nitriding is a case-hardening process whereby nitrogen is introduced
into the surface of a solid ferrous alloy by holding the metal at
a suitable temperature in contact with a nitrogenous gas, usually
ammonia. The nitriding temperature for all steels is between 495
and 565°C (925 and 1050°F).
for nitriding are:
To obtain high
To increase wear resistance and antigalling properties
To improve fatigue life
To improve corrosion resistance
To obtain a surface that is resistant to the softening effect of
heat at temperatures up to the nitriding temperature.
Because of the absence of a quenching requirement, with attendant
volume changes, and the comparatively low temperatures employed
in this process, nitriding of steels produces less distortion and
deformation than either carburizing or conventional hardening.
Of the alloying elements commonly used in commercial steels, aluminum,
chromium, vanadium, tungsten, and molybdenum are beneficial in nitriding
because they form nitrides that are stable at nitriding temperatures.
Molybdenum, in addition to its contribution as a nitride former,
also reduces the risk of embrittlement at nitriding temperatures.
Other alloying elements, such as nickel, copper, silicon, and manganese,
have little, if any, effect on minding characteristics.
Although at suitable temperatures all steels are capable of forming
iron nitrides in the presence of nascent nitrogen, the nitriding
results are more favorable in those steels that contain one or more
of the major nitride-forming alloying elements. Because aluminum
is the strongest nitride former of the common alloying elements,
aluminum-containing steels (0.85 to 1.50% Al) yield the best nitriding
results in terms of total alloy content. Chromium-containing steels
can approximate these results if their chromium content is high
enough. Unalloyed carbon steels are not well suited to gas nitriding
because they form an extremely brittle case that spalls readily,
and the hardness increase in the diffusion zone is small.
steels can be gas nitrided for specific applications:
low-alloy steels 7140 (Nitralloy G, 135M, N, EZ)
Medium-carbon, chromium-containing low-alloy steels of the 4100,
4300, 5100, 6100, 8600, 8700, and 9800 series
Hot-work die steels containing 5% chromium such as H11, H12, and
Low-carbon, chromium-containing low-alloy steels of the 3300, 8600
and 9300 series
Air-hardening tool steels such as A-2, A-6, D-2, D-3 and S-7
High-speed tool steels such as M-2 and M-4
Nitronic stainless steels such as 30, 40,50 and 60
Ferritic and martensitic stainless steels of the 400 and 500 series
Austenitic stainless steels of the 200 and 300 series
Precipitation-hardening stainless steels such as 13-8 PH, 15-5 PH,
17-4 PH, 17-7 PH, A-286, AM350 and AM355.
Aluminum-containing steels produce a nitrided case of very high
hardness and excellent wear resistance. However, the nitrided case
also has low ductility, and this limitation should be carefully
considered in the selection of aluminum-containing steels. In contrast,
low-alloy chromium-containing steels provide a nitrided case with
considerably more ductility but with lower hardness. Tool steels,
such as H11 and D2, yield consistently high case hardness with exceptionally
high core strength.
Prior Heat Treatment. All hardenable steels must be hardened and
tempered before being nitrided. The tempering temperature must be
high enough to guarantee structural stability at the nitriding temperature:
the minimum tempering temperature is usually at least 30°C (50°F)
higher than the maximum temperature to be used in nitriding.
In certain alloys, such as series 4100 and 4300 steels, hardness
of the nitrided case is modified appreciable by core hardness: that
is, a decrease in core hardness results in a decrease in case hardness.
Consequently, in order to obtain maximum case hardness, these steels
are usually provided with maximum core hardness by being tempered
at the minimum allowable tempering temperature.
and Double-Stage Nitriding. Either a single- or a double-stage process
may be employed when nitriding with anhydrous ammonia. In the single-stage
process, a temperature in the range of about 495 to 525°C (925
to 975°F) is used, and the dissociation rate ranges from 15
to 30%. This process produces a brittle, nitrogen-rich layer known
as the white nitride layer at the surface of the nitrided case.
The first stage
of the double-stage process is, except for time, a duplication of
the single-stage process. The second stage may proceed at the nitriding
temperature employed for the first; stage, or the temperature may
be increased to from 550 to 565°C (1025 to 1050°F): however,
at either temperature, the rate of dissociation in the second stage
is increased to 65 to 80% (preferably, 75 to 80%). Generally, an
external ammonia dissociator is necessary for obtaining the required
higher second-stage dissociation.
the use of a higher temperature during the second stage:
Lowers the case
Increases the case depth
May lower the core hardness depending on the prior tempering temperature
and the total nitriding cycle time
May lower the apparent effective case depth because of the loss
of core hardness, depending on how effective case depth is defined.
After hardening and tempering, and before nitriding, parts should
be thoroughly cleaned. Most pans can be successfully nitrided immediately
after vapor degreasing. However, some machine-finishing processes
such as buffing, finish grinding, lapping, and burnishing may produce
surfaces that retard nitriding and result in uneven case depth and
distortion. There are several methods by which the surfaces of parts
finished by such methods may be successfully conditioned before
One method consists of vapor degreasing pans and then abrasive cleaning
them with aluminum oxide grit or other abrasives such as garnet,
or silicon carbide, immediately prior to nitriding. Any residual
grit must be brushed off before pans are loaded into the furnace.
Pans should be handled with clean gloves.
A second method
consists of preoxidizing the pans in an air atmosphere at approximately
330°C (625°F). This may be done as a separate operation,
or it may be incorporated as part of the healing portion of the
nitriding cycle if suitable precautions are taken.
After loading and sealing the furnace at the start of the nitriding
cycle, it is necessary to purge the air from the retort before the
furnace is heated to a temperature above 150°C (300°F).
This prevents oxidation of parts and furnace components, and, when
ammonia is used as the purging atmosphere, avoids production of
a potentially explosive mixture. Nitrogen is preferred in place
of ammonia for purging, but the same precautions should be taken
to avoid oxidation of parts, except when preoxidation is intentionally
included as part of the cycle.
purging cycle using anhydrous ammonia follows:
and start flow of anhydrous ammonia gas at as fast a flow rate as
is practical with first step.
Set furnace temperature control at 150°C (300°F) simultaneously.
Heat furnace to this temperature but do not exceed.
When the furnace has been purged to the degree that 10% or less
air and 90% or more ammonia are present in the retort, the furnace
may be heated to the nitriding temperature.
It is not feasible to incorporate preoxidation as part of the cycle
unless nitrogen is available as a purging medium at the end of the
320°C (625°F) oxidizing stage. Under no circumstances should
ammonia be introduced into a furnace containing air at 330°C
(625°F) because of the explosion hazard.
Purging is employed also at the conclusion of the nitriding cycle
when the furnace is cooled from the nitriding temperature. It is
common practice to remove the ammonia remaining in the retort with
nitrogen to reduce the amount of ammonia that would otherwise be
released into the immediate area when the load is removed. Dilution
of the ammonia lessens the discomfort to employees working near
the furnace. The introduction of nitrogen into the retort can be
delayed until the nitrided parts have cooled to below 150°C
Ammonia for Purging. Advantages of nitrogen as a purging gas include
its safety, ease of handling, and ease of control. The use of nitrogen,
however, requires additional equipment, including piping.
no additional equipment and is relatively safe when properly handled;
mixtures of 15 to 25% ammonia in air, however, are explosive if
ignited by a spark.
Rates. The nitriding process is based on the affinity of nascent
nitrogen for iron and certain other .metallic elements. Nascent
nitrogen is produced by the dissociation of gaseous ammonia when
it contacts hot steel parts.
rates of dissociation can be used successfully in nitriding, it
is important that the nitriding cycle begin with a dissociation
rate of about 15 to 35% and that this rate be maintained for 4 to
10 h. Depending on the duration of the total cycle, temperature
should be maintained at about 525°C (975°F).
is supplied at a flow rate to achieve a minimum of four (4) atmosphere
changes in the retort per hour. This initial cycle develops a shallow
white layer from which diffusion of nitrogen into the main case
with dissociation rate of 15 to 35%, it is normal to control this
rate entirely by the flow rate of ammonia. At a dissociation rate
of 75 to 80%, however, it is necessary to introduce completely dissociated
and Dimensional Changes. Distortion in nitriding may result from:
Relief of residual
stresses from prior operations such as welding, hardening, machining,
and so forth
Stress introduced during nitriding due to inadequate support in
the furnace, or too rapid or nonuniform heating or cooling.
Stress is introduced by the increase in volume that occurs in the
case. This change causes a stretching of the core, which results
in tensile stresses that are balanced by compressive stresses in
the case after the parts have cooled to room temperature. The magnitude
of the permanent set in the core and case is affected by yield strength
of the material, thickness of the case, and by the amount and nature
of the nitrides formed.
Stabilizing Treatment. In nitrided pans, there is a balance between
compressive stresses in the case and tensile stresses in the core.
If this balance is upset by grinding off a part of the case, slow
dimensional changes may occur as the stresses approach equilibrium.
To prevent these changes, nitrided pans are first ground almost
to the final dimensions, then heated to 565°C (1050°F) for
1 h. and finally finish ground or lapped. Parts nitrided and not
ground after nitriding have excellent dimensional stability.
Finishing Costs. The amount of distortion resulting from nitriding
is small compared to that resulting from other case-hardening processes,
which involve quenching to form martensite. Consequently, the increase;
cost of the nitriding operation and of steel suitable for nitriding
often can be offset by the savings resulting from finishing to size
prior to nitriding.