R.NIT+® covers all gas nitriding and related processes in the premium segment. R.NIT+® advantages include maximum flexibility, best marks for wear and corrosion resistance, and optimized properties for gliding and rolling stress.
Detailed descriptions of our procedures are presented in our technical data sheets that are available for download.
During gas nitriding, nitrogen is provided in the form of ammonia gas. The process is performed in shaft and hood-type furnaces within a temperature range between 500°C and 600°C. Due to the chemical decomposition of the ammonia at the component, the nitrogen diffuses into the surface and a diffusion and compound layer is formed. It must be observed that it is not possible to nitride passivated metallic surfaces and particular attention must be paid to cleaning the component before treating it. Steels with a low to medium alloy content can be nitrided.
The process duration and the temperature have a direct influence on the result. With the targeted process control, the hardness, the nitriding hardness or hardening depth and the layer thicknesses can be influenced. Nitrocarburisation means that the surface is additionally concentrated with carbon. A post-oxidation is also possible, which can be performed during the cooling procedure or separately. A modern process control system guarantees highest reproducibility with supervision and documentation. Standard processes offered by RUBIG are short-term nitriding, normal nitriding and long-term nitriding. We, of course, also perform customer-specific nitriding treatments. Please contact us regarding your nitriding requirements.
The heat treatment nitriding belongs to the thermo-chemical heat treatment procedures. The surface of the workpiece is changed chemically by embedding nitrogen by means of a diffusion. Nitrocarburising is the process of adding nitrogen to carbon in the process gas. Nitrogen results in an increased hardness since it forms a compound with elements such as aluminium, chrome or vanadium (special nitrides). During the process, there is no structural change within the core (martensite formation as with case hardening and vacuum hardening procedures) which results in minimum deformations of the workpiece. A further increase of hardness is achieved with the interstitial embedding of nitrogen in the grid structure of the iron. The percentage of the overall increase of hardness is, however, low. With iron, nitrogen can form a new phase which predominantly grows on the surface of the workpiece, i.e. the so-called compound layer. This layer shows similar characteristics to those of a ceramic layer. It results in an improved wear resistance and a slightly increased corrosion resistance. This layer is mainly composed of two nitride types, i.e. Fe4N (γ‘ – nitride) and Fe2-3N (ε-nitride).
How far the nitrogen can penetrate into the steel surface depends on the time and the temperature of the process. The nitrogen depth is frequently measured with a hardness profile measurement; i.e. the nitriding depth is measured. Components can be treated with the following nitriding procedures: Salt bath nitriding (PQP procedure, Tenifer, ...), gas nitriding and plasma nitriding. Salt bath nitriding is not offered by RUBIG due to the environmental concerns.
If a higher improvement of the corrosion resistance is required, a targeted oxidation can be performed on the surface of the workpiece following the nitriding process. The compound layer created during nitriding is transformed on the surface into iron oxide. With the chemical stability of the iron oxide compound, a corrosive attack is hindered.
Steels suitable for the nitriding process are:
It is important that the tempering temperature of the workpieces must be above the nitriding temperature. Corrosion-resistant steels can be nitrided in plasma plants. Regarding these materials, particular attention must be paid to the resulting material characteristics (hardness, nitriding hardening depth and corrosion resistance). If you require more detailed information, we are pleased to advise you.
If the nitrided layer alone does not meet your requirements, a further improvement of the corrosion resistance and of the friction coefficient can be achieved with a post-oxidation (directly during the nitriding process or as a separate process). With the targeted post-oxidation, the iron nitride layer at the surface of the workpiece is transformed into an iron oxide layer due to an oxidising atmosphere (e.g. by adding H2O or O2). With the correct selection of temperature, time and atmosphere, the corrosion resistance can, compared to a nitrided layer, be increased by a factor of 10. The surface of oxidising workpieces is anthracite-coloured and has a smaller friction coefficient than nitrided surfaces.