In the plasma nitride processing that uses direct current glow discharge, the sample is heated and ruggedness is generated on the sample surface because the ions in plasma bombard against the sample surface. To minimize the ruggedness, we heated the sample by an external electric heater during plasma nitride processing. The sample material was SACM 645 steel. NH3
gas was used for nitriding. We examined the generation of nitrified layers and surface roughness under various conditions of NH3
gas composition from 20 to 60%, nitriding temperature from 743 to 843K and nitriding time from 1.8 to 43.2ks. In general, higher nitriding temperature led to higher surface hardness. Moreover, the total case depth hardened by nitriding became from 0.2 to 0.3mm. Longer nitriding time led to higher surface hardness and the total case depth hardened by nitriding became from 0.075 to 0.35mm. No layer of nitrides of iron was generated until the NH3
gas composition became 40%. When the NH3
gas composition was 60%, 1.3μm layer was generated and the surface roughness was Rz 0.24μm. No nitride was identified by X-ray diffraction. However, higher NH3
gas composition led to larger half value width of α-Fe at peak. This suggested that nitrides mainly composed of minute nitride chromium were being precipitated.