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David Pye is the owner and operator of Pye Metallurgical Consulting in Meadville, Pa. He has 22 years of experience in captive and commercial heat treating and metallurgical laboratories. Mr. Pye also has 17 years experience in furnace sales.

# Comparisons Between Hot-Wall, Cold-Wall Plasma-Ion Nitriding Systems

August 23, 2012
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The following information provides comparisons between hot-wall and cold-wall plasma-ion-nitriding systems.

Question: At what temperature is plasma started?
Cold wall: Room temperature
Hot wall: At a suitable elevated temperature usually around 200°C

Question: Why is plasma started at those temperatures?
Cold wall: The cold wall uses a constant D.C. system, which requires plasma general voltages around 600-800V. This means that there is a very serious risk of mechanical and metallurgical damage to the surface of the work by working so close to the arc discharge region.
Hot wall: The hot wall utilizes a partial-pressure condition using hydrogen or nitrogen as a thermal conductance gas. The vacuum retort is heated only by external heaters and not by plasma voltage. This means that the input voltage is not as high, 400-500V, and is away from this arc discharge region.

Question: Why is there a difference in plasma-generation voltages?
Cold wall: To generate the necessary amperage to ensure that sufficient heat causes the workpiece to heat up. The general Kwh would be approximately 1 amps/sq ft or 10 amps/m2 at the necessary partial pressure.
Hot wall: Because the part is already preheated, the power input would be at a suitable partial pressure of 0.093 amps/square foot to 0.372 amps/square foot (1-2 amps/m2). The normal power would be approximately 1-2 amps/m2, but we size between 1-4 amps so the power rating is oversized normally.

Question: How does the heat-up rate compare between cold and hot wall?
Cold wall: The cold wall usually requires more time for heat-up than hot wall.
Hot wall: The hot wall heat-up of the port would usually be about 15 faster than cold wall.

Question: Why pulse the power input?
Cold wall: With a constant voltage input, there is a constant heat output. If you reduce voltages to reduce temperature, you change the current density and the cathode fall thickness (glow seam). Therefore, you must begin to change other parameters.
Hot wall: With a pulsed voltage, high voltage can be used without risk of overheating this part or taking the part to the point of arc discharge. This means that the other parameters need not be changed.

Question: What happens to the heat?
Cold wall: With a cold-wall system, the released electron is hotter than the ion. This means that the electron goes back to the furnace wall (anode) and creates heat. The heat-up of the wall will continue, hence the need for water cooling of that wall to dissipate the heat.
Hot wall: The hot-wall furnace combined with the pulse technology uses external blowers as a means of preventing heating the wall to excessive temperatures. The wall temperature can safely go up to temperatures of around 650°C without any concern of a heat buildup.

Question: What happens if there is a need to push the glow seam into a deep-blind hole?
Cold wall: Increasing the operating pressure causes a corresponding increase in current density followed by an increase in part surface temperature.
Hot wall: Using the hot-wall/pulsed-power combination with the same pressure\/temperature combination and the same voltage/current relationship as the cold wall, the plasma energy can be maintained by  varying the duty cycle (pulse variation), even with a changing voltage and current density.

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