|
Automation Benefits
A properly controlled atmosphere with dissociation monitoring allows tools to be heated or cooled at rates that are very close to those obtained in a conventional salt or lead bath environment without suffering the distortion or contamination side effects. As discussed in our section - "FNC criteria", to obtain a repeatable
and controlled nitrocarburizing procedure, it is imperative to have properly controlled heat transfer criteria to assure successfully completing the entire process. Five factors have been established as being the most critical for tooling applications.
Let us examine these one at a time.
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 subsequent batches in the bed. For these reasons it is important to ask and know how the tools are cleaned prior to processing. Our automated process includes and automatic first station alkaline bath and dry treatment at elevated temperature (below 212 degrees F) for a predetermined amount of time depending on the load.
back to menu
Particle diameter
The diameter of the aluminum oxide particles have the greatest influence on heat transfer. The objective is to keep the particle size as small as possible without causing and electrostatic effect to develop and cause problems. To large of particles do not allow the heat to be properly transferred, while too small of size may develop electrostatic properties that cause the particles to
clump together negating the desired transfer rate. In practice the optimum particle diameter which is used for TherMaLLifeÒ
is 100µm or 3940µin.
back to menu
Bed material density
The concluded optimum density for bed material appears to be around 1280 to 1600 kb/m3 (80 to 100lbs/ft3) Use of too high of density material for the bed can dynamically affect the heat coefficient transfer rate and also require more power (heat & velocity) to attain fluidization. Electrostatic effects come into play if too low of density material is
used.
back to menu
Fluidization Velocity of gases
To assure maximum heat transfer, an optimum maintained flow rate of the required gas/gases must be employed in conjunction with particle densities and diameters as specified in the two preceding criteria. Too high of a velocity generates particle entrapment and a higher than required consumption of fluidizing gas/gases which lower heat transfer rates. On the other hand, too low of
velocity also leads to poor heat transfer and a serious lack of uniformity in processing. By automating and monitoring the controls and flow requirements of this criteria, TherMaLL ifeÒ
assures that optimum flow rate is maintained on a real time basis. Read outs and warning devices provide constant surveillance of this critical criteria. Without these benchmarks, tooling applications will show periodic successes and failures for the same applications.
The following graph shows the relationship of velocity to heat-transfer coefficient with a = f(1/tc). This data clearly shows that optimum gas/gases velocity is essential to maximizing heat transfer rate.
Temperature controls
Temperature Controls
Automated controls of this criteria require accurate burner controls along with strategic placement of thermocouples in each bed to provide the flat line temperature curve that is so critical to provide:
1. Maximization of heat transfer per recipe of the steel being treated.
2. Assurance that the bed temperature does not reach to high of temperature.
After determination of the correct processing temperature, which must below the last temper temperature, the nitrocarburing process is set to the specific and required recipe. If the bed temperature is not maintained and falls too much, the nitrocarburizing of the steel is not maximized. The depth of the compound layer may also prove to have insufficient composition
characteristics. On the other hand, too high of temperature could result in the possible annealing of the steel. The TherMaLLifeÒ
automated process automatically controls the burners for heating the bed by monitoring the strategically placed thermocouples in the bed. The results of this monitoring are then graphed until a a flat line desired temperature is attained once the steel's original temperature and the bed have acclimated to one another.
| 
