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FNC Criteria

	The FNC Phenomenon
	Gas velocities v/s temperatures
	Heat transfer factors
	Gas composition & percentages
	Summary

The FNC Phenomenon

Fluidization, contrary what you may be led to believe, is not a new technology.  An 1879 patent discusses baking minerals under bed conditions and draws attention to the outstanding uniformity of temperature it affords.

Fluidization consists of making a bed of dry finely-divided particles, usually aluminum oxide, behave like a liquid.  This is accomplished by moving gases through the medium's particles which separates them microscopically as the gases are fed through the bed.  The majority of beds used for heat treatment are of the aggregative or bubbling bed type.  The smooth or bubbly properties of the solids and fluids will determine the fluidization quality.  On the other hand, the size of the bubbles and heterogeneity in the bed influences the rate of solid mixing.  These factors include bed geometry, gas flow rate, type of gas distributor, and internal vessel features such as baffles, screens, and heat exchangers.  It can easily be seen that the success of fluidization, carbonitriding, and quality of treatment is influenced by many needed control factors.  TherMaLLifeÒ has these controls..

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Gas velocities v/s temperatures

If you plunge your hand into a fluidized bed medium (preferably one that is not heated), the sensation is the same as placing your hand in a bucket of water.  When an aluminum oxide medium is fluidized in a bed, and a light object is introduced in the bed, it will float, the same as in a liquid.  The required flow of gas is inversely proportional to the temperature of the bed and decreases rapidly with temperature.

The graph below shows the effect of temperature on gas flow rate with a   .1mm diameter particles that have an apparent assumed density of 2.0  The higher the temperature the lower the needed gas flow to maximize fluidization.  You must also keep in mind that heat transfer rate coefficient is affected by gas velocity.  This will be explained later on this page.  Also we must not forget  that the fluidization needs to be maintained at a temperature that is below the critical temperature of the steel (last heat treat temper temperature). This prevents the possibility of compromising the initial heat treat tool hardness.  The below chart shows the importance of various controls for repeatability.

As temperature increases		required gas flow decreases
Too Low of temperature clumping		reduced heat transfer coefficient
Too High of temperature cavitation		reduced heat transfer coefficient
Too High of temperature		steel hardness may be affected

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Heat Transfer Factors

Three main factors affect the heat transfer rate in a fluidized bed.  It is not enough to just maintain temperature.  All of these items are also critical to optimization of the process and the development of the correct recipe for each specific tool steel.

• Cleanliness of the tool steel
• Particle Diameter
• Bed Material Density
• Fluidization Velocity of the gas/gases

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Cleanliness of the tool steel - First and foremost is the cleanliness of the part or tool steel.  Usually a mild to aggressive alkaline bath at elevated temperature is recommended to remove both water and oil base materials on the surface and any other areas that will contact the bed media..  If these trace elements are not entirely removed the nitrocarburizing is compromised. In addition the bed media is also contaminated that jeopardizes current and subsequent batches in the bed.  For these reasons it is important to ask and know how the tools are cleaned prior to processing.  It is also imperative that USED tooling have the tool shop or die caster remove all of the solder by polishing or using NaOH material. Something as simple as this can cause immediate failure of the nitrocarburizing as well as inconsistent unexplained results.

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Particle Diameter - This parameter has the greatest influence on heat transfer.  Tests have shown that the medium should be as small as possible.  In practice the optimum size is 100 micro mm (3940 micro inches). 

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Bed Material Density - It has been determined that the governing physical property of the bed material is the density.  The apparent optimum value is around 1280-1600 kg/cu m or about 80-100lb/cu ft.  Denser materials tend to give a lower heat transfer coefficient and require more gas flow for fluidization.  Sometimes this condition can be exacerbated to an electrostatic effect if too low of density material is used.  Importantly other properties such as thermal conductivity and specific heat do not enter into the equation and are relatively unimportant.

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Fluidization Velocity of Gas/Gases - To obtain the maximum heat transfer, it is essential that an optimum flow rate be used.  This generally is between two and three times the minimum fluidization velocity.  Too high of velocity leads to particle entrapment, a higher consumption of fluidizing gas/gases, and poor heat transfer.  Too low of velocity gives poor heat transfer and lack of uniformity of processing. TherMaLLifeÒ  automates and computer regulates this and all the other important requirements to assure a repeatable highly controlled ferritic nitrocarburizing process.
Conventional methods control this critical phenomena by using a large number of screws in the bottom of the bed.  The passage of the gas between the threads of these control how much gas is introduced into the system.  These screws loosen and/or tighten over time which drastically affect the heat transfer rate between the threads.  Because of the labor intensive aspect of the maintenance of these screws, which require that the bed be totally emptied, adjustments are often neglected or bypassed.  It is important to investigate this especially if you see unexplained variations in processing.
Our TherMaLL ifeÒ multi-station bed does not use screws.  We incorporate a series of adjustable ceramic screens and eclipse valves with flow meters and sensors which by computer programming provide the exact control of the gas velocities to provide optimization of temperature and velocity.

The following graph shows the relationship of velocity to heat-transfer coefficient with a = f(1/t_c).  This data clearly shows that there is a definite optimum gas/gases velocity that maximizes the essential heat transfer rate.

As gas velocities get to high		cavitations may occur
As gas velocities get to low		clumping may occur

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Gas Composition and percentage - When all of the above have been taken into consideration, the most important element of the equation for a correct recipe is the gases used and percent amounts of each.

Summarizing - Many different criteria and controls are necessary to produce a repeatable and correct recipe for performing ferritic nitrocarburizing - of tool steels. Some of these controls are cleanliness of the tools, temperature of the bed, velocity of the gas/gases, composition of the gases, and time at proper temperature.  Without exact controls for each of these parameters, it is very difficult to have a repeatable process..

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A special thank you to Ray W. Reynoldson
for permitting us to source this information from his book and literature
"Heat Treatment in Fluidized Bed Furnaces"