Heat treatment in plasma
Plasma nitrocarburizing is a reasonable alternative to plasma nitriding for parts with extreme wear and tear. With this process, thicker compound layers with higher wear resistance can be produced in surfaces. In addition to nitrogen, this process variant also diffuses carbon. The thick compound layer produced on the surface during nitrocarburizing contains mainly ε carbonitrides Fe2-3CN, whose formation is favored by the increased carbon content in the furnace atmosphere.
Optimally suited for treatment are unalloyed and low-alloy steels with low carbon contents.
Advantages of the process
- Final cleaning of the components at the beginning of the plasma treatment
- increases toughness of the hardened surface layer
- forms a thick bonding layer with excellent running properties
- reduces the adhesive wear
- reduces the risk of chipping
- Layers are less brittle and porous than with nitrocarburizing or gas carbonitriding
- Minimization of rework
- no subsequent cleaning necessary
Plasma nitrocarburizing is often classified as a process variant of plasma nitriding with a carbon component and included under the generic term plasma nitriding, because the basic mechanisms of action are identical.
The individual performance and functionality of both processes can be read here:
Nitrocarburizing comprises (like nitriding) different process variants in gas, salt bath or plasma. Compared to salt bath nitrocarburizing and gas nitrocarburizing, plasma nitrocarburizing is characterized by lower treatment temperatures and special environmental friendliness. Furthermore, high-alloy steels can also be treated.
In the case of low-alloy steels, the achievable increase in hardness is small, but improved corrosion protection is achieved and adhesive wear is reduced. Oxidation (treatment and cooling in an oxidizing atmosphere) following plasma nitrocarburizing can additionally increase corrosion protection.
Physical principles of plasma nitriding carburizing
As with nitriding, nitrocarburizing creates a surface compound layer in the plasma that is only a few µm thick. Underneath this lies a diffusion zone up to several hundred µm (thick). The formation of a characteristic compound layer is the main objective of nitrocarburizing – because it results in improved corrosion resistance and wear resistance.
Alloying elements of the respective steel form nitrides, carbides and carbonitrides together with the nitrogen and carbon particles released from the treatment medium. The elements manganese (Mn), chromium (Cr), molybdenum (Mo), tungsten (W), tantalum (Ta), vanadium (V), niobium (Nb), titanium (Ti) tend to form carbides, the elements aluminum (Al), chromium (Cr), zirconium (Zr), niobium (Nb), titanium (Ti), vanadium (V) tend to form nitrides. Therefore they are suitable alloying elements for steels for nitrocarburizing in plasma.
The formed compounds are called “phases”. These phases are thermally and mechanically very stable. Furthermore, they are characterized by their high hardness, which results in a certain brittleness.
The high intrinsic hardness of the resulting carbonitrides, together with tensions between the structural components, leads to the special surface hardness of nitrocarburized workpieces. Compared to nitriding, compound layers of nitrocarburized parts show up to 200 HV 0.005 higher values of microhardness at the same treatment time and temperature. Thus, a remarkable increase in the service life of workpieces nitrocarburized in plasma can be achieved. This can be seen in the reduction of friction coefficients, reduced adhesion, abrasion and triboxidation tendencies and improved fatigue strength.
The suitability for the use of nitrocarburized workpieces at high temperatures is due to the tempering resistant microstructure. As with nitriding in carbon-free treatment media, nitrocarburizing increases the corrosion resistance compared to the base material. Thicker compound layers with uniform formation and low pore content generally favour this resistance to corrosive attack.
The corrosion resistance of unalloyed and low-alloy steels is increased by plasma nitrocarburizing. The corrosion resistance of high-alloy stainless steels is reduced by plasma nitrocarburizing because the extremely corrosion-resistant passive layer is sputtered off. This in turn makes it possible to treat passivated steels and is a special feature of the plasma process compared to those in gas and salt bath.
Due to the high dimensional accuracy of plasma nitrocarburizing, no reworking is necessary in most cases, in contrast to case hardening. Accordingly, in contrast to other hardening processes, the complete hardening depth with the particularly hard outer layer is retained.
There are no additional financial or time expenditures after the heat treatment. In addition, oxidation can be used as a subsequent step to increase the corrosion resistance of low and moderate alloyed materials. The combination of the properties of the formed compound layer with the increased surface hardness is the reason for the increased resistance to wear.
Typical fields of application for nitrocarburizing are crankshafts of motor vehicles, control cams, drive axles of windscreen wipers or hydraulic cylinders.