Nitriding of steel – process, nitrided layer, properties and applications

Nitriding of steel is a thermochemical treatment process that involves saturating the surface layer of a material with nitrogen. Nitrogen delivered to the surface in a suitably active form penetrates deep into the steel through diffusion and interacts with iron and alloying elements that exhibit a high affinity for nitrogen.
As a result, a characteristic nitrided layer, whose microstructure and properties differ from the core material. In properly selected steels, the process enables very high surface hardness, increased wear resistance, and favorable shaping of the component's tribological and fatigue properties.
One of the most important features of nitriding is that the process eliminates the need for austenitizing the steel and subsequent rapid hardening immediately after nitriding. This allows for smaller dimensional changes and deformations than in many technologies that utilize surface hardening.

What is steel nitriding?

Nitriding is a diffusion process. It creates conditions on the surface of the component that allow for the delivery of active nitrogen, its adsorption on the surface, and subsequent diffusion into the material.
The process can be simplified as:

NITROGEN SOURCE → SURFACE ACTIVATION → NITROGEN PENETRATION → DIFFUSION → NITRIDE FORMATION → NITRIDED LAYER

Unlike classical hardening, nitriding does not involve the formation of austenite and its subsequent transformation into martensite. The hardening mechanism is primarily related to the presence of nitrogen in solution and the formation of very fine precipitates of iron nitrides and alloying elements.

Steel nitriding mechanism

The process mechanism depends on the technology used. In gas nitriding, the nitrogen source is most often ammonia, while in plasma nitriding, the surface interacts with the active components of the nitrogen-containing plasma.
In classic gas nitriding, processes involving ammonia dissociation, nitrogen adsorption, and its penetration into the material occur on the steel surface. Some nitrogen can then diffuse into the steel.
In a simplified technological notation, the decomposition of ammonia can be represented as:

2NH ₃ ⇌ 2[N] + 3H ₂

The symbol [N] in this notation denotes nitrogen available for surface processes. The actual mechanism of surface reactions and nitrogen transport is more complex, so simple equations such as "Fe + N → FeN" should not be considered a complete description of the nitriding process.

What happens to nitrogen once it enters steel?

Once incorporated into a material, nitrogen can exist in solution and participate in the formation of appropriate phases and precipitates. Elements with a high affinity for nitrogen, such as chrome, vanadium and aluminum.
Depending on the chemical composition of the steel and the process parameters, very fine alloy nitride precipitates may form. Their presence can significantly strengthen the diffusion layer.
Therefore, steels specially designed for nitriding contain elements that allow for obtaining a very hard and stable layer.

Structure of the nitrided layer

The layer formed during nitriding is not uniform throughout its entire depth. Typically, the following can be distinguished:

BOND LAYER → DIFFUSION LAYER → CORE

The union layer – the so-called white layer

The outermost part of the nitrided layer is referred to as union layerand in metallographic practice also known as the "white layer". The name comes from its appearance on a properly prepared metallographic specimen.
This layer may include, among others:

• γ′ phase – Fe₄N,
• phase ε – Fe_2–3N.

The share of individual phases and the thickness of the compound layer depend on the nitriding potential, temperature, process time, atmospheric composition and substrate material.
A compound layer can provide favorable tribological properties and resistance to certain types of wear, but excessive thickness or poor structure can increase susceptibility to cracking and chipping. Therefore, the technology's goal is not always to achieve the thickest possible compound layer.

Diffusion layer

Beneath the compound layer is a diffusion layer . In this region, the nitrogen content gradually decreases with increasing distance from the surface.

In alloyed steels, nitrogen can form fine precipitates with elements that exhibit a high affinity for nitrogen. These precipitates are crucial for the high hardness of the diffusion layer.

The diffusion layer is of particular importance for:

• surface hardness,
• wear resistance,
• hardness distribution deep into the material,
• state of residual stresses,
• behavior of the element under variable loads.

Nitrogen and hardness profile in the nitrided layer

Nitrogen content is not uniform throughout the entire depth of the layer. The highest concentration occurs near the surface and then decreases towards the core.
The hardness profile is similar. Depending on the steel grade and layer structure, very high hardness occurs in the surface area, then gradually decreases to a value corresponding to the core material.
Therefore, when assessing a nitrided element, not only a single surface hardness value is important, but also microhardness profile as a function of distance from the surface.

Steel nitriding temperature

Nitriding is performed at temperatures below the range of classical austenitization of steel. The exact temperature depends on the process variant used, the material type, and the required structure and depth of the layer.
In industrial practice, many steel nitriding processes are carried out in the range of approximately 400-600 ° C, but this is not a universal limit for all nitriding technologies.
Temperature has a significant impact on diffusion. Increasing it accelerates nitrogen transport within the material, but also influences precipitation processes, the microstructure, and the properties of the substrate.
Therefore, the parameters should be selected as follows:

TEMPERATURE + TIME + NITROGEN ACTIVITY + STEEL COMPOSITION + REQUIRED LAYER

Nitriding time

Nitriding is a diffusion process, so time is crucial for the depth of the layer. Depending on the technology and requirements, the process can take from relatively short cycles to many hours, and even much longer if deep layers are required.
There is no single universal relationship "number of hours = specific depth" because the rate of layer growth depends on, among other things, temperature, chemical composition of the material, microstructure and atmospheric activity.
Due to the diffusion nature of the process, increasing time does not lead to a linear increase in layer depth.

Gas nitriding

Gas nitriding is one of the classic industrial nitriding methods. Components are placed in a furnace where a controlled atmosphere containing ammonia is generated.
Processes take place on the surface leading to the supply of nitrogen, which then diffuses into the material.
By controlling the temperature, time, atmospheric composition and the degree of ammonia dissociation, you can influence:

• nitrogen saturation kinetics,
• construction of the trade union layer,
• depth of the diffusion layer,
• hardness profile,
• final surface properties.

Nitriding potential

In modern gas nitriding control, an important parameter is nitriding potentialIt describes the ability of the atmosphere to deliver nitrogen to the surface and allows for more conscious control of the formation of individual phases in the layer.
Controlling the nitriding potential allows us to limit the uncontrolled growth of the compound layer and shape its structure depending on technical requirements.

Advantages of gas nitriding

• well-known industrial technology,
• the ability to process multiple elements simultaneously,
• the ability to control the composition of the atmosphere,
• good repeatability with proper process control,
• possibility of obtaining relatively deep diffusion layers.

Limitations of gas nitriding

• long time required for deeper layers,
• the need to control the process atmosphere,
• the possibility of the formation of an excessive or undesirable union layer,
• the need to safely operate an installation using ammonia.

Plasma nitriding – ion nitriding

Plasma nitriding, also known as ion nitriding, uses a glow discharge in a gas at reduced pressure. The workpiece is part of an electrical system, and the resulting plasma contains active components that interact with the surface.
The processes occurring on the surface include, among others, ion bombardment, activation and cleaning of the surface, and the supply of nitrogen enabling the formation of a nitrided layer.

Advantages of plasma nitriding

• great ability to control the character of the layer,
• the ability to limit or control the trade union layer,
• the possibility of selective protection of surfaces that are not to be nitrided,
• surface activation by plasma interaction,
• can be used in many demanding tool and machine applications.

Limitations of plasma nitriding

• more complex technological equipment,
• the need to control the discharge and atmosphere parameters,
• process sensitivity to detail geometry and electric field distribution,
• possibility of edge effects,
• the need for appropriate preparation and arrangement of elements.

Liquid nitriding

Nitriding can also be performed in liquid salt baths. Depending on the technology and bath composition, the process can result in the saturation of the surface with nitrogen or a simultaneous interaction of nitrogen and carbon.
For this reason, it is necessary to distinguish the classical concept of nitriding from the processes nitrocarburization and other types of thermochemical treatment using salt baths.

Advantages of bathing processes

• intense environmental impact on the surface,
• relatively quick layer creation in specific technologies,
• uniform effect of the bath on the accessible surfaces of the element.

Limitations of bathing processes

• chemical safety requirements,
• the need to control the composition of the bath,
• waste management requirements,
• environmental restrictions depending on the technology used and the salt composition.

Nitriding and nitrocarburizing

Nitriding i nitrocarburization are not identical concepts. In nitriding, the basic element introduced into the layer is nitrogen. In nitrocarburizing, the surface is saturated primarily with nitrogen, with a simultaneous addition of carbon.
Differences in the composition of the atmosphere or bath affect the structure of the compound layer, process kinetics, and surface properties. Therefore, these technologies should not be considered interchangeable.

Parameters influencing nitriding

The influence of the chemical composition of steel on nitriding

Not all steels respond to nitriding in the same way. The elements that form stable nitrides are particularly important.

Chromium – Cr

Chromium has an affinity for nitrogen and can participate in the formation of precipitates that contribute to the strengthening of the layer. It is an important component of many structural and tool steels subjected to nitriding.

Aluminum – Al

Aluminum has a very high affinity for nitrogen. This makes it a characteristic addition to classic steels specifically designed for nitriding. The formation of fine precipitates associated with aluminum allows for very high hardness of the layer.

Molybdenum – Mo

Molybdenum is an important component of many heat-treatable and nitridable steels. It influences hardenability, tempering resistance, and the material's behavior at elevated temperatures.

Vanadium – V

Vanadium forms stable compounds with nitrogen and can contribute to the strong strengthening of the diffusion layer.

Titanium – Ti

Titanium has a very high affinity for nitrogen and forms the highly stable TiN nitride. However, its effect on the process depends on the specific alloy composition and microstructural state.

Nitriding steels

Nitriding is performed on steels specifically designed for this process, as well as on selected structural, tool, and stainless steels. However, the final hardness and character of the layer depend strongly on the chemical composition.
Particularly important groups include:

• Cr-Al-Mo steels intended for nitriding,
• Cr-Mo and Cr-Mo-V steels,
• steels for thermal improvement,
• hot work tool steels,
• selected stainless steels, using appropriate surface activation technology.

38HMJ – classic nitriding steel

38HMJ It is one of the most characteristic structural steels associated with nitriding. The presence of chromium, molybdenum, and aluminum allows for a hard nitrided layer.
The material is used for critical machine parts requiring a combination of a durable core and a hard and wear-resistant surface.
In practice, before nitriding, the element should have a properly prepared core, most often obtained by prior heat treatment.

31CrMoV9 and other Cr-Mo-V steels

Steels containing chromium, molybdenum and vanadium can respond very well to nitriding due to the presence of nitride-forming elements and the good stability of the substrate properties.
They are used, among others, in machine elements, shafts, transmission parts, tools and other components exposed to wear and high mechanical loads.

Nitriding of tool steels

Nitriding is also used for selected tool steels, especially alloy tool steels containing chromium, molybdenum and vanadium.
The process can increase the resistance of tool surfaces to wear and abrasion, and in appropriate applications, extend their durability.
Typical applications include:

• matrices,
• forms,
• hot work tools,
• pins,
• forming elements,
• parts of plastics processing systems.

Nitriding of stainless steels

Nitriding of stainless steels requires a special approach due to the presence of a passive layer rich in chrome, which makes the classic saturation of surfaces with nitrogen difficult.
Plasma technologies enable surface activation, which is why they are also used for processing selected stainless steels.
Temperature selection is particularly important for stainless steels. Under certain conditions, chromium nitride precipitation can reduce the amount of chromium remaining in solid solution near the surface, thereby impairing corrosion resistance.
Therefore, in applications where high hardness and high corrosion resistance are required at the same time, specially controlled low-temperature processes are used.

Preparing the element for nitriding

Nitriding is usually one of the final operations in a component's production cycle. Therefore, core properties should be determined before the process begins.
A typical process line may include:

PRETREATMENT → TEMPERING → MECHANICAL TREATMENT → STRESS RELIEF IF REQUIRED → FINISHING → NITRIDING

The detailed sequence depends, of course, on the material, dimensions of the element and the required accuracy.

Why is the core often heat treated first?

Nitriding primarily improves the properties of the surface layer. It does not replace heat treatment, which is responsible for achieving the required strength of the entire cross-section.
Therefore, structural steels are often subjected to nitriding before thermal improvementThis allows you to achieve:

• durable core,
• appropriate plasticity,
• stable microstructure of the substrate,
• and then a very hard surface layer in the nitriding process.

Early tempering temperature

If the element has been previously heat treated, the temperature tempering should be selected taking into account the subsequent nitriding temperature.
The aim is to limit undesirable changes in the core properties during many hours of the element staying at the nitriding temperature.

Hardness of the nitrided layer

Nitriding can achieve very high surface hardness. However, there is no single value characteristic for all nitrided steels.
Hardness depends on:

• chemical composition of steel,
• the type and amount of elements forming nitrides,
• process temperature and time,
• nitrogen activity,
• building the trade union layer,
• microstructure of the diffusion layer,
• measurement methods and measurement depth.

In steels specifically intended for nitriding, the surface microhardness may exceed 1000 HV, but values ​​of this order should not be considered as a universal result of nitriding of every steel grade.

Depth of the nitrided layer

Layer depth is the result of diffusion and therefore strongly depends on the process time and temperature. The steel composition and the required definition of effective layer depth are also important.

In practice, both relatively shallow layers and diffusion layers reaching much greater depths are used. However, a single range, e.g., 0,1–0,6 mm, should not be considered the technological limit of the entire nitriding process.

Properties of the nitrided layer

Properly designed nitriding can lead to a number of favorable surface properties:

• very high surface hardness,
• increased wear resistance,
• increased resistance to seizure under appropriate tribological conditions,
• favorable distribution of residual stresses,
• improving fatigue behavior in many applications,
• high dimensional stability compared to processes requiring subsequent rapid hardening.

Nitriding and corrosion resistance

The effect of nitriding on corrosion resistance is not the same for all materials and technologies.
A properly formed compound layer, especially in nitrocarburizing processes and processes supplemented with oxidation, can improve the surface corrosion resistance in certain environments.
However, it cannot be generally accepted that all nitriding increases the corrosion resistance of all steels. In stainless steels, inappropriately selected temperatures can lead to the release of chromium nitrides and a reduction in corrosion resistance.

Nitriding and wear resistance

The high hardness and nature of the microstructure of the layer mean that nitriding is widely used in components exposed to friction and wear.
However, actual durability depends not only on hardness. The following factors also play a role:

• construction of the trade union layer,
• depth of the diffusion layer,
• hardness and strength of the substrate,
• surface roughness,
• contact pressures,
• type of movement of interacting surfaces,
• lubrication,
• operating temperature.

Nitriding and fatigue life

The process can have a positive effect on fatigue life by strengthening the surface layer and creating favorable compressive stresses.
However, the effect depends on the part geometry, surface quality, layer depth, core properties and the presence of notches and other stress concentrators.

Deformations during nitriding

One of the significant advantages of nitriding is that it does not require rapid cooling from the austenitizing temperature immediately after the process. This reduces the risk of large deformations compared to hardening, induction hardening and/ or flame hardening.
However, this does not mean that dimensional changes are completely absent. Nitrogen diffusion, the formation of new phases, residual stresses, and prolonged heating can cause slight dimensional changes, which must be taken into account for high-precision components.

Nitriding and surface hardening

Nitriding and carburizing

Carburizing and nitriding are thermochemical treatment processes, but they use different elements and different mechanisms for final hardening of the layer.

Quality control of the nitrided layer

Evaluating the quality of nitriding should involve more than measuring the hardness on the surface itself.
Depending on the application, the following are tested, among others:

• surface microhardness,
• microhardness profile deep into the material,
• effective layer depth,
• thickness and structure of the compound layer,
• microstructure of the diffusion layer,
• layer uniformity,
• surface condition,
• dimensional changes,
• in demanding applications also resistance to wear, fatigue or corrosion.

The most common problems during nitriding

• Excessively thick union layer – may increase the susceptibility of the surface to brittleness and chipping.
• Too small depth of the diffusion layer – insufficient time or inappropriate process parameters.
• Uneven nitriding – may result from the surface condition, element geometry or inhomogeneous process conditions.
• Incorrect core hardness – nitriding will not repair an incorrectly prepared substrate microstructure.
• Dimensional changes – small, but important in parts with very tight tolerances.
• Deterioration of the corrosion resistance of stainless steel – possible with incorrect selection of temperature and technology.

Application of nitriding

Nitriding is used in components requiring a hard, wear-resistant surface while maintaining a strong core and relatively small dimensional changes.

Machine industry

• embankments,
• axles,
• pins,
• sleeves,
• gears,
• transmission components,
• guides,
• parts of hydraulic systems.

Automotive industry

• shafts and drive system components,
• gears,
• pins,
• elements of mechanisms exposed to wear and fatigue.

Tool industry

• molds for plastics,
• matrices,
• pins,
• hot work tools,
• forming and guiding elements.

Construction of special machines

• elements requiring high wear resistance,
• precision machine parts,
• elements operating under high surface pressure,
• parts requiring the combination of a hard surface with a strong core.

The most important advantages of nitriding

• very high hardness possible to obtain in properly selected steels,
• high surface resistance to wear,
• possibility of improving fatigue behavior,
• no need to harden the element immediately after nitriding,
• relatively small dimensional changes,
• possibility of machining finished or almost finished parts,
• the ability to control the layer structure by selecting technology and parameters.

Limitations of nitriding

• long time required for deep layers,
• strong dependence of the final hardness on the chemical composition of the steel,
• the need to properly prepare the core before the process,
• the possibility of the formation of an undesirable compound layer,
• the need for thorough surface preparation,
• technological limitations depending on the geometry of the part and the method used.

Nitriding steel – the most important information

Nitriding of steel is a thermochemical treatment process that creates a very hard and wear-resistant surface layer without the need for austenitizing and subsequent rapid hardening of the component. The process is based on the delivery of active nitrogen to the surface and its diffusion into the material. The resulting layer may consist of an outer compound layer containing primarily γ′-Fe ​​phases.₄N and ε-Fe_2–3N and the underlying diffusion layer. Particularly good results are achieved with steels containing elements with a high affinity for nitrogen, including chromium, aluminum, and vanadium. For this reason, special steels for nitriding have been developed, a classic example of which is 38HMJ.
The properties of the layer are not determined by a single universal range of temperature, hardness, or depth. The result is a function of the steel grade, initial microstructure, temperature, time, nitrogen activity, and the nitriding method used.
Thanks to the combination of high surface hardness, wear resistance and relatively small dimensional changes, nitriding is one of the most important surface treatment technologies used in the production of critical machine elements, tools, molds, shafts, gears and precision parts.

Methods of heat and thermochemical treatment of steel

Solution treating is just one of the processes used to modify the microstructure and properties of steel. Depending on the desired effect, normalizing, stress relieving, hardening, tempering, etc., as well as thermochemical processes such as carburizing, nitriding, and others, are also used.

Steel heat treatment methods

• Normalizing steel – grain refinement and microstructure unification.
• Stress relief annealing – reduction of residual stresses.
• Softening of steel – reducing hardness and improving susceptibility to further processing.
• Full annealing – austenitization and slow cooling.
• Isothermal annealing – transformation carried out while holding at a properly selected temperature.
• Ball annealing – spheroidizing – change in the morphology of cementite or other carbides.
• Recrystallization annealing – removal of the effects of crushing and restoration of plasticity.
• Homogenizing annealing – reducing segregation and homogenizing the material composition.
• Hardening and heat treatment – shaping the microstructure and properties by hardening and tempering.
• Tempering steel – controlled modification of steel properties after hardening.
• Induction hardening – surface induction heating and rapid cooling.
• Flame hardening – surface hardening after local heating with a flame.
• Oversaturation – obtaining a supersaturated solid solution by an appropriate thermal cycle.
• Aging – controlled phase separation leading to material strengthening.

Methods of thermochemical treatment of steel

• Nitriding of steel – diffusional enrichment of the surface layer with nitrogen.
• Carburizing steel – diffusional enrichment of the surface with carbon.
• Carbonitriding – simultaneous enrichment of the surface layer with carbon and nitrogen.
• Nitrocarburization / nitrocarburization – enrichment of the layer primarily with nitrogen and carbon.
• Drilling – diffusion saturation of the surface with boron.
• Diffusion chrome plating – diffusion enrichment of the layer with chromium.
• Diffusion aluminizing – diffusion enrichment of the layer in aluminum.