Potential-Controlled Nitriding

Nitreg® is a modern heat treating 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.

NITREG® Advantage

  • control of the thickness of the compound (white) layer and its properties
  • elimination of closed nitride networks within the diffusion zone
  • control of case depth
  • control of surface hardness
  • no distortion
  • green technology, no waste pollution
  • family of derivative and related processes

Nitreg®  Treated Gears

Material: Afnor 27MC5JV

What is nitriding?

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.

The main objective of nitriding is to increase the hardness of the component’s surface by enriching it with 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. Salt baths are not used by Nitrex due to that method’s environmental dangers and our concern for personal safety of the operators. Besides, the process has few advantages, quick heating of the workpiece being the only one worth mentioning.

(b) gas nitriding which uses ammonia (NH3). 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.

(c) plasma nitriding where molecular nitrogen (N2) is split into ions in an electromagnetic field. Plasma nitriding is used by Nitrex for selected applications and the process.

Comparison table - advantages and disadvantages of different nitriding methods

Conventional Gas Salt Bath Plasma (Ion)
Cleaning (Before) Clean
Clean Relatively Clean Very Clean
Cleaning (After) Not required Not required Strongly Required
Not Required
Heating Time
Short Short Very Short Long
Positioning of Parts
Simple Simple Simple Very Complex /
Requires Skill & Experience
Nitriding of
Stainless Steel
Possible Not Possible Possible Possible
Operation of Equipment Very Simple /
Fully Automated
Simple Very Complex /
Requires Advanced Skills
Temperature Control &
Excellent Good Good Difficult / Insufficient /
Overheat Possible
Control of Nitriding Potential Yes No No No
Control of % of ε and γ’ Possible No No Possible
Nitriding with No White Layer Possible No No Possible
Porosity Control Possible No No Possible
Repeatability of Results Excellent
(regardless of load)
(repetitive loads only)
(repetitive loads only)
(repetitive loads only)
Equipment Maintenance
Simple Relatively Complex Complex Very Complex
Degree of Pollution Very Low High Extremely High Very low


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 µm). Underneath the white layer we have a diffusion case or diffusion zone. Both together comprise what is generally referred to as the case. However depending on the material and its original pre-process hardness there will be very significant differences between the properties of these layers.

Controlled vs Uncontrolled Nitriding

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® 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 our ability to produce a variety of white layer / diffusion case combinations is shown in the following chart:

Nitrided case combinations – PDF

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

In conclusion, Nitreg® 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


Potential-Controlled Nitrocarburizing

Related technologies:

Nitreg®-C is based on the proven Nitreg® potential-controlled nitriding technology, whose ability to control processes and produce the desired properties of the nitrided layer has revolutionized the whole concept and practice of nitriding. This was achieved mainly by introducing superior process control and providing excellent properties to the nitrided layer. Nitreg®-C has shorter process cycle times than Nitreg® for the same expected specifications and is carried out predominantly on carbon and low alloy materials.

Nitreg®-C is a nitrocarburizing process, which involves the diffusion of carbon and nitrogen into the steel surface simultaneously. The purpose of this treatment is to create a hardened superficial layer, enhancing wear and corrosion resistance, or improved fatigue resistance of treated steel or cast iron parts, without distortion of shape or dimensional changes. In order to maintain a proper concentration of nascent nitrogen and carbon at the surface of metal, Nitreg®-C technology utilizes the Nitriding and Carburizing Potentials (KN and KC) control concept.

It is often specified in industrial applications on the merit of its being an environmentally friendly but equivalent alternative to salt bath nitrocarburizing.

The advantage of a KN & KC 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, but also strongly depend on the relative level of porosity developed in the WL.

KN control is essential in producing the desired WL configuration.

Example of various porosity levels achieved through KN control


Nitreg® -C Treated Hose Connector Housings

Material: 12B10 free machining steel

NITREG®-C Advantage:

  • accelerates formation of the compound (white) layer on low-carbon unalloyed steel
  • features low operational costs by utilizing more economic gas compositions
  • increases and stabilizes the ε-phase content in most types of steels
  • Green technology, no waste pollution


Potential-Controlled Nitriding of Stainless Steel

Related technologies:


Stainless steels respond to nitriding differently than other ferrous alloys, and there are also significant differences within the stainless group as well. The primary reason for this is that depending on the chemistry of the steel it will behave differently with respect to the kinetics of layer formation, and it is a rather difficult process to control. In other words, unless you know what you are doing you may end up with nothing, or too little, or too much case depth or white layer, or even damaged parts.

NITREG®-S is a process in which any stainless steel may be nitrided, with complete control over the formation of nitrided layers.

All types of stainless steel can be nitrided; martensitic, austenitic or PH materials inclusively.

NITREG®-S Advantage

    • Attains excellent wear resistance
    • Improves fatigue strength
    • Prevents galling
    • Does not alter chemical composition of alloy
    • Has no effect on the steel’s non-magnetic nature
    • No change in the color, shape or size
    • Uniformly hardened even small bores, tight grooves and sharp edges
    • Green technology, no waste pollution

Nitreg®-S  Treated Pitch Gears

Materials: 17-4PH, 13-8Mo, 15-5PH and 416


Surface Hardening Process For Stainless Steel

Related technologies:

Stainless steels have excellent corrosion resistance but possess relatively low strength and wear resistance. NANO-S can now expand their application range to include components that are subjected to intensive wear.

NANO-S™  is a surface hardening process that improves the wear and galling resistance of stainless steel components without affecting the inherent corrosion resistance. The process diffuses nitrogen and/or carbon into the surface of the steel, creating a new phase structure, the S-Phase, which provides extremely high hardness. Because NANO-S™ is a fully controlled process, there is no formation of chromium nitrides/carbides and consequently no loss of corrosion inhibiting properties. The treatment produces a hardened layer that is ductile and up to 25 μm deep (0.001”).

Achieved at low temperatures less than 932°F (500°C), NANO-S™ does not induce distortion, which eliminates final machining. Finished parts are uniformly hardened even inside small bores, tight grooves and at sharp edges.

NANO-S™ Avantage

  • Attains excellent wear resistance
  • Improves fatigue strength
  • Retains intrinsic corrosion properties
  • Prevents galling
  • Does not alter chemical composition of alloy
  • Has no effect on the steel’s non-magnetic nature
  • No change in the color, shape or size
  • Uniformly hardened even small bores, tight grooves and sharp edges
  • Green technology, no waste pollution

Ball Valves

Refinery application

Ball Valve Refinerys

Frictional wear caused by metal to metal contact is significantly reduced with NANO-STM


In-process Post-Nitriding/Nitrocarburizing Oxidation

Related technologies:

When resistance to atmospheric corrosion and an attractive black finish are the predominant requirements, ONC® is the appropriate process.

Its objective is to transform the very top portion of the WL obtained with either Nitreg® or Nitreg®-C technologies into a complex spinel type structure consisting mostly of Fe3O4 type of iron oxide.

Such a post-nitriding oxidation treatment has a net effect of enhancing the corrosion resistance of an already nitrided component. This integrated process (i.e. Nitreg® + ONC® or Nitreg®-C + ONC®) simultaneously enhances corrosion and wear resistance of steel, while giving the surface an attractive dark or black appearance, expressly desired by many customers.

ONC®, applied in combination with the Nitreg® potential-controlled nitriding process or the Nitreg®-C potential-controlled nitrocarburizing process, is a clean technology that in many instances can replace chrome plating and salt bath nitriding with their inherent problems of pollution and cost.

Depending on the type of steel, parts treated with the Nitreg®-ONC process can easily pass well over 200 hours of salt-spray test per ASTM B117 before the first corrosion spot appears. Click here to view a comparison of metallurgical and corrosion test results obtained on three materials treated with the Nitreg®-ONC process.

ONC® Advantage

  • Improved corrosion resistance
  • Attractive black surface finish
  • Inherent wear resistance

Nitreg® + ONC® Treated Gas Spring Rods

1st corrosion spot after 400 hours of salt spray


Gas nitriding of titanium alloys

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

The technology is not widely known since the applications involved are also very specific. For best results consult with our engineers who will advise you on certain unique aspects of the manufacturing sequence of operations.

Nitreg®-Ti  Treated Communication Cable Connectors
Underwater application


Post-Nitriding/Nitrocarburizing Oxidation

Blac-Tride® is a proprietary Nitrex Metal post-oxidation treatment performed after the Nitreg® nitriding process and Nitreg®-C nitrocarburizing process for simultaneous improvement of wear properties and corrosion resistance. The flat black color of Blac-Tride® provides a uniform, aesthetically pleasing appearance while maintaining the mechanical and tribological properties of the nitrided or nitrocarburized component.

Blac-Tride® is available exclusively to clients of our heat treating services and is used for products across a wide range of markets from brake discs, shafts, screws, firearm components to sporting goods.

Plasma (Ion) Nitriding

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 Nitreg® 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

Ask us for an advice about which nitriding method is better suited to your situation.
If your drawing calls for “ion” or “plasma” nitriding we are at your service.

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.

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.