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/m2at 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.
Comparisons Between Hot-Wall, Cold-Wall Plasma-Ion Nitriding Systems
By David Pye

David Pye is the owner and operator of Pye Metallurgical International Consulting, Saint Anne's on Sea, Lancashire, U.K. He has 25 years of practical experience in captive and commercial heat treatment, metallurgical laboratory operation and industrial furnace sales. He also has teaching experience on a very wide range of heat-treatment and metallurgical subjects.
He can be reached at pye_d@ymail.com.
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