7/28/2019 Atmsferas Protectoras

1/6

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.

7/28/2019 Atmsferas Protectoras

2/6

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.

7/28/2019 Atmsferas Protectoras

3/6

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.

7/28/2019 Atmsferas Protectoras

4/6

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.

7/28/2019 Atmsferas Protectoras

5/6

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.

7/28/2019 Atmsferas Protectoras

6/6

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