Nitriding or ferritic nitrocarburizing of steels and cast iron
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Reviewed by A3TS on July 29, 2026
PRINCIPLE AND DIFFERENT CONFIGURATIONS
Nitriding is a family of surface treatments that involves enriching the surface of steel with nitrogen to induce hardening through the formation of nitrides. Other elements, such as carbon, oxygen, or sulfur, may be added to modify the properties.
The treatment is generally carried out in the ferritic phase at a temperature below 590°C followed by slow cooling. But a variant, not detailed here, is performed in the austenitic phase up to temperatures of 680 to 700°C. The holding time is between 1 hour and several days.
The nitriding treatment forms a combination layer (or white layer) composed of epsilon ε (Fe2N1-x) or gamma prime ϒ' (Fe4N) nitrides or a mixture of the 2, and a diffusion layer where finely dispersed nitrides are formed which, depending on their composition, generate a strong increase in hardness.
The coating formed by the ε-nitride process exhibits interesting tribological properties. To promote its formation, a small amount of carbon is added to the nitriding medium. This process is known as nitrocarburizing.
Oxygen can be introduced during the treatment or after treatment to form a layer of iron oxide (Fe3O4) to help resist corrosion. This treatment is called Oxynitriding or Oxynitrocarburizing. If the layer is worn away by abrasion, the protection against corrosion is that of a classic nitriding.
Sulfur can be introduced into the nitriding medium to improve friction properties by increasing the surfaces’ capacity for adaptation. This treatment is called sulfonitriding or sulfonitrocarburizing.
When carbon, oxygen, and sulfur are added to the nitriding medium in addition to nitrogen, the process is called oxysulfonitrocarburizing. This process improves both friction properties and corrosion resistance by altering the composition and morphology of the diffusion layer.
The hardening of the diffusion layer depends on the presence of alloying elements such as chromium, aluminum, molybdenum, or vanadium in the alloy. The hardness of the diffusion layer in non-alloy steel does not exceed 350 HV. A quenching and tempering pretreatment is beneficial because the availability of alloying elements is enhanced by the presence of carbides that are much smaller than those found after annealing.
TYPICAL CHARACTERISTICS OF THE TREATMENT
The hardness of the combination layer is about 1000HV;
The thickness of the combination layer varies from 0 to 40µm;
The hardness of the diffusion layer ranges from 350 HV to over 1,500 HV if alloying elements such as chromium, aluminum, molybdenum, and vanadium are present.
The residual surface stresses are in compression of the order of -500MPa which increases the resistance to mechanical fatigue;
The target treatment depth ranges from 0.03 to 0.5 mm in typical cases. In certain extreme applications, depths of up to 1 mm are targeted. The conventional NHD nitriding depth is controlled in accordance with the NF EN ISO 18203 standard;
The surface fatigue strength is increased because the stressed surface area has a high yield point. The treatment depth must be sufficient to cover the stressed area;
The corrosion resistance is improved by the formation of a surface layer of iron oxide and by an impregnation after treatment with resin, wax or oil.
The friction properties are mainly improved by the formation of the combination layer;
The deformations after treatment are very low because the nitriding temperature is chosen to be 50°C lower than the tempering temperature. However, a slight volume expansion of a few microns is noted due to the introduction of nitrogen;
In some cases, savings are possible (easier in ion nitriding by masking) and allow localized treatments.
Materials Processed
Carbon steels or cast irons. Hardness is present only in the surface layer (800 to 1000 HV).
Structural steels such as 42CrMo4 steel or 40CrMnMo8 pretreated steel with a yield strength of 1,100 MPa. The surface hardness is in the range of 600 to 800 HV.
Steels with high nitrogen affinity, such as 33CrMoV13 or 41CrAlMo7-10. Surface hardness can exceed 1000 HV.
Tool steels such as X37CrMoV5-1 steel, which is used to manufacture dies and punches for hot forging or injection mold cavities. The hardness can exceed 1200 HV.
Austenitic or ferritic stainless steels to improve hot friction properties. The surface hardness exceeds 1000 HV. It should be noted that nitriding these steels under conventional conditions degrades their corrosion resistance. Austenitic or austenitic-ferritic stainless steels treated using specialized “S-phase” processes, which form expanded austenite, allow for surface hardening without degrading corrosion resistance.
BENEFITS
Increased wear resistance
Improving Fatigue Resistance
Improving the tribological properties of composite layers (seizure inhibitor, low coefficient of friction)
Improved Corrosion Resistance in Oxynitriding or Oxynitrocarburizing
Geometric variations are very small; the use of parts that require no further machining after heat treatment is feasible, provided that the nitriding process is performed on a previously stabilized material.
APPS
Tooling parts to limit friction and resist abrasion (molds, cavities, dies...)
Standard mechanical parts (axles, shafts, rods, pistons, pinions, crankshaft...) for wear resistance
Parts that must resist corrosion in ambient humidity or in lubricated environments (cylinder rods)
Parts exposed to a corrosive atmosphere for a short period of time (storage).
Transmission parts (sprockets, gears).
Brake discs designed to minimize wear particles caused by abrasion.
LIMITATIONS
In certain applications, the composite layer poses a risk of brittleness, requiring its removal by grinding or polishing.
An excessively high nitrogen flux drives carbon to the grain boundaries. This can cause the formation of angel hair (carbides or carbonitrides) in the diffusion layer, which weakens the material. These defects are unacceptable for certain applications.
The impact resistance of nitrided layers is often reduced.
The limited treatment depth restricts the use of nitriding when contact pressures are very high.
PROCESS PARAMETERS
The processing temperature ranges from 350°C to 590°C, with a holding time of between 1 hour and several days, depending on the desired result.
Implementation TECHNOLOGIES
Gas nitriding, gas nitrocarburizing: the treatment atmosphere, established between 350 and 590°C following a heating phase under nitrogen, consists of ammonia (NH₃) and an ammonia-nitrogen mixture to which various gases may be added: activators such as nitrous oxide (N₂O) and carbon dioxide (CO₂), air (in which case it is referred to as oxynitriding) or reducing agents: carbon monoxide (CO), endothermic gases, or exothermic gases in the case of nitrocarburizing. Current industrial processes are controlled and regulated. Reduced-pressure versions of these processes exist and are carried out in vacuum-purged furnaces at pressures ranging from 200 to 300 mbar. Gaseous emissions rich in ammonia must be burned before being released into the outside atmosphere.
Salt bath nitrocarburizing: use of salts based on alkali chlorides and cyanates, activated by the addition of air and additives (specific to the process developers), attemperatures between 550 and 590°C (ideally 570–580°C). These processes are not intended to produce deep diffusion layers. Their reaction rates are very fast. The disadvantages are related to the use of molten salts (recovery of waste and cleaning of treated parts).
Plasma-assisted nitriding and nitrocarburizing (often referred to as ion nitriding): The reaction takes place in a nitrogen-hydrogen plasma within a vacuum-purged reactor equipped with a high-frequency arc generator (with opposite polarity applied to the workpieces and the furnace walls). The ionized molecules produce activated molecules through collision, which enhance the reaction kinetics. The advantages include a wide range of possible temperatures—particularly toward lower temperatures—and the flexibility of results (presence and nature of the compound layer), as well as the ease of protecting specific areas using metal masks. Reaction kinetics are rapid for the shallowest penetration depths. Charge preparation is more delicate and less suited to mass production than other processes.
Oxidizing finishes: Allprocesses that produce a conversion layer can include an oxidation step at the end of the nitriding cycle by holding the workpiece in an oxidizing atmosphere at a temperature of approximately 450°C, which develops a thin conversion layer that transforms the nitrides into black Fe₃O₄ oxides. This layer contributes to improved corrosion resistance, which can reach 450 to 600 hours in salt spray testing on layers free of microporosity. The detrimental effect of microporosity can be eliminated by impregnating the surfaces with cold-curing organic products. In gaseous processes, oxidation occurs through the introduction of an oxidizing gas (N₂O, water vapor, air); in salt bath processes, oxidation occurs through sequential immersion in an oxidizing salt bath (nitrate-based).
Once the white layer has been removed, nitrided layers can serve as a reinforcing undercoat for thin-film coatings such as PVD and PACVD (duplex treatment).
