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How is the dew point measured in the furnace atmosphere and why is it the largest factor in determining the quality of the atmosphere for most heat treating applications?

Dew point, or the moisture content of the atmosphere, is measured using a hygrometer. The traditional manual type is based on the principle of condensation and requires manually pumping the sample and visually recording the temperature of the fog that appears in the test device's chamber. More recent continuous monitoring devices are also available that can give digital readouts of the dew point and can be recorded in a data log.

The dew point is the most important property of the atmosphere that needs to be monitored and controlled because it determines the reducing power of the atmosphere; that is, assuming other factors such as hydrogen content, temperature and time remain unchanged. It's also one of the factors that affects the decarburization and oxidation of the alloy being processed.

What safety precautions and steps do we need to take to implement nitrogen/hydrogen atmosphere systems?

All of the safety steps that are recommended by NFPA 86 for atmospheres containing flammable gases are also observed for nitrogen/hydrogen systems. Procedures are detailed in NFPA 86 for the proper introduction and removal of flammable gases from a heat treat furnace. There are also specific distance requirements for the location of the hydrogen supply storage trailers and cryogenic tanks.

In addition, all mixing and flow control systems must follow NFPA guidelines for safety interlocks and purge requirements. If you need help determining the safety criteria to follow, contact us or give us a call at 800-654-4567, code 688.

How do I determine the right flow rate and atmosphere composition for the nitrogen/hydrogen system if my current 24 inch wide sintering belt furnace is using 2500 scfh of an endothermic atmosphere?

The operating flow rate required for an open ended belt furnace, such as those used in powder metal sintering and brazing, largely depends on the cross sectional area of the entrance and exit ends of the furnace. It doesn’t depend on the length or the internal volume of the furnace. After the furnace gets filled up by the atmosphere, the velocity of the atmosphere in the furnace and that exiting the furnace can be determined by dividing the flow by the cross sectional area.

Typically, the velocity should be sufficient to flush out the moisture and lubricant vapors from the furnace, plus be sufficient enough to prevent air (oxygen) from entering the furnace. A general rule of thumb for the flow rate is about 75 to 85 scfh per inch of belt width for door openings less than 3 inches. A good leak-free 24 inch belt furnace can operate well with a total flow rate of about 1800 to 2000 scfh.

It is also important to note that there is an optimum flow rate. Going above that rate may create turbulent flow and a venturi effect at the door. This can make things worse by entraining and sucking in higher amounts of air into the furnace. Air Products application engineers, can help you determine the optimum flow rate so you can operate the most efficiently. Contact us or give us a call at 800-654-4567, code 688.

We typically cool our reactive metals in a helium atmosphere to minimize the cooling time required. Is there a helium/argon mixture that may allow comparable cooling times?

Most companies use 100% helium as an atmosphere for reactive metals and as a cooling medium when high rates of cooling are required. However, a few years ago, Air Products developed a blend of helium and argon/nitrogen that equals and possibly exceeds the cooling rates obtained from 100% helium. Given the relatively lower costs for argon, this can significantly reduce the total gas costs. 

Read a technical paper on the subject, by Minfa Lin, Ph.D., Air Products Senior Principal Research Engineer, that will give more detailed information. If you interested in further exploring this and other atmosphere and cooling technologies with Air Products, please contact us or call 800-654-4567, code 688.

Our sintered parts have a blue, black or brown color. How can we improve our part color?

The discoloration of the sintered, powder metal part could be the result of oxidation occurring in the hot zone or in the cooling zone. Plus, it may be caused by high dew point or high oxygen levels—or both. Careful examination of the nature and extent of the oxidation together with an oxygen and dew point analysis of the atmosphere should help you find the cause and location of the oxidation. If the oxidation is happening in the last cooling zone due to infiltration of air from the exit end, increasing the nitrogen flow to the cooling zone can help dilute the oxygen content. Plus, if the nitrogen is used as a curtain, it can help prevent the ingress of air into the furnace.

For additional help with this and other trouble shooting techniques, contact us or call us at 800-654-4567, code 688.

How much nitrogen gas should we use in our cooling zone?

For typical, open ended, belt furnaces, you can introduce nitrogen at 20% of the total flow to the front end and 20% to the exit end. Into the hot zone, you can use about 60% as mixed gases (nitrogen plus hydrogen). Additional care should be taken so the atmosphere is not sucked out from the furnace. The atmosphere should be gently pushed out of the furnace. You can achieve this through proper design and control over the exhaust hoods and the makeup air in the plant. For specific atmosphere ratio recommendations to your operation, contact us or call us at 800-654-4567, code 688.

How can I benefit from a nitrogen-based system if I’m already getting good parts at a reasonably low cost? Are there any downsides?

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 the carbon monoxide and carbon dioxide emissions.

Please keep in mind that nitrogen-based atmosphere systems are generally economical when you have more than one furnace. Additional gas piping and a mixing panel are required to supply, monitor and control the atmosphere in the furnace.

To learn whether or not a nitrogen-based system makes sense for your operation, contact us or call us at 800-654-4567, code 688.

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