Metals and Materials Processing FAQs

Carbon steel components have been routinely annealed or heat treated in nitrogen-hydrogen based atmospheres to relieve stress, alter microstructure and/or improve surface appearance for a number of years. The flow rate and composition of the atmosphere to be used for annealing components in furnaces are usually determined by a trial and error approach.

Although the composition of nitrogen-hydrogen based atmosphere introduced into a furnace does not change with time, the true reducing or oxidiing 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. 

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, depending on composition, process them in a highly reducing atmosphere with a dew point lower than –40°C and a minimum of 25% hydrogen.

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 the 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.

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.

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.

Figure 1: 

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 5,5 bar and reading 28,3 m³/h will really only be delivering 21,5 m³/h if it is operated at 2,8 bar.  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.

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.

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 (<– 65° C 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.

Since the reducing potential of a hydrogen-based furnace atmosphere is defined by the ratio of pH₂O, the first answer that comes to most people’s minds is “yes.” And, in some cases, they are correct. Lower dew point readings (lower pH₂O) lead to more reducing conditions and, in many cases, better furnace atmosphere performance. However, there are situations where that is not always the case. One example of that is hydrogen-based belt furnace atmospheres where the dew point can reach values drier than –45°C or even –50°C under certain conditions. The reducing potential of this atmosphere is more than enough for the typical parts processed, but it can lead to unnecessarily strong reducing conditions that decrease belt life. Another example might be a brazing atmosphere that is too reducing and prone to excessive braze flow. Air Products’ new Atmosphere Humidification System allows for precise and consistent moisture additions to furnace atmospheres for just the right amount of moisture to improve belt life performance and/or braze flow while still maintaining adequate reducing conditions for the sintering or brazing operations being performed. 

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.

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 –40°C and a minimum of 25% hydrogen.

For sintering and brazing atmospheres in a continuous belt type furnace with open ends, you must follow EN 746 Standard for industrial thermo processing equipment like Ovens and Furnaces. Typically, atmospheres containing greater than 5% hydrogen plus carbon monoxide plus methane (in total, whereas the methane content should be below 1%) in nitrogen are considered flammable. In fact, any mixed atmosphere -even if it contains less than 5% combustible components- is considered “indeterminate” and must be treated as if it were flammable, especially at higher furnace temperatures but below the self-ignition point.

EN 746-3 recommends you satisfy the following conditions before introducing any flammable or indeterminate atmosphere into the furnace:

• The furnace must be hotter than 750°C.
• The furnace must be purged with an inert gas until the atmosphere analysis indicates an oxygen level below 1% . General recommendation is to use five volume changes of inert gas flow.
• There must be visible indication of purge flow. Plus, purge piping should have normally open solenoid valves.
• The atmosphere system should be designed with interlocks so the flammable gases are shut off using normally closed solenoid valves in the event of power failure, a temperature drop below 750°C, or insufficient flow of the main atmosphere component.

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 oxidizing 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.

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 1080˚C, 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.

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.

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 10m³/h of atmosphere, as an example, 40% or 4m³/h will be nitrogen, according to the ratios above. The remaining 60% or 6 m³/h will be made up of dissociated methanol. Since one liter of methanol dissociates into approximately 1,67 m³ of gas, 3,6l/h of methanol would be needed to dissociate into the required 6m³/h of atmosphere

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.

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 CₓHᵧ 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.

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.

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

• Independence from natural gas and 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.

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 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.

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 1 to 1,5 m³/h per cm of belt width is enough for door openings less than 7,5 cm.
• 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.

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 –40°C and a minimum of 25% hydrogen.

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 CₓHᵧ 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.

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.

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 (<– 65° C 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.

There are many aspects of a flow control or blend panel that require periodic maintenance for proper functionality—especially those related to its safe operation. You should check the operation of the solenoids to help verify that the combustible gas flow is automatically turning off and the inert gas purge is automatically turning on as intended. They should be tested in accordance with recommended maintenance frequency—typically every six months. Plus, you should rebuild the solenoids as needed. It’s also important to check the purge timer setpoint to help confirm that it can adequately purge the furnace. And you should verify and document the low-flow alarm setpoints on the inert gas purge and process flows. These are just some of the items that should be reviewed on a regular basis.

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.

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.

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

• Independence from natural gas and 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.

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 1 to 1,5 m³/h per cm of belt width is enough for door openings less than 7,5 cm.
• 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.

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 CₓHᵧ 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.

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.

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 –40°F and a minimum of 25% hydrogen.

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 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.

In its liquid state, nitrogen is -195 degree Celsius! 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.

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 oxidizing 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.

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.

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 5,5 bar and reading 28,3 m³/h will really only be delivering 21,5 m³/h if it is operated at 2,8 bar.  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.

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.

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 oxidizing 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 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.

In its liquid state, nitrogen is -195 degree Celsius! 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.

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 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.

Yes, it's true. In fact, NFPA 86C no longer exists. The requirements for "Industrial Furnaces Using a Special Processing Atmosphere," formally defined NFPA 86C have been incorporated into NFPA 86 as of the 2003 version. Now, NFPA 86 (2019 version available as of this writing) addresses the safety requirements for many types of industrial furnaces, including those typically used for atmospheric and vacuum heat treating (referred to as Class C and D). The previous contents of NFPA 86C are now primarily found in Chapter 13 of NFPA 86.

Many operators of such furnaces use nitrogen gas as both a purge gas and as a component of the process atmosphere. Nitrogen gas is supplied to the process from on-site storage tanks using ambient air to provide the heat to vaporize the liquid nitrogen. This supply mode economically provides very high purity gas at flexible flow rates and does not require any external utilities that may fail in an emergency.

NFPA 86 requires that users of these furnaces include a low temperature alarm panel to indicate an overdraw condition on the ambient air vaporizers used for emergency purging. It additionally requires that tanks containing purge media, such as liquid nitrogen storage tanks be provided with low-level audible and visual alarms to ensure adequate purge volume to provide time for an orderly furnace shut-down. Air Products' nitrogen supply monitoring system is designed to help you comply with these requirements.

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.

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.

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 14 barg from a standard 16,5 barg 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. 

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 3m³ is required for a 5 barg quench pressure (approx. 72 psig), it would require 18m³ 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  3m³ with a minimum operating pressure level of approximately 12 barg (175 psig). A tank with a 15 barg 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.

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.

This is a question that comes up frequently when troubleshooting for oxidation in a continuous furnace atmosphere. The rising price of nickel, and therefore stainless steel, has made belt life longevity more important than ever. While many variables—including the belt alloy, initial break-in procedure, wire gauge and tracking—impact the life of a stainless steel belt, you can realize dramatic improvements by adjusting the sintering atmosphere.

Air Products' atmosphere process technology has been shown in field service to extend the life of stainless steel belts used in sintering powder metal parts. In general, the atmosphere provides a protective oxide coating on the stainless steel belt while remaining carbon-neutral to your parts. The oxide layer reduces the carbon and nitrogen pickup and helps maintain the desired mechanical properties of the belt. In industry service, the use of this technology has resulted in extending the life of stainless steel mesh belts from 25% to more than 50% over the life that is typically experienced in N₂-H₂ sintering atmospheres. The results of extended belt life: reduced maintenance, less furnace downtime and fewer belts to replace.

Many processing variables such as powder size, composition and purity; size distribution; and carbon content affect sintered components’ final properties. The type and amount of lubricants, compaction densities, and furnace parameters—temperature, time at temperature, cooling rates, and belt loading—also influence the end results. Most of these variables are determined during the design stage of the component.

The sintering atmosphere is often overlooked as a variable. The atmosphere properties can vary over time. Controlling an atmosphere system’s variables can improve the consistency of sintered properties. The primary variables in an atmosphere system are the atmosphere composition, purity, flow rates and distribution, pressure inside the furnace, exit velocity, stability (external influences), and the door openings.

Nitrogen-based atmospheres have been successfully proven for a wide range of heat treatment processes over many years. They have been adopted as the industry standard due to their ability to produce the right atmosphere composition to ensure high quality parts and do not produce the well known decarburizing problems associated with endothermic generated atmospheres.

Sintered parts should exit the furnace with a shiny, bright finish. If they don’t, that is a sign of a problem in your process. Oxygen or air may be infiltrating the furnace at the front entrance. Also, if the oxidizing potential in the preheat zone is too high, it can cause oxidation on the powder metal part surface. This oxidized surface reduces as the part travels through the highly reducing atmosphere in the hot zone, causing it to lose its shiny finish and appear dull and matte. In addition to a dulled finish, you may notice lower surface hardness due to the surface decarburization that resulted from the oxidation.

To help solve this problem, you can add a flame curtain at the front end of the furnace. The curtain should be attached to the door to provide full coverage of the front entrance, plus the flame should be directed downwards. You can also control the dew point in the preheat zone so it’s oxidizing enough to facilitate de-lubrication, but does not oxidize the metal.

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.

For sintering and brazing atmospheres in a continuous belt type furnace with open ends, you must follow NFPA 86 Standard for Ovens and Furnaces. Typically, atmospheres containing greater than 4% hydrogen in nitrogen are considered flammable. In fact, any mixed atmosphere—even if it contains less than 4% hydrogen—is considered “indeterminate” and must be treated as if it were flammable. 

NFPA 86 recommends you satisfy the following conditions before introducing any flammable or indeterminate atmosphere into the furnace:

• At least one zone of the furnace must be hotter than 1400°F.
• The furnace must be purged with an inert gas until the atmosphere analysis indicates it’s below 50% of its LEL (lower explosive limit). General recommendation is to use five volume changes of inert gas flow.
• There must be visible indication of purge flow. Plus, purge piping should have normally open solenoid valves.
• The atmosphere system should be designed with interlocks so the flammable gases are shut off using normally closed solenoid valves in the event of power failure, a temperature drop below 1400°F, or insufficient flow of the main atmosphere component.

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 oxidizing 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.

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 1080˚C, 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.

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 CₓHᵧ 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.

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.

Traditional HVOF (High Velocity Oxy-Fuel) systems use a few types of fuels for combustion, typically kerosene, methane (natural gas), propane, propylene and hydrogen. While each fuel has its benefits, hydrogen offers some unique advantages. Because of its higher thermal conductivity, hydrogen achieves the best heat transfer from the flame to the powder particles, despite having an overall lower flame temperature than the hydrocarbons. Excess hydrogen in the flame also creates a reducing atmosphere, which lowers oxide production. Since the stoichiometric reactants of hydrogen and oxygen completely combust, unburned residuals are not deposited on the coating. As the lightest gas with the highest speed of sound properties, hydrogen has the highest potential particle velocity—allowing for higher particle adhesion. Plus you don’t need heating pads in the winter to ensure sufficient fuel flow to your booth as you do with other fuels.

In its liquid state, nitrogen is -195 degree Celsius! 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.

There are only a few types of fuels to use for combustion in traditional HVOF (High Velocity Oxy-Fuel) systems, i.e., hydrogen, kerosene, methane (natural gas), propane and propylene. While each fuel has some distinct advantages, hydrogen offers some unique advantages:

• Because of its higher thermal conductivity, hydrogen achieves the best heat transfer from the flame to the powder particles despite having an overall lower flame temperature in comparison with the conventional hydrocarbon fuels.
• The performance of the HVOF process depends on the type of fuel, stoichiometric ratio, and combustion pressure, as well as gun design features. The ability to run rich hydrogen flows creates a reducing atmosphere, which lowers oxide production and further improves the quality of the coating.
• Due to the complete combustion of the stoichiometric reactants of hydrogen and oxygen, there are no unburned residuals deposited on the coating.

In addition, hydrogen can be delivered at sufficient pressures in tubes and bulk liquid tanks that do not require heating pads during the winter months in order to assure sufficient fuel flow to your HVOF booth.