Handbook Heat Treating (1991) WW Part 9 potx

Fe + CO 2 € FeO + CO (Eq 15) water vapor and carbon dioxide are oxidizing gases, and hydrogen and carbon monoxide are reducing gases. Ultimately, the quantity of reducing gas or oxidizing gas formed may become great enough for one to cancel the effects of the other. By proper control of these reactions, a neutral, reducing, or oxidizing effect may be produced. The opposing reactions may be controlled according to the water gas reaction, which is as follows: CO + H 2 O € CO 2 + H 2 (Eq 16) The gases that enter into the water gas reaction react with the surface of steel to cause oxidation or reduction, depending on the equilibrium condition corresponding to the temperature and composition of the system. At 830 °C (1525 °F), the oxidizing potentials of carbon dioxide and water vapor are equal, and the reducing potentials of carbon monoxide and hydrogen are equal. At this temperature, therefore, the equilibrium constant of the water gas reaction has a value of unity. Above 830 °C (1525 °F), carbon dioxide is a stronger oxidizing agent than water vapor, and hydrogen is a stronger reducing agent than carbon monoxide. Below 830 °C (1525 °F), the reverse is true. Consider the reactions: C + CO 2 → 2CO (Eq 17) CO + H 2 O € CO 2 + H 2 (Eq 18) C + H 2 O € CO + H 2 (Eq 19) and their equilibrium constants, which are, respectively: 2 1 [] []² CO K CO = (Eq 20) 2 2 22 [][] [][] COHO K COH = (Eq 21) 2 3 2 [] [][] HO K COH = (Eq 22) Calculations for the data in Table 2 were developed, assuming that the hydrogen content of the system remains constant at 40% and the CO + CO 2 content remains constant at 20%. Figure 4 shows the equilibrium conditions for steels with carbon concentrations from 0.10 to 1.20%. Table 2 Variation of equilibrium constants, composition, and dew point of an H 2 -H 2 O-CO-CO 2 system The hydrogen content is assumed to remain constant at 40% and the CO + CO 2 content is assumed to remain constant at 20%. Temperature Equilibrium constants Composition, % Dew point °C °F K 1 K 2 K 3 CO 2 H 2 O °C °F 650 1200 3.770 0.51 1.922 4.5 4.5 +32 +90 705 1300 0.942 0.66 0.695 2.6 3.4 +24 +75 760 1400 0.348 0.83 0.363 1.2 2.0 +18 +65 815 1500 0.125 1.02 0.127 0.5 1.02 +7 +45 870 1600 0.050 1.22 0.061 0.2 0.49 -3 +27 925 1700 0.022 1.44 0.003 0.1 0.25 -12 +10 Fig. 4 Dew point for equilibrium conditions with carbon steel of various carbon concentrations Ammonia Vapor. Ammonia dissociates in the endothermic reaction: 2NH 3 → N 2 + 3H 2 (Eq 23) with the dissociated ammonia containing 75% hydrogen and 25% nitrogen. Dissociation occurs when ammonia vapor is heated and passed over the proper catalyst. Dissociated ammonia then is cooled and often passed through a purifying molecular-sieve absorption system to remove undissociated ammonia and water vapor. Ammonia vapor also may be used as an additive to suitable carbon-bearing carrier atmospheres for carbonitriding, or it may be used directly for nitriding processes. In these instances, partial ammonia dissociation occurs inside the furnace, allowing nitrogen to react with heated steel surfaces to form hard nitrides. Lithium Vapor. Lithium vapor reacts with water vapor in the furnace atmosphere to form lithium oxide and hydrogen: 2Li + H 2 O → Li 2 O + H 2 (Eq 24) Lithium vapor also combines with any free oxygen present in the furnace atmosphere to form lithium oxide: 4Li + O 2 → 2Li 2 O (Eq 25) The lithium oxide formed by these reactions oxidizes some of the carbon monoxide present in the atmosphere, causing the release of a certain amount of lithium: 2Li 2 O + CO → Li 2 CO 3 + 2Li (Eq 26) The lithium liberated can then pick up more oxygen. The practical application of this vapor as a furnace atmosphere has been in forging furnaces as a protection against scaling of the surface of steel heated to forging temperature. Sulfurous Gases. Sulfurous gases in a furnace atmosphere are basically deleterious and are to be avoided. These gases result from the presence of sulfur compounds in industrial fuels, furnace refractories, and cutting oils on work being processed. Sulfur compounds occur as hydrogen sulfide (H 2 S); sulfur dioxide (SO 2 ), or sulfur trioxide (SO 3 ); mercaptans; thiophene (C 4 H 4 S); and metallic sulfates. When sulfur is present in reducing atmospheres, it is generally found as hydrogen sulfide from the following reaction: SO 2 + 3H 2 € H 2 S + 2H 2 O (Eq 27) C 4 H 4 S + 6O 2 → 4CO 2 + 2H 2 O + SO 2 (Eq 28) 2SO 2 + 3Fe + 2Ni → Fe 2 O 3 + NiS + NiO + FeS (Eq 29) When sulfur is present in atmospheres in which high-nickel steels are heated, both nickel sulfide and nickel oxide are formed, and an alligator surface results. In addition to damaging parts being treated, this effect accelerates the failure of high-nickel, high-chromium, heat-resistant alloy furnace parts, trays, and fixtures. In general, the presence of sulfur in a furnace atmosphere accelerates the rate of scaling, and this rate increases as temperature increases. Classifications of Prepared Atmospheres Most prepared atmospheres are commonly referred to in the field by their generic names, or, in some instances, by trade names. The American Gas Association has classified the commercially important prepared atmospheres into six groups on the basis of method of preparation or on the original constituents employed. These groups are designated and defined as follows: • Class 100, exothermic base: Formed by partial or complete combustion of a gas- air mixture; water vapor may be removed to produce a desired dew point • Class 200, prepared nitrogen base: An exothermic base with carbon dioxide and water vapor removed • Class 300, endothermic base: Formed by partial reaction of a mixture of fuel gas and air in an externally heated catalyst-filled chamber • Class 400, charcoal base: Formed by passing air through a bed of incandescent charcoal • Class 500, exothermic-endothermic base: Formed by complete combustion of a mixture of fuel gas and air, removing water vapor, and re- forming the carbon dioxide to carbon monoxide by means of reaction with fuel gas in an externally heated catalyst-filled chamber • Class 600, ammonia base: This can consist of raw ammonia, dissociated ammonia, or partially or completely combusted dissociated ammonia with dew point regulated These broad areas of classification are subclassified and numerically designated to indicate variations in the method by which they are prepared. This subclassification is differentiated by replacing the two zeros of the six basic designators by one of the following two-digit numbers that indicate some special preparation of the furnace atmosphere: • 01: Use of a lean air-gas mixture • 02: Use of a rich air-gas mixture • 03 and 04: Preparation of the gas was completed within the furnace itself without the use of a separate machine or generator • 05 and 06: Original base gas was subsequ ently passed through incandescent charcoal before admission to the work chamber • 07 and 08: Addition of a raw hydrocarbon fuel gas to the base gas before admission to the work chamber • 09 and 10: Addition of a raw hydrocarbon fuel gas and raw dry anhydrous ammonia to the base gas before admission to the work chamber • 11 and 12: Addition of a combusted mixture of chlorine, hydrocarbon fuel gas, and air to the base gas before admission to the work chamber • 13 and 14: Base gas has had all sulfur or all sulfur and odors removed before admission to the work chamber • 15, 16, 17, and 18: Addition of lithium vapor to the base gas before admission to the work chamber • 19 and 20: Preparation of the gas was completed within the furnace itself with the addition of lithi um vapor • 21 and 22: Some additional special treatment was given to the base gas before admission to the work chamber • 23 and 24: Addition of steam and air in conjunction with a catalyst within the generator to convert CO to CO 2 , which is then removed • 25 and 26: Addition of steam in conjunction with a catalyst within the generator to convert CH 4 to H 2 and CO 2 , which is then removed This classification system provides for a large number of possibilities. In practice, only a few of the possible atmosphere classifications are industrially important. Table 3 lists significant furnace atmospheres and typical applications. Table 3 Classification and application of principal furnace atmospheres Nominal composition, vol% Class Description Common application N 2 CO CO 2 H 2 CH 4 101 Lean exothermic Oxide coating of steel 86.8 1.5 10.5 1.2 . . . 102 Rich exothermic Bright annealing; copper brazing; sintering 71.5 10.5 5.0 12.5 0.5 201 Lean prepared nitrogen Neutral heating 97.1 1.7 . . . 1.2 . . . 202 Rich prepared nitrogen Annealing, brazing stainless steel 75.3 11.0 . . . 13.2 0.5 301 Lean endothermic Clean hardening 45.1 19.6 0.4 34.6 0.3 302 Rich endothermic Gas carburizing 39.8 20.7 . . . 38.7 0.8 402 Charcoal Carburizing 64.1 34.7 . . . 1.2 . . . 501 Lean exothermic-endothermic Clean hardening 63.0 17.0 . . . 20.0 . . . 502 Rich exothermic-endothermic Gas carburizing 60.0 19.0 . . . 21.0 . . . 601 Dissociated ammonia Brazing, sintering 25.0 . . . . . . 75.0 . . . 621 Lean combusted ammonia Neutral heating 99.0 . . . . . . 1.0 . . . 622 Rich combusted ammonia Sintering stainless powders 80.0 . . . . . . 20.0 . . . Furnace Atmosphere Hazards Furnace atmospheres constitute one of the major safety hazards involved in heat treating. Generally, these hazards fall into three groups: fire, explosion, and toxicity. Fire. When an atmosphere contains more than 4% of combustible gases, it is classified as flammable. Included in this percentage is a practical safety margin that should never be ignored. Caution: The combustible gases H 2 , CO, CH 4 , and other hydrocarbon fuel gases should never be admitted to a furnace chamber at temperatures below 760 °C (1400 °F) without proper inert gas purging (per NFPA standard 86C). Explosion. At some point, mixtures of air and combustible gas will explode when ignited. When a furnace chamber is properly gassed with the chamber temperature at or above 760 °C (1400 °F), it is likely that combustible gases will burn before creating an explosion hazard. An adjacent cold chamber or vestibule can then be flared as the atmosphere flows from the furnace to the vestibule until it is free of oxygen from the air. The vestibule can then be closed. The positive flow of atmosphere through the furnace and adjoining cold chamber or vestibule can then be burned. An ignited effluent from an atmosphere furnace is an immediate visual sign that a safe condition prevails. Toxicity. Caution: Many of the gases making up furnace atmospheres are toxic. Burning them at the furnace exits reduces their chemistry to the products of combustion. These products should then be vented outside the building to avoid dilution of the available oxygen supply within the building. Caution: Ventilation of the building containing atmosphere generators and atmosphere heat-treating furnaces is a major safety consideration. Furnace Atmosphere Hazards (Ref 1, 2, 3, 4, 5, 6, 7). Furnace atmospheres (endo, exo, dissociated ammonia, dissociated alcohols, and nitrogen based) normally consist of a mixture of gases, which may be flammable, toxic, asphyxiant, or a combination of these. Explosion, fire, and poisoning are potential hazards. National Fire Protection Association standard 86C covers various safety considerations pertaining to continuous conveyor belt furnaces used in the thermal processing industry. Four intrinsic dangers are associated with producing and using the common atmosphere gases. A mixture of atmosphere gas and air can accumulate in a confined area and explode. Relatively small quantities of atmosphere gas can unexpectedly burn or flash out of control. Personnel may be poisoned by carbon monoxide, ammonia, or methanol. Simple asphyxiation is possible when asphyxiants are present in high concentrations. Table 4 describes the characteristics of the common sintering atmosphere constituent gases. Most of the major constituents are flammable; four are toxic; and four are simple asphyxiants. Carbon monoxide, ammonia, and methanol are both flammable and toxic. The percentage by volume of the flammable ingredients ranges from 75% hydrogen in dissociated ammonia atmosphere to only a few percent in purified exothermic and nitrogen-based atmospheres. Table 4 Potential hazards and functions of heat treating atmosphere-constituent gases Potential hazard Gas Flammable Toxic Simple asphyxiant Atmosphere function Nitrogen . . . . . . Yes Inert Hydrogen Yes . . . Yes Strongly reducing Carbon monoxide Yes Yes . . . Carburizing and mildly reducing Carbon dioxide . . . Yes Yes Oxidizing and decarburizing Natural gas Yes . . . Yes Strongly carburizing and deoxidizing Ammonia Yes Yes . . . Strongly nitriding Methanol Yes Yes . . . Carbon monoxide and hydrogen generating Dangers are present even when heat treating with atmospheres that contain relatively small percentages of hazardous ingredients because the gases may accumulate or concentrate. Under typical operating conditions, however, the active ingredients in nitrogen systems are diluted below the level that is flammable, even if all the nitrogen is replaced by air. Explosion, burning, poisoning, and asphyxiation hazards are thus reduced, but not eliminated. The explosive ranges of typical atmosphere constituents are: Atmosphere constituents Concentration in air, % Hydrogen 4.0-74 Carbon monoxide 12.5-74 Methane 5.3-14 Ammonia 15.0-28 Methanol 6.7-36 Any mixtures between the high and low limits of flammability will burn when ignited, and, under certain conditions, detonate or explode. The destructive power of the pressure wave from an ignited flammable mixture depends on the amount of gas and the heat of combustion of the fuel gas, the combustion mode (deflagration or detonation), and the configuration of the confinement space. The energy released is either absorbed by the surroundings or destroys them. The explosive potential of the large volumes of flammable gases used in sintering furnaces poses an important safety consideration. To ensure the safety of sintering and heat treating, the equipment and systems that handle gases must be designed, operated, and maintained to prevent accumulation of explosive mixtures. Dangerous accumulations that cannot be readily detected by personnel can occur. Properly designed safety systems, maintained and operated by well-trained and competent personnel, substantially reduce explosion hazards. Generally, sufficient volumes of atmosphere gas flowing through the furnace leave the charge and discharge doors, mix with air, and burn uniformly and completely. However, several conditions may exist that prevent the atmosphere gas from burning off properly. Sometimes, a combustible mixture of air and gas may form in the furnace throats or vestibules. It may burn rapidly and forcefully exhaust, or flash, flames and hot gases through the furnace door. Unprotected personnel in the vicinity may be burned. Eyes are particularly sensitive to damage. Although flashes of this type are more common than explosions, they are potentially less destructive. Nonetheless, personnel who approach these areas without safety glasses, protective face shields, gloves, and flameproof clothing risk serious flash burns. If safe methods are established and used for cleaning jam-ups, inspecting the furnace interior, and removing products from the furnace, this hazard is substantially reduced. Protective guards and shields used for routine operation are no substitute for the required protection of personnel working in the vicinity of the furnace doors. However, the possibility of flashback under nonflammable, nitrogen-rich atmospheres is remote. Ammonia, carbon monoxide, and methanol, which are highly toxic, are used routinely in the sintering and heat-treating industries. Liquid ammonia produces dissociated ammonia or ammonia vapor for nitriding, and carbon monoxide is a constituent of exothermic, endothermic, and dissociated methanol gases. Methanol is used to produce dissociated methanol, which consists of hydrogen and carbon monoxide. Concentrations of less than 1 2 % ammonia or carbon monoxide in air are considered fatal in less than 1 2 h of exposure. The physiological effects of various concentrations of these gases are given in Tables 5 and 6. Table 5 Physiological effects of ammonia Concentration, Physiological effects ppm 20 First perceptible odor 40 Slight eye irritation in a few individuals 100 Noticeable irritation of eyes and nasal passages after a few minutes of exposure 400 Severe irritation of the throat, nasal passages, and upper respiratory tract 700 Severe eye irritation; no permanent effect if the exposure is limited to less than 1 2 h 1700 Serious coughing, bronchial spasms; less than 1 2 hr of exposure may be fatal 5000 Serious edema, strangulation, asphyxia; almost immediately fatal Table 6 Physiological effects of carbon monoxide Concentration, ppm Physiological effects 100 Allowable for an exposure of several hours 400 Can be inhaled for 1 h without appreciable effect 600 Causes a barely appreciable effect after 1 h of exposure 1000 Causes unpleasant symptoms, but not dangerous after 1 h 1500 Dangerous for exposure of 1 h 4000 Fatal for exposure of less than 1 h Threshold limit values, published by the American Conference of Governmental and Industrial Hygienists, lists carbon monoxide, carbon dioxide, ammonia, and methanol as commonly used toxic chemicals. Concentrations as small as 35 ppm of ammonia, 400 ppm of carbon monoxide, and 250 ppm of methanol are harmful; therefore, only short exposures are allowed. Protection from poisoning by raw ammonia or methanol vapor or from contact with liquid ammonia or methanol is provided partly by the design of the storage and delivery system. Suppliers are excellent sources of detailed safety information on ammonia and methanol systems and ammonia dissociators. Carbon monoxide is not quite as toxic as ammonia; however, because is is odorless, it poses a greater safety hazard. Lethal concentrations can collect in isolated areas and remain undiscovered until personnel are overcome by the fumes. According to Ref 2, ". . . in concentrations far short of the asphyxiation level, carbon monoxide is still dangerous, especially if exposure is prolonged. Carbon monoxide (poisoning) can be a contributing factor to death that ultimately results from other causes, such as fatal accidents, and is a health as well as safety problem." Carbon monoxide interferes with the ability to breathe, and, as a result, the ability of the body to utilize oxygen. It is over 200 times more attractive than oxygen to the blood hemoglobin that delivers oxygen throughout the body. A relatively small amount of carbon monoxide, therefore, depletes the body of a large amount of oxygen. To ensure personnel safety, piping between the generator and the furnace must be leaktight, and all gas that enters the furnace must be either burned off or properly vented. The highly toxic nature of carbon monoxide requires use under carefully controlled conditions. The safest and most convenient disposition of carbon monoxide is to ensure complete mixing of the furnace atmosphere effluent with air to combustible proportions and subsequent ignition of the mixture. Products of combustion are far less toxic but should still be properly vented. In some applications, carbon monoxide cannot be burned directly because it has been diluted in inert gas to a level that is nonflammable when mixed with air. Special precautions must be taken to vent and dilute it to below toxic limits. To ensure the continued effectiveness of the venting, carbon monoxide levels near the furnace setup must be monitored continuously. Consequently, apparatus using or emitting gas containing carbon monoxide that will not burn when mixed with air should not be operated unless the user is willing to accept the added liability. Additionally, special gas- monitoring and disbursing equipment should be installed. Personnel should also be familiar with the early symptoms of carbon monoxide poisoning and trained in appropriate first aid. Early symptoms include slight dizziness, weakness, or headache. In the later stages of poisoning, the victim's lips and skin turn a characteristic cherry red. Finally, the danger of asphyxiation is not intrinsic to the gases used in the heat treating industry, but is a concern. Asphyxiation can be caused by a harmless gas (for example, one that has no significant physiological effect) if it is present in high concentrations in the air. Normal oxygen content in air is about 21 vol%. Minimum oxygen content should be 18 vol% under normal atmospheric pressure. References cited in this section 1. W. McKinley and H.S. Nayar, Safety Considerations in Sintering Atmospheres, Prog. Powder Metall., Vol 35, 1979 2. J.T. Holtzberg, Requirements for Monitoring Carbon Monoxide, Ind. Heat., March 1980 3. "Industrial Furnaces Using a Special Processing Atmosphere," ANSI/NFPA 86C, National Fire Protection Association, Dec 1983 4. "Threshold Limit Values of Substances in Workroom Air," Paper presented at the American Conference of Governmental and Industrial Hygienists, 1979 5. "Safety and Health Standard 29CFR 1910," Occupational Safety and Health Administration, June 1981 6. F.T. Bodurtha, Industrial Explosion, Prevention and Protection, McGraw-Hill, 1980 7. Handbook of Industrial Loss Prevention, Factory Mutual Engineering Company, McGraw-Hill, 1967 Exothermic-Based Atmospheres Exothermic gases (class 100) have been used extensively for many years as lower-cost prepared furnace atmospheres. Exothermic atmospheres are divided into two basic classes: rich and lean. Rich exothermic atmospheres (class 102) have moderate reducing capabilities of 10 to 21% combined carbon monoxide and hydrogen, and lean exothermic atmospheres (class 101), usually with 1 to 4% combined carbon monoxide and hydrogen, have minimal reducing qualities. Rich Exothermic Atmospheres The principal uses of rich exothermic furnace atmospheres include clean heat treating of certain ferrous and nonferrous applications. Among these are annealing and tempering of steel, brazing of copper and silver, and sintering of powdered metals. Reducing properties of rich exothermic atmospheres may be varied to make them suitable for specific processes. Figure 5 indicates the usual operating range of the gas generator and reflects changes (by dry volumetric measurement) in the following constituents of the product gas at any particular setting: carbon dioxide, carbon monoxide, hydrogen, and unburned methane. The remainder of the mixture is nitrogen. Because these atmospheres have a carbon potential below 0.10%, steel heat treating is generally limited to processes for low-carbon steels to minimize decarburizing, or processes where decarburization is unimportant. Water vapor is present in substantial quantities and may be removed partially by initial cooling and refrigerant drying to an equivalent 5 °C (40 °F) dew point. This procedure may be followed by further dehydration with an adsorbent desiccant dryer for final few points of -40 to -50 °C (-40 to -60 °F), as applications require. Fig. 5 Exothermic atmosphere composition versus air-to-fuel ratio (natural gas) Gas Production. Rich exothermic gas is produced by combustion of a hydrocarbon fuel such as natural gas or propane with the air-to-fuel ratio closely controlled. This air-gas mixture is burned in a confined combustion space to maintain a reaction temperature of at least 980 °C (1800 °F) for sufficient time to permit the combustion reaction to reach equilibrium. Heat is obtained directly from combustion. The resultant gas is then cooled to remove part of the water vapor formed by burning and to permit convenient transportation and metering. In this process, the simplified theoretical reaction of methane with air is: CH 4 + 1.25O 2 + 4.75N 2 → 0.375CO 2 + 0.625CO + 0.88H 2 + 4.75N 2 + 1.12H 2 O + heat (Eq 30) where 1 volume of fuel and 6 volumes of air yield 6.63 volumes of product gas mixture, with water vapor removed. In practice, exothermic gas generators are seldom operated with an air-to-gas ratio lower than about 6.6 to 1, to prevent formation of soot as a result of incomplete reaction. Trace percentages of unreacted methane also exist in the product gas. [...]... Endothermic 40 39 19 2 0.05 0.1 N2-5% H2 95 5 0.001 N2-3% CH3OH 25 75 0.001 75 25 0.001 Dissociated ammonia + H2O 25 75 3 90 10 2 Exothermic 75 9 7 3 6 88 10 2 Endothermic 40 39 19 2 0.05 0.1 95 5 0.001 40 39 19 2 0.05 0.2 N2-endothermic 87 8 4 1 0.01 0.05 76 16 7 1 0.005 0.05 N2-8% H2-2% CH4 90 8 1 1 0.005 0.01 Dissociated ammonia 25 75 0.001 Endothermic 40 39 19 2 0.05... CH4 Nitrogenhydrogen 90 100 0-10 Nitrogenmethanol Protective H2 Nominal composition, % 91 100 0-6 03 Dissociated ammonia 75 Nitrogenhydrogen 60 -90 1040 Exothermic 70-80 10-16 8-11 Nitrogenhydrogen 95 5 Dissociated ammonia 25 75 Endothermic 40 40 20 Nitrogenhydrogen 95 5 Dissociated ammonia 25 75 Nitrogenmethanol 85 10 5 Hardening Endothermic 40 40 20 Nitrogenmethane 97 1 1 1 Carburizing... and carbon monoxide Many heattreating atmosphere applications can be converted from combustible to noncombustible compositions in conjunction with the switch to a commercial nitrogen-based system Because elemental components are available, commercial nitrogen-based heat- treating practice may start with a pure nitrogen purge to eliminate oxygen from the surface During the heat- treating cycle, a very reactive... steel sheet, tube, wire CO2 80 12 8 0.01 0.5 N2-5% H2 Carbon steel rod Exothermic-purified 95 5 0.001 Exothermic-purified 100 0.01 0.5 Exothermic-endothermic blend 75 15 8 2 0.01 0.5 N2-1% C3H8 97 1 1 1 0.001 0.01 N2-5% H2-3% CH4 90 7 2 1 0.001 0.01 N2-3% CH3OH 91 6 3 0.001 0.01 Copper wire, rod 3 11 99 1 0.001 Exothermic-lean 86 3 11 100 0.001 Dissociated ammonia 25 75 0.001... reaction to reach equilibrium The resultant gas is then cooled to partially condense water vapor formed by burning and to permit convenient transport and metering In this process, the simplified theoretical reaction of methane fuel with air is: CH4 + 1.9O2 + 7.6N2 + 0.1CO + 0.1H2 + 7.6N2 + 1.9H2O + heat → 0.9CO2 (Eq 31) where 1 volume of fuel and 9. 5 volumes of air yield 8.7 volumes of product gas mixture,... psig), the air-gas mixture is passed through a fire-check valve to the heated catalyst, which is contained in a pressuretight retort The retort is externally heated, usually by natural gas For a completely reacted gas of consistent analysis, the temperature inside the catalyst bed should be approximately 98 0 to 1040 °C (1800 to 190 0 °F) Furthermore, the ratio of the diameter of the retort to its length... chamber to be reactivated, the recirculated gas is heated to about 230 °C (450 °F) either electrically or by way of a gas-heated retort After having passed through the desiccant material in the chamber being reactivated, the water vapor that is picked up by the recirculated gas stream from the desiccant is partially condensed by way of water-cooled heat exchanger At this point in the closed circuit,... achieve proper part chemistry Table 9 summarizes the compositions of reactive atmosphere alternatives, both with respect to major constituents and to trace impurities Carbon-controlled atmospheres transfer carbon from the atmosphere to the surface of the metal part ( C ) through controlled gas-metal reactions The two most common reactions are: 2CO € C + CO2 CO + H2 € C + H2O (Eq 38) (Eq 39) Because carbon... as a pure, dry, inert gas that provides an efficient purging and blanketing function within the heat- treating furnace The nitrogen stream is often enriched with a reactive component, and the resulting composition and flow rate are determined by the specific furnace design, temperature, and material being heat treated Although there is similarity in the components of commercial nitrogen-based atmosphere... most common applications using a carbon-controlled atmosphere include carburizing and carbonitriding of machined parts, neutral hardening, electrical-lamination decarburization annealing, powder-metal sintering, and carbon restoration of hot-worked or forged materials Table 7 describes heat- treating applications that are presently using nitrogen-based atmosphere systems The major components of the generated . Health Standard 29CFR 191 0," Occupational Safety and Health Administration, June 198 1 6. F.T. Bodurtha, Industrial Explosion, Prevention and Protection, McGraw-Hill, 198 0 7. Handbook of. of methane fuel with air is: CH 4 + 1.9O 2 + 7.6N 2 → 0.9CO 2 + 0.1CO + 0.1H 2 + 7.6N 2 + 1.9H 2 O + heat (Eq 31) where 1 volume of fuel and 9. 5 volumes of air yield 8.7 volumes of. Sintering Atmospheres, Prog. Powder Metall., Vol 35, 197 9 2. J.T. Holtzberg, Requirements for Monitoring Carbon Monoxide, Ind. Heat. , March 198 0 3. "Industrial Furnaces Using a Special