Design aspects of Plasma Nitriding

Process

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Nitriding, nitro-carburising are temperature treatments
that produce thin, hard layers on the steel surface to
improve its wear , corrosion resistance.

In general terms harder a piece of steel for making

components, tools etc., less will be wear .

Component like gear ; should be as hard as possible so

that it will not wear out.

Unfortunately in steels, high hardness has other

consequences, one of those is loss of ductility. Hard steel
tend to be brittle.

Gear out of hard, wear resistant steel, if shocked say while

changing gear for instance, the gear teeth would fracture
and fall off.
Solution : produce a hard wear resistant surface
outside with tough shock resistant core

Both treatments diffuse nitrogen into the surface of

steel to form layer.
Thermo chemical treatment

Thermo chemical treatment involves the introduction of

alloying elements (such as carbon and nitrogen) into the
surface of steel through diffusion at elevated
temperatures to produce a hardened surface case and a
soft, tough and strong core within a steel.

Thermochemical treatment processes mainly include

nitriding and carburising.
The nitriding process
The generic term nitriding refers to a thermochemical treatment with
which the surface zone of ferrous materials is enriched with nitrogen.

When nitrogen diffuses into the surface zone, it is at first dissolved

interstitially in the iron matrix. If the nitrogen concentration exceeds the
solubility limit of 2.5 weight percent, a single- or multi-phase nitride
layer is formed.

This treatment is preferably carried out in the temperature range

between 400 and 600°C.

By nitriding steel, steel surface hardens to form nitrides (AlxN, CrxN,

VxN,…)

The familiar nitriding processes provide not only excellent corrosion

protection but also outstanding protection against wear whilst also
allowing the dynamic characteristics of components made of ferrous
materials to be improved.
Schematic illustration of gas nitriding
If only nitrogen is incorporated in the surface zone, the
process is referred to as “nitriding”.

If at the same time carbon diffuses into the surface zone

as a result of the addition of a carbon source to the
nitriding medium, the process is called “nitrocarburizing”.

Both methods are used primarily for providing wear

protection.

With the selective addition of oxygen to the nitriding

atmosphere, the process is referred to as “oxynitriding”.

This process variant is used to produce a porous nitride

layer in the nitriding zone, which is important for the
adhesion of an oxide layer applied subsequently.
The Fe-N equilibrium phase diagram
The effects of introducing nitrogen (N) into the α-Fe
lattice are shown in the Fe-N equilibrium phase diagram.

It is evident that there is a limited interstitial solid

solubility of N in α-Fe, with a maximum of 0.1 wt% N at
585ºC.

As is evident from the phase diagram, a new phase is

precipitated, namely γ’- Fe4N as the concentration of
interstitial N in the α-Fe matrix increases beyond 0.1 wt%
N and at a temperature below 585ºC. In the composition
range of 5.7 – 5.9 wt% N, γ’- Fe4N forms as a single
phase field

Increasing the N concentration higher than 5.9 wt% N,

results in the precipitation of another iron nitride phase
namely ε-Fe2-3N
Around ~7.6 wt% N, the ε-Fe2-3N iron nitride exists as a
single phase field. Above this concentration of ~7.6 wt%
N, there exists another phase of iron nitride, that is δ –
Fe2N. This iron nitride phase exists in a narrow band of N
concentration between approximately ~11.1 wt% N and
11.35 wt% N and below ~500ºC.

In the compound layer normally, N enters the steel

surface by a process of diffusion by using one of many
possible nitriding techniques.

All this assumes that there is a sufficient N potential at

the surface and that there are no impediments to the N
entering the steel lattice, such as impervious oxide
layers. As the N content of the surface reaches the
solubility limit of 0.1 wt% N, then γ’- Fe4N starts to
precipitate.
With further diffusion of N the volume of γ’- Fe4N
precipitates increases such that eventually the surface
forms a continuous layer of γ’- Fe4N at a N concentration
of around 5.7 wt% N.

With further build up of N beyond 5.9 wt% N, ε-Fe2-3N will

start to precipitate.
This results in the formation of a dual phase consisting
of γ’- Fe4N and ε-Fe2-3N and is commonly referred to as
the compound layer.
In some nitriding applications the compound layer can be
up to 50 μm thick which can lead to spalling and possible
seizure of components. Depending on the application of
the treated component, this compound layer can be
removed by mechanical grinding in order to avoid
potential failures. It should be noted that the formation of
the compound layer gives rise to a reduced diffusion
coefficient for N in α-Fe
Carburising

Carburising involves changing the carbon content of the

surface, followed by a quenching process to convert the
surface layers to martensite.

There are different methods of carburising: gas

carburising, salt bath carburizing and pack carburising.

Plasma carburising is normally carried out in a vacuum

furnace at a temperature range of 950 to1050ºC by
diffusion of deposited carbon ions on the surface.

The carbon ion containing plasma is produced by glow

discharge in a mixture of hydrocarbon plus hydrogen for
dilution.
Nitrocarburizing

A thermo chemical process in which nitrogen and carbon

are diffused into the surface of a ferrous metal.

It produces a thin case consisting of a ceramic iron-

nitrocarbide layer and an underlying diffusion zone
where nitrogen and carbon are dissolved in the matrix.

The treatment is mainly used to improve wear and fatigue

properties and to enhance corrosion resistance.
Common areas of application include gears and engine
parts

2NH3 = 2N+ 3H2

CO2 +H2 = CO +H2O
CO +H2 = C +H2O
After gaseous nitrocarburizing samples are usually
quenched in oil or gas.

Oil quenching is the fastest of the two methods. The

quenching intensity mainly affects the properties of the
diffusion zone.

In low alloy steels quick cooling leads to more nitrogen

left in solid solution in ferrite and hardness is increased.

In high alloy steels the hardness of the diffusion zone

depends mainly on precipitation hardening and
quenching rate plays a less vital role.
Quick quenching also results in higher compressive
residual stresses in the surface region, which improves
fatigue resistance.

Shortened production time is also an advantage.

Too fast quenching can however lead to cracks. Slower

cooling rate is preferred if distortions need to be
minimized.

Nitrocarburizing can be followed by a post-oxidation step

which is performed to enhance the corrosion resistance of
the material. The goal is to create a continuous layer of
protective magnetite, Fe3O4, on the surface of the
compound layer. The oxide layer thickness is typically in
the range of a micrometer and it gives the surface a black
appearance
Porosity

The outer part of the compound layer contains pores.

Longer process time and high nitrogen potential
increases the amount of porosity.

On low alloy steels, it is common that 30–40 % of the

compound layer is porous. Increasing the alloy content
causes the porosity to decrease, likely because the
nitrogen activity is lowered by alloying.

Pores form by the pressure of gaseous nitrogen as

interstitial nitrogen in the compound layer recombines to
nitrogen molecules (2N = N2).

The phenomenon occurs particularly at grain boundaries

and other discontinuities
The Diffusion Zone

Beneath the compound layer is the diffusion zone,

classically this zone consists of interstitial N dissolved in
the α-Fe lattice and Fe alloy carbo-nitrides.

It is shown that the high surface hardness obtained by

nitriding is attributable to the precipitation of fine alloy
nitrides.

The latter are a consequence of the presence of strong

nitride forming elements, such as Al, Cr, Mo, Ti, Mn, Si
and V in the steel substrate.

Hardness in the diffusion zone depends on the amount of

alloying elements in the steel.
Layer structure of the nitrided zone

The surface zones of nitrided ferrous materials are

generally composed of two distinct parts. Directly on
the surface is the compound layer which is typically 2 to
50 μm thick, hard and chemically resistant. Beneath it is
the tougher diffusion layer with a thickness of 0.1 to 0.8
mm .

The compound layer which consists of iron nitrides

and/or carbonitrides determines the ceramic character
of the surface. The compound layer assumes one of the
three following forms, according to the depth-dependent
concentration distributions of nitrogen and carbon:
• a γ’-compound layer (γ’-nitride: Fe4N)
• an ε-compound layer containing more nitrogen and/or
carbon (ε-nitride: Fe2-3N,
ε-carbonitride: Fe2-3NC)
• a mixed-phase compound layer (γ’-nitride and ε-nitride).

γ’-compound layers are tougher than ε-compound layers,

but they grow more slowly
The composition of the compound layer may also be
modified by the presence of special nitrides and a more or
less pronounced porous zone .

The considerable range of control options for the plasma-

based nitriding process enables the growth of the
compound layer to be optimized for a specific application.

As a rule, a mixture of both nitride phases is obtained.

Depending on the process control parameters, nearly

single-phase γ’ (plasma nitriding) or ε-compound layers
(plasma nitrocarburizing) can be produced.

Both types of compound layer are characterized by high

resistance to wear. As the nitrogen content increases,
hardness, corrosion resistance and ceramic character
increase and ductility decreases.
Structure of the compound and diffusion layers
 Nitriding Processes

Enrichment with nitrogen or with nitrogen and carbon

can be achieved by means of:

• a gas mixture (gas nitriding)

• a molten salt (salt bath nitriding)

• a low-energy plasma (plasma nitriding).

Sketch of gas nitriding chamber and equipment.
Schematic of a simple ammonia gas nitriding furnace
Gas Nitriding Unit
The nitriding chamber and fittings must be made from a
material that will not react significantly with the nitriding
gas.

The interior of the chamber and the parts to be

treated are then heated to the nitriding temperature (520-
700ºC) via heating elements located inside the nitriding
chamber.

By using a circulating fan located at an appropriate position

inside the chamber, the temperature can be kept uniform.

Ammonia gas NH3, controlled by a flow meter and needle

valve, is then allowed to flow into the nitriding chamber for
a specific time. The hot metal components catalytically
dissociate the ammonia gas according to the reaction,
Schematic illustration of gas nitriding
The hot metal components catalytically dissociate the
ammonia gas according to the reaction,

NH3 = N + 3 H

A percentage of the constituent atomic N reacts with

the surface of the components to form iron nitrides,
FeNx.

The remainder of atomic N reverts back to its molecular

state, described by

2 N = N2

In addition, atomic hydrogen (H) recombines as per

2 H = H2
Liberation of nascent N
Once re-combination has occurred, reactions with the
surface of the components are less likely due to the size
of the molecules in comparison with the metallic space
lattices.

It is then necessary to purge the remaining H2, N2 and

diluted ammonia gas by replacing it with fresh ammonia
gas.

The level of ammonia dissociation is determined by

utilising the fact that ammonia is water soluble where N2
and H2 are not.

Thus, an evaluation of the flow rate level is readily

obtained by using a water filled pipette
Gas nitriding does have its disadvantages; the gas
nitriding process uses ammonia, which in
concentrations of 15-26% in air, produces a flammable
environment.

This scenario can have severe ramifications especially

in a commercial environment if a leak should occur. In
addition, if a leak was to occur in the nitriding chamber
the dry ammonia gas would be in direct contact with
moist air, which produces a corrosive mixture.

It is for this reason that components of a gas nitriding

chamber must be periodically examined for any signs of
fatigue or corrosion and if discovered must be replaced.
Treatment cycle times for gas nitriding systems can be
of the order of 90 hours for significant nitriding depths,
but treatment times between 40 and 60 hours are more
typical.

Long nitriding times increase the overall cost associated

with nitriding a component such as an automotive
crankshaft when compared to other case hardening
methods.

A direct effect of the long treatment times used in gas

nitriding is the formation of thick compound layers. In
some cases this layer can be up to 20-50 μm thick which
can de-laminate during the components operation. The
removal of this compound layer to prevent spalling from
taking place can incur costs comparable to the gas
nitriding treatment itself
The health and environmental considerations associated
with ammonia gas nitriding ultimately limit its commercial
viability.

Even though ammonia gas is not considered harmful in

low concentrations it can cause irritation and discomfort
for personnel working in the environment, however, at high
concentrations which are present during gas nitriding the
hazards can be fatal in the event of a large leak.

In addition, ammonia is fatal for aquatic species even in

minute quantities which constitutes a significant
environmental hazard.
 Gas Nitriding

• Thermo chemical treatment producing enhanced surface

properties from the bulk properties

• Case hardening results from diffusion of N into substrate

(solid solution) and precipitation of nitrides (FeN and alloy
elements nitrides) when holding the metal at suitable
temperature (generally below 575°C)

• Ammonia (NH3) is nitrogenous gas typically used since it is

metastable at nitriding temperature and decomposes on
contact with iron

• Quenching is not required for production of a hard case

Limitations of Gas Nitriding
Salt Bath Nitriding / Liquid Nitriding

A mixture of molten potassium and sodium cyanide salts

are the essential ingredients in salt bath liquid nitriding.
Typically, the salt mixture consists of 60-70% sodium
salts and 30-40% potassium salts. These salts form a
molten eutectic when raised to within a temperature
range of between 535ºC and 595ºC for 24-48 hours.

It is essential that the nitriding salts are free from

moisture before melting takes place, otherwise an
explosion may occur.

In addition, during the melting phase the bath cover

should be in place to prevent splash and splatter of the
molten salts from the bath.
Once the nitriding salts have been thoroughly melted,
nitriding can take place.

The operating temperature is generally around 565ºC

and treatment times can vary from ~ 5-50 hours
depending on the system and desired results.

Various techniques exist which can accelerate this

nitriding process, such as adding sulphur to the salt
bath, pressurising the salt bath and aerating the salt
bath
Sketch of a typical molten salt bath nitriding unit.
One major drawback of molten salt bath nitriding is the
high maintenance required. It is recommended that the
nitriding salts be completely changed every 3-4 months
to minimise corrosion of the bath.

This raises the issue of waste disposal of these toxic

chemicals. In addition, the salt bath composition needs
to be analysed weekly to ensure correct quantities of
the constituent chemicals.

It is also recommended that regular desludging of the

salt bath be undertaken to minimize contamination of
the bath.
Salt bath liquid nitriding requires poisonous cyanide
containing salts which pose a serious health and
environmental risk if an accident or leak should occur.

Appropriate safety measures also need to be taken in

terms of personal protective equipment and adequate
ventilation to minimise corrosion and health effects
 Salt bath nitriding
Salt bath nitriding has been used for decades in various branches of industry. The
nitrogen penetrating the surface zone is derived from a liquid medium consisting of
molten salts.

The temperature of this salt bath is usually between 400 and 600°C.

Cyanate, which is used to this day, undergoes catalytic decomposition on the steel
surfaces at these temperatures to form cyanide, carbonate and adsorbed nitrogen.

Due to the formation of the carbonates, this method is only able to produce nitride
layers containing carbon. Thus it is always a nitrocarburizing process.

In order to avoid carryover of the highly toxic bath constituents, the tools or
components must be washed thoroughly after treatment.

Used salt bath materials must be disposed of in an environmentally acceptable

manner.

It is an old technology with a negative impact on the environment, which is increasingly

being replaced by gas or plasma nitriding in industrial use.
 Salt Bath Nitriding

The process is called salt bath nitriding because the parts to be nitrided are immersed
into a salt bath containing molten salt combinations. The salt mixtures originally had 60
– 70% by weight NaCN and 30-40% KCN [1]. There is in addition, a few percent of
carbonates Na2CO3 and cyanates NaCNO. The process relies on the decomposition of
cyanade to cyanate.
At operation, a desired level of cyanate should be 45%. This is accomplished by aging
the bath at a temperature of approximately 570 °C and for around 12 hours. It is
important to keep this level of cyanates along the whole bath to avoid differences of
hardness and nitrided layer thickness in the steel parts, thus, to do this, air is injected
into the bath to control the cyanate level [1].
Another problem that may easily arise, is the dissolution of the iron crucible in the bath.
This leads to oxidation and pitting of the steel charge. To overcome this problem the
crucibles of the salt baths today are made of titanium. The normal temperatures for
this process are between 550 – 570 °C and a maximum average process time of about
2 hours and up to maximum 4 hours. During immersion time the salt bath gives off
carbon and nitrogen according to the following expressions[1]:

4NaCNO ---2NaCN + Na2CO3 + CO + 2N

3Fe + 2CO ----- Fe3C + CO2
The process using this type of salts was developed by DEGUSSA and is called the
Tufftride process (Teniferbehandlung in German).
A different variation of the process is called the “Sulfinuz Treatment” [1]. In this
process sodium sulphide (Na2S) is also a component of the salt, which will liberate
sulphur that will be included in the nitrided layer enhancing the antifriction properties of
the outer compound layer.
After any of these processes, quenching in warm water will give a better result. This
will create a supersaturated solid solution of nitrogen in a-iron and thus increase the
fatigue resistance of the part. On the other hand, this operation reduces the
toughness of the nitrided layer [1] which should also be considered.
Salt baths became very popular because the process led to an outer surface
compound layer of g’-nitride (Fe4N) and e-nitride (Fe3N) which are not as brittle as z-
nitride (Fe2N) and are used to enhance the wear resistance, the friction properties and
the corrosion resistance of the steel surface. The formation of these layers is due to
the reaction between oxygen-saturated-cyanates which are active compounds which
were formed by aeration of the bath which oxidized the cyanides, and the steel surface
[6].
Although very beneficial, for years salt baths have been used with some problems, i.e.
it was hard to control the chemical composition of the salt and had big environmental
problems such as complicated regulation for disposal of waste salts and the disposal of
rinse water which is also environmentally hazardous. These complications made the
development of the baths to slow down and in many cases be replaced by gas nitriding
[6].
Due to this, there has been a necessity for developing a novel salt bath composition
that having the same nitriding capabilities was environmentally friendly. Cyanate-base
baths were developed. In this type of baths, the environmental problem was solved
since the disposal process and control of the salt reactivity with the help of a developed
generator was much more easier than that with the cyanade baths. On the other hand,
it took some time for the process to be fully developed and go into industrial use since
the chemical reactivity of the bath was difficult to control [6
Direct Current Plasma Nitriding (DCPN)

Also known as Ion nitriding and Glow-Discharge nitriding,

components to be treated are placed on a conducting
metal plate (cathode) inside a vacuum chamber.

The metal plate and the samples to be treated are

subjected to a high cathodic potential of up to 1500V,
where the metal chamber walls form the anode of the
system.

This cathodic potential is responsible for heating the

workload and for producing the plasma environment. Once
evacuated to a satisfactory base pressure, N2 and H2 gas
mixtures are typically introduced into the chamber using
flow meters and an appropriate treatment pressure is
established.
Normally, H2 is added to the treatment gas mixture to aid
in the cleaning process of the samples to be treated.

By establishing a potential across the low pressure gas,

excitation and ionisation of the molecular N2 and H2 gas
mixture takes place.

The ionisation gives rise to an emission of visible light

which can be seen through a viewing screen and is
commonly referred to as the glow-discharge.

Once a glow-discharge is established, the ionised

particles are accelerated towards the negatively charged
cathode and the samples to be treated.
Upon collision with the samples, the charged particles
impart kinetic energy which provides the heating
required.

This heating mechanism is usually efficient enough to

negate any need for external heaters.

The temperature is monitored with a thermocouple and

the power supply bias can be adjusted such that the
samples remain at the nitriding temperature.
Typical chamber for Glow-Discharge Ion
nitriding, also known as DC plasma nitriding
 Plasma Nitriding Process

• Low pressure plasma process

– Vacuum filled with H2 and
N2 at pressure between 0.1
to 10 torr, with a large DC
voltage between the work
space as the cathode and
furnace as the anode
– DC glow discharge ionizes
the gas an ion hit the work
surface and ion nitriding
happens.
An electric glow discharge tube featuring its most
important characteristics: (a) An anode and cathode at
each end (b) Aston dark space (c) Cathode glow (d)
Cathode dark space (also called Crookes dark space,
or Hittorf dark space) (e) Negative glow (f) Faraday
space (g) Positive column (h) Anode glow (i) Anode
dark space.
Current Voltage characteristics of gaseous discharge
Apart from the complete removal of environmental
hazards compared with gas and salt bath nitriding,
there are many process and system based advantages
offered by using plasma based processing.

Some of these advantages include reduced gas and

energy consumption, reduced nitriding cycle times,
reduced distortion and consequently reduced post-
treatment polishing and finishing.

In addition, plasma based processing enables greater

nitriding uniformity and control of the sample surface
properties such as brittleness
Despite these advantages, the large bias voltage applied
to the samples to be treated in DCPN can lead to
problems in maintaining a uniform temperature in
components with different mass.

Other known problems which exist in this process are

hollow cathode and edge effects. Hollow cathode effects
occur when parts to be treated are located close to each
other or contain deep holes of small diameter, where the
plasma from each part or wall overlap and produce high
localised currents and temperatures which in turn can
melt or overheat the parts to be treated
Attempts to address these shortcomings have involved
the use of auxiliary heating and the use of pulsed biased
power.

These approaches have met with some success,

although high cathodic potential is still applied directly
to the parts to be treated.

Conventional DC systems, where a high cathodic

potential is made directly on the parts to be treated, are
only efficient for the treatment of simple homogeneous
loads.

DCPN systems therefore have inherent shortcomings

when more complex loads are treated due to difficulties
in maintaining uniform temperatures.
 Plasma Nitriding Process

• Low pressure plasma process

– Vacuum filled with H2 and
N2 at pressure between 0.1
to 10 torr, with a large DC
voltage between the work
space as the cathode and
furnace as the anode
– DC glow discharge ionizes
the gas an ion hit the work
surface and ion nitriding
happens.
Typical process parameters

• primary gases used in nitriding: nitrogen,

hydrogen

• primary gases used in nitrocarburizing:

nitrogen, hydrogen, carbon dioxide or methane

• additive gas: argon

• temperature: 350 to 600°C

• gas pressure: 50 to 500 Pa

• gas consumption: from 20 l/h (lab scale) to 500 l/h
(industrial plant)

• treatment time: 0.5 to 60 hours

• plasma power: 500 A at 0 to 800 V.

 Surface Modification using Nitrogen Plasma

• Improves the surface

hardness
– Gear wheels
– Crank shafts
– Dies etc.
• Reduced Corrosion
• Improved High-
temperature Strength
• Increased part surface
life
 Properties of Plasma Nitrated Surfaces
• Improvement in surface hardening

• Greater wear resistance

• Longer fatigue life

• Corrosion Resistance

• Process can be applied to wide variety of metal.

– Cast Iron
– Stainless Steel
– Titanium

• Plasma treatment is preferred over conventional

treatment because if reduces the treatment time
which takes about 50 hrs.
Plasma Nitriding Systems
Plasma Nitriding Steps 1:

Care has to be taken with bore holes and hollow spaces,

since the components can become overheated by
hollow discharges that occur. Optimal setting of the gas
pressure prevents hollow discharges.

After the chamber is closed, it is first evacuated to

under 10 Pa with a vacuum pump.

The chamber is then flooded with nitrogen to just below

atmospheric pressure. The partial vacuum keeps the
closed chamber air-tight, so the parts cannot be
oxidized by air entering the system.
An integrated heater and circulation of the nitrogen
ensures uniform heating of the batch to approximately
500°C.

With large parts, a dwell time at this temperature must be

provided to allow the parts to heated properly.

Afterward, the chamber is evacuated again and the treatment

gas (a nitrogen-hydrogen mixture) is introduced.

At a pressure of 10 to 500 Pa the voltage is slowly increased.

The gas discharge ignites at approximately 500 V (abnormal

glow discharge) and spreads evenly over the entire surface of
the parts.
New (Recently developed) voltage generators use a
pulsed unipolar or bipolar direct current voltage. In this
phase, an additional cleaning of the surface takes place
as a result of the intense ion bombardment (sputtering).

The increased kinetic energy of the heavy ions such as

nitrogen, argon and carbon dioxide striking the surface
atomizes the uppermost atomic layers of the components
and ensures optimal cleaning of the surface in
preparation for the subsequent diffusion process.

The final treatment temperature selected (usually about

570°C) depends on the material.

The parts are exposed to the abnormal glow discharge at

constant regulated temperature for up to 20 hours .
The vacuum pump continuously removes the used gas
and this is replenished with fresh gas.

The low working pressure keeps the gas consumption

low (100 to 400 litres per hour).

After the end of the treatment, the chamber is flooded

with nitrogen and the circulation fan carries the heat to
the chamber wall. The walls are cooled by externally
mounted fans. Once the batch temperature is below
100°C, the chamber can be opened and the parts
removed.
It should be highlighted that although conventional
plasma nitriding can effectively increase the hardness
and dry wear resistance of stainless steel, such
improvements is at the price of its corrosion resistance.

When this treatment temperature is higher, the level of

chromium at the surface of the material is depleted.

During these treatments, the hardening process

reduces the level of corrosion resistance, due to the
depletion of chromium in solid solution.
This chromium depletion layer is very prone to corrosion.
The formation of precipitates only occurs when plasma
nitriding is carried out at temperatures above 500ºC in
order to accelerate nitrogen diffusion.

This highlights the need to use lower treatment

temperatures, successfully addressing this problem.

Attention has been paid to plasma nitriding at

temperatures below 500°C in order to maintain the
properties that already excel in this material, which is
corrosion resistance, and improve those that do not
perform so well.
Carburizing of Steels

In the carburization process for a steel, carbon is

introduced into the surface of a low-carbon steel,
typically containing 0.1 to 0.2 wt% carbon.

The carbon content of the carburized surface layer or

case normally is increased to about 0.8 to 1.0 wt% C.

The carbon is introduced into the steel at temperatures

above the upper transformation temperature, where the
steel is FCC-austenitic.

Carburizing is widely used in industry for machine

components such as gears, bearings, and shafts where
surface resistance to wear and contact and bending
fatigue is required.
Carburizing is also used for heavy-duty gears where
good fracture toughness is needed in addition to wear
and fatigue resistance.

The steel must be in the austenitic condition where

there is appreciable interstitial solid solubility of carbon
in austenite.

The solubility of carbon in iron varies with temperature,

and in Fe-C alloys ranges from about 0.8% at 723°C to
about 2.0% at 1148°C.

Alloying elements can affect the solubility of carbon in

iron. Below 723°C, carbon has a very low solubility in
BCC ferrite (about 0.02% maximum), and thus
carburizing is not possible.
Carburizing is usually done in the 850 to 950°C range,
although temperatures as low as 790°C and as high as
1095°C can be used.

The high-temperature limit is due primarily to the

limitations of the furnace equipment.

After carburizing, the workpiece is usually quenched to

produce a martensitic structure in the case.

Thus, the carburizing treatment can produce a steel with a

hardened surface (case) and a core with low strength,
high ductility, and toughness typical of a low-carbon steel.
In gas carburizing the workpieces are heated in contact
with carbon containing gases such as the hydrocarbons,
methane, ethane, and propane.

The carburizing gases are diluted with an endothermic

carrier gas which consists mainly of nitrogen (N2) and
carbon monoxide (CO) along with smaller amounts of
carbon dioxide (CO2), hydrogen (H2), and water (H2O).

Of these gases, N2 is inert and acts only as a dilutent.

The carrier gas serves to control the amount of carbon

supplied to the steel surface and prevents the formation
of soot residue.
During gas carburizing, carbon is transferred from the
carburizing atmosphere surrounding the steel workpiece
to its surface.

The carbon then diffuses slowly into the bulk of the

workpiece, setting up a carbon concentration gradient
below the surface.

Because the carbon content at the surface is higher than

below the surface, carbon diffuses from the surface
toward the center of the workpiece.
Carbonitriding

is a modified form of carburizing and is not a form of

nitriding.

The modification in carbonitriding consists of adding

ammonia (NH3) to the carburizing gas so that nitrogen
diffuses in the steel case along with carbon.

Carbonitriding is usually carried out at a lower

temperature and for a shorter time than gas
carburizing, and so a thinner case is usually produced
than by carburizing.
Carbonitriding is principally used to produce a hard,
wear-resistant case in steels, normally from 0.075 to
0.75 mm thick. Nitrogen increases the hardenability of
steel, and so a carbonitrided case has higher
hardenability than a carburized case on the same steel.

Also, since nitrogen is an austenite stabilizer, high

nitrogen levels can result in retained austenite,
particularly in alloy steels.

Maximum hardness and less distortion can be attained

by carbonitriding since less drastic oil quenching than
for carburizing can be used.
 Plasma carburizing

• Plasma primarily assists in the mass transfer of

carbon atoms to the surface of the sample.

• Carburizing of steel has been done using

hydrocarbons gas at 1-20 torr in a DC discharge. This
process required extra heat treatment of the part at
1050°C

• Plasma treatment is conducted for a short period of

time. A longer heating time is need in order to diffuse
carbon into steel.
 Schematic of Plasma Carburizing Unit

• Work piece is heated up to

process temperature by an
external heater in a vacuum
furnace.

• Methane or propane is
introduced into the furnace
to a pressure 2 to 3 torr.

• A DC voltage is applied
between the work piece or
cathode and a glow
discharge is generated
Active Screen Plasma Nitriding

In recent years, an innovative Active Screen (Through

Cage) Plasma Nitriding (ASPN) system is developed.

ASPN has been the focus of much interest, since it

claims to avoid fundamental problems associated with
DC Plasma Nitriding, these include a reduction of arcing
damage and hollow cathode issues, and most
importantly, the ability to treat parts with a large range of
geometries within the one batch.

Here, high cathodic potential is applied to a screen

surrounding the parts to be treated which is the new
cathode for the system, rather than directly on the load.
A schematic of a laboratory based ASPN system
DCPN

ASPN
Samples are placed on the base plate which is enclosed
by the large metal screen made from expanded mesh.

The base plate and samples to be treated are then

allowed to float or is subjected to a small negative bias of
100 to 200V.

The rationale behind this innovation was the claim that

the active species in plasma nitriding were highly
energetic neutrals rather than ions and so there is no
need to form the plasma directly onto the parts to be
treated.

In this new technology the role of the plasma generated

on the active screen is as follows: (1) to heat up the load
by radiation thereby providing uniform temperature
distribution throughout the load and (2) to generate active
neutral species.
It is demonstrated in the laboratory system, where the
physical distance between the electrically floating load
and the active screen was of the order of 12 mm, that the
nitriding response of low alloy steels was equivalent to
DC plasma nitriding using a 500V DC bias without the
common problems of arcing damage, hollow cathode and
edge effect.
Comparing the nitriding response between DCPN samples
treated with 500V bias and electrically floating ASPN
samples in the same nitriding chamber which was similar to
that shown in Figure.

The distance from the sample surface and the active screen
top lid was less than 15 mm in the ASPN experiments.

It is shown that the DCPN samples had a non uniformly

nitrided surface due to edge effects. On the other hand, the
ASPN samples had a uniform matt grey surface finish after
the nitriding treatment.

After a standard metallographic analysis, it is concluded

that the surface hardness and composition of the ASPN
samples were similar to that of the uniform central area of
the DCPN samples without the undesirable sample edge
effects
ASPN technology claims to have significant advantages
over many of the commercially available plasma based
nitriding processes.

The key innovation of ASPN is the removal of the need to

supply a high cathodic potential to the load.

The rationale behind this innovation rests on the claim that

the mechanism of nitrogen mass transfer is not ions, but
either highly energetic neutrals or sputter deposition of the
active screen material onto the samples.

There is therefore no requirement to form the plasma

directly on the parts to be treated. Consequently, the load
is allowed to float or is subjected to a small negative bias
potential. However, there is debate as to the mechanisms
for nitriding in ASPN.
In particular, are energetic neutrals really the active
species in ASPN, both at the laboratory and commercial
scale? Does sputtering of the active screen material on
to the samples play a role in the nitriding process?

What level of bias is required on the load to get a

satisfactory response?

Do the optimal nitriding parameters used depend on the

load type?