The main objective of nitriding is to increase the hardness of the component’s surface by enriching it with nitrogen.

Nitriding is a process of diffusing nitrogen atoms into the metal’s surface. Nitrogen is plentiful on Earth, however, in nature it exists as a two-atom molecule, chemically inert and too large to penetrate the surface. Hence nitriding technologies focus on the source of nascent (atomic) nitrogen.

Regardless of the method, nitriding is a process of diffusing nitrogen into the metal and such diffusion, once individual atoms of nitrogen have penetrated the surface, continues as long as the temperature is high enough, and there is a fresh supply of nascent nitrogen on the surface. In other words, the diffusion is basically the same in all nitriding, while the difference lies in the supply of nitrogen. The latter has a fundamental influence on the resultant properties of the surface
 

Traditional Nitriding Methods  

The three traditional nitriding methods practiced on an industrial scale are:

(a) salt bath (liquid) nitriding, where the source of nitrogen (and also carbon) is molten salt,
(b) gas nitriding which uses ammonia (NH3), and
(c) plasma nitriding where molecular nitrogen (N2) is split into ions in an electromagnetic field.  

Nascent nitrogen is obtained from ammonia in gas nitriding. The conventional version of the process relies on the measurement of the dissociation rate of ammonia into its constituent gases – nitrogen and hydrogen. A simple device called a buret (or more properly burette) is employed for periodically checking the dissociation rate and an adjustment to the flow of ammonia is made as required, usually in a manual fashion.  

Materials  

Generally speaking all ferrous alloys, including stainless steels, cast irons, and even titanium alloys, are capable of being nitrided. However, the various alloys have different characteristics with regard to surface conditions, the natural speed of diffusion and propensity to form nitrides. It is important to understand that even a properly run nitriding process will produce significantly different results on dissimilar materials. Consequently, some users may have insurmountable difficulties, particularly if their methodology is primitive and/or their knowledge and experience inadequate.

Nitriding Effect - Properties of Nitrided Layers

A surface exposed to a nitriding medium will generally form two distinct layers. The outside layer is called a compound layer (or white layer) and its thickness generally falls between zero and 0.001” (25 mm). Underneath the white layer we have a diffusion case or diffusion zone. Both together comprise what is generally referred to as the case. However, as mentioned earlier, depending on the material and its original pre-process hardness there will be very significant differences between the properties of these layers. A full description of these phenomena is well outside the scope of this website. We encourage our visitors to contact us if specific such information is required. Meanwhile we will concentrate our attention on explaining the “why and how” of controlling the properties of the nitrided surface, particularly with regard to the compound layer.

Control

The images of two Vickers hardness tester indentations shown below illustrate the difference between a controlled and uncontrolled process. The specimen on the left was produced in a traditional process and the cracking of the surface is indicative of the brittleness of the layer. The one on the right is a product of a Nitreg® potential-controlled process where, in spite of the same hardness, cracks have not formed. The Nitreg® treated component is therefore more resilient with high toughness of the compound layer.

Such superior result can only be achieved by controlling the nitrogen concentration in the substrate and the modern approach is control of nitriding potential (Kn). Proper understanding and application of the principles that tie nitriding potential (Kn), temperature and time are the cornerstone of the Nitreg® technology. An example of its ability to produce a variety of white layer / diffusion case combinations is shown in the following chart:

Nitrided case combinations - Acrobat®

Gradually the ability to control the nitriding potential is becoming a requirement as set forth by specifications such as AMS 2759/10.

Potential-controlled nitriding is a modern process, capable of meeting the metallurgical requirements of all nitriding specifications that may have been originally written for salt bath, plasma or traditional gas nitriding. The ability to control the concentration of nitrogen in the surface allows the user to control the growth of the compound layer virtually independently from developing a desirable diffusion zone. This approach facilitates not only meeting any specification requirements but it also makes it possible to improve on them by allowing tighter tolerances to be satisfied, particularly with regard to the thickness and properties of the compound layer.

Summary of Benefits:

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Nitreg®–C is a controlled version of nitrocarburizing. It allows for precise Kn control during the process. The additions of carbon bearing gases to the nitriding atmosphere help to increase the relative content of the epsilon phase.

The advantage of a Kn controlled technology is best evidenced when increased wear and/or corrosion resistance is sought. Such properties of the nitrided case are not only influenced by the thickness and relative phase composition of the WL. They also strongly depend on the relative level of porosity developed in the WL.

The Kn control is essential in producing the desired WL configuration

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The rules applying to nitriding of stainless steel or refractory alloys are no different than those for the other groups of steels, with one exception.

The exception is the proprietary de-passivation stage that allows for a removal of oxides of alloying elements such as Cr, Ni and others which, if not removed, will effectively block the nitriding process. The same way they block the rust formation on the stainless steel surface.

All types of stainless steel can be nitrided. The martensitic, austenitic or PH materials inclusive.

Example of a nitrided stainless steel part.

Potential-Controlled Nitriding of Titanium Alloys

Titanium alloys, used mostly in the aerospace and defense industries can also be successfully gas nitrided leading to an increased wear resistance and providing an attractive golden finish.

Example of a nitrided titanium alloy part.

As an alternative to gas nitriding, nitriding plasma (ion) nitriding process has been developed to overcome the shortcomings of the earlier traditional uncontrolled gas nitriding processes and to offer certain operational advantages that gas nitriding does not have.

Plasma is essentially a gas nitriding treatment in which the method of delivering nitrogen atoms to the surface of nitrided components is quite different from the standard gas nitriding processes. It occurs at a very low pressure and under high voltage.

From the metallurgical, tribological and mechanical properties standpoint the properties of nitrided case obtained with potential-controlled nitriding and well controlled plasma technology are comparable.

Plasma offers certain distinct advantages such as:

• The ease of masking the component surface where nitriding is to be avoided.
   
• Ability to nitride low density powder metallurgy parts

Vacuum Carburizing

Vacuum carburizing is a state-of-the-art thermal process where carburizing is effected under very low pressures. First the parts are heated in vacuum to above the transformation temperature of the alloy. Then they are exposed to carbon-carrying gas, or gas mixtures, under partial pressure. Nitrex has developed a revolutionary process called "Pulse- Pressure", a method quickly becoming the industry standard.

Relative to conventional carburizing, the main advantages of the method are:

• repeatable results to within ±0.001" (±25µm),
   
• significantly reduced size changes and distortion,
   
• improved fatigue strength,
   
• better control of the surface layer chemistry,
   
• the process is environmentally friendly.

The basic aspects or carburizing in general are described in the Conventional Carburizing section.

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Vacuum Carbonitriding

It is a thermal process of simultaneously diffusing both carbon and nitrogen into ferrous alloys under partial pressure. This leads to an extremely hard and wear resistant surface. Vacuum carbonitriding is a significant improvement over conventional gas carbonitriding. The process contains all of the inherent benefits of vacuum carburizing, but also has the additional benefit of precise computer control of surface ammonia content. Furthermore, this process does not require any additional refractory burn-outs so not only is the end product of higher quality, but it is often less expensive than with competing conventional gas processes.

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