Gas nitriding was published by AF ry in Germany in 1923. The workpiece is placed in a furnace and NH3 gas is directly introduced into the nitriding furnace at 500-550°C for 20-100 hours to decompose NH3 gas into atomic (N) gas and (H) gas for nitriding treatment. The main purpose is to produce a wear-resistant and corrosion-resistant compound layer on the surface of the steel. Its thickness is about 0.02-0.02m/m. Its properties are extremely hard Hv 1000-1200 and extremely brittle. The decomposition rate of NH3 varies depending on the flow rate and temperature. The larger the flow rate, the lower the decomposition degree, the smaller the flow rate, the higher the decomposition rate, the higher the temperature, the higher the decomposition rate, and the lower the temperature, the lower the decomposition rate. NH3 gas is thermally decomposed at 570°C as follows: NH3 →〔N〕Fe + 3/2 H2
Gas nitriding has low efficiency of nitriding due to decomposition of NH3, so generally, steel suitable for nitriding is selected, such as steel containing nitriding elements such as Al, Cr, and Mo, otherwise nitriding can hardly be carried out. Generally, JIS, SACM1, new JIS, SACM645, and SKD61 are used for toughening treatment, also known as quenching and tempering. Because Al, Cr, Mo, etc. are elements that increase the transformation point temperature, the quenching temperature is high, and the tempering temperature is also higher than that of ordinary structural alloy steel. This is because temper brittleness occurs during long-term heating at the nitriding temperature, so quenching and toughening treatment is applied in advance. NH3 gas nitriding, because of the long time, the surface is rough, hard and brittle, and it is not easy to grind. It is also not economical because of the long time. It is used for nitriding of the feed pipe and screw rod of the plastic injection molding machine.
