FEATURE | Industrial Gases/

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 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 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 CO A common constituent in generated atmospheres changed (by adding carbon, 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 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 (CH4), (C3H8) and butane available for use in (Table (C4H10). 1). In most instances, and case A constituent of many furnace atmospheres used to aid in heat 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 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 ) 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 . 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

• 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 , 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. It is depends to a great extent on the:

CO and water vapor (H2O) or to form only at lower temperatures (900-1350°F) CO2 and hydrogen (H2) depending on that CO will supply carbon (Eq. 3) in • Type and size of furnace the relative percentages of each in the the form of soot in the so-called carbon- • Presence or absence of doors and/or furnace atmosphere. reversal reaction. Soot causes most of the curtains maintenance-related issues with gas gen- • Environment (especially drafts)

(2) CO2 + H2 = CO + H2O erators and heat-treating furnaces. • Size, loading, orientation and nature of (Water-Gas Reaction) the work being processed

(3) 2CO = C + CO2 • Metallurgical process involved Water vapor and CO2 both appear in this equation, and we can use this fact to Reactions Involving Nitrogen In all cases, the manufacturer’s recom- control the carbon potential of a furnace Below about 1850°F, molecular nitrogen mendations should be followed for gas atmosphere. In simplest terms, dew-point (N2) will not react with the surface of steel introduction, purging and removal since analyzers look at the H2O/H2 ratio in the or stainless steel. However, atomic nitro- the original equipment manufacturer has water-gas reaction. Infrared analyzers and gen (N), which does not normally occur taken these factors into account during oxygen-probe devices look at the CO/CO2 in a furnace atmosphere unless it is pur- the design of the equipment. ratio in the water-gas reaction. posely introduced by the addition of am- National Fire Protection Association

Water vapor is a strongly decarburizing monia (NH3), will react by being absorbed (NFPA) Standard 86, “Standard for Indus- gas. Any constituent such as CO2 will have into the steel surface. trial Furnaces Using a Special Processing a tendency to form water vapor, therefore, Atmosphere,” applies to all furnaces, and

CO2 must also be closely controlled. In ad- Reactions Involving Hydrocarbons the procedures listed within this standard dition, to prevent decarburization by water Methane and other hydrocarbons (pro- must be followed. vapor, the CO and H2 must be present in pane and/or butane) are carburizing A “rule of thumb” to remember is that to amounts to satisfy the equilibrium condi- agents. At elevated furnace temperatures, purge air out of a furnace prior to introduc- tion at each temperature. methane (CH4) breaks down into carbon tion of a combustible furnace atmosphere Water vapor and CO2 oxidize and de- (C) and H2. The higher the furnace tem- requires a minimum of fi ve volume changes carburize steel. Hydrogen is formed when perature, the greater the tendency for CH4 of the chamber (Table 5). This is to ensure IndustrialHeating.com - October 2009 47 FEATURE | Industrial Gases/Combustion

that the oxygen content of the chamber is below 1% prior to the introduction of the atmosphere.

Important Cautions In order to interpret furnace-atmosphere data correctly it is im- portant to understand the whole picture, including knowing how the data was collected as well as understanding the exact furnace operating conditions at the time the data was collected (e.g., zone temperatures and gas fl ows, furnace pressure, exhauster settings, fan rotation and speed, etc.). Part two of this article will discuss atmosphere-control techniques. IH

References: 1. Herring, D. H., Understanding Furnace Atmospheres, Atmosphere Op- eration and Atmosphere Safety, Heat Treating Hints, Vol. 1 No. 7. 2. Mr. Thomas Philips, Air Products & Chemicals (www.airproducts. com), private correspondence.

For more information: Contact the author at The HERRING GROUP, Inc., P.O. Box 884, Elmhurst, IL 60126; tel: 630.834.3017; fax: 630.834.3117; e- mail: [email protected]; web: www.heat-treat-doctor.com

For over 50 years, we have provided Electrical Safety Equipment for Industry. For complete info, contact us at [email protected]. Additional related information may be found by searching for Visit our website at www.protectioncontrolsinc.com. these (and other) key words/terms via BNP Media SEARCH at www. industrialheating.com: oxidation, carburization, endothermic, exothermic, nitrogen/methanol. hydrocarbon, chemical equilib- rium, dew point, NFPA 86

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