Gas 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).
Principal reasons for nitriding are:
- To obtain high surface hardness
- 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.
The following steels can be gas nitrided for specific applications:
- Aluminum-containing 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 H13
- 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.
Single-Stage 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
To summarize, the use of a higher temperature during the second stage:
- Lowers the case hardness
- 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 nitriding.
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.
Furnace Purging. 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.
A typical purging cycle using anhydrous ammonia follows:
- Close furnace 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
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 (300°F).
Nitrogen versus 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.
Ammonia requires 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.
Dissociation 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.
Although various 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).
Typically ammonia 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 structure proceeds.
When nitriding 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 ammonia.
Distortion 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.
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
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.