atmósferas protectoras
TRANSCRIPT
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Lecture35:AtmosphereinFurnaces
Contents:Selectionofatmosphere:Gasesandtheirbehavior:PreparedatmospheresProtectiveatmospheresapplicationsAtmospherevolumerequirementsAtmospheresensorsKeywords:Heattreatment,furnaces,atmosphere,annealing,sintering,heating
Selectionofatmosphere:Thesurroundinginthethermalenclosure(furnace)istermedatmosphere.Theatmosphereconsistsof
gasesandisusuallyair.However,insomeheattreatment,thermomechanicalprocessing,sinteringetc
specialtype
of
atmosphere
is
required
to
Preventoxideformation,iftheheatingmaterialispronetooxidation. Decarburizesteel. Controlthesurfacechemistryofsteelwhichmeanstheelementsmustnotbeoxidizedor
reducedduringheating.
Produceblueingeffectinsteel.Theblueingeffectimpartsawearresistantandoxidationresistantsurfacefinish.
reduceoxidesformedonthesurface. Makethesurfacehardbyallowingcarburizingornitriding.
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Gasesandtheirbehavior:
1. Nitrogenistheprimarycomponentofatmosphericair(78.1%).Oxygenis20.9%.Rest1%couldbe
other
gases,
which
is
of
very
little
significance.
Nitrogen
is
considered
to
be
chemically
inert
andisusedasacarriergasforreactivefurnaceatmosphere,forpurgingetc. Athigh
temperatures,nitrogenmayshowreactionwithMo,Ti,CrandCo.
2. Hydrogenisareducinggasandisusedwherereducingatmosphereisrequired.Itmaybeusedtopreventoxidationofiron
FeO+H2=Fe+H2O
Fe3O4+H2=H2O+3FeO
Hydrogencan
also
be
used
to
decarburize
the
steel
for
certain
applications.
At
the
material
temperatureisgreaterthan973K,thefollowingreactionoccurs:
C+2H2=CH4
Hydrogenmaybeabsorbedbythemetalatelevatedtemperaturesandcausehydrogen
embrittlement.
3. Carbonmonoxide:itisalsoareducinggasandisusedtocreateareducingatmosphere
4. Carbondioxide:itisamildoxidizinggas.Itformsoxideswithironatelevatedtemperatures.Attemperaturesgreaterthan540 ,thefollowingreactionmayoccur
Fe+CO2 FeO+CO
andattemperatureslower than540 ,thefollowingreactionmayoccur
3FeO+CO2 Fe3O4+3CO
Decarburizationmayalsoresultbythereactionlike
Fe3C+CO2 3Fe+2COand
C+CO2 2CO
5. ArgonandHelium:Bothareinertgasesandareusedtomaintaininertatmosphere.Insomeapplicationsargonisusedforpurging.
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6. Steam:itisusedtoprovide blueingeffectinsteelbetween573Kand923K.BlueingeffectisduetotheformationofeitherFe2O3,Fe3O4,orFeO.Theformationofoxideofirondependson
temperature,andratiobypartialpressureofH2OtopartialpressureofH2
intheatmosphere.
DewpointsquantifiestheconcentrationofH2Ovaporintheatmosphere.Dewpointisa
temperatureat
which
gas
is
saturated
with
water
vapour
(100%
relative
humidity).
In
a
furnace,
watergasreactioncontrolstheconcentrationofH2,H2O,COandCO2accordingtothefollowing
reaction:
CO+H2O=CO2+H2.
PreparedatmospheresForhealtreatmentandotherpurposesatmospherecomprisingofgasesispreparedaccordingtothe
requirement.American
Gas
Association
(AGA)
has
classified
atmospheres
in
6
groups,
on
the
basis
of
methodofpreparation
a) Exothermicbase(AGA100):Preparedeitherbypartialorbycompletecombustionofgaseousfuelwithair.Watervapourmayberemovedtoproduceadesireddewpoint.Theatmosphereis
amixtureofCO+CO2+H2+H2O+N2. InaleanexothermicatmospheretheratioofCO2/COis
greaterthanarichexothermicone.
Thisatmosphereispreparedbyburningamixtureofhydrocarbonfuelandair.Thecombustion
productsarepassedtoacondensertoremovewater.Thecombustionproductsarefurther
driedbyusinganabsorbentsuchasactivatedaluminaoractivatedsilica.Thedriedatmosphere
isthen
transferred
to
the
furnace.
Typicalapplicationsincludebrightannealingofsteel,copper,sinteringofnonferrousmetal
powders,andironpowders
b) Preparednitrogenbase(AGA200)Theyareexothermicatmospheresandproducedbycombustionofamixtureofairandfuelgas.
CarbondioxideandH2Oareremovedfromproductsofcombustion.
Theyareusedtoheattreatlowcarbon,mediumcarbonandhighcarbonsteels.N2atmosphere
cannotbeusedfordecarburization.
c) Endothermicbaseatmospheres(AGA300).Endothermicbaseatmospheresarepreparedbyusingaleanmixtureofhydrocarbonfuelwith
air,i.e.massofairislessthanstoichiometric(theoretical)amountrequiredforcomplete
combustion.TheobjectiveistoproduceCOandH2.
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Asamountofairislessthanstoichiometeric,acatalystandextraamountheatarerequiredtofacilitate
combustion.Theendogasiscooledimmediatelytopreventthefollowingreaction:
2CO CO C
Typicalapplicationsincludebrightannealingofsteelofanycarboncontentwithoutdecarburizationor
carburization,heattreatmentofsteelofanycarboncontent,brightcopperbrazing,andcarriergasfor
gascarburizingorcarbonitriding.
Considercompletecombustionofmethane
CH4+2(O2+3.76N2)=CO2+7.52N2+2H2O
Fortheratioof
9.52,theatmosphereconsistsofCO2,N2andH2.Howeverifthesaidratio
islessthan9.52e.g.1.88,thenaccordingtofollowing,reaction,
2CH4+(O
2+3.76N
2)=2CO+4H
2+3.76N
2
theatmospherecomprisesofCO,H2andN2.Thevolumeofatmospherewouldincreaseby3mole
volumes.Theatmospherewouldcompriseof20.4%CO,40.8%H2and38.8%N2.
d)Charcoalbaseatmospheres(AGA400)
Itisproducedbyfollowingreaction
2C+O2+3.76N2=2CO+3.76N2.
Theoreticallyatmospherewouldconsistof34%COand66%N2.Duetomoistureandvolatilesin
charcoaland
incomplete
combustion
of
carbon,
the
reaction
would
produce
CO2,
CO,
H2,
CH4
and
N2
.
Normallytheatmosphereisneutraltohighercarbonsteelsbutthecarbonpotentialmaybeincreased
byaddingnaturalgas.
Thecharcoalbasedatmosphereisusedforhardening,annealingandnormalizinghighcarbonsteels
withoutscaleformationordecarburization.
e)ExothermicEndothermicbaseatmospheres(AGA500)
Theyarepreparedbycombustingamixtureofairandfuel.POCisdehydrated,andapredetermined
quantity
of
hydrocarbon
fuel
is
added.
The
mixture
is
made
to
react
in
presence
of
a
catalyst.
Typicalapplicationincludescarburizingandcarbonitriding. Duetothecostofproduction,these
atmospheresarenotveryoftenused.
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f)Ammoniabaseatmospheres(AGA600)
Ammoniadissociationisusedtopreparehighestpuritynitrogenwhichisfreefromoxygen.
Liquidammoniaisvaporizedintoaheatexchangerandisfedtodissociateinareactorcalledas
dissociater.Thedecompositionofammoniatonitrogenandhydrogenbeginsataround3003200C.Rate
ofdecompositionincreasesastemperatureincreases..
Itsprimaryuseisforbrightannealingmetalssuchassilicontoobtainelectricalproperties.Ferrousand
nonferrousmetalsarebrightannealedinammoniaatmosphere.
Brightsilverbrazingandcopperbrazingofsteelareotherapplications.
ProtectiveatmospheresapplicationsComposition(vol%)
Atmosphere
Lean
exothermic
N2
86.8
CO2
10.5
CO
1.5
H2
1.2
CH4
Dewpoint
4.5
Applications.
Bright
annealingof
Cu,sintering
offerrites
Rich
exothermic
71.5 5.0 10.5 12.5 5 10 Bright
annealing
lowCsteel,
silicon
steels/Cu
brazing,
sintering
Dissociated
NH3
25 75 50to+60 Brazing
sintering
bright
annealing
Endothermic 4045 00.5 20 3440 0.51 10to+10 Hardening,
carburizing
withCH4,
sintering
brazing
Nitrogen
H2
99.9
99.9
60
68
Natural
for
annealing
Reducing,
sintering
Ar orHe : Thesearepureandinertgasesandareusedtopreventoxidationduringweldingof
stainlesssteel,aluminumetc.andheattreatmentofspecialsteels.
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AtmospherevolumerequirementsItdependson
i) Typeandsizeoffurnaceft.
es.
ed.
xit.
ii) Environmentandpresenceofdraiii) Thenatureandsizeofworkpieciv) Metallurgicalprocessinvolvv) Presenceorabsenceofcurtains atentranceande
AtmosphericsensorsItisimportanttomeasuretheconcentrationofvariouscomponentsoftheatmosphereduringthe
treatmentinthefurnace.TheconstituentsofatmosphereareCO,CO2,H2,H2O,N2andhydrocarbon
gasessuchasCH4.Themainobjectiveoffurnaceatmosphereistopreventdecarburization,hydrogen
embrittlement,oxidation,surfaceblueingandsootformation.
ORSATanalysiscanbeusedtodeterminethecompositionofO2,COandCO2.ORSAT analysisis
describedinlecture10.
Amongothertechniquesgaschromatography,thermalconductivity,oxygensensors,dewpointarealso
used.Detailsaboutthesensorscanbeobtainedinreferencesgivenattheendofthelecture.
References:
Y.V.Deshmukh:
industrial
heating.
2004.
Furnaceatmospheresandcarboncontrol:ASMCommitteeonfurnaceatmospheres