no distortion. This process was invented by Bernhardt Berghaus of Germany who later settled in Zurich to escape Nazi persecution. After his death in late 1960s the process was acquired by Klockner group and popularized globally. Plasma nitriding is often coupled with physical vapor deposition (PVD) process and labeled Duplex Treatment, with enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to produce a smooth jetblack layer of oxides which is resistant to wear and corrosion. Since nitrogen ions are made available by ionization, differently from gas or salt bath, plasma nitriding efficiency does not depend on the temperature. Plasma nitriding can thus be performed in a broad temperature range, from 260 °C to more than 600 °C. For instance, at moderate temperatures (like 420 °C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties. In the plasma nitriding processes, nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like hydrogen or argon are also used. Indeed, argon and H2 can be used before the nitriding process during the heating of the parts to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant, since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process H2 gas is also added to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding (see for instance ). Materials for nitriding Examples of easily nitridable steels include the SAE 4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring nitriding to take place at a lower temperature than the last tempering temperature. A fine-turned or ground surface finish is best. Minimal amounts of material should be removed post nitriding to preserve the surface hardness. Nitriding alloys are alloy steels with nitride-forming elements such