Endothermic Atmospheres Used for Heat Treating Why Keeping CO Content of a Furnace Mustd

Endothermic atmospheres used for heat treating Why keeping CO content of a furnace mustD. beSCOTT stable MACKENZIE,at a value of 20 PH.D.,percent FASMis critical for  accurateHOT carbon control of the endothermicSENIOR atmosphere. RESEARCH SCIENTIST–METALLURGY Endothermic atmospheres used for heatHOUGHTON treating INTERNATIONAL INC. D. Scott MacKenzie, Ph.D., FASM WhySEAT keeping CO content of a furnace must be stable at a value of 20 percent is critical for accurateHoughton carbon International, control Inc.of the endothermic atmosphere. Endothermic atmospheres used for heat treating D.During Scott heat MacKenzie, treatment, Ph the.D. part, FASM experiences different temperature regimes (Figure ). Atmospheres areWhyEndothermic used keeping in heat CO treating content atmospheres to of protect a furnace the mustpart from be stableused scaling at aatfor value the elevatedheat of 20 percent temperaturestreating is critical used for during accurateaustenitizing.Houghton carbon International, The control atmospheres Inc.of the endothermicrange from simple atmosphere. wrapping the part in stainless steel or tantalum foilWhy for keeping small tool CO room content parts, ofto aprotective furnace atmospheres must be containingis very important,inert or protective as this value gases. is used to determine the carbon D.During Scott heat MacKenzie, treatment, Ph the.D. part, FASM experiences different temperaturepotential regimes in the ( carbonFigure controller. ). Atmospheres Onarestable anused industria at in heata valuel treatingscale, of these 20to protectpercent protective the is part gasses critical from are sc for usuallyaling at mtheixtures elevatedFor atmosphereof carbon temperatures generators,monoxide used air(CO), during and the hydrocarbon are mixed austenitizing.Houghton International, The atmospheres Inc. range from simple wrapping the part in stainless steel or tantalum haccurateydrogen (H carbon2), nitrogen control (N2), of carbon the endothermic dioxide (CO2) and traceand amounts introduced of water into a vapor.retort. ForThese nitrogen-methanol, the nitrogen gasesDuringfoil for are smallheat produced treatment, tool room using the parts, either part to experiences endothermic protective atmospheresdifferent generators temperature withcontaining andnatural methanol regimes inert gas orare ( Figurepropaneprotective injected ). asAtmospheresdirectly gases.the carbon into the furnace. For endother- atmosphere. mic generators, the air:gas ratio for natural gas is typically 2.8 - 3.1 source,Onare anused industria or in injection heatl treatingscale, directly these to protect inprotective the furnacethe part gasses usingfrom are scn itrogenusuallyaling at and mtheixtures methanolelevated of carbon temperaturesmixtures. monoxide In thisused (CO),article, during austenitizing. The atmospheres range from simple wrapping (theoreticalthe part in 2.4:1),stainless while steel for aor generator tantalum using propane, the air:gas wehydrogenuring are going heat (H treatment,2 ),to ndiscussitrogen the the part(N 2formation), experiences carbon dof ioxidedifferent endothermic (CO temperature2) and atmospheres trace ratio amounts is for7.5 - the8.1of (theoreticalwaterprotection vapor. 7.2:1). of These parts The difference between the theo- foil for small tool room parts, to protective atmospheres containing inert or protective gases. Dgasesduringregimes are austenitization. produced(Figure 1). Atmospheresusing either are endothermic used in heat treatinggenerators to pro with- reticalnatural vales gas and or thosepropane typically as the used carbon in the shop are associated with [INSERTsource,Ontect anthe industria partor FIGUREinjection froml scaling scale, 1]directly atthese the elevated inprotective the furnace temperatures gasses using are used n itrogenusually during andm ixturesreaction methanol of kinetics. carbon mixtures. For monoxide nitrogen-methanol, In this (CO),article, the mixture is a bit more austenitizing. The atmospheres range from simple wrapping the part difficult as methanol is a liquid. To obtain the proper atmosphere, wehydrogen are going (H2 ),to ndiscussitrogen the (N 2formation), carbon dofioxide endothermic (CO2) and atmospheres trace amounts for theof waterprotection vapor. of These parts gasesduringEndothermiin stainless are austenitization. producedsteelc atmospheres or tantalum using foil either are for commonly small endothermic tool room used parts,generators durin to gprotec the with -heat thenatural treatment total volumegas ofor steel.willpropane be It 40% is as usednitrogen. the carbonas a One gallon of methanol dis- tive atmospheres containing inert or protective gases. sociates to form 240 standard cubic feet of CO and H , so for 1,000 source,carrier gasor injectionfor atmosphere directly additions in the furnace for carburizing using nitrogen or carbonitriding. and methanol Using mixtures. an endothermic In this article, 2 generator,[INSERTOn an industrial FIGUREeither naturalscale, 1] these gas protective or propane gases is are used. usually If nitrogen mixtures- methanolSCF of atmosphere is injected needed in the for afurnace, furnace, then 400 standard cubic feet weof carbon are going monoxide to discuss (CO), hydrogen the formation (H ), nitrogen of endothermic (N ), carbon dioxatmospheres- of nitrogen for theis required, protection and 600of parts standard cubic feet of CO and H are thenEndothermi methanolc atmospheres is the carbon are source. commonly2 The predominant used2 during reactionsthe heat treatment for producing of steel. endothermic It is used asgas a 2 duringide (CO2 ),austenitization. and trace amounts of water vapor. These gases are produced required. Therefore, 2.5 gallons per hour (600 SCF needed/240 SCF arecarrierusing shown either gas below:endothermicfor atmosphere generators additions with naturalfor carburizing gas or propane or carbonitriding. as per gallon CHUsingOH =an 2.5 endothermic gallons per hour) are needed. [INSERT FIGURE 1] 3 generator,the carbon source, either or natural injection gas directly or propane in the furnace is used. using If nitrogennitrogen -methanol is injected in the furnace, thenEndothermiand methanol methanolc mixtures. atmospheres is the carbon In this are article,source. commonly we The are predominantgoing used to durin discussg reactionsthe the heat treatment for producing of steel. endothermic It is used asgas a Nitrogen- Gas Species Formula Natural Gas Propane Naturalarecarrierformation shown gas Gas:of below:for endothermic atmosphere atmospheres additions for for the carburizing protection of or parts carbonitriding. Using an endothermic Methanol Carbon Monoxide CO 20% 23% 20% generator,during austenitization. either natural gas or propane is used. If nitrogen-methanol is injected in the furnace, Endothermic atmospheres are commonly used during the heat Hydrogen H2 40% 31% 40% then methanol is the carbon source. The predominant reactionsCarbon for Dioxideproducing endothermicCO2 gas0.30% 0.30% 0.30% treatment of steel. It is used as a carrier gas for atmosphere additions Natural Gas: Water Vapor H2O < 0.1% < 0.1% < 0.1% arefor carburizingshown below: or carbonitriding. Using an endothermic generator, Methane CH4 < 0.1% - - either$ natural' gas or propane is' used. If nitrogen-methanol is injected 2𝐶𝐶𝐻𝐻 + 𝑂𝑂 ↔ 2𝐶𝐶𝐶𝐶 + 4𝐻𝐻 Propane C3H8 - < 0.1% - in the furnace, then methanol is the carbon source. The predominant Propane: Nitrogen N2 40% 46% 40% Naturalreactions forGas: producing endothermic gas are: Table 1: Typical gas compositions from different hydrocarbons used for generating $ ' ' endothermic atmospheres. 2Natural𝐶𝐶𝐻𝐻 + Gas:𝑂𝑂 ↔ 2𝐶𝐶𝐶𝐶 + 4𝐻𝐻 Propane: ENDOTHERMIC GENERATORS Propane:* +$ 1 ' ' ' ' The endothermic generator, regardless whether used with natural 𝐶𝐶2𝐶𝐶𝐻𝐻𝐻𝐻 ++ 𝑂𝑂𝑂𝑂 ↔↔23𝐶𝐶𝐶𝐶𝐶𝐶++44𝐻𝐻𝐻𝐻 2 gas or propane, consists of several important parts: the retort; cata- Methanol:Propane: lyst; heating method (gas or electric); insulation; shell, gas-safety 1 equipment; and temperature control. Automated gas control is often 𝐶𝐶Methanol:*𝐻𝐻+ + 𝑂𝑂' ↔ 3𝐶𝐶𝐶𝐶 + 4𝐻𝐻' added (and recommended). 2 An endothermic atmosphere generator (Figure 2) is operated

Methanol: at very high temperatures, typically 1,950°F (1,065°C) for natural * 1 ' gas, and 2,050°F (1,120°C) for propane generators. To handle these 2*𝐶𝐶𝐻𝐻+ 𝑂𝑂𝐻𝐻 ↔' 2𝐶𝐶𝐶𝐶 + 4𝐻𝐻 ' 𝐶𝐶 𝐻𝐻All +of these𝑂𝑂 methods↔ 3𝐶𝐶𝐶𝐶 produce+ 4𝐻𝐻 an atmosphere containing CO and elevated temperatures, a high temperature, heat resisting alloy like All of these2 methods produce an atmosphere containing CO and H2. Nitrogen appears because it Methanol:H2. Nitrogen appears because it is either from the atmosphere in HT is used. The retort is supported by a flange, and allowed to grow isthe either case of from endothermic the atmosphere generators, in or the as acase carrier of gasendothermic in the case of generators, in the vertical or as direction. a carrier A gas schematic in the ofcase a small generator is shown * ' 2nitrogen-methanol𝐶𝐶𝐻𝐻 𝑂𝑂𝐻𝐻 ↔ 2𝐶𝐶𝐶𝐶 atmospheres.+ 4𝐻𝐻 The nominal composition of the in Figure 3. gas produced using these methods are shown in Table 1. The differ- Inside the retort is the catalyst. This catalyst is often 1-inch cubes All of these methods produce an atmosphere containing CO and H2. Nitrogen appears because it isence either in the from amount the atmosphereof CO produced in theusing case the ofdifferent endothermic methods generators, of alumina or impregnated as a carrier with gas nickel.in the Magnesiacase (MgO) cubes impreg- * ' 2𝐶𝐶𝐻𝐻 𝑂𝑂𝐻𝐻 ↔ 2𝐶𝐶𝐶𝐶 + 4𝐻𝐻 All of these methods produce an atmosphere containing CO and H2. Nitrogen appears because it is either from the atmosphere in the case of endothermic generators, or as a carrier gas in the case

Figure 1: Typical sequence for heat treating, including carburizing.

26 gearsolutions.com The advantage of an oxygen probe, is that it is accurate and fast. A direct read-out of the carbon potential of the atmosphere is common. The probe has a high temperature range suitable for high temperature carburizing. Little maintenance is required. The retort is either heated using gas or electric heating elements. Radiant tubes are used for natural gas, and are preventhung vertically the formation to reduce of soot indistortion, the temperature and to range allow of 1,300°Fthe thermal to expansionsion ofof Thethethe radiantradiantretort is tubes.tubes. either Electric heated900°F (700°C usingheatinging - gas500°C). elementselements or electric Soot formscan heating also by thebe elements. “carbonused. Typical reversal” Radiant temperature reaction tubes are control used for natural gas, and are hung vertically to reduce distortion, and to allow the thermal expansion of equipment, consistingCO of Cthermocouples + CO2, in which sootfor processis formed temperature, from the reaction excess of carbon temperature control thethermocouple,thermocouple, radiant tubes. processprocess Electricmonoxide controllers,controllers, heat toing form elements excessexcess carbon dioxide.temperaturetemperature can also These be heatinstrumentinstrumentused. exchangers Typical andand cantemperature datadata either logginglogging control equipmentequipment equipment,are usually consistingincluded.be ofwater-cool, thermocouples or air-cooled. for Modernprocess practice temperature, is to use air-cooledexcess temperature heat control thermocouple, processexchangers. controllers, excess temperature instrument and data logging equipment If the retort is heated by natural gas, additional control equipment consisting of fire safety areIf the usually retort included. is heated byThe natural retort is gas, either additional heated using control gas or equipmentelectric heating consisting elements. of fire safety equipment (manual Radiantshut--off tubes valve are or used MSOV, for natural fire--eyes gas, andand are high hung-- and vertically lowlow--pressure to switches)switches) areare Ifgenerally the retort require is heatedd forreduce by safe natural distortion, gas operation. gas, andadditional to Theallow gas controlthe supply thermal equipment is expansion shut off consisting of(tripping(tripping the radi of - thethe fire MSOV)MSOV) safety inin thethe equipmentevent of any (manual malfunction shutant tubes.-off such valve Electric as: or heating MSOV, elements fire-eyes can and also highbe used.- and Typical low- pressuretem- switches) are generally required forperature safe gas control operation. equipment, The consisting gas supply of thermocouples is shut off (tripping for process the MSOV) in the

event of any malfunctiontemperature, such as: excess temperature control thermocouple, process con- • Excess retorttrollers, temperature excess temperature.. instrument and data logging equipment • High gas pressureare usually.. included. If the retort is heated by natural gas, additional control equipment •• ExcessLow gas retort pressure temperature.. . • High gas pressureconsisting. of fire safety equipment (manual shut-off valve or MSOV, • No pilot detectedfire-eyes.. and high- and low-pressure switches) are generally required • Figure 2: Three retort endothermic generator used to produce endothermic• LowLow gasretort pressure temperaturefor safe. gas operation... The gas supply is shut off (tripping the MSOV) atmospheres. (Courtesy: Surface Combustion, Inc., Maumee, Ohio.)• No pilot detectedin the. event of any malfunction such as:

• Low retort temperatureExcess retort. temperature. Similar gas safety equipmentHigh gas is usedpressure. on the gas train supplying the retort with gas, separate from

thethe temperaturetemperature controlcontrol gasgasLow supply.supply.gas pressure. Similar gas safety equipmentNo pilot is useddetected. on the gas train supplying the retort with gas, separate from Atmosphere Control ofLow Generated retort temperature. Gas theAtmosphere temperature Control control of gas Generated supply. Gas Similar gas safety equipment is used on the gas train supplying The equilibrium composition of the gas depends on the water gas reaction: AtmosphereThe equilibrium Control compositionthe of retort Generated with of thegas, gasseparateGas depends from theon temperaturethe water gas control reaction: gas supply.

The equilibrium compositionATMOSPHERE of the CONTROLgas depends OF onGENERATED the water gasGAS reaction: The equilibrium composition of the gas depends on the water gas reaction: '' '' '' 𝐶𝐶𝐶𝐶 + 𝐻𝐻 𝑂𝑂 ⇔ 𝐶𝐶𝑂𝑂 + 𝐻𝐻 This means that there are four gas species that can be used for proper control of the atmosphere. ' ' ' Practically, there are reallyThis meansonly two that𝐶𝐶𝐶𝐶 reactions there+ 𝐻𝐻 are𝑂𝑂 fouravailable⇔ gas𝐶𝐶𝑂𝑂 species+ for𝐻𝐻 control:that can be used for This means that thereproper are four control gas of species the atmosphere. that can Practically,be used for there pro areper really control only of the atmosphere. two reactions available for control: Practically, there are really only two reactions available for control:

1 𝐶𝐶𝐶𝐶 + 𝑂𝑂'' ⇔ 𝐶𝐶𝑂𝑂'' 2 Figure 3: Schematic representation of an endothermic generator showing general 1 ' ' assembly. Piping or flame safety equipment is not shown. 𝐶𝐶𝐶𝐶 + 𝑂𝑂 ⇔ 𝐶𝐶1𝑂𝑂 These two reactions𝐻𝐻 are''𝑂𝑂 combined⇔2 𝐻𝐻'' + into𝑂𝑂' 'the water gas reaction nated with nickel are often used with propane as the MgO resists heat above. From these last two reactions, that2 means that we can control TheseThese twotwo reactionsreactions areare combinedcombined intointo tthehe waterwater gasgas reaction1reaction above.above. FromFrom thesethese lastlast twotwo better than alumina. Sometimes 1-inch spheres, impregnated with the furnace atmosphere𝐻𝐻' by𝑂𝑂 monitoring⇔ 𝐻𝐻' + and𝑂𝑂' controlling either the nickel are used, as the spheres offer a greater surfacereactions,reactions, area. Nickel thatthat actsmeansmeans water thatthat vapor wewe cancan in the controlcontrol atmosphere thethe furnacefurnace or by controlling atmosphere2atmosphere the carbon byby monitoringmonitoring dioxide. andand controllingcontrolling as the catalyst to improve the kinetics of the naturalTheseeithereither gas twothethe or waterwaterpropanereactions vaporvapor are inDewin combined thethe point atmosphere,atmosphere, is ofteninto tusedhe oror water tobyby control controllingcontrolling gas reactionthe carbon thethe above. carboncarboncontent From dioxide.dioxide.of atmo these- last two reaction to form CO and H. Plain refractory cubesreactions, are generally that placed means sphere that wegenerators can control or furnaces. the furnace However, atmosphere the use of dew by point monitoring instru- and controlling 2 DewDew pointpoint isis oftenoften usedused toto controlcontrol thethe carboncarbon contentcontent ofof atmoatmospheresphere generatorsgenerators oror furnaces.furnaces. at the air/gas mixture entrance to delay the reactioneither and the preheat water the vapor ments in the has atmosphere, waned with theor byimplantation controlling of morethe carbon modern dioxide. control gas to the operating temperature. Once at the operatingHowever,However, temperature, thethe useuse ofofmethods dewdew pointpoint such instrumentsinstruments as carbon probes hashas and wanedwaned infrared withwith analyzers. thethe implantationimplantation The relation ofof- moremore modernmodern the gas reacts with air to form the desired atmosphereDewcontrolcontrol pointcomposition. methods,methods, is often suchsuch usedship asas toof carboncarbon dewcontrol point probesprobes the to thecarbon andand percentage infraredinfrared content of analyzers.analyzers.carbonof atmo issphere shown TheThe inrelationshipgeneratorsrelationship Figure 4. or ofof furnaces. dewdew pointpoint toto Once the gas has reached the operating temperatureHowever,thethe percentagepercentage andthe usehas ofof ofcarboncarbon Onedew thing pointisis shownshown to instruments remember, inin FigureFigure is has that .. OneOne waned these thingthing charts with toto remember,remember,arethe onlyimplantation valid isis with thatthat of athesethese more chartscharts modern areare onlyonly reacted, the gas exits the generator, where it is controlimmediately methods, passed such 20 aspercent carbon CO. probes Other charts and infrared need to be analyzers. calculated Theor developed relationship for of dew point to through a heat exchanger to fix the compositionthe of percentage the gas, and toof carbonCO concentrations is shown in atFigure values .other One than thing 20 to percent remember, CO. The is classical that these charts are only June 2018 27 valid with a 20 percent CO. Other charts need to be calculated or developed for CO concentrations at values other than 20 percent CO. The classical dew point instrument is the ALNOR Dewpointer (Figure 1) – these instruments are commonly found in the heat treat shop. 180 Theyvalid are with still dewa 20in point uspercente today.instrument CO. Other is the charts ALNOR need Dewpointer to be calculated (Figure 5) — or these developed for CO 160 validconcentrations with ainstruments 20 percentat values are CO. other commonly Other than charts found20 percent needin the toCO.heat be treatThecalculated classicalshop. They or dew developedare point instrument for CO is the 1400 [INSERTconcentrations FIGUREstill inat usevalues 4] today. other than 20 percent CO. The classical dew point instrument is the 140 ALNOR Dewpointer (Figure 1) – these instruments are commonly found in the heat treat shop. [INSERTALNORThey are Dewpointer FIGUREstill Thein us amount e5] today. (Figure of carbon 1) – dioxide these instrumentspresent in the areatmosphere commonly is also found in the heat treat shop. 120 They are stillused. in Thisuse istoday. often done using infrared analyzers. These often are 1500 100 The[INSERT amount FIGUREcalled of carbon three 4] gasdioxide analyzers. present The advantagein the atmosphere of three gas is analyzers, also used. is This is often done using [INSERT FIGUREthat the three 4] gases measured are CO, CO2, and CH4. This enables F infrared analyzers. These often are called three gas analyzers. The advantage of three gas ° 80 [INSERT FIGUREthe user to 5] optimize the generator (or nitrogen-methanol system) analyzers,[INSERT isFIGURE that the 5]thr ee gases measured are CO, CO2 and CH4. This enables the user to Depoint, Depoint, 60 1600 The amountsetting of carbon to first dioxide achieve apresent 20 percent in theCO. Onceatmosphere that is accomplished, is also used. This is often done using optimize histhe generator charts and (or graphs nitrogen indicating-methanol various system) carbon setting potential to first rela achieve- a 20 percent CO. 40 OnceTheinfrared amount that analyzers.is accomplished, of carbon These dioxide often the chartspresent are called and in thegraphs three atmosphere gas indicating analyzers. is variousalso The used. advantagecarbon This potentialis oftenof three done gas using infrared analyzers.tionships Thesewill accurately often are indicate called carbonthree gaspotential analyzers. as a function The advantage of three gas 20 relationshipsanalyzers, ofis willeitherthat accuratelythe dew thr pointee gases orindicate carbon measured carbondioxide. are potential CO, CO as2 aand function CH4. Thisof either enables dew the point user or to carbon analyzers, is that the three gases measured are CO, CO2 and CH4. This enables the user to 0 optimize his generator (or nitrogen-methanol system) setting to first achieve a 20 percent CO. 1700 dioxide. The infrared gas analyzer operates by measuring the absorption 1800 optimize hisof generatorinfrared light (or through nitrogen the- gasmethanol sample. Thesystem) gases CO,setting CO , andto first CH4 achieve a 20 percent CO. -20 1900 Once that is accomplished, the charts and graphs indicating2 various carbon potential TheOncerelationships infrared that isabsorb gasaccomplished, will analyzer light accurately at specific operates the indicate wavelengths.charts by measuringcarbonand graphsA detector potential the indicating measuresabsorpti as a function variousonthe ofenergy infrared ofcarbon either light potential dew through point or the carbon gas -40 sample.relationships Thepresent gases will accuratelyatCO, the CO wavelength2 and indicate CH of4 interest absorbcarbon and lightpotential compares at very as itaspecific tofunction the refer wavelengths. of- either dew A pointdetector or carbon 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 dioxide.1 1.1 % Carbon measuresdioxide. the ence energy condition present of no at absorption. the wavelength A typical of modern interest, infrared and comparesanalyzer it to the reference Figure 4: Relationship of dew point to the carbon potential. These valuesconditionThe are infrared ofis no shown gas absorption. analyzer in Figure operates A6. typical by m measuringodern infrared the absorptianalyzeron is ofshown infrared in Figure light through . the gas correct with a 20 percent CO concentration. Thesample. infrared The gasgasesThe analyzerrelationship CO, CO operates 2between and CH bythe4 absorbmeasuringcarbon potential light the at andabsorptivery carbon specificon dioxide of wavelengths.infrared light Athrough detector the gas Thesample.measures relationship The the gases energy between CO, present CO the2 and carbonat isthe CHshown wavelength 4potential absorb in Figure light and of7. Again,carbonatinterest, very it dioxidespecificis and important compares is wavelengths. shown it into Figurethe A referen detector . Again,ce it is important to reiterate, that these graphs are only valid with 20 percent CO in the atmosphere. measurescondition theof noenergy absorption. present A at typical tothe reiterate, wavelength modern that theseinfrared of interest, graphs analyzer are and only compares isvalid shown itin to Figure the referen . ce Usecondition of other of valuesno absorption. of CO in A the typicalwith furnace 20 m percentodern or generator COinfrared in the atmosphe analyzeratmosphere. reis willUseshown produce in Figure erroneous . results. OneThe of relationship the most recently between developed the carbonof other methods potential values offor COand controlling in carbon the furnace dioxide furnace or gen is atmospheres- shown in Figure is the . Again,oxygen it orTheis carbonimportant relationship probe. to reiterate, Inbetween this method, that the thesecarbonerator the graphscarbon potentialatmosphere are potential onlyand will carbon valid produceis related with dioxide erroneous 20to thepercent is shown oxygen CO in partialin Figure the atmosphere.pressure . Again, it associatedisUse important of other with to values reiterate,the reversible of CO that in these reaction:theresults. furnace graphs One or areof generatorthe only most valid recently atmosphe with developed 20re percent will produce CO in theerroneous atmosphere. results. UseOne of of other the most values recently of CO developed in themethods furnace methods for or controllinggenerator for controlling atmosphe furnace furnace atmore will- atmospheres produce erroneous is the oxygen results. Oneor carbon of the probe.most recently In this method, developedspheres the methodscarbon is the oxygen potential for controlling or carbon is related probe. furnace Into thisthe atmospheres oxygen partial is thepressure oxygen or carbon probe. In this method,method, the carbon the carbon potential potential is related is related to the to oxygen partial pressure associated with the reversible reaction:the oxygen partial pressure associated with associated with the reversible reaction: the reversible reaction: 1 𝐶𝐶𝐶𝐶 ⇔ 𝑂𝑂' + 𝐶𝐶𝑂𝑂' 2 67 𝑝𝑝 23 1 9 𝐾𝐾 = ' ' ' 𝐶𝐶𝐶𝐶 ⇔1 𝑂𝑂 + 𝐶𝐶8 𝑂𝑂 [INSERT FIGURE 6] 2𝑝𝑝7'8 𝑝𝑝67 ' 𝐶𝐶𝐶𝐶 ⇔ 𝑂𝑂 + 𝐶𝐶𝑂𝑂 2 𝑝𝑝67 [INSERT FIGURE 7] 𝐾𝐾23 = 9 67 23 9'𝑝𝑝 𝐾𝐾 = 8 [INSERT FIGURE 6] 𝑝𝑝'78 𝑝𝑝67 [INSERT FIGURE 6] 𝑝𝑝78 𝑝𝑝678 [INSERT FIGURE 7] [INSERT FIGURE 7]

1 %𝐶𝐶 ≈ 78 <𝑝𝑝 Figure 5: Classic ALNOR Dewpointer used to control carbon potential by the use of water vapor. In other words, the carbon potential of

In other words, the carbon potentialthe atmosphere of the atmosphere is inversely1 is relatedinversely to the related to the square root of square root of the partial pressure of oxygen. the partial pressure of oxygen. This again assumes%𝐶𝐶 ≈ that1 7 8the CO content of the atmosphere is This again assumes%𝐶𝐶 ≈ that<𝑝𝑝 the CO content of constant, and at approximately 20 percent (23 percent𝑝𝑝7 for8 propane generated atmospheres). In other words, the carbon potentialthe atmosphere of the atmosphere is constant< isand inversely at approxi related- to the square root of AnInthe otheroxygen partial words, probepressure the or carbonofcarbon oxygen. potentialprobe matelyThis consists ofagain 20 the percent ofassumesatmosphere platinum (23 percent that electrodesis the inversely for CO propane- content separated related of theto by the atmospherean square yttrium root dopedis of zirconia tube. The probe is insertedgenerated into theatmospheres). furnace or generator. An air supply of approximately tconstant,he partial and pressure at approximately of oxygen. This 20 percent again assumes (23 percent that forthe propane CO content generated of the atmospheres).atmosphere is constant, and at approximately 20 Anpercent oxygen (23 probe percent or carbon for propane probe con generated- atmospheres). An oxygen probe or carbon probesists consists of platinum of platinum electrodes electrodes separated byseparated by an yttrium doped Anzirconia oxygen tube. probe The or probe carbon is insprobeertedan yttrium-dopedconsists into the of furnace platinum zirconia or tube.electrodesgenerator. The probe separatedAn air supply by a nof yttrium approximately doped zirconia tube. The probe is insertedis inserted into theinto furnacethe furnace or orgenerator. generator. An An air supply of approximately air supply of approximately 0.5 CFH of air is supplied to the probe as a reference. The dif- ferential oxygen partial pressure between the furnace atmosphere and the reference atmosphere sets up a voltage across the probe. By measuring this small voltage (measured in millivolts), the carbon poten- Figure 6: Modern multi-gas infrared analyzer. (Image of Ultramat 23 courtesy: Siemens Corporation.) tial can be determined. Thus, the furnace

28 gearsolutions.com 3.0 The advantage of an oxygen probe, is that it is accurate and fast. A direct read-out of the carbon potential of the atmosphere is common. 1500 2.5 The probe has a high temperature range suitable for high temperature carburizing. Little maintenance is required. The primary disadvantage of the oxygen probe is that it assumes 2.0 a fixed CO content (typically either 20 percent for natural gas or 23 percent for propane). If the CO content is not at this fixed value, then 2 1.5 the readings obtained by the probe are erroneous. The oxygen probe % CO 1600 is also a ceramic tube and prone to thermal or mechanical shock. It must be routinely replaced at roughly yearly intervals, depending on 1.0 the application. Heavy carburizing can shorten the life of the probe.

1700 However, the accuracy, ease of use and lack of maintenance, generally 0.5 outweigh the disadvantages. 1800 1900 CONCLUSION 0.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 In this article, we have discussed the production of endothermic Carbon % atmospheres used in the hardening of steel. We also discussed the control of the carbon potential of the associated furnace or generated Figure 7: Relationship between CO and carbon potential. 2 atmosphere. Probably the most important takeaway from this discus- sion, is that the CO content of the furnace must be stable at a value of atmosphere carbon potential can be controlled by air and natural 20 percent (or 23 percent for propane generators) for accurate carbon gas additions by monitoring the voltage potential across the probe. control of the endothermic atmosphere. The use of a portable infra- In modern instruments, this is done internally using microproces- red gas analyzer can make troubleshooting of atmosphere problems sors. In many ways, it operates in a similar fashion to the O2 sensor much simpler and more accurate. Should you have any questions in a car for proper combustion. regarding this article, please contact the writer.

ABOUT THE AUTHOR D. Scott MacKenzie, Ph.D., FASM, is senior research scientist-metallurgy at Houghton International Inc. For more information, go to www.houghtonintl.com.

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