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Carburizing, Nitrocarburizing, and Carbonitriding

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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  • How do I calculate the amount of methanol I'll need for my carburizing atmosphere?
    Dave Mitchell
    Principal Engineer, Equipment and Process Technology

    Carburizing and other carbon control atmospheres all require a source of CO to facilitate the diffusion of carbon into the surface of the metal. One source is through endothermic atmosphere generation, in which air and natural gas are reacted in an external generator to form a gas composed of 20% CO, 40% H2, and 40% N2, with trace amounts of CO2 and moisture.

    Another source of CO is the introduction of a blend of 40% nitrogen and 60% methanol into the furnace, which forms a gas of the same composition produced endothermically. The heat of the furnace dissociates the methanol (CH3OH) into CO and H2, which then blends with the nitrogen. Here's how to calculate the amount of methanol needed. For 1000 scf of atmosphere, as an example, 40% or 400 scf will be nitrogen, according to the ratios above. The remaining 60% or 600 scf will be made up of dissociated methanol. Since one gallon of methanol dissociates into approximately 240 scf of gas, 2.5 gallons of methanol would be needed to dissociate into the required 600 scf of atmosphere.
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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.
    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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  • 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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  • 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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  • 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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  • 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.

    The paper below will give you an overview of the different systems for nitrogen generation and the appropriate oxygen concentrations for different products and materials in furnaces. For most materials, although free oxygen is not tolerable, some oxidising impurities may be permitted. In these cases, the on-site production of high purity nitrogen for the heat treatment of very sensitive materials may be commercially viable. Examples and descriptions of these systems will also be provided in the paper. Click here to view our expert paper.
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  • When does on-site nitrogen generation make sense versus liquid nitrogen delivery?
    Steve Ruoff
    Metals Processing Segment Manager

    The amenability of on-site gas generation involves 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 be amenable if the volumes, pressure and purity are sufficient to allow gas storage that covers peak flows. Also, the lower the purity requirement, the greater the amenability—although high purity is amenable at higher volumes. Other factors include local power cost and pressure required. There are no firm rules defining when to switch from delivery to an on-site. Different on-site options are available to meet your nitrogen requirements, including pressure swing adsorption, membranes or cryogenics. Count on Air Products’ extensive experience in on-site technologies to help you determine your optimal supply mode. Call 800-654-4567 for an assessment.
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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

    When 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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  • 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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  • Can I remotely monitor what’s happening in my furnaces and other process equipment while I’m away from my plant?
    Chris Ward Chris Ward
    Engineering Associate

    With the proper instrumentation and controls, you can securely monitor and control your heat treating or thermal process from nearly anywhere in the world! This is possible using a variety of hardware and communication methods, including Internet, dial-up, and cell phones. Alarm and warning notifications can also be proactively delivered to you so you can react to upsets, trends, and events before it’s “too late.” It’s important to identify the key parameters, equipment and instrumentation you want to monitor, and then select the hardware and software that best match your needs. Contact Air Products’ team of remote process monitoring and control specialists at 800-654-4567 for an assessment and recommendations as to how to get started.
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  • My sintered PM parts come out of the furnace "sooty." How do I prevent this?
    Mark Wells
    Metals Industry 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.
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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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