is a surface-hardening heat treatment that introduces nitrogen into
the surface of steel at a temperature range (500 to 550°C, or
930 to 1020°F), while it is in the ferrite condition. Thus,
nitriding is similar to carburizing in that surface composition
is altered, but different in that nitrogen is added into ferrite
instead of austenite. Because nitriding does not involve heating
into the austenite phase field and a subsequent quench to form martensite,
nitriding can be accomplished with a minimum of distortion and with
excellent dimensional control.
of nitriding is generally known, but the specific reactions that
occur in different steels and with different nitriding media are
not always known. Nitrogen has partial solubility in iron. It can
form a solid solution with ferrite at nitrogen contents up to about
6%. At about 6% N, a compound called gamma prime (?), with
a composition of Fe4N is formed.
contents greater than 8%, the equilibrium reaction product is e
compound, Fe3N. Nitrided cases are stratified. The outermost surface
can be all ? and if this is the case, it is referred to as
the white layer. Such a surface layer is undesirable: it is very
hard profiles but is so brittle that it may spall in use. Usually
it is removed; special nitriding processes are used to reduce this
layer or make it less brittle. The e zone of the case is hardened
by the formation of the Fe3N compound, and below this layer there
is some solid solution strengthening from the nitrogen in solid
reasons for nitriding are:
To obtain high
To increase wear resistance
To improve fatigue life
To improve corrosion resistance (except for stainless steels)
To obtain a surface that is resistant to the softening effect of
heat at temperatures up to the nitriding temperature
Nitrided steels are generally medium-carbon (quenched and tempered)
steels that contain strong nitride-forming elements such as aluminum,
chromium, vanadium, and molybdenum.
The most significant hardening is achieved with a class of alloy
steels (nitralloy type) that contain about 1% Al. When these steels
are nitrided the aluminum forms AlN particles, which strain the
ferrite lattice and create strengthening dislocations. Titanium
and chromium are also used to enhance case hardness although case
depth decreases as alloy content increases.
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 nitriding characteristics.
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.
steels can be gas nitrided for specific applications:
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 HI1, HI2, 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.
Process methods for nitriding include:
gas (box furnace or fluidized bed),
liquid (salt bath),
plasma (ion) nitriding.
The advantages and disadvantages of these techniques are similar
to those of carburizing. However, times for gas nitriding can be
quire long, that is, from 10 to 130 h depending on the application,
and the case depths are relatively shallow, usually less than 0.5
mm. Plasma nitriding allows faster nitriding times, and the quickly
attained surface saturation of the plasma process results in faster
diffusion. Plasma nitriding can also clean the surface by sputtering.
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. Quenching is not required for the production of a hard
case. The nitriding temperature for all steels is between 495 and
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. Some
growth occurs as a result of nitriding but volumetric changes are
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.
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 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.
process, known also as the Floe process, has the advantage of reducing
the thickness of the white nitrided layer.
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; 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.
purpose of double-stage nitriding is to reduce the depth of the
white layer produced on the surface of the case. Except for a reduction
in the amount of ammonia consumed per hour, there is no advantage
in using the double-stage process unless the amount of white layer
produced in single-stage nitriding cannot be tolerated on the finished
part or unless the amount of finishing required after nitriding
is substantially reduced.
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.
Operating Procedures. After hardening and tempering and before nitriding,
parts should be thoroughly cleaned. Most parts can be successfully
nitrided immediately after vapor degreasing.
Bright nitriding is a modified form of gas nitriding employing ammonia
and hydrogen gases. Atmosphere gas is continually withdrawn from
the nitriding furnace and passed through a temperature-controlled
scrubber containing a water solution of sodium hydroxide (NaOH).
Trace amounts of hydrogen cyanide (HCN) formed in the nitriding
furnaces are removed in the scrubber thus improving the rate of
The scrubber also establishes a predetermined moisture content in
the nitriding atmosphere reducing the rate of cyanide formation
and inhibiting the cracking of ammonia to molecular nitrogen and
hydrogen. By this technique control over the nitrogen activity of
the furnace atmosphere is enhanced and nitrided parts can be produced
with little or no white layer at the surface. If present, the white
layer will be composed of only the more ductile Fe4N (gamma prime)
Pack nitriding is a process analogous to pack carburizing. It employs
certain nitrogen-bearing organic compounds as a source of nitrogen.
Upon heating, the compounds used in the process form reaction products
that are relatively stable at temperatures up to 570°C.
Slow decomposition of the reaction products at the nitriding temperature
provides a source of nitrogen. Nitriding times of 2 to 16 h can
be employed. Pans are packed in glass ceramic or aluminum containers
with the nitriding compound, which is often dispersed in an inert
Ion (or Plasma) Nitriding
Since the mid-1960s, nitriding equipment utilizing the glow-discharge
phenomenon has been commercially available. Initially termed glow-discharge
nitriding, the process is now generally known as ion, or plasma,
nitriding. The term plasma nitriding is gaining acceptance.
Ion nitriding is an extension of conventional nitriding processes
using plasma-discharge physics. In vacuum, high-voltage electrical
energy is used to form a plasma, through which nitrogen ions are
accelerated to impinge on the workpiece. This ion bombardment heats
the workpiece, cleans the surface, and provides active nitrogen.
versatile, the process provides excellent dimensional control and
retention of surface finish. Ion nitriding can be conducted at temperatures
lower than those conventionally employed. Control of white-layer
composition and thickness enhances fatigue properties. The span
of ion-nitriding applications includes conventional ammonia- gas
nitriding, short-cycle nitriding in salt bath or gas, and the nitriding
of stainless steels.
lends itself to total process automation, ensuring repetitive metallurgical
results. The absence of pollution and insignificant gas consumption
are important economic and public policy factors. Moreover, selective
nitriding accomplished by simple masking techniques may yield significant
Comparison of Ion Nitriding and Ammonia-Gas Nitriding
Ammonia-gas nitriding produces a compound zone that is a mixture
of both epsilon and gamma-prime structures. High internal stresses
result from differences in volume growth associated with the formation
of each phase. The interfaces between the two crystal structures
are weak. Thicker compound zones, formed by ammonia-gas nitriding,
limit accommodation of the internal stresses resulting from the
Under cyclic loading, cracks in the compound zone can serve as initiation
points for the propagation of fatigue cracks. The single-phase gamma-prime
compound zone, which is thin and more ductile, exhibits superior
fatigue properties. Reducing the thickness of the ion-nitrided compound
zone further improves fatigue performance. Maximization occurs at
the limiting condition, where compound zone depth equals zero.
The bulk of the thickness of the nitride case is the diffusion zone
where fine iron/alloy nitride precipitates impart increased hardness
and strength. Compressive stresses are also developed, as in other
nitriding processes. Hardness profiles resulting from ion nitriding
are similar to ammonia-gas nitriding but near-surface hardness may
be greater with ion nitriding, a result of lower processing temperature.
Disadvantages of Ion Nitriding. Ion nitriding achieves repetitive
metallurgical results and complete control of the nitrided layers.
This control results in superior fatigue performance, wear resistance,
and hard layer ductility. Moreover, the process ensures high dimensional
stability, eliminates secondary operations, offers low operating-temperature
capability and produces parts that retain surface finish.
Efficient use of gas and electrical energy
Total process automation
Selective nitriding by simple masking techniques
Process span that encompasses all sub-critical nitriding
Reduced nitriding time
The limitations of ion nitriding include high capital cost, need
for precision fixturing with electrical connections, long processing
times compared to other short-cycle nitrocarburizing processes,
and lack of feasibility of liquid quenching for carbon steels.