasting.........................................................................................331

6.7.2VacuumMeltingandCasting.................................................................................332

6.8.1TiAlloys.................................................................................................................336

6.8.2MeltingandCastingTiAlloys...............................................................................336

6.8.3SurfaceFilmsonTiAlloys....................................................................................339

7.1 ShrinkagePorosity...........................................................................................................341

7.1.1GeneralShrinkageBehaviour................................................................................341

7.1.2SolidificationShrinkage.........................................................................................342

7.1.3FeedingCriteria......................................................................................................348

7.1.4Feeding:TheFiveMechanisms.............................................................................352

7.1.5InitiationofShrinkagePorosity.............................................................................365

7.1.6GrowthofShrinkagePores....................................................................................381

7.1.7ShrinkagePoreStructure........................................................................................382

9.4

9.4.3YieldStrength......................................................................................................477

9.4.4TensileStrength...................................................................................................490 9.5

9.6

9.6.1HighCycleFatigue.............................................................................................498

9.6.2LowCycle,HighStrainandThermalFatigue...................................................506 9.7

9.10.1InternalOxidation..............................................................................................512

9.10.2Corrosion...........................................................................................................514

9.10.3PittingCorrosion...............................................................................................515

9.10.4FiliformCorrosion.............................................................................................517 9.10.5Inter-granularCorrosion....................................................................................518

CASTINGMANUFACTURE

10.2.3TheNo-FallRequirement...............................................................................547

10.3 Rule3:AvoidLaminarEntrainmentoftheSurfaceFilm(TheNon-Stopping, Non-ReversingCondition)..........................................................................................550

10.3.1ContinuousExpansionoftheMeniscus..........................................................550 10.3.2ArrestofForwardMotionoftheMeniscus....................................................551

10.3.3WaterfallFlow:TheOxideFlowTube...........................................................552

10.9.4ControlledQuenchingUsingPolymerandOtherQuenchants....................620

SECTION2FILLINGSYSTEMDESIGN

15.3.1Greensand(Clay

16.1.3Two-StageFilling(PrimingTechniques)........................................................825

16.1.4VerticalStackMoulding..................................................................................826

16.1.5HorizontalStackMoulding(HProcess).........................................................826

16.1.6PostscripttoGravityFilling............................................................................827

16.2 HorizontalTransferCasting........................................................................................827

16.2.1LevelPour(SidePour)....................................................................................828

16.2.2ControlledTiltCasting....................................................................................829

16.2.3Roll-overasaCastingProcess........................................................................835

16.2.4Roll-overAfterCasting(SometimesCalledInversionCasting)....................836 16.3 Counter-Gravity...........................................................................................................838

16.3.1Low-PressureCasting......................................................................................842

16.3.2LiquidMetalPumps........................................................................................846

16.3.3DirectVerticalInjection..................................................................................853

16.3.4ProgrammableControl....................................................................................855

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Preface

Inthisfirstupdateofthe Handbook,themajorrevisionsareprobablythoserelatingtorunningsystemdesigninwhichthe vestigesoffillingdefectshavefinallybeeneliminatedfromcastings.

Thus,thepowerfulbenefitsofcontactpouring(inwhichtheuniversalconicaltrumpetdecoratingalltraditional fillingsystemsisnoweliminated)isfinallyshowntohavebeenhugelyunderestimatedbyanumberoffoundries.Contact pouringhasprobablybeenthemostimportant(andthemostsimpleandzero-cost)initiativetorevolutionisequalityin castings.Inaddition,theadoptionofvariousformsoftangentialfilterdesignstogateshasfinallyeliminatedtheproblem oftheentrainmentofprimingbubbles.Theseresidualbubbleshavelongimpairedthebenefitsofpreviousfillingsystems.

GravitypouringhasnowadvancedtothepointatwhichIfindmyselfhavingtoadmitthatitstartstothreatenmy cherishedandfavouredcastingproductionsystem:countergravity.

Thisisseentobeespeciallytrueforthoselow-pressuresystemswhichusearefractoryliningforthepressurised furnace.Ionlyrecentlydiscoveredthehugelydamagingemissionofbubblesfromtheseliningsduringdepressurisation ofthefurnace.Thisproblemhasclearlybeenamajorsourceofimpairedcastingsinthelow-pressurecastingindustryand hashamperedthisindustrysinceitsbeginnings.

Theuseofmypneumaticpumpisdescribedforthefirsttime.Itwouldlowercostsandsolvemostoftheproblemsof thisindustry.Thus,Icontinuetostandbycountergravityastheoptimumcastingsystemwhereitcanbeused.Myhopeis thatitwillbeteamedupwithagoodmeltingandmetalhandlingsystem.Onlycarefulfoundrydesignwillminimise bifilmpopulationsinmetals.Onlywhencastingscanbeproducedsubstantiallyfreefrombifilmswillweenjoythe fullbenefitsofcastings,andmetalsingeneral,resistanttohottearing,cracking,blisters,corrosionpittingandattack ofgrainboundaries,plusthebenefitsofextraordinarymechanicalproperties,potentiallyeliminatingfuturefailureby fractureorfatigue.

Theseareheadypredictions.However,earlyresultsinfoundriesarealreadyindicatingthatbeautifuldefect-freecastingswithrevolutionarymetallurgicalbenefitsappeartoberoutinelyattainable.Despitechallengesfromtheundoubtedly uniquebenefitsofsuchnewprocessesasadditivemanufacture,myhopeforthefutureforcastingsisbasedontheadoptionofsimpleprincipleswhichcouldnotonlysecurethefutureofourcastingindustry,butimprovethewelfareand environmentofallofuswhoselivesdependonit.

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Introduction

CASTINGSHANDBOOK,2NDEDITION,2015

When CastingsHandbook firstappearedin2011,Ihadnotexpectedtoreviseitsosoon.However,thelatestfindings requirepublicisingasquicklyaspossible thereisstillalongwayfortheindustrytogo!Themessageofthebook, summarisedinthesection“Bifilm-freeProperties”,isthataqualityimprovementofastonishingscaleispossible now.WhenIfirststartedtoexperimentwithnovelfillingsystemsforcastings,therewerenaturallymanydisappointments.However,thosedaysarelonggone.Theconceptsofentrainmentandbifilmcreationlaidoutinthebookare nowproven.Somefoundriesarealreadybeingdesignedtotakeadvantageofauniqueandeasilyaffordablequality revolutionandscrapreduction.Moreneedtofollow.Therisksareminimalandtherewardsaregreat.

Withregardtoimprovedcastingtechniqueswhichcanreduceoreveneliminatetheusualvastpopulationsofbifilms inourmetals,Ihavealwaysbeenawareofthepotentialbenefitofcontactpouring,buthadcompletelyunderestimatedits effects.Itachievesmiraculousimprovementstocastingsbyeliminatingthe50%airmixingstep.Contactpouringis stronglyrecommendedinthisvolumeasamajorbutlow-coststepforward.

Theultimatestepforwardiscountergravitycastingwhichshouldalwaysbetargetedifpossible.However,althoughI discussthetraditionalpumpingtechniques,Ipresenthereforthefirsttimemynewpneumaticpump.Itisanotherlowcost,reliabletechniquewhichenjoysuniquelylowturbulenceandmightallowarapidtakeupofthisuniquetechnology.

Turningtotheseriousnessofthecurrentpositionincasting,andinmetallurgicalengineeringasawhole,thefactthat mostcurrentmetalscanfailbycrackingshouldalertustotheglaringinconsistencyinourmetallurgicalthinkingbecause manyofourmetalsandalloysareductile,sofailurebycrackingshouldbeimpossible.Inaductilemetalanattemptto propagateacrackshouldmerelyresultinthecracktipblunting,preventingpropagation.

Intheabsenceofanyotherviablealternativemechanism,thatmetalsdocrackisastrongindicationthatcrackspreexistinmetalsintheformofbifilmsformedintheliquidstateduringpouringofthemetaltomakeacasting.Thepoor practiceisalmostuniversallyassociatedwithpouringmethodswhichensurethatthemoltenmetalismixedandemulsifiedwithatleast50volume%ofairduringitsjourneyintothemould.Idefyanyonetomakearespectablecastingfrom suchadisgracefullyinappropriateandinepttechnique.Thisbookpresentsthecasethatthisneednotbeso;metalsneed notcontainbifilms,andthusneednotcontainthoseGriffithcrackswhichcaninitiatefailurebycracking.Toachievethis, wesimplyhavetoimproveourcastingtechnology.

Itremainsthecasethatbifilmsarestilllamentablyresearched,sothatthisbookhastoresorttosiftingthroughinconclusiveandfragmentaryevidencefromresearcherswhowerenotlookingforbifilms.Unfortunately,researchersup untilnowhavenotbeenawareoftheirpresence,andcertainlydidnotsuspecttheiroverwhelminginfluenceontheirresults.Althoughawelcomestartisbeingmadebyafewworkers,Iremainimpatientformoredefinitiveresearchtobe carriedout.

Inthemeantime,whileresearchersslowlygetaroundtoprovingthebackgroundtheory,foundersneednotwaitfor answers.Practicallowbifilmcastingtechniqueshavealreadybeendevelopedandaredescribedhere.Theypromisethe qualityimprovementandcostreductionthatthecastingindustrysobadlyneeds.

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Acknowledgements

ItisapleasuretoacknowledgethesignificanthelpandencouragementIhavereceivedfrommanygoodfriends. JohnGrassihasbeenmyclosefriendandassociateatAlotech,thecompanypromotingthenew,excitingablation castingsprocess.KenHarrishasbeenaninexhaustiblesourceofknowledgeonsilicatebinders,aggregatesandrecycling. HisassistanceisclearinChapter15.Clearly,thecastingindustryneedsmorechemistslikehim.BobPuhakkahasbeen thefirstregularuserofmycastingrecommendationsfortheproductionoflargealuminiumandsteelcastings,whichhas providedmewithinspirationalconfirmationofthesoundnessofthetechnologydescribedinthisbook.Inaddition,the practicalfeedbackandwarmfriendshipoverseveralyearsfromthoseattheUKsteelfoundryFurnissandWhiteis apleasuretorecord.MuratTiryakiogluhasbeenaloyalsupporterandcritic,andprovidestheelegantlywritten publicationsthathaveprovidedwelcomescientificunderpinning.Hehasprovidedgenerousandinvaluablehelpwith theimportantsection TheStatisticsofFailure.Naturally,manyotheracknowledgementsaredeservedamongfriends andstudentswhoseresearchhasbeenaprivilegetosupervise.Idonottaketheseforgranted.Evenifnotlistedhere, theyarenotforgotten.

TheAmericanFoundrySocietyisthankedfortheuseofanumberofillustrationsfrom Transactions

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CASTINGMETALLURGY

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THEMELT

1

Someliquidmetalsmaybereallypureliquid.Suchmetalsmayincludepureliquidgold,possiblysomecarbonmanganesesteelswhileinthemeltingfurnaceatalatestageofmelting.These,however,arerare.

Manyliquidmetalsareactuallysofullofsundrysolidphasesfloatingaboutthattheybegintomorecloselyresemble slurriesthanliquids.Thisslurrytypenaturecanbeseenquiteoftenassomemetalsarepoured;themeltoverflowsthelip ofthemeltingfurnaceasthoughitwereacementmixture.Intheabsenceofinformationtothecontrary,thisawful conditionofaliquidmetalshouldbeassumedtobecorrect.Thusmanyofourmodelsofliquidmetalsthatareformulated toexplaintheoccurrenceofdefectsneglecttoaddressthisfact.Astechniqueshaveimprovedoverrecentyears,therehas beengrowingevidencefortherealinternalstructureofliquidmetals,revealingmeltstobecrammedwithdefects.Some ofthisevidenceisdescribedinthischapter.Muchevidenceappliestoaluminiumanditsalloyswherethegreatesteffort hasbeenfocussed,butevidenceforsomesteelsandallNi-basealloysisalreadyimpressiveandisgrowingsteadily.

Itissoberingtorealisethatmanyofthestrength-relatedpropertiesofmetalscan only beexplainedbyassumingthat theoriginalmeltwasfullofdefects.Classicalphysicalmetallurgyandsolidificationsciencethathasconsideredmetals asmerelypuremetalscurrentlycannotexplainaspectsoftheimportantpropertiesofcastmaterialssuchastheeffectof dendritearmspacing;itcannotexplaintheexistenceofporesandtheirareadensity;itcannotexplainthereasonforthe crackingofstrong,crack-resistantprecipitatesformedfromthemelt.Thesekeyaspectsofcastmetalswillbeseento arisenaturallyfromtheassumptionofapopulationofaparticulartypeofdefect:thebifilm.

Anyattempttoquantifythenumberandsizedistributionofthesedefectsisanon-trivialtask.McClainandco-workers (2001)andGodlewskiandZindel(2001)havedrawnattentiontotheunreliabilityofresultstakenfrompolishedsections ofcastings.Atechniqueforliquidaluminiuminvolvesthecollectionofinclusionsbyforcingupto2kgofmeltthrougha finefilter,asintheporousdiscfiltrationanalysisandPrefiltests.Themethodovercomessomeofthesamplingproblemby concentratingtheinclusionsbyafactorofabout10,000times(EnrightandHughes,1996andSimardetal.,2001).The layerofinclusionsremainingonthefiltercanbestudiedonapolishedsection.Thetotalquantityofinclusionsisassessed astheareaofthelayerasseenunderthemicroscope,dividedbythequantityofmeltthathaspassedthroughthefilter.The unitisthereforethecuriousquantitymm2kg 1.(Itistobehopedthatatsomefuturedatethisunhelpfulunitwill,by universalagreement,beconvertedintosomemoremeaningfulquantitysuchasvolumeofinclusionspervolumeofmelt. Inthemeantime,thestandardprovisionofthediameterofthefilterinreportedresultswouldatleastallowareaderthe optiontodothis.)

Togainsomeideaofthehugerangeofpossibleinclusioncontents,animpressivelydirtymeltmightreach 10mm2kg 1,whereasanalloydestinedforacommercialextrusionmightbeinthe0.1–1range,foilstockmightreach 0.001,andcomputerdiscs0.0001mm2kg 1.Forafilterof30mmdiameter,thesefiguresapproximatelyencompassthe rangeofvolumefraction10 3 (0.1%)downto10 7 (0.1partpermillionbyvolume).

OthertechniquesforthemonitoringofinclusionsinAlalloymeltsincludeLiquidMetalCleannessAnalyser (LiMCA)(SyvertsenandEngh2001),inwhichthemeltisdrawnthroughanarrowtube.Thevoltagedropappliedalong thelengthofthetubeismeasured.Theentryofaninclusionofdifferentelectricalconductivityintothetubecausesthe voltagedifferentialtorisebyanamountthatisassumedtobeproportionaltothesizeoftheinclusion.Thetechniqueis generallythoughttobelimitedtoinclusionsapproximatelyinthe10–100 mmrangeorso.

Althoughwidelyusedforthecastingofwroughtalloys,theauthorregretsthattheLiMCAtechniquehastobeviewed withgreatreservation.Inclusionsinlightalloysareoftenoxidebifilmsuptoa10mmdiameter,aswillbecomeclear.

SuchinclusionsdofindtheirwayintotheLiMCAtube,wheretheytendtohang,caughtupatthemouthofthetube,and rotateintospiralslikeaflagtiedtothemastbyonlyonecorner.Thesearetornfreefromtimetotimeandsedimentinthe bottomofthesamplingcrucibleoftheLiMCAprobe,wheretheyhavetheappearanceofaheapofspiralItaliannoodles (Asbjornsonn,2001).ItunfortunatethatmostworkersusingLiMCAhavebeenunawareoftheseseriousproblems. Becauseoftheairenfoldedintothebifilm,thedefectsintheLiMCAprobehaveoftenbeenthoughttobebubbles,which, probably,theysometimespartlyareandsometimescompletelyare.Onecanseetheconfusion.

Ultrasonicreflectionshavebeenusedfromtimetotimetoinvestigatethequalityofmelt.Theearlyworkby MountfordandCalvert(1959)isnoteworthy,andhasbeenfollowedupbyconsiderabledevelopmenteffortsinAlalloys (Mansfield,1984),Nialloysandsteels(Mountfordetal.,1992).Ultrasoundisefficientlyreflectedfromoxidebifilms (almostcertainlybecausethefilmsaredouble,andtheelasticwavecannotcrosstheintermediatelayerofair,and thereforeisefficientlyreflected).However,thereflectionsmaynotgiveanaccurateideaofthesizeofthedefectsbecause theirirregular,crumpledformandtheirtumblingactioninthemelt.Thetinymirror-likefacetsofalarge,scrambled defectreflectsbacktothesourceonlywhentheyhappentorotatetofacethebeam.Theresultisageneralscintillation effect,apparentlyfrommanyminuteandseparateparticles.Itisnoteasytodiscernwhethertheimagescorrespondto manysmallorafewlargedefects.

NeitherLiMCAnorthevariousultrasonicprobescandistinguishanyinformationonthetypesofinclusionsthatthey detect.Incontrast,theinclusionscollectedbypressurised(forced)filtrationcanbestudiedinsomedetail,althougheven heretheareasoffilmdefectsareoftendifficulttodiscern.Inadditiontofilms,manydifferentinclusionscanbefoundas listedin Table1.1.

Nearlyalloftheseforeignmaterialswillbedeleterioustoproductsintendedforsuchproductsasfoilorcomputer discs.However,forshapedcastings,thoseinclusionssuchascarbidesandboridesmaynotbeharmfulatall.Thisis becausehavingbeenprecipitatedfromthemelt,sotheyareusuallythereforeinexcellentatomiccontactwiththe matrix.Thesewell-bondednon-metallic phasesaretherebyunabletoactasinitiatorsofotherdefectssuchasporesand cracks.Conversely,theymayactasgrainrefiners.Furthermore,theircontinuedgoodbondingwiththesolidmatrixis expectedtoconferonthemaminorornegligibleinfluenceonmechanicalproperties.(However,weshouldnotforget thatitispossiblethattheymayhavesomeinfluenceonoth erphysicalorchemicalpropertiessuchasmachinabilityor corrosion.)

Generally,therefore,thisbookconcentratesonthoseinclusionsthathaveamajorinfluenceonmechanicalproperties, andthatcanbetheinitiatorsofotherseriousproblemssuchasporesandcracks.Thustheattentionwillcentreon entrainedsurfacefilms whichexhibitunbondedinterfacesinthemeltandleadtoaspectrumofproblems.Usually,these inclusionswillbeoxides.However,carbonfilmsarealsocommon,andoccasionallynitrides,sulphidesandother compounds.Thatthesefilmsarenecessarilyentrainedintothemeltasdoublefilmsisakeyfeatureoftheirstructure.This aspectoftheirmake-upwillbeacentralfeatureofthebook.

Table1.1TypesofInclusionsinAlAlloys

InclusionType

Carbides Al4C3

Boro-carbides Al4B4C

TitaniumborideTiB2

Graphite C

PossibleOrigin

PotcellsfromAlsmelters

Borontreatment

Grainrefinement

Fluxingtubes,rotorwear,entrainedfilm

Chlorides NaCl,KCl,MgCl2,etc.Chlorineorfluxingtreatment

Alphaalumina a-Al2O3

Gammaalumina g-Al2O3

MagnesiumoxideMgO

Spinel MgOAl2O3

Entrainmentafterhigh-temperature melting

Entrainmentduringpouring

HigherMgcontainingalloys

MediumMgcontainingalloys

Thepressurisedfiltrationtestscanfindmanyoftheseentrainedsolids,andtheanalysisoftheinclusionspresenton thefiltercanhelptoidentifythesourceofmanyinclusionsinameltingandcastingoperation.However,theonlyinclusionsthatremainundetectablebutareenormouslyimportantarethenewlyentrainedfilmsthatoccuronacleanmelt asaresultofsurfaceturbulence.Thesefilmsarecommonlyentrainedduringthepouringofcastings.Theyaretypically only20nmthick,andsoremaininvisibleunderanopticalmicroscope,especiallyifdrapedaroundapieceofrefractory filterthatwhensectionedwillappearmanythousandsoftimesthicker.Theonlydetectiontechniqueforsuchinclusions isthelowlyreducedpressuretest.Thistestopensthefilms(becausetheyarealwaysdoubleandcontainair,aswillbe explainedindetailinChapter3)sothattheycanbeseen.Metallographicsections(orradiographs)ofthecasttestpieces clearlyrevealthesize,shapeandnumbersofsuchimportantinclusions,ashasbeenshownbyFoxandCampbell(2000). Thetestwillbediscussedindetailagainandagain.

1.1 REACTIONSOFTHEMELTWITHITSENVIRONMENT

Aliquidmetalisahighlyreactivechemical.Itwillreactbothwiththegasesaboveitand,ifthereisanykindofslagor fluxfloatingontopofthemelt,itwillprobablyreactwiththat,too.Manymeltsalsoreactwiththeircontainerssuchas cruciblesandfurnacelinings.

Thedrivingforcefortheseprocessesisthestrivingofthemelttocomeintoequilibriumwithitssurroundings.Its successinachievingequilibriumis,ofcourse,limitedbytherateatwhichreactionscanhappenandbythelengthoftime available.

Thusreactionsinthecrucibleorfurnaceduringthemeltingofthemetalareclearlyseentobeseriousbecausethereis usuallyplentyoftimeforextensivechanges.Hydrogenbeingpickedupfromdamprefractoriesiscommon.Similar troublesareoftenfoundwithmetalsthataremeltedinfurnacesheatedbytheburningofhydrocarbonfuelssuchasgasor oil.

WecandenotethechemicalcompositionofhydrocarbonsasCxHy andthusrepresentthestraightchaincompounds suchasmethaneCH4,ethaneC2H6 andsoon,oraromaticringcompoundssuchasbenzeneC6H6 etc.(Othermore complicatedmoleculesmaycontainotherconstituentssuchasoxygen,nitrogenandsulphur,notcountingimpurities whichmaybepresentinfueloilssuchasarsenicandvanadium.)

Forourpurposes,wewillwritetheburningoffueltakingmethaneasanexample:

Clearly,theproductsofcombustionofhydrocarbonscontainwater,sothehotwastegasesfromsuchfurnacesare effectivelywet.

Evenelectricallyheatedfurnacesarenotnecessarilyfreefromtheproblemofwetenvironment:anelectricresistance furnacethathasbeenallowedtostandcoldoveraweekendwillhavehadthechancetoabsorbconsiderablequantitiesof moistureinitsliningmaterials.Mostfurnacerefractoriesarehygroscopicandwillabsorbwaterupto5or10%oftheir weight.Thiswaterisreleasedintothebodyofthefurnaceoverthenextfewdaysofoperation.Ithastobeassumedthat theusualclay/graphitecruciblematerialscommonlyusedformeltingnonferrousalloysarequitepermeabletowater vapourand/orhydrogenbecausetheyaredesignedtobeapproximately40%porous.Additionally,hydrogenpermeates freelythroughmostmaterials,includingsteel,atnormalmetallurgicaloperatingtemperaturesofaround700 Cand higher.

ThemoisturefromliningsoratmospherecanreactinturnwiththemetalM:

Thusalittlemetalissacrificedtoformitsoxide,andthehydrogenisreleasedtoequilibrateitselfbetweenthegasand metalphases.Whetheritwill,onaverage,enterthemetalorthegasabovethemetalwilldependontherelativepartial pressureofhydrogenalreadypresentinbothofthesephases.Themolecularhydrogenhastosplitintoatomichydrogen (sometimescalled‘nascent’hydrogen)beforeitcanbetakenintosolution,asisdescribedbythesimplerelation: