Nitriding – Process, Methods, Structure & Applications

Nitriding is a thermo-chemical treatment of steel, saturating the surface of steel with a very thin layer of nitrogen to create a case-hardened surface. This layer is very hard and resistant to wear, which is why nitriding is often used as a final treatment of tools and machine components. 

Process, temperature and duration of nitriding

The steel workpiece is immersed in an environment containing free nitrogen atoms at a high temperature, but lower than A_C1. Nitrogen deposits on the steel surface as a result of interstitial absorption in the form of nitrides or carbonitrides, and over time diffuses deeper into the steel. 

Nitriding is carried out in the temperature range of 500–600°C, most often 520–560°C. Depending on the duration of the process, short term or long term nitriding can be distinguished. 

Short term nitriding lasts from several minutes to a few hours (tool steels in particular are often short-term nitrided). Long term nitriding can last even several dozen hours. Both are generally carried out in the same temperature. When nitriding at a constant temperature, the nitrided layer strength depends on the duration of the operation. For some steels, nitriding for several minutes is as good as nitriding for a much longer time.

After nitriding, the steel is cooled slowly, which causes hard nitride needles to precipitate in the structure.

Structure after nitriding

In the iron–nitrogen system, four distinct phases occur:

• Phase α (nitroferrite) is a solid solution of nitrogen in iron, with a nitrogen content of 0.015% at room temperature and 0.42% at the eutectoid temperature.
• Phase γ (nitroaustenite) is a solid solution of nitrogen in γ-iron, existing at temperatures above 595°C. Upon slow cooling, it decomposes into a eutectoid of phases α and γ′.
• Phase γ′ is iron nitride, with a chemical composition of approximately Fe₄N.
• Phase ε is iron nitride containing more nitrogen, with a chemical composition of approximately Fe₂N. It may also contain carbonitrides.

Long-term nitriding produces a continuous zone of γ′ + ε nitrides and carbonitrides, beneath which lies the diffusion zone. Short-term nitriding does not form a continuous nitride zone, but only a diffusion zone of ferrite supersaturated with nitrogen and precipitations of nitrides and carbonitrides of the γ′ + ε phases. Both the diffusion zone and the nitrides and carbonitrides of the γ′ + ε phases increase wear resistance. Moreover, the truly hard nitride and carbonitride layer is only 0.020–0.025 mm thick. As a result of long nitriding, this hard layer does not thicken significantly, but penetrates deeper, while the subsurface layers become porous and brittle. Therefore, short-term nitriding is sufficient to improve wear resistance. The nitride and carbonitride layer is very hard – its hardness reaches 800–1200 HV. 

Below is shown the structure of alloy steel nitrided in a heat-treated condition. On the outside, a thin layer of γ′ + ε phases can be seen, followed by a dark-etched sorbitic layer rich in nitrogen and nitride precipitates, which transitions into a lighter sorbitic core.

Nitriding steel grades

Nitriding steels are structural steels designed specifically for nitriding. Without nitriding, these steels are generally not used. Here is a brief description of the most important alloys:

• 41CrAlMo7 / 1.8509 – a chromium–aluminium nitriding steel with molybdenum addition, used in the automotive industry (e.g. diesel fuel pump parts) and in mechanical engineering (e.g. gears). Not suitable for welding.
• 34CrAlNi7-10 / 1.8550 – a nickel–aluminium nitriding steel, used in the automotive, mechanical, and energy industries for fasteners, piston rods, cylinders, etc. Poor weldability.
• 31CrMo12 / 1.8515 – a chromium–molybdenum nitriding steel, used in the automotive, mechanical, and energy industries for parts operating at elevated temperatures up to 600°C – spindles, gears, shafts, fasteners. Not suitable for welding.
• 31CrMoV9 / 1.8519 – a chromium–molybdenum–vanadium nitriding steel, with slightly worse properties than 1.8509, but without aluminium.

All of the above steel grades are nitrided at the end of the technological process after hardening and tempering, as a result of which they obtain a wear- and abrasion-resistant surface.

Tool steel grades suitable for nitriding

For tool steels, nitriding is an optional final treatment. Saturating the subsurface layers of tools with nitrogen significantly increases their wear resistance. Such nitriding is carried out at 480–600°C, usually 520–560°C, and lasts from several minutes up to 1 hour, producing layers about 0.01–0.05 mm thick. This increases hardness to 800–1200 HV and extends the tool life by 2 to 8 times.

Nitriding should not reduce the hardness of the tool core, therefore the nitriding temperature must be at least 30°C lower than the previous tempering temperature (called heat improvement). Steels tempered at low and medium temperatures are not subjected to nitriding. Thus, nitriding is applied to tools:

• Cutting tools made of high-speed steels, which are hardened from very high temperatures and, despite tempering at 550–570°C, retain high hardness of 58–62 HRC, e.g. S6-5-2 / 1.3343,
• Shearing tools made of high-chromium steels for cold working, which have similar properties to those mentioned above, e.g. 100MnCrW4 / 1.2510 or X165CrV12 / 1.2201,
• Hot wrought steel tools made of steels tempered at 530–650°C, e.g. 1.2567.

Anti-corrosion nitriding of carbon steels

Nitriding of carbon steels and many alloy steels not listed above, although possible, is not applied because it does not significantly improve hardness. The hardness of a nitrided carbon steel layer is in the range of 300–350 HV.

Some textbooks also describe anti-corrosion nitriding carried out on carbon steel at 600–850°C for between several minutes and 2 hours. The thickness of the nitrided layer is 0.02–0.04 mm. In current practise, nitriding is mainly carried out as a case-hardening process.

Methods of nitriding

Several nitriding methods are distinguished, depending on the nitriding medium.

Powder nitriding – here, nitriding is carried out using a powder composed, for example, of 80% calcium cyanamide CaN(CN), 15% sodium carbonate Na₂CO₃, 3% ferromanganese, and 2% silicon carbide SiC. Such nitriding lasts from 30 minutes to even 10 hours. Rarely used.

Gas nitriding is the most widely used type of nitriding. The nitriding medium is usually a stream of dissociated ammonia at 500–600°C. Iron, acting as a catalyst, breaks ammonia down into free hydrogen and nitrogen atoms. In this method, in addition to ammonia, pure nitrogen must be present to regulate the nitriding rate. Dissociated ammonia alone results in varying nitriding rates, without repeatable results. The dissociation of ammonia should not be lower than 20%, as lower dissociation produces overly brittle layers, and not higher than 70%, as higher dissociation results in soft case. The optimal dissociation level is 40–50%. Using this method, on high-speed steel tools such as 1.2581, a nitrided layer up to 0.02 mm can be obtained within 30 minutes. A danger of this method is toxic fumes. Maintaining optimal dissociation reduces the amount of toxic substances.

Salt-bath nitriding (o molten salt nitriding) is carried out in cyanamide baths at 540–580°C. This method produces less bright tool surfaces than the gas method but is less toxic. An example composition of salts for nitriding: nitriding salt mixture consisting of 30% BaCl₂, 10% KCl, 10% NaCl, 50% CaCl₂ and an activating salt consisting of 3–5% BaCl₂, 5–7% KCl, 5–7% NaCl, 9–11% charcoal C, and 74–79% nitrate (which consists of about 55% calcium cyanamide CaCN₂).

Ion / plasma nitriding is performed in an atmosphere of ionized nitrogen (pure nitrogen or a nitrogen–hydrogen mixture). The steel parts to be nitrided are placed in a retort (serving as the anode) and connected to the negative pole. The applied voltage is 500–1500 VDC, and the pressure of the nitriding atmosphere is reduced. The high voltage ionizes the gas at the cathode, i.e. at the nitrided object. Collisions of nitrogen ions with the surface of the treated part release heat sufficient for nitriding. The process can be controlled by adjusting the pressure, voltage, and chemical composition of the gas. Nitrided layers obtained by this method are more resistant to wear and fatigue and are much more ductile compared to layers obtained by other nitriding methods.

Fluidized-bed nitriding – the object is placed in a fluidized bed of sand or alumina. The fluidized bed is created by solid particles kept in suspension by hot gas flowing from below. The gas nitrides the object immersed in the bed.

Role of nitriding in the technological process

Nitriding is the last operation in the technological process. Before nitriding, the objects are subjected to hardening and high-temperature tempering. After nitriding, the objects cannot be ground, due to the very thin protective nitrided layer.

Before nitriding, steel objects should be clean and carefully degreased. It should be remembered that nitriding usually increases the dimensions of small objects (e.g. tools) by about 0.05%.