Furnace Atmosphere Considerations for Fasteners

Transcription

1 132 Furnace Atmosphere Considerations for Fasteners by Richard D. Sisson, Jr. & Daniel H. Herring A critical consideration in the heat treatment of fasteners is the type, consistency, and control of the furnace atmosphere. The purpose of a furnace atmosphere varies with the desired end result of the heat treating process. The atmospheres used in the fastener industry have one of two common purposes: To protect the fasteners being processed from harmful chemical reactions that could occur on their surfaces (such as oxidation or carburization), that is, to be passive (or chemically inert) to the metal surface. To allow the surface of the fastener to be changed (by adding carbon, or nitrogen, or both), that is, to be reactive (or chemically active) to the metal surface. Types of Furnace Atmospheres Many types of furnaces atmospheres are available for use in heat treating (Table 1). Endothermic gas, Nitrogen/Methanol and Methanol (only) systems are the most common atmosphere types for hardening and case hardening operations used in the fastener industry. For tempering of fasteners, an air atmosphere is typically used if the presence of a tightly adherent oxide surface (or skin ) will not affect the fastener s performance; otherwise an inert gas (or vacuum) is selected. Table 1. Common Types of Furnace Atmospheres Type Symbol Remarks Air Typically used in tempering operations Argon Ar An inert gas Carbon Dioxide CO 2 A common constituent in generated atmospheres Carbon Monoxide CO A common constituent in generated atmospheres Custom Blends Examples include alcohols and combinations of nitrogen and hydrocarbon gases Generated Atmospheres Endothermic, Exothermic, Dissociated Ammonia Helium He An inert gas Hydrocarbon Gases Typically used as additions or enriching gases to furnace atmospheres; common types include methane (CH 4 ), propane (C 3 H 8 ), and butane C 4 H 10 ). A constituent of many furnace atmospheres used to aid in heat transfer and Hydrogen H 2 react with oxygen present. Nitrogen N 2 A blanketing gas that is not truly inert Oxygen O 2 Oxidizing to a hot steel surface Products of Combustion Produced from a mixture of a hydrocarbon fuel gas and air the atmosphere typically consists of high amounts of carbon dioxide and water vapor. Steam H 2 0 Water vapor. Most often used to impart an protective oxide layer. Sulfur Dioxide SO 2 Used in the heat treatment of magnesium alloys. Synthetic Atmospheres Nitrogen and Methanol (methyl alcohol) Vacuum The absence of an atmosphere

2 133 Endothermic Gas Atmospheres Endothermic gas generators are common equipment in the heat treat shop. The main components of an Endothermic Generator (Fig. 1) are relatively simple consisting of a: Heated reaction retort with catalyst Air-gas proportioning control components Pump to pass the air-gas mixture through the retort Cooler to freeze the reaction and prevent soot formation Endothermic gas (also called Endo or RxÔ gas) is produced when a mixture of air and fuel is introduced into an externally heated retort at such a low air-to-gas ratio, that it will normally not burn. The retort contains an active catalyst, which is needed for cracking the mixture. Upon leaving the retort, the gas is rapidly cooled to avoid carbon reformation (in the form of soot) before it is sent into the furnace. The Endothermic gas composition (Table Nos. 2 & 3), by volume, varies depending on the type of hydrocarbon gas feed stock. Table 2. Compositional Ranges for Endothermic Gas Gas Constituent Percentage (based on natural gas) (1) CH 3 OH + N 2 CO + H 2 + N Percentage (based on propane) N % 40.9% CO 19.6 % 23.3% CO % 0.1% H % 35.5% CH % 0.2% Dew point ºC (ºF) -7ºC / +10ºC -23ºC/-26ºC (+20ºF/+50ºF) (-10/-15ºF) (Air/Gas) Ratio 2.6:1 7.8:1 Nitrogen/Methanol or Nitrogen/Hydrogen Atmospheres This chemical reaction typically takes place inside the furnace as the liquid methanol and gaseous nitrogen are metered in through a special injector called a sparger, that atomizes the liquid and sprays it into the chamber, usually onto a hot target such as the furnace fan. The equivalent of 4 KW of heat are required per gallon to crack the methanol. One gallon per hour (3785 ml/h) of methanol liquid produces 6.8 m3/h (241 cfh) of dissociated methanol. For some neutral hardening fastener applications a gas is produced with a lower carbon monoxide value than an Endothermic equivalent atmosphere (Table 3). The most common problems with methanol or nitrogen/methanol systems have to do with difficulties in furnace control; the failure to properly atomize large droplets which do not properly decompose. Also, methanol is corrosive to nickel alloys used for the internal furnace components (e.g. fans, radiant tubes, belts, etc.) Endothermic gas is used for neutral hardening as well as a carrier gas for gas carburizing and carbonitriding. It is generally produced so that its composition is chemically inert to the surface of the steel and can be made chemically active by the addition of enrichment (hydrocarbon) gas that is usually done at the furnace. An Endothermic equivalent gas atmosphere can be obtained by cracking methanol (methyl alcohol), a liquid, and combining it with nitrogen (Equation 1), using a blend of 40% nitrogen and 60% methanol (dissociated). Figure 1. Endothermic Gas Generator Schematic Piping Arrangement

3 134 Other types of blended atmospheres (Table 4) produced with nitrogen and/or hydrogen are less common but have been used in some applications; the resultant atmosphere may not contain carbon dioxide or carbon monoxide. Gas Reactions The gas reactions involved can be classified into four general categories: Oxidation reactions Reduction reactions Carburizing reactions Decarburizing reactions Reactions Involving Oxygen In the presence of oxygen, steel will oxidize. This tendency increases in severity as the temperature is raised. In addition, oxygen will decarburize steel. If steel is to be kept bright during heat treatment and free of decarburization, free oxygen (O 2 ) in the furnace atmosphere must be eliminated. Reactions Involving Water Vapor The Water Gas Reaction (Equation 2) is the most important furnace atmosphere chemical reaction. This equation involves the major constituents of the gas atmosphere as it controls the reactants formed on each side of the equation. The equal sign indicates chemical equilibrium, that is, the reaction can go either way, to form carbon monoxide (CO) and water vapor (H 2 O) or to form carbon dioxide (CO 2 ) and hydrogen (H 2 ) depending on the relative percentages of each in the furnace atmosphere. (2) CO 2 + H 2 = CO + H 2 O (Water Gas Reaction) Water vapor (H 2 O) and carbon dioxide (CO 2 ) both appear in this equation, and due to this, we can control the carbon potential of a furnace atmosphere. Dew point analyzers look at the H 2 O / H 2 ratio in the water gas reaction, infrared analyzers and oxygen probe devices look at the CO/ CO 2 ratio in the water gas reaction. Water vapor (H 2 O) is a strongly decarburizing gas and, therefore, any constituent such as carbon dioxide (CO 2 ) will have a tendency to form water vapor, because of this carbon dioxide (CO 2 ) must also be closely monitored. In addition, to prevent decarburization by water vapor, the carbon monoxide (CO) and hydrogen (H 2 ) must be present in amounts to satisfy the equilibrium condition at each temperature. Water vapor (H 2 O) and carbon dioxide (CO 2 ) will oxidize and decarburize steel. Hydrogen (H 2 ) is formed when water vapor oxidizes iron. Therefore, to prevent oxidation and to keep iron bright, a definite excess of H 2 over H 2 O vapor is required for each temperature. Reactions Involving Carbon Dioxide When a hydrocarbon fuel is burned in air, carbon dioxide (CO 2 ) is one of the reaction products. Carbon dioxide oxides iron at elevated temperatures. To prevent oxidation, it is necessary to have an excess of carbon monoxide (CO). Therefore, carbon monoxide is a desirable constituent, to prevent oxidation. Carbon dioxide is not only oxidizing to steel but it is extremely decarburizing. To prevent decarburization, CO 2 Table 3. Nitrogen Methanol Atmosphere Field Data [1] Flow Data [2] % N 2 % H 2 % CO 2 % CO % CH 4 Dew Point, C ( F) Nitrogen/Methanol C to +17 (+30 to with Natural Gas and/or Air ) Enrichment Notes: 1. A 2000 lb/hr (900 kg/h), 48 (1.2 m) wide electrically heat mesh belt conveyor furnace operating at carbon potential settings between 0.20% %C. 2. Approximate gas flows: m3/h nitrogen ( cfh); 3 l/h methanol (190 cfh); 6 9 m3/h natural gas ( cfh); m3/h (40 50 cfh) air.

4 135 must be controlled very closely. The actual amount depends upon the carbon monoxide content, temperature and the carbon content of the steel. Reactions Involving Carbon Monoxide Carbon monoxide is a strong carburizing agent. The reversible reaction of carbon monoxide (CO) to form carbon (C) and carbon dioxide (CO 2 ) is of particular interest in a furnace atmosphere. CO has a high carbon potential and becomes increasingly more stable at elevated temperatures. It is only at lower temperatures (480 C C) that carbon monoxide will supply carbon (Equation 3) in the form of soot in the so-called carbon reversal reaction. Soot causes most of the maintenance related issues with gas generators and heat treating furnaces. (3) 2CO = C + CO 2 Reactions Involving Nitrogen Below about 1000 C molecular nitrogen will not react with the surface of steel or stainless steel. Atomic nitrogen (N) however, that does not normally occur in a furnace atmosphere unless it is purposely introduced by the addition ammonia (NH 3 ) will react by being absorbed into the steel surface. Reactions Involving Hydrocarbons Methane and other hydrocarbons (propane and/or butane) are carburizing agents. At elevated furnace temperatures, methane (CH 4 ) breaks down into carbon (C) and hydrogen (H 2 ). The higher the furnace temperature, the greater the tendency for methane to break down. Because of this tendency, methane and other hydrocarbon gases are introduced into the furnace to help change the atmosphere from neutral to one with a high carbon potential (the driving force of carbon into the surface of the steel). Atmosphere Volume Requirements During operation, the volume of protective atmosphere required for safe use in a particular heat treating furnace and the ability to properly control that atmosphere depends to a great extent on the: type and size of furnace; presence or absence of doors and/or curtains, environment (especially drafts); size, loading, orientation, and nature of the work being processed, and metallurgical process involved. In all cases, the manufacturer s recommendations should be followed for gas introduction, purging and removal since the original equipment manufacturer has taken these factors into account during the design of the equipment. National Fire Protection Association (NFPA) Standard 86, Standard for Industrial Furnaces Using a Special Processing Atmosphere, applies to all furnaces and the procedures listed within this standard must be followed. A rule of thumb to remember is that to purge air out of a furnace prior to introduction of a combustible furnace atmosphere requires a minimum of five (5) volume Table 4. Comparison of Synthetic Furnace Atmospheres Atmosphere Type % H 2 % N 2 % CO Dew Point, C ( F) Hydrogen Pure to -85 (-95 to 120) Dissociated Ammonia (DA) Generated to -75 (-40 to 50) Nitrogen-DA Blended > -58 (-50) Endothermic Generated to -23 (+40 to -10) Nitrogen-Endo Blended < 0 Nitrogen Hydrogen Blended (-60)

5 136 changes of the chamber (Table 5). This is to ensure that the oxygen content of the chamber is below 1% prior to the introduction of the atmosphere. Table 5. Volume Changes Required for Safe Purging of Furnaces Number of Volume Changes Percentage of Air Still Remaining Control of Furnace Atmospheres Good quality control must be ensured due to the fact that the composition of the furnace atmosphere is constantly changing, so measurement and control devices must be used. In particular, for atmospheres designed to run neutral, avoiding decarburization and/or carburization is critical to the proper functionality of the fastener. This is accomplished by making sure that one or several of the following control methods is monitoring and/or controlling the process: Dew point analysis; Infrared analyzer (single or multiple gas analyzers); Oxygen (Carbon) probe The trend today is to use multiple measurement tools to obtain the most accurate snapshot of the atmosphere in real time. Whether neutral or case hardening, a number of variables determine how well a furnace does its job. Throughout the entire cycle it is critical to the process that we control the: Percentage of carbon dioxide, oxygen, and water vapor; Ratio of enriching gas (or air) to carrier gas. For example, surface carbon can be controlled within ± 0.10% by measuring one or more of these constituents. Dew point Control The temperature that water vapor starts to condense is known as the dew point. In simplest terms then, a dew point analyzer measures the amount of water vapor present in atmosphere of the furnace (Table 6). This information can then be used to determine the carbon potential of the atmosphere (Table 7). It is considered an indirect measurement technique if it involves pulling a gas sample from the furnace into the instrument. Table 6. Typical Dew Point Levels Dew Point, ºC (ºF) Water Vapor (ppm) +8 (+46) 10,590-4 (+25) 4, (0) 1, (-40) (-90) 3.4 Note: 1% moisture = 127 ppm water vapor = +40ºF dew point Table 7. Dew Point versus Surface Carbon (%) Dew Point, ºF (ºC) 815ºC (1500ºF) 870ºC (1600ºF) 925ºC (1700ºF) +30 (-1) (+4) (+ 10) If performed properly dew point analysis is a simple and accurate atmosphere measurement technique and indicates the condition inside the atmosphere generator or heat treat furnace. It will tell you if the reaction is stable or unstable (constant dew point or changing dew point over time). It can tell you when the catalyst bed in your endothermic gas generator is starting to soot and if there is a water leak, an air leak or non-uniformity ( breathing ) of the atmosphere inside your furnace. The types of dew point analyzers available include capacitance sensors, chilled mirrors and fog chambers (AlnorÒ, Dew Cup). If the sample temperature is less than the dew point of the gas, condensation can be a problem for all dew point devices. One solution is to heat trace the sample lines. If the ambient temperature exceeds 40ºC (105 F) (as it often does in heat treat shops), the instruments will not give accurate readings unless special precautions are taken. Dew point control can also be measured using an oxygen probe arrangement (Fig. 2). Infrared Control Infrared analysis uses the infrared spectrum (light for example is in the middle of the spectrum in the visible range) to analyze a gas sample and determine the percentage of that constituent in the furnace atmosphere. Single gas

6 137 Oxygen Probe Control Figure 2. Endothermic Generator Dew Point Control Scheme via Oxygen Probe (Illustration Courtesy of Super Systems, Inc.) (carbon monoxide) or multiple gas (carbon dioxide, carbon monoxide, methane) analyzers detect the presence of these gases in the furnace atmosphere. Another indirect way of measuring the carbon potential of the atmosphere is to measure the amount of carbon dioxide in the furnace. Today, three (3) gas infrared analyzers are used to monitor the carbon monoxide, carbon dioxide, and methane contents of generators (Table 8) and furnaces (Fig 3). Individual gases absorb infrared radiation of very specific wavelengths. The amount of absorption increases with gas concentration. The unit operates under the principle that a gas sample passes through a cell where a heated wire emits infrared energy of known wavelength. The sensor converts measured infrared energy into an electrical signal. Usually these values are compared to the values obtained with a reference gas. Infrared analyzers are known for their fast response and are easily calibrated. The oxygen (or carbon) probe is a in-situ device, which looks similar to a thermocouple for measuring temperature and typically sits inside the furnace, inside the generator above the catalyst bed or in a separate heated well into which the furnace atmosphere is pumped. In whatever location, the oxygen probe measures changes in the furnace atmosphere. A difference in partial pressure of oxygen in the furnace atmosphere and the partial pressure of oxygen in the room air induces a voltage (called an electromotive force or EMF) across the electrodes in the probe. At any given temperature, there is a known relationship between the voltage output and the oxygen potential of the atmosphere. The oxygen potential can be directly related to the carbon potential. Therefore the carbon potential of the furnace atmosphere can be controlled by monitoring the furnace temperature and the probe output. The oxygen probe uses a conductive ceramic sensor, for example manufactured from zirconium oxide (Zr2O3) that can be mounted in-situ inside the furnace (Fig. 4). Operating range is between 650ºC (1200ºF) and 980ºC (1800ºF). Oxygen probes can be used for a variety Table 8. Typical Field Data for an Operating Endothermic Gas Generator [1], [2] Constituent 1ST Quarter 2nd Quarter 3rd Quarter 4th Quarter % CO % CO % CH Generator Dew Point, F +39 (+4) +39 (+4) +40 (+4) +39 (+4) Dew Point at Furnace Inlet, C ( F) +37 (3) +38 (3) +38 (3) +38 (3) Zonal Dew Point (Z1 Z4), C ( F) [3] +40 to +42 (+4 to +6) +40 to +42 (+4 to +6) +40 to +42 (+4 to +6) +40 to +42 (+4 to +6) Notes: m3/h (3000 CFH) output 2. Natural gas feedstock 3. Neutral hardening

7 138 Figure 3. Furnace Infrared (3-Gas) Control Scheme (Illustration Courtesy of Super Systems, Inc.) Figure 4. Combination Furnace Oxygen Probe and Infrared Control (Courtesy of Super Systems Inc.) of gases but they need to be calibrated for the specific atmosphere used. They are fast response devices but tend to be subject to contamination by carbon or zinc. When used in carbonitriding applications, the presence of ammonia will shorten the life of the probe. Typical oxygen probe data for a mesh belt conveyor furnace (Table 9) running at the rate of 1815 kg/h (4000 pounds/hour) shows deviations from setpoint values due to non-equilibrium conditions present within the furnace. Table 9. Control Settings Mesh Belt Conveyor Furnace Steel Process Setpoint Values (Oxygen Probe), % C Actual Values (Oxygen Probe), % C 10B21 Carbonitriding Neutral hardening Important Cautions Knowing how the data was collected as well as understanding the exact furnace operating conditions at the time (e.g. zone temperatures and gas flows, furnace pressure, exhauster settings, fan rotation and speed, etc.) are all important components in fully understanding and interpreting furnace atmosphere data correctly. In more than one instance, data was being analyzed and adjustments made based on improper sample port locations, instrumentation that was not properly calibrated and/or sample ports not extending fully into the furnace chamber. Mistakes such as these can be devastating to both equipment and part quality. References: Herring, D. H., Understanding Furnace Atmospheres, Atmosphere Operation and Atmosphere Safety, Heat Treating Hints, Vol. 1 No. 7. Mr. Thomas Philips, Air Products & Chemicals ( private correspondence. Mr. James Oakes, Super Systems, Inc. ( private correspondence. Daniel H. Herring is President of The HERRING GROUP, Inc. Elmhurst, IL while Richard D. Sisson, Jr. is George F. Fuller Professor and Director of Manufacturing & Manufacturing Engineering, Mechanical Engineering Department, Worcester Polytechnic Institute, Worcester, MA.