铝土矿和红柱石经热处理后的耐火性的比较.docx

铝土矿和红柱石经热处理后的耐火性的比较.docx

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铝土矿和红柱石经热处理后的耐火性的比较

Effectofthermaltreatmentondamagemechanicalbehaviourofrefractorycastables:

Comparisonbetweenbauxiteandandalusiteaggregates

M.GhassemiKakroudia,b,E.Yeugo-Fogaingc,C.Gaultb,M.Hugerb,

andT.Chotardb,

aDepartmentofCeramicEngineering,UniversityofTabriz,Tabriz51666-16741,IslamicRepublicofIran

bGrouped’EtudedesMatériauxHétérogènes（GEMH）,ENSCI,Limoges,France

cMatériauxàFinalitésSpécifiques,EA3834,InstitutdesSciencesdel’IngénieurdeToulonetduVar,Av.G.Pompidou,BP56,83162LaValetteduVarCedex,France

Received15January2008; 

revised20March2008; 

accepted28March2008. 

Availableonline6June2008.

Abstract

Duringservicelife,refractorycastablesaresubjectedtodifferentsolicitations.Theknowledgeoftheirdamagebehaviourundersuchsolicitationsishighlyneededforabetterunderstandingofmechanisms,whichinducethefinalruptureofstructuresmadewithrefractories.Sincethesematerialsareoftenusedaswallsofmetallurgicaltools,thermalgradientsinsuchstructurecanleadtoseveremechanicalstressesintheouterlayeroftherefractorypartwhichisataratherlowtemperature.

Thisstudydealswiththemechanicalpropertiesatroomtemperature（bytensiletest）oftworefractorycastablestreatedatdifferenttemperatures（110 °C,250 °C,500 °C,700 °C,900 °Cand1100 °C）inordertoreproducethethermalgradientinwallsofmetallurgicaltools.Tworefractorycastablesareconsidered:

anultra-lowcementcontentbauxite-basedmaterial（Bau-ULCC）andalowcementcontentandalusite-basedmaterial（And-LCC）.

Keywords:

Tensiletest;Mechanicalproperties;Thermalexpansion;Refractories;Castables

ArticleOutline

1.Introduction

2.Materialsandexperimentalprocedures

2.1.Materialandsamplepreparation

2.2.Dilatometry

2.3.Ultrasonicmeasurements

2.4.Tensiletest

3.Resultsanddiscussion

3.1.Young'smodulusvaluesatroomtemperature

3.2.Microstructuralevolutionduringafirstthermalcycleupto1500 °C

3.3.Young'smodulusevolutionduringthermalcyclesatlowertemperatures

3.4.Mechanicalbehaviourintensionafterthermaltreatment

3.5.Sourceofdamageinthecastables

4.Conclusion

Acknowledgements

References

1.Introduction

Theuseofmonolithicrefractoriesinvariousindustries（metallurgical,cement,etc.）iscontinuouslyincreasingforthelast20years[1]and[2].Duringtheirservicelife,refractorycastablesaresubjectedtoseveresolicitations,especiallyfromathermomechanicalpointofviewandaredegradedbyacombinationofseveralmechanisms,mainlythermalshock,abrasion,corrosionandmechanicalimpact.Thebehaviourofthesematerialsfacetothosemechanismsisinfluencedbytheevolutionofmanyfactorssuchastheirchemicalcomposition,theirmicrostructureaswellastheirphasetransformation,whichoccurathightemperatureduringfiringprocess,and/orinservice[3]and[4].

Thephysicalpropertiesofarefractoryconcretearehighlytemperature-dependent.Thisisprimarilycausedbythecomplexhydrationanddehydrationreactionsofcalciumaluminatecement[5],[6]and[7].

Theelaborationofmonolithicscontainingcalciumaluminatecementscontainsseveralstepssuchasmixing,placingandconsolidation,curinganddryoutandfinallyuseinservice.Eachofthesestepswithinthecastableplacingchainareintimatelylinkedtotheinitialhydrationprocessofthecalciumaluminatecement（CAC）[8]and[9].

Therefractorycastablesgenerallypresentcomplexheterogeneousmicrostructureswhichcanprovidestronginternalstressesbythermalsource.Becauseofthemismatchbetweenthepropertiesofphasesandmainlybetweentheircoefficientsofthermalexpansion,theserviceconditionscanconsiderablyaffecttheirinitialmicrostructuralstateandthustheirthermomechanicalproperties[10]and[11].

Previousstudieshavealreadybeenperformedinthefieldofthehightemperaturebehaviourofrefractorycastables[3]and[12].Thispaperdealswithresultsofanexperimentalapproachdevelopedtocharacterisethemicrostructuralchangesanddamageprocesses,whichoccurinsuchmaterialsduringthefirstheating.

2.Materialsandexperimentalprocedures

2.1.Materialandsamplepreparation

Twocommercialcastablesareconsidered.Thefirstoneisalowcementandalusitecastable（And-LCC）madeofandalusiteaggregates,fumedsilica,aluminaandofacalciumaluminatecement.Thesecondisanultra-lowcementbauxitecastable（Bau-ULCC）madeofbauxiteaggregates,fumedsilica,aluminaandofthesamecement.Bothmaterialsarecharacterisedbythesamefumedsilicacontent.InBau-ULCC,thealuminacontentistwotimeshigherthaninAnd-LCC.Table1showsthechemicalcompositionsofthecastablessuppliedbythemanufacturer.Thehighdifferencebetweenthesilicacontentsofthetwomaterialsismainlyduetothehighsilicacontentinandalusiteaggregatescomparedtobauxiteones.Forbothcastables,themaximumaggregatesizeisabout5 mm.Thematerialswerecuredduring24 hat110 °C.Fig.1showspicturesofpolishedsectionsofcuredmaterials.Aftermachining,somesampleshavebeenfiredat250 °C,500 °C,700 °C,900 °Cand1100 °Cinordertosimulateseveralthermalhistoriesbeforecharacterisation.Thesetemperaturelevelshavebeenfixedaccordingtothetemperaturerangerepresentingtherefractorycastablesinspecificindustrialapplications.Firingthermalcyclesarecharacterisedby5 °C/minheatingandcoolingratesandbya5 hisothermaldwellatthemaximumfiringtemperature.

Table1.

Chemicalanalysisandcharacterisationsdataofthetworefractories

Castabletype

And-LCC

Bau-ULCC

Aggregatetype

Andalusite

Bauxite

Al2O3（wt.%）

58

85

SiO2（wt.%）

37.5

10

CaO（wt.%）

2.3

1.1

Fe2O3（wt.%）

0.9

1

Maximumaggregatesize（mm）

5

5

Waterrequirement（wt.%）

4.5–5.5

4.2–5.2

Openporosity（vol.%）

6

10

Apparentdensity（kg/m3）

2600

2970

Full-sizetable

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Full-sizeimage（122K）

Fig.1. Microstructureofstudiedrefractories:

（a）And-LCC;（b）Bau-ULCC.

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2.2.Dilatometry

Inordertostudythermalexpansionmismatcheffects,samplesofmatrixandofaggregateshavebeenprepared.Thesamplesofthematrixwerepreparedbycastingandthesamplesofaggregates（grainsizelessthan200 μm）,wereshapedbypressing.Thethermalcycles（heating/cooling）werecarriedoutwithaslopeof5 °C/min.

Theinfluenceofthetemperatureonamaterialcauses,ingeneral,variationsofitsapparentvolume.Theknowledgeofthesevariationsmakesitpossibletocharacterisemanyphysicalphenomenawhichoccurwithinamaterialduringagivenheattreatment.DilatometrictestswerecarriedoutbyadilatometerADAMELDI.Thesamplesusuallyusedareofdimension10 mm × 5 mm × 5 mm.

2.3.Ultrasonicmeasurements

Anultrasonictechniquebasedonacontinuousinsitumeasurementofthevelocityoflongitudinallongbarmodewavesinthematerialhasbeenusedtomonitortheevolutionoftheelasticmodulusversustemperatureonbothmaterials[13]and[14].Fig.2isaschematicrepresentationoftheultrasonicdevice.Thedeterminationoftheultrasonicvelocityisbasedonthemeasurementoftheroundtriptimeτbetweentwosuccessiveechoesinthesample.Theultrasonicpulseistransmittedfromthetransducertothesamplethroughawave-guide.ThemeasurementofthetimeτbetweentwosuccessiveechoeswithinthesampleallowstocalculatethewavevelocityandthentoobtainthevalueoftheYoung'smodulusbyEus = ρ（2L/τ）2,whereLandρaresamplelengthanddensity,respectively.

Full-sizeimage（33K）

Fig.2. Experimentalset-upusedforYoung'smodulusmeasurementathightemperaturebylongbarmodeultrasonicpulsetechnique.

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2.4.Tensiletest

TensiletestshavebeenperformedwithanINSTRON8862electro-mechanicaluniversaltestingmachineatroomtemperature.Fig.3presentsaschematicofthetensiletestdevice.Thestrainismeasuredbytwoextensometersequippedbysiliconcarbiderodswhichareplacedontwooppositefacesofthespecimen.Theextensometergaugelengthis25 mm.

Full-sizeimage（53K）

Fig.3. Schemeofthetensiletestdevice.

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Thelowvaluesofthedisplacementatruptureexhibitedbythesematerials（3–5 μm）requiredagoodcontrolofthethermalstabilityoftheextensometers（±0.1 °C）.Therefractorysamplesareconstitutedofacylindricalrod（18 mmindiameter）gluedtotwometallicparts.Thegeometryispreciselyadjustedthankstoafinalcylindricalmachiningstepofthetotalassembly.Thetensiletestsarecarriedoutuntilrupturewithaconstantdisplacementvelocityof0.04 mm/minwithintermediateunloadingatseverallevelofstress.ToaccuratelydeterminetheYoung'smodulusfromtensiletestresults（ET）,theearlyslopeofthefirstloadingstepofthestress–straincurveshasbeenevaluated.Fig.4illustratesanexampleofstress–straincurveobtainedonAnd-LCCtreatedat110 °CandthemethodtodeterminetheYoung'smodulusattheveryfirsttime.

Full-sizeimage（45K）

Fig.4. Stress–straintensilecurvesofAnd-LCCatroomtemperatureaftertreatmentat110 °C.ZoomoftheinitialpartofthecurvesthatillustratestheearlydeterminationoftheYoung'smodulusET.

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3.Resultsanddiscussion

3.1.Young'smodulusvaluesatroomtemperature

Foreachmethod,exceptfortheultrasonicone,atleastfivesamplesweretested.Table2presentstheresultsoftheYoung'smodulusmeasuredatroomtemperaturebythetwotechniques（ultrasonicmeasurementandtensiletest）onbothcastables.Theobtainedvaluesa