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Considerations in Heat Treatment Part One: Furnace Atmospheres Daniel H FEATURE | Industrial Gases/Combustion Considerations in Heat Treatment Part One: Furnace Atmospheres Daniel H. Herring – The HERRING GROUP, Inc., Kromschröder BIC Elmhurst, Ill. Burner - Courtesy of Hauck Manufacturing Company A key heat-treat consideration is the creation of an atmosphere within the furnace that is neutral to the parts being processed. This can be done a number of ways and depends on the temperature of the process and the carbon content of the parts. critical consideration in main components of an endothermic gen- Endothermic gas – also called endo or hheat treatment is the type, erator (Fig. 1) consist of: Rx™ gas – is produced when a mixture of consistency and control of air and fuel is introduced into an exter- A the furnace atmosphere. • Heated reaction retort with catalyst nally heated retort at such a low air-to-gas The purpose of a furnace atmosphere var- • Air-gas proportioning control compo- ratio that it will normally not burn. The ies with the desired end result of the heat- nents retort contains an active catalyst, which treating process. The atmospheres used in • Pump to pass the air-gas mixture is needed for cracking the mixture. Leav- the heat-treat industry have one of two through the retort ing the retort, the gas is cooled rapidly to common purposes: • Cooler to “freeze” the reaction and pre- avoid carbon reformation (in the form of vent soot formation soot) before it is sent into the furnace. • To protect the components being pro- Table 1. Common types of furnace atmospheres cessed from harmful chemical reactions Type Symbol Remarks that could occur on their surfaces (such Air Typically used in tempering operations as oxidation or carburization) – that is, to be passive (chemically inert) to the Argon Ar An inert gas metal surface. Carbon Dioxide CO2 A common constituent in generated atmospheres • To allow the surface of the parts to be Carbon Monoxide CO A common constituent in generated atmospheres changed (by adding carbon, nitrogen or Examples include alcohols and combinations of nitrogen and Custom Blends both) – that is, to be reactive (chemi- hydrocarbon gases cally active) to the metal surface. Generated atmospheres Endothermic, exothermic, dissociated ammonia Helium He An inert gas Types of Furnace Atmospheres Typically used as additions or enriching gases to furnace atmospheres. Many types of furnace atmospheres are Hydrocarbon Gases Common types include methane (CH4), propane (C3H8) and butane available for use in heat treating (Table (C4H10). 1). In most instances, hardening and case A constituent of many furnace atmospheres used to aid in heat Hydrogen H hardening operations use endothermic 2 transfer and react with oxygen present. gas or nitrogen/methanol systems. Most Nitrogen N2 A blanketing gas that is not truly inert tempering operations are performed in air Oxygen O Oxidizing to a hot steel surface atmosphere as long as the presence of a 2 tightly adherent oxide surface (“skin”) will Produced from a mixture of a hydrocarbon fuel gas and air, the Products of combustion atmosphere typically consists of high amounts of carbon dioxide and not affect the part’s performance. Other- water vapor. wise, an inert gas (vacuum) is selected. Steam H20 Water vapor is most often used to impart a protective oxide layer. Sulfur Dioxide SO Used in the heat treatment of magnesium alloys. Endothermic Gas Atmospheres 2 Endothermic gas generators are common Synthetic atmospheres Nitrogen and methanol (methyl alcohol) equipment in the heat-treat shop. The Vacuum The absence of an atmosphere IndustrialHeating.com - October 2009 45 FEATURE | Industrial Gases/Combustion The endothermic-gas composition (Tables 2 & 3), by volume, varies depending on Air Burnoff Natural the type of hydrocarbon-gas feed stock. gas or Filter Endothermic gas is used for neutral propane Mixer pump hardening and as a carrier gas for gas car- Carburetor burizing and carbonitriding. It is generally Back pressure regulator produced so that its composition is chemi- To furnace Nominal cally inert to the surface of the steel and composition Cooler can be made chemically active by the ad- 40% H2 dition of enrichment (hydrocarbon) gas, 20% CO/CO2 Insulated 40% N2 which is usually done at the furnace. reaction chamber Nitrogen/Methanol or Nitrogen/ Retort Hydrogen Atmospheres External heat supplied by An endothermic equivalent gas atmo- Catalyst gas or electric sphere can be obtained by cracking liquid methanol (methyl alcohol) and combin- ing it with nitrogen (Eq. 1), using a blend of 40% nitrogen and 60% methanol (dis- sociated). Fig. 1. Endothermic gas generator schematic piping arrangement → (1) CH3OH + N2 CO + H2 + N2 Table 2. Compositional ranges for endothermic gas This chemical reaction typically takes Gas constituent Percentage (based on natural gas) Percentage (based on propane) place inside the furnace as the liquid N2 40.9 % 40.9% methanol and gaseous nitrogen are me- CO 19.6 % 23.3% tered in through a special injector called CO 0.4 % 0.1% a sparger, which atomizes the liquid and 2 sprays it into the chamber, usually onto H2 38.9 % 35.5% a hot target such as the furnace fan. The CH4 0.2 % 0.2% equivalent of 4 KW of heat is required per Dew point +20/+50ºF -10/-15ºF gallon to crack the methanol. One gallon (Air/Gas) Ratio 2.6:1 7.8:1 per hour (3,785 ml/hour) of methanol liq- uid produces 241 cfh (6.8 m3/hour) of dis- [1] sociated methanol. Table 3. Nitrogen/methanol atmosphere fi eld data For some neutral-hardening applica- % Flow data[2] % N % H % CO % CH Dew Point, °F (°C) tions, a gas is produced with a lower car- 2 2 2 CO 4 bon monoxide value than an endothermic Nitrogen/methanol with natural 37-46 38-42 0.4–1.1 11.8–14.1 6-11 +30 to +65 (0 to +17) equivalent atmosphere (Table 3). gas and/or air enrichment The most common problems with ni- Notes: A 2,000 lb/hour (900 kg/hour), 48-inch-wide (1.2-m) electrically heated mesh-belt conveyor furnace trogen/methanol systems have to do with operating at carbon potential settings between 0.20-0.45%C. Approximate gas fl ows: 600-800 cfh (17-23 m3/hour) the failure to properly atomize. Large drop- nitrogen, 190 cfh methanol (3 l/hour), 200-300 cfh natural gas (6-9 m3/hour), 40-50 cfh (1.0-1.5 m3/hour) air. lets do not properly decompose, resulting in diffi culties in furnace control. Also, Table 4. Comparison of synthetic furnace atmospheres methanol is corrosive to nickel alloys used Atmosphere Type %H2 %N2 %CO Dew Point, °F (°C) for the internal furnace components (e.g., Hydrogen Pure 100 0 0 -95 to -120 (-70 to -85) fans, radiant tubes, belts, etc.). Other types of blended atmospheres Dissociated Ammonia (DA) Generated 75 25 0 - 40 to -50 (-40 to -75) (Table 4) produced with nitrogen and/or Nitrogen-DA Blended 90 10 0 > -50 hydrogen are less common but have been Endothermic Generated 40 40 20 +40 to -10 (3 to -23) used in some applications. The resultant Nitrogen-Endo Blended 12 82 6 < 0 atmosphere may not contain carbon diox- Nitrogen-Hydrogen Blended 3–75 97–25 0 -60 (-51) ide (CO2) or carbon monoxide (CO). 46 October 2009 - IndustrialHeating.com Gas Reactions water vapor oxidizes iron. Therefore, to Table 5. Volume changes required for The gas reactions involved can be classi- prevent oxidation and to keep iron bright, safe purging of furnaces fi ed into four general categories: a defi nite excess of H2 over H2O vapor is Number of Percentage (%) required for each temperature. volume changes of air remaining • Oxidation reactions 0.1 90.48 • Reduction reactions Reactions Involving Carbon Dioxide 0.2 81.87 • Carburizing reactions CO2 is one of the reaction products when 0.3 74.08 • Decarburizing reactions a hydrocarbon fuel is burned in air. CO2 0.5 60.65 oxidizes iron at elevated temperatures. To 1.0 36.79 Reactions Involving Oxygen prevent oxidation, it is necessary to have 2.0 13.53 In the presence of oxygen, steel will oxi- an excess of CO. Therefore, to prevent 3.0 4.98 dize. This tendency increases in severity as oxidation, CO is a desirable constituent. 4.0 1.83 the temperature is raised. In addition, oxy- CO is not only oxidizing to steel but it 2 5.0 0.67 gen will decarburize steel. If steel is to be is extremely decarburizing. To prevent kept bright during heat treatment and free decarburization, CO2 must be controlled to break down. Because of this tendency, of decarburization, free oxygen (O2) in the very closely. The actual amount depends CH4 and other hydrocarbon gases are in- furnace atmosphere must be eliminated. upon the CO content, temperature and troduced into the furnace to help change the carbon content of the steel. the atmosphere from neutral to one with a Reactions Involving Water Vapor high carbon potential (the driving force of The water-gas reaction (Eq. 2) is the most Reactions Involving carbon into the surface of the steel). important furnace-atmosphere chemical Carbon Monoxide reaction. This equation involves the CO is a strong carburizing agent. The Atmosphere Volume Requirements major constituents of the gas atmosphere reversible reaction of CO to form carbon During operation, the volume of protec- as it controls the reactants formed on (C) and CO2 is of particular interest in a tive atmosphere required for safe use in a each side of the equation. The equal sign furnace atmosphere. CO has a high car- particular heat-treating furnace and the indicates chemical equilibrium – that is, bon potential and becomes increasingly ability to properly control that atmosphere the reaction can go either way, to form more stable at elevated temperatures.
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