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Browse previously asked/answered questions below.

  • Can you eliminate intergranular oxidation of your steel parts during carburizing?
    Zbigniew Zurecki
    Research Associate

    Internal or intergranular oxidation (IGO) occurs during carburization using endothermic or dissociated methanol atmospheres. Metals processors who eliminate or at least minimize IGO can gain substantial benefits including reduced grinding after carburizing, shortened carburizing cycles, and an improved fatigue-life of gears, shafts and other machine parts.

    Preventing oxidation of more reactive steel elements (Mn, Si, and Cr) can be achieved by carburizing in dry hydrocarbon atmospheres that contain no CO additions. Fortunately new technology offers an easier, less costly retrofit alternative. It has been shown during carburizing in the traditional integral quenching and pit furnaces, that an atmosphere of nitrogen containing just a couple percent of natural gas and/or propane produces the same IGO-free, hard cases as the vacuum carburizing process. These N2-HC atmosphere systems have been installed on existing, ambient-pressure furnaces, and industrial tests are in progress.

    If you are interested in the mitigating IGO and/or in the low-cost, atmospheric-pressure, N2-HC atmosphere solutions, please call Air Products at 800-654-4567.

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  • My sintered powder metal parts come out of the furnace sooty. How do I prevent sooty parts?
    Shane Chunko - Applications Engineer Shane Chunko
    Applications Engineer 

    To resolve a sooting problem, you must first identify the type of soot. There are three main types: adherent soot; loose, granular soot; and shiny or oily soot. All are associated with hydrocarbons from either lubricants or enriching hydrocarbon gas. Adherent soot looks like a stain and is difficult to remove. It is generally produced by the pyrolysis of lubricant in the preheat zone. Loose, granular soot appears as a black snow on the top of the parts and is produced from lubricant vapors in the hot zone. Shiny soot appears as a uniform black coating on exposed surfaces. The catalytic cracking of natural gas on the parts produces this type of soot.

    Once the type of soot is known, the problem can be resolved by evaluating factors such as atmosphere flow, flow balance, preheat dew point, belt speed, belt loading, temperature profile, part density, percent lubricant, and furnace condition.

    If you are having a process issue that you think may be related to a sooting problem, please call Air Products at 800-654-4567 and mention.

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  • Is my gas purity adequate for my process?
    Don Bowe Don Bowe
    Sr. Applications Engineer

    Industrial gases (such as nitrogen, hydrogen, and argon) for furnace atmospheres are characterized by their very high purity (>99.995%). Typical impurity levels are much less than 10 parts per million by volume (ppmv) oxygen and less than 3 ppmv moisture (<– 90° F dew point). This purity is typically adequate for many processes involving a wide array of materials. Some materials, though, due to their high reactivity, may require additional purification to reach even lower levels of impurity, especially with gases supplied via bulk or tube trailer supply modes. Some facilities install in-line purifiers as an added precaution against impurities picked up from the houseline. In-line purification typically involves the removal of oxygen and moisture. Sometimes with argon supply, it is necessary to remove trace nitrogen impurities. The choice of purifier is dependent on the gas and the type and amount of impurities to be removed.

    If you are having a process issue that you think may be related to gas purity, please call Air Products at 800-654-4567.

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  • How can I address a cooling issue within my process?
    In its liquid state, nitrogen is -320 degrees Fahrenheit! This makes it one of the most effective coolants available. Depending on your process, liquid nitrogen can provide temperature control, shorten cycle time, and improve product quality. Nitrogen is also a green product, as it leaves no residue and is sourced from the air we breathe. It’s used in many industrial processes and can be adapted to heat treating, machining, thermal spray, and many other applications that have problems related to excess heat.

    If this is an issue for your applications, please give us a call at 800-654-4567, to learn how nitrogen can help improve your process.
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  • We anneal and passivate 300 series stainless tube and sometimes get a “grey” coloration on the ID—can we keep it bright?
    Zbigniew Zurecki
    Research Associate

    Formation of oxide film is a function of hydrogen/water partial pressure ratio, temperature, and time, all of which can change significantly at the furnace exit. An elevated dew point due to air ingress reacting with hydrogen, combined with reduced temperature, will lead to oxidation if the time within the exit area is sufficiently long. Additionally, the ID surface of the tubing may cool more slowly than the OD, causing inconsistent oxidation. Countermeasures include increasing the tubing’s travel speed, increasing the hydrogen flow rate over all tubing surfaces, and applying a nitrogen curtain at the exit to dilute air ingress and assist in cooling. There are technological, safety, and cost considerations associated with each countermeasure. Contact Air Products’ technical specialists for assistance at 800-654-4567.
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  • How can I benefit from a nitrogen-based system if I’m already getting good parts at a reasonably low cost?
    Tom Philips
    Sr. Principal Applications Engineer

    There are numerous benefits of using a nitrogen-based atmosphere system, including:

    • Independence from natural gas and related maintenance of generators.
    • Increased flexibility to change atmosphere compositions and flow rates to suit the process and material requirements.
    • Improved safety due to automatic nitrogen purge capabilities.
    • Elimination of toxic components such as carbon monoxide and ammonia, associated with the use of endothermic generators and ammonia dissociators.
    • Minimizing the amount of hydrogen needed to get the correct reducing power—due to nitrogen’s low dew point.
    • Reduced carbon footprint and also significantly reduced carbon monoxide and carbon dioxide emissions.

    To see if a nitrogen-based system makes sense for your operation, call us at 800-654-4567.

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  • My ceramic carrier tiles are deteriorating faster than expected. Could my atmosphere be affecting it?
    Robert Kelly
    Principal Applications Specialist

    Refractories are affected by atmospheres in several ways. Although stable at room temperature, a number of oxides are reduced in the presence of hydrogen or free carbon at elevated temperatures—thus shortening their lives. The customer's process and desired output dictate the design atmosphere. However, crystallography of the ceramic material will have a major impact on its resistance to that atmosphere. By understanding the effects of atmosphere gases on refractories and by selecting refractories that are more stable at operating temperatures and in the presence of specific gas species, you can enhance the performance of your furnace. Air Products' engineers can work with you to optimize your process. Give us a call at 800-654-4567 to schedule an audit of your operation.
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  • What are some considerations to properly select a vacuum furnace surge tank?
    Don Bowe Don Bowe
    Sr. Applications Engineer

    We are increasingly asked about surge tank sizing for vacuum furnaces. The shift towards faster quenching through higher pressure backfills has made surge tank selection – size and pressure rating – more critical.

    First, you need to determine the required tank operating pressure that will provide the necessary furnace backfill pressure and time to backfill. There are tradeoffs between the tank size, its pressure rating, the resulting stored volume of gas and the cost of the tank. The gas supply system also must be able to provide adequate pressure to refill the tank. There are natural pressure level break points from standard cryogenic based supply systems, such as 200 psig from a standard 250 psig rated liquid cryogenic tank.

    Be sure that the ASME approved surge tank is rated for the pressure that you are using and that it is adequately protected from over pressurization. Also, if you’re using a cryogenic supply system, make sure it has a low temperature alarm to prevent embrittlement of the carbon steel surge tanks.

    The volume of a surge tank is usually referred to in terms of its gallons of water displacement. Since there are 0.134 cubic feet (ft3) per gallon, a 1,000 gallon surge tank has a volume of 134 ft3) . Therefore, for each atmosphere of pressure [(14.7 pounds per square inch (PSI)] there are 134 standard cubic feet (SCF) of gaseous volume available for the backfill. For example, 134 SCF of gaseous volume is available at 14.7 PSIG, 268 SCF at 29.4 PSIG and so on.

    A surge tank needs to be able to store the proper volume of gas at an adequate pressure level above the backfill pressure of the furnace. For instance, using simple ideal gas laws, if 100 ft3) is required for a 5 barg quench pressure (approx 72 PSIG), it would require 600 SCF of gas for a backfill from full vacuum. That’s assuming a minimum pressure of 6 bar is required to provide an adequate flow rate to backfill within the desired time, The resulting surge tank would need to be about 750 gallons with a minimum operating pressure level of approximately 12 barg (175 psig). A tank with a 200 psig maximum allowable working pressure (MAWP) rating would be recommended and the actual size would be based on how much overdesign might be desired. A smaller tank could be used with a much higher operating pressure.

    With this information as a background and a consultation with an applications engineer, you should be able to determine the required pressure and tank size to properly backfill your furnace. To learn more, contact us online or give us a call at 800-654-4567 and mention code 749.
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  • Why is control necessary for a protective atmosphere?
    John Dwyer
    Sr. Principal Industry Engineer

    The simplest nonreactive atmosphere for thermal processing of a metal or material is a pure inert gas or vacuum, yet neither offers protection against trace impurities such as O2, H2O, and CO2, which are almost invariably present in the heat treating furnace atmosphere.

    The problem of trace impurities is exacerbated as the temperature increases. Depending on the process and material, even small variations in the temperature or impurity level can shift a reducing or neutral atmosphere to an oxidizing one, with a negative impact on the quality of the treated parts.

    To counteract the temperature effect, reactive species (H2 and CxHy) can be added to scavenge impurities and maintain the required potential for the material being processed. Control systems have gained increasing acceptance to regulate the amount of reactive species added. However, ensuring proper control is not simply a matter of installing elaborate equipment; it also requires accurate knowledge of which variables must be controlled and how close the control must be in any given case.
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  • Can we conserve energy and increase savings by converting to a synthetic nitrogen/hydrogen atmosphere?
    Tom Philips
    Sr. Principal Applications Engineer

    In one word—yes. You can lower costs and reduce waste by converting from a generated atmosphere such as endothermic or dissociated ammonia to a synthetic nitrogen/hydrogen atmosphere.

    Here’s how:

    • Using and paying for the atmosphere only when your furnace is in production, rather than paying for fixed output volumes with generators—even if you use less than the set volume.
    • Reducing the hydrogen concentration to a range of 2%–10% while maintaining the high reducing potential that results from the very low dew point of nitrogen.
    • Zoning the atmosphere by adding only the required gas blend and volume independently in the different zones of the furnace.

    To find out which atmosphere system can provide the greatest benefits to your operation, call us at 800-654-4567 to talk to an Air Products applications engineer.

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  • Why would I want to cryogenically treat tool steels? Does it really affect the microstructure?
    Minfa Lin
    Senior Principal Research Engineer, Ph.D.

    After austenitizing and quenching, tool steels are sometimes subjected to cold treatment at approximately –80°C, followed by tempering. Primarily, the cold treatment is done to increase strength, improve dimensional or microstructure stability, and improve wear resistance. These benefits are due to the transformation of retained austenite to martensite. Some studies show that lowering the cold treatment temperature below
    –100°C does not significantly increase the amount of retained austenite-to-martensite transformation—therefore it does not result in additional benefits. However, other studies show that compared to cold treatment (–80°C), cryogenic treatment (–190°C) further improves wear resistance. For help determining which subzero quenching process can help you produce the best parts, call us at 800-654-4567.
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  • How can I provide customer documentation proving my heat treat process was controlled while treating their products?
    Quality programs that require information about how you process a part for your customers are becoming more common. Understanding what variables you control and what effect they have on your parts is an important step in starting this effort. Variables such as temperature, time, atmosphere flow rates and composition, and utility consumption are good places to start tracking.

    A monitoring system makes this task easier day to day and increases the accuracy of recorded data. Air Products' PURIFIRE® process management system automates data monitoring and collection, and provides additional benefits such as remote monitoring of your process, alarming to indicate problems, and custom report generation for customer documentation. Our engineers help you determine what variables are important for you to monitor and then customize a system that fits both your specifications and those of your customers.

    Benefits such as reduced scrap, elimination of manual data collection, faster problem troubleshooting, and increased product quality can enhance your customer relationship and help your bottom line.

    For more information please call us at 800-654-4567.
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  • What is dezincification and how does it apply to thermal processing of brass?
    Mark Lanham
    Applications Engineer

    Dezincification is typically defined as the leaching of zinc from copper alloys in an aqueous solution. In thermal processing of brasses (and other zinc-containing alloys), dezincification is the removal of zinc from the metal substrate during thermal processes, like brazing and annealing, typically due to the very low vapor pressure of zinc in the alloys. Dezincification can result in excessive furnace dusting, zinc vapors alloying with other metals, and in extreme cases, loss of alloy properties.

    While eliminating dezincification is not always possible, it can be reduced during thermal processing. Controlling temperature, time at temperature, and the furnace atmosphere's reducing potential can help minimize dezincification and improve your thermal processing. However, understanding which variables to change can be a challenge. Air Products' industry specialists, experienced in thermal processing, can help pinpoint the variable(s) that you can regulate to help lower costs and improve productivity by minimizing dezincification.

    Call for an assessment or audit at 800-654-4567.
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  • Can nitrogen reduce costs and improve operations in my dissociated ammonia (DA) atmosphere?
    John Kaiser
    Applications Engineer


    Nitrogen-DA dilution can be a cost-effective alternative to 100% DA. Since many materials being processed do not require the 75 percent hydrogen content in DA, you can reduce your atmosphere cost by using less costly nitrogen to dilute your DA. The use of nitrogen also provides an economical means for purging in addition to a lower cost for furnace idling. Also, using hauled-in hydrogen with nitrogen to replace DA can be cost-competitive and completely eliminate ammonia—a more toxic, more expensive gas.

    Air Products applications engineers can help you compare atmosphere costs and
    recommend ways to reduce atmosphere consumption to further reduce your total cost of
    ownership. To find out more, contact us online or give us a call at
    800-654-4567; mention code 749.

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  • I know my flowmeter tells me that I have a certain gas flow rate, but how can I be sure?
    Flowmeters must be sized properly for each particular application, type of gas, gas pressure, and operating range. First, make sure that your flowmeter is calibrated for the specific gravity of the gas that you are metering. Check the label or the glass tube of the flowmeter or call the manufacturer to be sure. Second, operate the flowmeter only at the pressure for which it was calibrated. As an example, a variable-area flowmeter calibrated for 80 psi and reading 1000 scfh will really only be delivering 760 scfh if it is operated at 40 psi. This is a 24% error! Third, for best accuracy and to allow room for adjustment, size the flowmeter so that your normal flow rate falls within 30%–70% of full scale. These three steps will help ensure that you have good control over your gas flows and, ultimately, your process.

    For a free copy of Gas Atmosphere Analysis Guidelines, please call 800-654-4567.
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  • I use high-pressure gas cylinders and am concerned about safety. Is there a better way?
    John Tapley
    Microbulk Business Development Manager

    Traditionally, high-pressure gas cylinders have been the supply mode for users in the low- to medium-volume range. This has left companies vulnerable to safety risks associated with moving cylinders and exposure to high pressure. Consolidating to a centralized microbulk system eliminates the need to handle cylinders and reduces the risk of product mix-up. Further benefits include decreased exposure to high-pressure containers and reduced traffic congestion with less frequent supplier deliveries.

    Air Products developed the microbulk supply option as a cost-effective, reliable alternative to high-pressure cylinders for nitrogen, argon, oxygen and carbon dioxide supply. In addition to efficient and flexible storage systems, innovative piping solutions are available to help you have a smooth transition from cylinders to microbulk.
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  • How can I identify the stain or discoloration on my part after a heat treating process?
    Jiyun Xu
    Senior Research Chemist

    Analytical techniques can help characterize a stain.

    If the stain is mainly inorganic (chip, scale or rust), an elemental analysis by X-ray fluorescence (XRF) or scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) will most likely be sufficient. However, if the stain is rather thin (a few monolayers) and surface sensitivity is an issue, X-ray photoelectron spectroscopy (XPS) can help determine the type of stain through elemental and chemical analysis.

    If the stain is organic in nature, such as oil, grease and surfactants, infrared (IR) spectroscopy would be the tool of choice.

    Air Products can help characterize your stains, whether by comparing the stain’s IR spectrum with Air Products’ library featuring over 17,000 compounds or by using our other analytical tools. Give us a call at 800-654-4567.
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  • How can I increase production flexibility and ensure the quality of my annealed components?
    Rob Edwards
    Heat Treatment Specialist   

    Carbon steel components have been routinely annealed or heat treated in nitrogen-hydrogen atmospheres to relieve stress, alter microstructure and/or improve surface appearance for a number of years. The flow rate and composition of nitrogen-hydrogen atmosphere to be used for annealing components in furnaces are usually determined by a trial and error approach. Once the atmosphere flow rate and composition that produces parts with acceptable quality have been determined, they are generally fixed for future annealing operations. Although the composition of nitrogen-hydrogen atmosphere introduced into a furnace does not change with time, the true reducing or oxidizing potential of the atmosphere inside the furnace changes continuously with time due to leaks and drafts in the furnace, desorption of impurities such as moisture from the surface of components or decomposition of lubricant present on the surface of components being annealed. This continuous change in reducing or oxidizing potential of the atmosphere inside the furnace provides a great difficulty to commercial heat treaters and parts producers to produce annealed components with good and consistent quality and compete effectively in the global market. Therefore, to provide operational flexibility to commercial heat treaters and parts producers in terms of (1) controlling the true reducing or oxidizing potential of the atmosphere inside the furnace and (2) improving quality of annealed components, Air Products has developed an advanced control system currently being marketed under the trade name Purifire® AN.

    Click here to find out more about Air Products' advanced control system (PDF, 79 K).

    You will need the free Adobe Acrobat Reader to view these documents.

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  • I’m experiencing intermittent oxidation in my furnace. Could leaks in the nitrogen houseline be the problem?
    Don Bowe Don Bowe
    Sr. Applications Engineer

    Yes, leaks in any pressurized high-purity gas line can cause intermittent oxidation. There are several possible causes. One is through retrodiffusion—the movement of impurities from the surrounding air to a high-pressure, low-impurity gas houseline. This is driven by concentration gradients, not pressure gradients, and is aggravated by changes in flow rate, pressure or piping temperature.

    Air Products industry specialists can help you determine the cause of your problem. Since the oxidation is intermittent, you’ll need to continuously monitor your nitrogen houseline for leaks with a trace oxygen analyzer. For combustible gas lines, a combustible gas sniffer can also be used. Once impurities are found, the source of the leak can be identified using various techniques, including soap bubble testing, static pressure testing or helium mass spectrometry. Leaks often occur in weld cracks, mechanical joints, valve packing and loose fittings.

    To help minimize wasted product and part oxidation, call us for a leak detection or full process audit at 800-654-4567.
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  • I have measured the oxygen level in my continuous furnace, and it's low, but my parts still come out oxidized. Why?
    Don Bowe Don Bowe
    Sr. Applications Engineer
    Guido Plicht
    Senior Research Engineer
    Europe

    This is a question that comes up frequently. When troubleshooting for oxidation in a continuous furnace atmosphere, it's important to measure both oxygen level and dew point. Here's why.

    The dew point is a measure of the moisture content of a gas and is the temperature at which water vapor in a sample gas starts to condense. Oxygen concentration is simply that—a measure of the partial pressure of oxygen.

    When a gas sample is extracted from the hot zone of a furnace for analysis, reactive gases like H2, CO, or CxHy have already combined with any O2 present to produce moisture and other gaseous components. As a result, depending on the furnace temperature and how the sample is obtained, your analyzer will often display a low oxygen level. In most applications, a low oxygen level and a low dew point are required to control the process and prevent oxidation.

    Click here to find the Gas Atmosphere Analysis Guidelines

    For further details contact Shawn Lainchbury tel: +44 (0) 1932 249 398
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  • How can we minimize infiltration of oxygen into open-ended continuous furnaces?
    Tom Philips
    Sr. Principal Applications Engineer

    Oxygen from air can diffuse or infiltrate your furnace from the front and exit ends, causing problems such as oxidation, decarburization, under-sintering or inadequate braze quality. Here are some methods to reduce oxygen infiltration:

    • Use adequate total atmosphere flow to have a slightly positive pressure inside the furnace. Typically, a flow of about 70 to 100 scfh per inch of belt width is enough for door openings less than 3 inches.
    • Install a flame curtain at the front end, preferably attached to the bottom of the door, with the flames shooting down onto the parts—ensuring complete coverage of the front end opening.
    • Install a good fiber type curtain with an additional nitrogen spray curtain at the exit end.
    • Ensure the exhaust stacks are separated from the furnace and don’t cause differential suction on the furnace’s atmosphere.

    For more information or an atmosphere audit by Air Products’ experienced engineers, call 800-654-4567.

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  • Is it true that NFPA 86C has changed?
    Mark Lanham
    Applications Engineer

    Yes, it's true. In fact, NFPA 86C no longer exists. The requirements for "Industrial Furnaces Using a Special Processing Atmosphere," formally defined in the 1999 version of NFPA 86C, have been incorporated into NFPA 86 as of July 18, 2003. Now, NFPA 86 combines the furnace safety requirements for all types of industrial furnaces, including Class A – Food and Baking Ovens, Class B – Melting Furnaces, Class C – Furnaces Using Special Processing Atmospheres, and Class D – Vacuum Furnaces.

    The previous contents of NFPA 86C are now primarily found in Chapter 11 of NFPA 86. A notable change is that NFPA 86 recommends that users of Class C furnaces include a low temperature alarm panel to indicate an overdraw condition on the ambient air vaporizers used for emergency purging. Previously, NFPA 86C required the use of a low temperature flow-restricting device that could potentially limit available purging capacity. Air Products' PURIFIRE® nitrogen supply monitoring system is designed to help you comply with this new recommendation.

    Users of furnaces with special processing and flammable atmospheres should fully understand the requirements and recommendations of NFPA 86 and determine how the changes from the old NFPA 86C may affect their furnace operations. For help in understanding these specifications or for more information about our PURIFIRE nitrogen supply monitoring system, contact us at 800-654-4567.
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  • What nitrogen purity do I need for my heat treatment process?
    Rob Edwards
    Heat Treatment Specialist

    That depends on your process. Nitrogen based atmospheres for metals processing have been successfully proven over many years, and due to the enormous range of requirements in furnaces for various materials and surface needs, the use of gas mixtures is now an industry standard. Different products can tolerate differing concentrations of oxidising components in the furnace atmosphere due to additional reducing or reactive components in the blend. For this reason, the use of on-site generated nitrogen with residual amounts of oxygen can be tolerated. By understanding your oxygen tolerance levels we can help you reduce your costs.

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  • Which nitrogen supply makes sense—on-site generation or liquid delivery?
    Steve Ruoff
    Metals Processing Segment Manager

    If your operation uses nitrogen, you may wonder when on-site generation makes sense instead of liquid delivery. Here’s some information to help you understand if on-site nitrogen generation may
    be the supply mode for you . . .

    The appropriateness of on-site gas generation depends on many factors—nitrogen flow and purity are the most important ones. Flows with a steady or sufficient baseline rate can be great fits for on-sites. Periodic or erratic flow patterns can also be appropriate if there is a suitable baseline flow or the volumes, pressure and purity are sufficient to allow
    gas storage that covers peak flows. Also, the lower the purity requirement, the greater the fit (although high purity is okay at higher volumes). Other factors you should consider include local power cost and the pressure that’s required.

    There are no firm rules defining when you should switch from delivery to an on-site system. However, with our extensive experience in on-site technologies and gas delivery, we can help you determine your optimal supply mode. For an assessment of your operation, give us a call at 800-654-4567; mention code 749.

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  • Can I determine if the oxidation in the cooling section of my continuous furnace is caused by air ingress or a water leak?
    Tom Philips
    Sr. Principal Applications Engineer

    A simple copper/ steel test can differentiate oxidation by air (O2) or water (H2O). The test is performed by sending a piece of clean bright copper strip alongside a piece of clean carbon steel strip through the continuous furnace and observing the oxidation on each test coupon. Take care to keep the furnace temperature below 1981˚F, the melting point of copper. The steel strip will discolor or oxidize if the atmosphere has an air or water leak; however, the copper strip will only oxidize if an air leak is present. You can use this test for nitrogen-based or generated type atmospheres like endothermic or dissociated ammonia. And it can be done without oxygen or dew point analyzers.

    For more details on this and other atmosphere troubleshooting tips, give us a call at 800-654-4567.
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  • Can I determine where the oxidation on my heat treated parts is coming from within my furnace?
    Mark Lanham
    Applications Engineer

    Dezincification is typicallyWhen checking a continuous furnace, oxidation in the preheat section has a matte or frosted appearance and is usually caused by air infiltration from the entrance of the furnace. Hot zone oxidation may cause scaly or blistered parts. This generally occurs from elevated moisture or oxygen levels due to improper atmosphere balance or water/air leaks in the cooling zone. Cooling zone oxidation typically results in a smooth, sometimes shiny discoloration—poor curtain design, excessive belt speed, water leaks, or insufficient atmosphere flow rates are possible causes.

    In batch furnaces, start by identifying the oxidant causing the problem. Flowing nitrogen and measuring the oxygen and moisture levels can give an indication of the oxidant involved. Then a review of typical leak sources, such as seals, fittings, unions, and weld joints, usually leads to discovery of the leak source.

    For help determining the oxidation causes or for atmosphere composition monitoring assistance, call 800-654-4567.
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  • What causes discoloration and oxidation of stainless steels in brazing, sintering or annealing processes?
    Tom Philips
    Sr. Principal Industry Engineer

    All grades of stainless steels are iron-based alloys with significant percentages of chromium. Typically, stainless steels contain less than 30% chromium and more than 50% iron. Their stainless characteristics stem from the formation of an invisible, adherent, protective and self-healing chromium-rich oxide (Cr2O3) surface film. While stainless steels are resistant to rusting at room temperatures, they're prone to discoloration by oxidation at elevated temperatures due to the presence of chromium and other alloying elements such as titanium and molybdenum.

    Factors that contribute to increased oxidation include high dew points, high oxygen and oxides of lead, boron and nitrides on the surface. For bright stainless steels, process them in a highly reducing atmosphere with a dew point lower than –40oF and a minimum of 25% hydrogen.

    For an audit and troubleshooting tips, call us at 800-654-4567.
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  • How can I reduce my costs for Al-annealing without reducing my quality?

    Rob Edwards
    Heat Treatment Specialist


    In the metals processing industry, heat treatment applications are required for producing parts with the desired mechanical and surface properties, as well as for stress relief after mechanical deformation. Today some companies use exothermic or endothermic generators or ammonia dissociates to create the necessary atmospheres. Compared with atmospheres composed of technical gases such as nitrogen and hydrogen, these generated gases have serious disadvantages.

    Click here to find the range of solutions which Air Products have to offer.
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  • What causes stainless steel to turn green in a continuous belt furnace?
    Zbigniew Zurecki
    Research Associate

    The green color that you see on stainless steel parts is chromium oxide (Cr2O3). It forms when there is too much oxygen and/or moisture in the furnace atmosphere, which is usually caused by a water leak, poor atmosphere tightness, or overly low flow rates of atmosphere gas. A dark green-brown color indicates significant levels of free oxygen inside furnace originated by a large air leakage.

    In addition to the traditional steel and copper test, some companies run a piece of stainless through the furnace to check for high moisture and oxygen levels. A better and more precise way of measuring moisture and oxygen levels is to install an oxygen analyzer and dew point meter. It's inexpensive and highly accurate. If a green oxide film is forming on your stainless steel parts, that's an indication that the furnace or atmosphere is not optimized.

    For a complete evaluation and audit of your process, give us a call at 800-654-4567.
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  • Can the increased temperature variability in my annealing furnace be causing the ductility variations in my product?
    Jake Fotopoulos
    Lead Process Controls Engineer

    It depends on the amount and location of the variability. Variability in the critical annealing parameters—temperature, dew point and atmosphere compositions—can have a dramatic impact on product quality. To help find the source of the variability, record the critical process parameters during production—larger than normal deviations in temperature can affect grain growth, hardness and ductility. Then you can correlate poor quality runs to data trends and identify what may be causing the change in properties.

    Installing a process control system to monitor and control these variables can help you reduce variability. A small investment in control technology can provide a large return in reduced production costs and improved quality. Our commercial engineers and extensive experience in process controls can help you improve your process consistency and save money.

    Call us at 800-654-4567 to schedule an assessment.
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  • How do I know if I’m wasting gas due to leaks in my gas piping?
    John Green
    Research Technician

    Gas piping leaks can result from various conditions, including improper thread sealing, missed brazed joints, defective piping, over pressurization, or even vibration and shocks. A pinhole leak can cost you tens of thousands of dollars per year, depending on the size, number and severity of the leak(s). There are many ways to detect leaks; for instance, using soap tests, pressure drop tests, mass spectrometry or thermal conductivity tests. They all have their place; however, they also often come with limitations in precision, speed, difficulty or cost.

    Air Products’ leak detection service can identify and repair costly leaks in your piping to help improve your part quality and bottom line.

    In a short video, various methods for identifying leaks are described in more detail. You can view it online at www.airproducts.com/experts2. If you’d like to speak to a specialist about a leak detection audit of your facility, give us a call at 800-654-4567, and mention code 833.

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  • For our vacuum furnace, how can we get adequate gas pressure to quench at pressures up to 20 bar?
    Steve Ruoff
    Commercial Technology Manager, Metals Processing

    There are a number of ways to address the challenge of high pressure gas quench in vacuum furnaces – and a variety of factors to consider in order to achieve the most economical high pressure gas supply solution.

    First, you need to know the furnace gas volume required for backfilling. Then, the corresponding surge tank must be properly sized, which requires a balance between the maximum tank operating pressure and its internal volume. This surge tank pressure is one of the key factors that influences the type of gas supply system that is best suited to your operation. Another factor to think about is the estimated monthly volume of gas you’ll use, which is dependent on the number of times all of the furnaces will require a backfill.

    Next, is a consideration of the cryogenic gas supply options. Cryogenic systems using high pressure liquid tanks generally result in the least amount of vented gas, but are capital intensive and are somewhat limited in pressure due to the critical point of the cryogen (i.e. liquid nitrogen is 473 psia, approximately 32 bar). High pressure liquid tanks generally are standardized at 400 and 600 psig. Switching batch-type high pressure systems utilize less costly standard pressure liquid supply tanks (250 psig), but can have high vent losses as the batch vessels vent down each time. These systems are also typically limited to about 450 psig (31 bar). High pressure liquid pumping systems also use standard pressure liquid tanks, with a cryogenic pump filling high pressure cylinder banks or hydril tubes. These systems have a much higher pressure range (up to as much as 2,300 psig) and if properly specified, have relatively low vent losses, however they often have the highest overall capital cost. Additional factors to consider as part of a complete evaluation include the maintenance costs for each type of system, along with the unit price for the gas.

    Air Products applications engineers can work with you to thoroughly understand your parameters. Then, they can help you evaluate the benefits and considerations to each type of supply, in order to deliver a system optimized to your operation.

    To contact us about your heat treating challenge, send us an e-mail or call 800-654-4567 (press 1 and mention code 749).

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  • What’s the best approach to select the hydrogen concentration for our nitrogen-hydrogen atmosphere for bright annealing of steels?
    Wehr-Aukland.jpg Anna Wehr-Aukland
    Ph.D., Senior Principle Research Engineer

    Bright annealing of steels requires conditions that are reducing to steel oxides. Traditionally, the Ellingham diagram has been used to predict the conditions that correspond to oxidation of pure metals or reduction of their oxides. This method can be used to predict the conditions that should be reducing to iron oxides and the oxides of the alloying elements added to steels, such as chromium oxide when stainless steels are considered. This traditional approach is not precise because it only uses thermodynamic data for pure metals and their oxides—it ignores the fact that iron and alloying elements form a solid solution. In addition, you can only determine the approximate equilibrium partial pressure ratio of hydrogen and water vapor for oxidation of a specific metal at a particular temperature.

    Alternatively, you can use more accurate and convenient diagrams for steels and other alloys, which are created with the help of modern databases and computer programs, such as FactSage™ (thermochemical software and database package developed jointly between Thermfact/CRCT and GTT-Technologies) or Thermo-Calc software. Using the oxidation-reduction curves, presented as dew point of pure hydrogen or nitrogen-hydrogen atmospheres versus temperature, you can quickly select the atmosphere for annealing steels without formation of oxides. The diagram in Figure 1 was calculated using FactSage. This diagram shows that oxidation-reduction curves for Fe-18%Cr and Fe-18%Cr-8%Ni systems representing stainless steels are higher than the corresponding Cr/Cr2O3 curves. For alloys (e.g. steels), you can achieve more precise calculations using thermodynamic data from both the pure substances (i.e. pure metals and oxides) and solutions databases. Such diagrams can be produced specifically for the steels of interest and variety of atmosphere compositions.

    These methods can help you troubleshoot and optimize your annealing operation by balancing hydrogen usage versus product quality. If you would like to learn more about these techniques, send us an e-mail or call 800-654-4567 (press 1 and mention code 749).

    Oxidation Reduction Chart

    Figure 1: Oxidation-reduction curves for pure chromium, Fe-18%Cr and Fe-18%Cr-8%Ni for the total pressure of 1 atm and hydrogen partial pressures of 0.5 atm and 0.05 atm. The hydrogen pressures correspond to the following atmosphere compositions: 100% H2, N2-50% H2 and N2-5% H2. (This diagram has been produced using thermodynamic data for solutions in addition to the data for pure elements and their oxides.) 

     

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