Under the influence of heat, plasma nitriding causes a chemical transformation of the surface layer. This occurs through the diffusion of nitrogen, which forms nitrides together with the material of the treated components. The result is a high surface hardness and significantly improved resistance to wear — properties that are of essential importance for many applications.
Compared to conventional hardening processes, the treatment of workpieces takes place at considerably lower temperatures. As a result, high dimensional accuracy can be guaranteed with this type of heat treatment. Costly post-processing of parts in the surface-hardened condition is eliminated or can be reduced to a minimum. This translates into additional cost savings within the process chain through plasma nitriding.
The treated components can often be manufactured to their final dimensions in the soft state and finished after plasma heat treatment with little or no reworking. Furthermore, very lightly tempered, quenched and tempered steel can be treated without loss of core strength.
Nitriding is in principle possible using various processes. In addition to plasma nitriding, bath nitriding and gas nitriding are well established. If a limited amount of carbon is introduced alongside nitrogen, the process is referred to as nitrocarburising.
When a general increase in hardness and improved corrosion protection is required for low-alloy workpieces, short-cycle nitriding is often used. In such cases, target values for hardness and nitriding hardness depth are often not specified. Where “maximum hardness and hardness depth” are the goal, industrial practice often refers to long-cycle nitriding.
Among hardening processes, plasma nitriding and plasma nitrocarburising hold a special position due to their reproducibility, environmental compatibility and energy efficiency. With the help of our extensive equipment portfolio, we can optimally tailor our processes to your requirements and offer a wide range of process variants.
Through targeted control of the layer structure, the treatment result achieved by plasma nitriding can be advantageously adapted to the specific loading conditions. Heat treatment in plasma takes place at significantly lower temperatures compared to conventional hardening processes. Mechanical post-processing is frequently no longer required, as distortion is minimised as a result.
Areas of Application
The fields in which plasma nitriding is applied are diverse. Possible areas of application include, among others:
- Gears & sprockets
- Moulds, forming tools, punches & dies
- Tools & tool holders
- Engine components, camshafts & crankshafts
- Transmission parts, axles, shafts & couplings
- Valves, nozzles & non-return valves
- Knives, grinding rollers & mixers
- Conveyor screws & pump impellers
- Components for mechanical engineering
Every ferrous material can be plasma nitrided. Structural steels benefit from improved wear and corrosion resistance. Sintered materials, despite their porosity, achieve improved running properties and greater resistance to abrasion. Higher-alloy steels with a high content of chromium and aluminium are particularly suited to applications with high component loads. Depending on the material and treatment, surface hardness values of over 1000 HV can be achieved. If only a specific area of the workpiece is to be treated, this is straightforward to implement in most cases. The effort required for partial nitriding is considerably less than with comparable heat treatment processes. Stainless steel can be treated in two ways. Where wear protection is the priority, a standard process exists for maximum hardness and nitriding hardness depth. If the surface is to become hard while retaining its corrosion resistance, special long-cycle low-temperature processes are available.
Physical Principles
The physical principles of plasma nitriding give rise to the characteristic features of both the process and the required plant technology:
Nitriding in plasma is a vacuum-assisted process. The parts to be treated form the cathode as a charge, while the furnace wall acts as the anode. After the loaded retort has been evacuated, an electric field is applied between the charge and the furnace wall. The process gas supplied splits in the electric field and becomes ionised. A conductive gas — the plasma — is formed. The nitrogen ions contained within it are accelerated towards the cathode due to the current flow and strike the workpiece surfaces with high energy. This leads to:
- Fine cleaning of the surfaces through sputtering of foreign atoms
- Dissolution of passive layers (e.g. on stainless steels and titanium)
- Activation of the surface
- Heating of the furnace charge to be nitrided
- Diffusion of nitrogen into the workpiece surface
Once the treatment temperature is reached, the holding time begins. This depends on the type of material and the desired nitriding hardness depth. Typical holding times in plasma nitriding are 12–50 hours. Compared to gas nitriding, plasma nitriding requires only approximately half the holding time.
After the required treatment time, pressure equalisation is achieved by flooding with a gas. The charge then cools down in a controlled manner and the finished workpieces can be removed at low temperature.
The Nitrided Layer and Its Properties
The nitrided layer consists of the outer compound layer (CL) and the diffusion layer (DL) beneath it. The compound layer is located at the surface. It is composed of iron nitrides — the nitrogen-richer ε-nitride Fe₂₋₃N and the iron-richer γ’-nitride Fe₄N. Compared to gas nitriding, the compound layer produced by plasma nitriding is more compact, lower in porosity and therefore possesses better layer properties.
Nitriding depth (Source: Plasmanitriertechnik Dr. Böhm GmbH)
Nitriding depth (Source: Plasmanitriertechnik Dr. Böhm GmbH)
Beneath the compound layer lies the diffusion zone (DL), which is composed of the base material with precipitated nitrides. The surface hardness achievable through plasma nitriding is higher the more nitride-forming elements are present in the steel. This explains why unalloyed steels achieve surface hardness values of only 250–300 HV, low-alloy steels 600–700 HV, and nitriding steels and high-alloy steels 800–1200 HV.
The characteristic value nitriding hardness depth (NHD) is defined as the distance from the surface at which core hardness +50 HV is present (in accordance with DIN 50190 Part 3). Typical NHD values are:
- up to 0.8 mm for unalloyed and low-alloy steels
- up to 0.15 mm for high-alloy steels and stainless steel
The achievable depth and the time required to reach it are predominantly determined by the steel used, the treatment temperature and the treatment duration.
Where particularly thick compound layers are required, plasma nitrocarburising is recommended as an alternative to plasma nitriding. To further increase the corrosion resistance of low and medium-alloy materials, there is also the option of carrying out post-oxidation. With the help of this additional step, the corrosion protection of the surface can be further enhanced following plasma nitriding. Further information on the process can be found here. In the following section, material-specific treatment results can be viewed.
