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

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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 experiencesendothermic 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 Endothermithen 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 carrierareusing 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, Endothermithenand 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 carrierareNaturalformation shown gas Gas:of below:for endothermic atmosphere atmospheres additions for for the carburizing protection of or parts carbonitriding. Using an endothermic Methanol during austenitization. Carbon Monoxide CO 20% 23% 20% generator, 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% thentreatment methanol of steel. is It the is used carbon as a carrier source. gas The for atmosphere predominant additions reactions Carbon for Dioxideproducing endothermicCO2 gas0.30% 0.30% 0.30% Natural Gas: Water Vapor H2O < 0.1% < 0.1% < 0.1% arefor carburizingshown below: or carbonitriding. Using an endothermic generator, Methane CH < 0.1% - - $ ' ' 4 either natural gas or propane is used. If nitrogen-methanol is injected 2 + ↔ 2 + 4 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. 2Natural + Gas: ↔ 2 + 4 Propane: ENDOTHERMIC GENERATORS Propane:* +$ 1 ' ' ' ' The endothermic generator, regardless whether used with natural 2 ++ ↔↔23++44 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 Methanol:*+ + ' ↔ 3 + 4' 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*+ ↔' 2 + 4 ' All +of these methods↔ 3 produce+ 4 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-methanol ↔ 2 atmospheres.+ 4 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- * ' 2 ↔ 2 + 4 All of these methods produce an atmosphere containing CO and H2. Nitrogen appears because it is either from the atmosphere in
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