Aerospace & Defense Technology - September 2021 - 5

Aerospace Manufacturing
Senior Product and Sales Manager at
PVA TePla America.
In addition, commercial heat treat
shops and high-volume part producers
can now select from multiple system
configurations that offer flexibility, efficiency,
repeatability, and throughput optimization.
As a result, global aerospace
OEMs and component manufacturers
are now leveraging these systems to run
a cleaner, more efficient operation.
In the aerospace industry, the number
and range of pulse plasma applications
is rapidly growing. Material-wise, it includes
the low-temperature pulse
plasma nitriding and nitrocarburizing
(350°C - 430°C) of tools and components
made of rust- and acid-resistant
stainless steel to increase wear resistance
without sacrificing corrosion resistance.
It also encompasses the high-temperature
pulse plasma nitriding (600°C -
900°C) of components made of titanium
or titanium alloys.
Applications range from low-temperature
pulse plasma nitriding and plasma
nitrocarburizing of turbine parts made
of high-alloy special steels, to pulse
plasma nitriding of aircraft undercarriage
components, to even the pulse
plasma nitriding of satellite components
made of high-alloy steels.
Pulse Plasma Nitriding Advantages
For steel and steel alloys, case-hardening
can be achieved by carburizing, nitriding,
cyaniding, or carbonitriding.
Although carburizing, and nitrocarburizing
of steel is a traditional approach,
the part has to be raised above the A1
temperature (727°C) on the Iron-Carbon
diagram, usually in the temperature
range of 900°C - 930°C. Since the solubility
of carbon is higher in the
austenitic state than the ferritic state, a
fully austenitic state is required for carburizing.
Along
with the high temperatures and
time-at-temperature associated with carburizing,
parts can be distorted. Therefore,
a post-carburizing heat treatment is
required, at a minimum, to reduce internal
part stresses. Depending on the part,
and its geometric tolerances, limited
machining may also be required.
An alternative to carburizing is nitriding,
a lower-temperature, time-depenIn
the aerospace industry, the number and range of pulse plasma applications is rapidly growing.
dent, thermo-chemical process used to
diffuse nitrogen into the surface of
metal. One method is salt bath nitriding.
In this process, liquid immersion is
required, and is typically conducted at
550°C to 570°C. The nitrogen donating
medium is a nitrogen-containing salt,
such as sodium cyanide, often greater
than 50% in concentration. However,
with post-salt bath nitriding there is a
high washing effort required to remove
the residual cyanide-based treatment.
In addition, there are disposal costs for
salt and washing lye, environmental
handling costs, as well as safety and operational
liabilities.
Gas nitriding (500°C) and gas nitrocarburizing
(540°C - 580°C) are universally
accepted procedures, and typically
require a high concentration of ammonia
(NH3), and a high amount of carrier
gas flow (normal pressure process) compared
with pulse plasma nitriding. The
elemental nitrogen gas constituent diffuses
into iron and forms hard nitrides.
Because of the reduced temperature
compared to carburizing, no quenching
is necessary, and therefore the chance
for distortion and cracking are lower.
Disadvantages of gas nitriding are that
it requires the use of flammable gasses
like ammonia, high gas consumption,
and it is not able to treat nitride rustand
acid-resistant steels.
With recent advancements in pulse
plasma nitriding, however, a new
level of precision and control is possible
which results in uniform and consistent
case hardening. Together with
the advantages of using environmenAerospace
& Defense Technology, September 2021 www.aerodefensetech.com
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tally friendly gasses only - in contrast
to the use of ammonia in gas nitriding
- plasma-based nitriding has become
a focal point for additional innovations
and a requirement for those that
seek a more environmentally and safe
solution.
In pulse plasma nitriding, parts are
loaded into a heated vacuum chamber.
After evacuating the chamber to a working
pressure of 50 to 400 Pa, on a supporting
fixture, it is covered by a bell
chamber. The chamber is evacuated to
below 10 Pascals prior to heating and a
pulsating DC voltage of several hundred
volts is applied between the charge
(cathode) and the chamber wall (anode).
The process gas in the chamber is then
ionized and becomes electrically conducting.
For this type of process, nitrogen
and hydrogen gas mixtures and
gasses with carbon additions, like
methane, are often utilized.
Depending on treatment time and
temperature, nitrogen atoms diffuse into
the outer zone of components and form
a diffusion zone. This can be as atomic
nitrogen, dissolved in the iron lattice, as
well as in the form of included nitrate
deposition.
Adding further precision, innovators
in pulse plasma have discovered methods
to optimize the process through
better control of the pulses. In the
PulsPlasma® process developed by PVA
TePla AG Industrial Vacuum Systems, for
example, a precision regulated gas mixture
of nitrogen, hydrogen, and carbonbased
methane is used. A pulsating DC
voltage signal of several hundred volts is
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Aerospace & Defense Technology - September 2021

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