Nitrocarburizing / Nitrocarburizing of steel – process, compound layer, properties and applications

Nitrocarburization, also referred to as nitrocarburation, is a process thermochemical treatment consisting in the simultaneous saturation of the surface layer of the material, primarily nitrogen and to a lesser extent coalThe process is primarily used to increase surface resistance to wear, seizure, contact fatigue, and – in appropriately executed variants – to improve corrosion resistance.
In industrial practice, it is of particular importance ferritic nitrocarburizing, FNC, carried out at a temperature at which the steel remains essentially in the ferritic range. This distinguishes the process from carbonitriding, usually carried out in the austenitic range and completed hardening.
During nitrocarburization nitrogen i carbon diffuse into the surface layer of steel and participate in the formation of a characteristic structure usually consisting of layers of compounds and the one located beneath it diffusion zone.
The process can be carried out using gas, bath, or plasma methods. The choice of technology depends on the type of material, the geometry of the component, the required layer thickness and structure, and the expected performance properties.

Nitrocarburization, nitrocarburization and carbonitriding – differences in nomenclature

Definitions nitrocarburization i nitrocarburation refer to a process in which the dominant component introduced into the surface layer is nitrogen, carbon acts as an additional component influencing, among other things, the formation of a layer of compounds.
This process should not be identified with carbonitriding.

Carbonitriding is therefore a process closer to nitriding than carburizing. However, the introduction of additional carbon can alter the kinetics of formation and the composition of the compound layer.

What is nitrocarburization?

During the process, the steel surface is exposed to an environment that provides active nitrogen i carbon.
Atoms of these elements are adsorbed on the surface and then penetrate into the material by diffusion.
As a result of the process, two main zones are created:

• layer of compounds located directly at the surface,
• diffusion zone located beneath the layer of compounds.

The structure of both zones depends on the chemical composition of the steel, temperature, time, nitrogen and carbon potential of the atmosphere and the technology used.

Layer of compounds after nitrocarburization

A thin layer is formed directly at the surface a layer of iron compounds with nitrogen and carbonIn technical literature, the term is also used compound layer.
Depending on the process parameters, the layer may contain mainly the following phases:

• ε – Fe_2–3(N,C),
• γ′ – Fe₄N.

In nitrocarburizing, it is often sought to obtain a layer with a high proportion of the ε phase, because the presence coal favors its stabilization.
The compound layer is primarily responsible for the surface's high resistance to adhesive wear and abrasion. However, its properties depend not only on its thickness but also on its phase composition, compactness, and porosity.
A thicker layer of compounds does not always mean a better wear layer. Excessive thickness or unfavorable porosity can increase surface brittleness and impair the component's behavior under certain types of loading.

Diffusion zone

Under the layer of compounds there is diffusion zone.
Nitrogen It penetrates much deeper than the thickness of the compound layer itself. It can exist in solid solution and form fine nitrides and carbonitrides with elements with a high affinity for nitrogen.
Of particular importance are:

• chrome Cr,
• aluminum Al,
• molybdenum Mo,
• vanadium V,
• titanium Ti.

The formation of fine nitride precipitates may increase hardness diffusion zone and influence the distribution of residual stresses.
The diffusion zone plays an important role in increasing fatigue resistance and supporting the thin, very hard surface layer.

The structure of the layer after nitrocarburization

Nitrocarburizing temperature

Ferritic nitrocarburizing is most often carried out in the range of approximately 500-600 ° C, the exact temperature depends on the material, technology and required layer properties.
Common industrial temperatures are found near 570-590 ° C.
The process is carried out below the transformation temperature A₁, thanks to which the steel matrix remains essentially ferritic and the classical austenitization characteristic for hardening.
This has significant technological implications. The lower process temperature reduces dimensional changes and the risk of distortion compared to processes requiring austenitizing and subsequent hardening.

Typical nitrocarburation parameters

The ranges provided are indicative. The appropriate cycle must be selected based on the steel grade, prior processing, component geometry, and required layer characteristics.

Gas nitrocarburizing

Gas nitrocarburizing is carried out in a furnace in a controlled atmosphere providing nitrogen i carbon.
One of the nitrogen sources may be ammonia, while the carbon component may come from appropriately selected carbon-containing gases.
Control of the atmosphere composition, temperature and time enables the control of the kinetics of growth of the compound layer and the diffusion zone.
The advantages of the gas method include the possibility of carrying out the process for a large number of elements and relatively precise control of the atmosphere parameters.

Bath nitrocarburizing

Bath nitrocarburizing is carried out in liquid salt baths enabling the delivery of active nitrogen and carbon to the surface of the treated material.
The method allows for very intense heat exchange and uniform environmental impact on the surface of elements with a complex shape.
After the actual nitrocarburization step, additional cooling, polishing and oxidation operations may be applied.
Bathing technology requires strict control of the bath composition and appropriate technological and environmental procedures.

Plasma nitrocarburizing

Plasma nitrocarburation uses a glow discharge in a gaseous atmosphere containing nitrogen and carbon supply components.
The workpiece can act as a cathode. Ions generated in the plasma interact with the surface, aiding its activation and transport of components.
The advantages of the plasma process include the ability to precisely control parameters and selectively protect surfaces that should not be treated.

Nitrocarburizing and nitriding

Carbonitriding is a process related to nitriding , but the presence of additional carbon affects the formation and stability of the compound layer.

Nitrocarburizing and carburizing

Carburizing is a process designed primarily to increase the content coal in the surface layer of low carbon steel.
After carburizing, appropriately selected hardening and usually low letting go, which creates a hard martensitic layer while maintaining a more ductile core.
In nitrocarburization, the primary surface hardening mechanism is not the formation of high-carbon martensite. These properties result primarily from the presence of a layer of compounds and nitrogen in the diffusion zone.

Why does nitrocarburizing not require classical hardening?

Ferritic nitrocarburizing is carried out below the temperature A₁Therefore, no austenite is formed, which would undergo the classic martensitic transformation during rapid cooling.
The surface hardness is the result of the formation of nitrides, carbonitrides and compound layer phases and the interaction of nitrogen in the diffusion zone.
This allows you to avoid one of the main sources of deformation typical of hardening.
However, this does not mean a complete absence of dimensional changes. Any heat or thermochemical treatment may cause some dimensional changes resulting from temperature, residual stresses, and structural changes.

The influence of alloying elements on nitrocarburizing

What steels can be nitrocarburized?

Nitrocarburizing can be used on many types of steel, not just special steels. steel for nitriding.
The process is used, among others, for:

• carbon structural steels,
• alloy structural steels,
• steel for thermal improvement,
• steel for nitriding,
• selected tool steels,
• cast iron, if the composition and required technology allow it.

In steels containing Cr , Mo , V or Al, it is possible to harden the diffusion zone particularly effectively due to the formation of fine nitrides.

Nitrocarburizing of steel after heat treatment

In many applications, the element is subjected to nitrocarburation beforehand. thermal improvement.
This allows the required core properties to be first obtained – appropriate strength, yield strength and ductility – and then the thin surface layer to be modified by nitrocarburizing.
Temperature of the previous tempering should be appropriately selected in relation to the subsequent nitrocarburization temperature, so that the surface process does not cause undesirable tempering of the core and loss of previously obtained properties.

Properties of the layer after nitrocarburization

Properly performed nitrocarburation can provide a favorable combination of surface properties.

Nitrocarburizing with oxidation

After nitrocarburation, additional carburation may be performed. surface oxidation.
A thin layer of iron oxides is then formed on the surface, in which magnetite Fe may be of particular importance.₃O₄.
Connection:

can increase corrosion resistance and give the surface a characteristic dark or black appearance.
Depending on the technology, polishing is also used between the nitrocarburization and final oxidation stages.

QPQ – Quench–Polish–Quench process

One of the developments in bathing technology is the process known as QPQ – Quench–Polish–Quench.
In simplified terms, it includes:

1. nitrocarburation surface,
2. first stage of cooling or oxidation treatment, depending on the specific technology,
3. polishing surface,
4. final stage of oxidation.

The goal is to combine high wear resistance with favorable tribological properties, good surface quality and increased corrosion resistance.
The name QPQ is associated with a specific family of technologies and should not be automatically used as a synonym for any nitrocarburizing process with subsequent oxidation.

Preparing the surface for nitrocarburization

The condition of the surface before the process is of great importance for the uniformity of the layer.
The surface should be free of:

• oils and greases,
• corrosion products,
• scale,
• technological pollution,
• coatings that limit access of active ingredients to the surface.

For some materials, additional surface activation may be necessary. This is particularly important for materials that create durable passive layers.

Influence of process time on the layer

Extending the nitrocarburization time allows for further diffusion nitrogen and increasing the depth of the diffusion zone.
This does not mean, however, that the process should be carried out for as long as possible.
Excessive time can lead to:

• excessive growth of the compound layer,
• increasing its porosity,
• deterioration of the mechanical properties of the surface,
• unnecessary increase in process costs.

The parameters should therefore result from the required characteristics of the layer, and not from the principle "the thicker the layer, the better".

Application of nitrocarburization

Nitrocarburization is used in the production of components exposed to friction, wear, seizure and variable loads.
Typical applications include:

• shafts and rollers,
• gears,
• bushings and pins,
• transmission components,
• cams,
• camshafts,
• elements of hydraulic and pneumatic systems,
• pump parts,
• guides,
• elements of working machines,
• parts of the automotive industry,
• selected tools and equipment.

Advantages of nitrocarburizing

• high surface wear resistance,
• increased resistance to abrasion,
• possibility of improving fatigue resistance,
• possibility of increasing corrosion resistance after appropriate additional treatment,
• relatively low process temperature,
• no need for classic hardening after ferritic nitrocarburization,
• lower risk of deformation than in processes requiring austenitizing and hardening,
• possibility of machining finished or almost finished dimensions of elements,
• can be used for many types of steel.

Limitations of nitrocarburization

• the layer is much shallower than the typical layer obtained by carburizing,
• the process does not replace proper heat treatment of the core,
• an excessively thick or porous layer of compounds may be unfavorable,
• the effectiveness of the process depends on the chemical composition of the material,
• Improper surface preparation may cause uneven saturation,
• process parameters must take into account the previous heat treatment of the component.

Inspection of the layer after nitrocarburization

The quality of the layer can be assessed by metallographic examination and measurements hardness and analysis of the structure and phase composition.
Among other things, the following are checked:

• thickness of the compound layer,
• depth of the diffusion zone,
• porosity of the compound layer,
• microhardness profile from the surface towards the core,
• layer uniformity,
• metallographic structure,
• tribological properties, if required,
• corrosion resistance in the case of processes also designed for this purpose.

Nitrocarburization, or nitrocarburization, is a thermochemical treatment consisting in the simultaneous introduction of primarily nitrogen and a smaller amount coal.
The most important industrial variant is ferritic nitrocarburizing FNC, conducted below temperature A₁, most often in the range of about 500–600°C.
The process creates a thin layer layer of compounds, containing mainly ε-Fe phases_2–3(N,C) and γ′-Fe₄N, and the one below it diffusion zone.
A layer constructed in this way can provide high resistance to wear and seizure, improved fatigue properties and – especially after additional oxidation – increased corrosion resistance.
Nitrocarburization should not be confused with carbonitridingIn nitrocarburation, the dominant component is nitrogen. The process is most often carried out in the ferritic range and does not require classical hardening. Carbonitriding, on the other hand, is a related process. carburizing, usually carried out in the austenitic range and completed with hardening.

Methods of heat and thermochemical treatment of steel

Carbonitriding 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.