Nitriding is a thermochemical treatment that generates significant changes to the tribological, mechanical and anti-corrosive properties of the surfaces of the treated parts, aimed at increasing the working life, reliability and performance of the same.
This is why surface treatments have been widely used and, over the years, they have evolved into new techniques, the most recent of which is plasma nitriding, which TAG carries out with the plant systems at the Dolzago production facility.
Nitriding using plasma technology has made it possible to successfully overcome certain typical problems generated by the old methodologies, such as salt bath nitriding and gaseous nitriding, which we will now examine in detail.
STARTING FROM THE TRADITIONAL METHODS…
Salt bath nitriding (Tenifer – QPQ – TF1) takes place at a temperature of approx. 570°C, and is mainly used for short length nitriding (a few hours) or bulk pieces and produces a layer measuring a few hundredths of centimetres in depth with mainly anti-seizure characteristics. The typical chemical composition of the bath generates a white coating of carbonitrides, porous and with mixed phases ɛ (Fe 2-3 N) / Fe 2-3 C x N y type and ɣ’ (Fe 4 N) type.
A specific application of liquid phase nitriding is the treatment of high-speed steel tools. The main purpose has always been to increase the resistance to fatigue by reducing the fragility, something that is very high in these types of steel. It is considered to be a low-cost treatment, however the cleaning and removal of salt from the pieces after nitriding, especially from the holes and grooves, generates high labour costs.
Plasma nitriding provides surface hardening of threading tools for screws and bolts, made of high-speed steel:
The possibility of carrying out the process using an extremely low percentage of nitrogen, combined with low temperatures, lets obtain a hardened layer that is completely free of porosity and which is very thin, compact and therefore highly resilient.
Unlike other treatments, the low plasma temperature allows the re-nitriding of the tool after regeneration (reworking and reinstating of the threaded profile after the first production cycle), guaranteeing a considerably longer useful life.
Likewise, the plasma process is successfully applied also to sintered steels (e.g.70 DDH2 Sint, Sint D30, etc.) at our production facilities: in this case the plasma used immediately forms a layer of compounds, to avoid the rapid diffusion of nitrogen within the porosity which is typical of such materials and subsequently avoid an excessive fragility and/or volumetric increase.
Gas nitriding, the second of the “traditional” methodologies, is carried out at essentially lower temperatures (500-550°C) compared to the bath method, but the process requires a considerably longer amount of time and can last from 20 to 100 hours, with morphology layers that are difficult to control due to the fact that, when using this process, it is not possible to reduce the nitrogen input below 30%. This generates a consequent white coating consisting of two very distinct phases, superimposed one over the other: the ɛ (Fe 2-3 N) phase and the ɣ’ (Fe 4 N) phase, very porous and fragile. The impact on post-treatment dimensional variations (deformations) is another aspect not to be underestimated.
… AND ARRIVING AT THE INNOVATIVE ONES: WHY PLASMA NITRIDING
- With plasma nitriding it is possible to vary the nitriding depth, the thickness and the type of layer of the surface compounds as required. It is therefore possible to obtain a single layer of ɣ’ (Fe 4 N) type or a mix of ɣ’ (Fe 4 N) and ɛ (Fe 2-3 C x N y ) or just ɛ (Fe 2-3 C x N y ) or just ɛ type (Fe 2-3 N) without incurring, unlike other processes, the typical layer porosity and fragility.
- For particular applications it is also possible to eliminate the white layer of the compounds or add a layer of magnetite as protection against corrosion.
- It is possible to carry out the process at temperatures below 500°C, hence making it possible to maintain “in the core” the hardness obtained after hardening or tempering: for example, for chromium eutectic steel Wrn °1.2379 (X155CrMoV121) the process temperature = 480°C.
- An important advantage is the pickling and surface activation caused by the sputtering. This leads to a marked reduction in treatment times and consequently in the dimensional variations compared to other processes, with excellent quality surfaces, as well as permitting the treatment of stainless steels and non-ferrous alloys, such as titanium alloys.
- Above all, it is possible to perform the nitriding process directly on the finished pieces, given that the deformations are practically insignificant if the pieces are free of their mechanical thermal stresses. This characteristic generates a marked reduction in the costs of the manufactured pieces thanks to the elimination of the grinding phase.