NITRIDING: DEFINITION, PROCESS PARAMETERS, PROS & CONS

NITRIDING PROCESS

The nitriding process was first developed in the early 1900s and continues to play an important role in many industrial applications.

Along with the derived nitrocarburizing process, nitriding is commonly used in the manufacturing of aircraft, bearings, auto parts, textile machinery, and turbine power generation systems.

Although wrapped in a bit of “alchemical mystery”, it is still the simplest surface hardening technology.

Nitriding

The secret of the nitriding process is that it does not require a phase change from ferrite to austenite, nor does it require an additional change from austenite to martensite.

In other words, the steel maintains the ferrite phase (or cementite, depending on the composition of the alloy) throughout the process.

This means that the molecular structure of ferrite (body-centered cubic lattice, or bcc) does not change its configuration or the face-centered cubic lattice (fcc) characteristics of austenite as in the more traditional method such as carburizing.

In addition, since only free cooling occurs, not rapid cooling or quenching, there wouldn’t be any further transformation from austenite to martensite.

Similarly, there is no molecular size change, more importantly, there is no size change.

The volume of the steel surface changes due to nitrogen diffusion, and is only slightly increased.

SEVERAL PROCESS PARAMETERS

Various process parameters must be considered to ensure successful nitriding in metallurgy and distortion:

• Nitrogen source

• Heat

• Time

• Steel composition

In gas nitriding, the nitrogen source almost always comes from the decomposition (or dissociation) of ammonia supplied by a storage system or a single bottle connected to the manifold.

Ammonia begins to break down when heated, usually from an external source in the furnace.

At the usual nitriding temperature of 500 to 570 °C (930 to 1060 ° F), ammonia is in an unstable thermodynamic state and decomposes as follows:

2NH3 ↔ 2N + 3H2

Generally, steel will have three reactions.

NH3 → 3H + N

2N → N2

2H → H2

The atomic nitrogen and hydrogen are unstable and will combine with other similar atoms to form molecules.

Nitrogen released diffuses into steel at nitriding temperature, but is so slow that it is not economically practical or efficient.

A temperature of 500 °C (930 °F) is considered as “economic” temperature.

Primary nitrogen has an affinity for steel and iron, and can easily diffuse into these two materials at high temperatures.

Higher the temperature, the faster and deeper the nitrogen diffusion.

Economical temperature is a temperature that can produce the optimum case depth without adversely affecting the core performance of the processed steel.

DISSOCIATION OF GAS AT THE SELECTED NITRIDING TEMPERATURE

At a temperature of approximately 500 °C (930 °F), there is a problem with the stability of ammonia, resulting in a dissociation rate greater than 98%, thus forming a protective gas without any nitriding effect .

Although the surface of the workpiece and the furnace wall have a beneficial catalytic effect, the dissociation process of ammonia is very slow.

Therefore, the amino nitriding atmosphere used for steel treatment rarely contains less than 20% ammonia, usually as high as 50%, so their degree of dissociation is far from equilibrium.

The remaining ammonia content plays a decisive role in the nitriding effect.

During this period, nitrogen diffuses into steel according to the reaction:

NH3 → N (α) + (3/2) H2

which occurs in the boundary layer.

Nitrogen transfer is relatively low, and the hydrogen released by ammonia molecules determines the speed of the process.

Therefore, as mentioned above, the nitriding time is quite long, up to 120 hours.

The nitriding effect of nitriding atmosphere is defined by its degree of dissociation.

High dissociation always means close to equilibrium, and the nitriding effect is low.

In processing, a constant degree of dissociation (for example, 30%) is usually used, but sometimes a two stage procedure is followed, involving changes in temperature and degree of dissociation.

However, the measured ammonia content is not equal to the actual dissociation degree.

In the ammonia dissociation, two ammonia molecules are broken down into one nitrogen molecule and three hydrogen molecules.

This increase in volume dilutes the ammonia content, as does the additional gas, which must be taken into account when determining the actual degree of dissociation.

The nitriding effect can be more easily determined by the nitriding potential.

The nitriding potential is generally used to describe the nitriding capacity of an atmosphere of ammonia.

Your control allows the formation of a predictable casing depth and structure suitable for a variety of steels, including some tools and stainless steel grades.

A higher Np value will produce a higher concentration of nitrogen on the surface and a steeper concentration gradient.

The lower potential allows the formation of a nitrided shell without the brittle compound layer (white) on high alloy steel.

ADVANTAGES

Better retention of hardness at elevated temperatures.

Greater fatigue strength under corrosive conditions.

Less warping or distortion of pans treated.

Higher endurance limit under bending stresses.

Greater resistance to wear and corrosion.

Greater surface hardness.

DISADVANTAGES

Reaction kinetics heavily influenced by surface condition on oily surface or one contaminated with cutting fluids will deliver poor results.

Surface activation is sometimes required to treat steels with a high chromium content – compare sputtering during plasma nitriding.

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