Nitriding Effects of Ammonia Flames on Iron-based Metal Walls

Effects of NH₃/O₂/N₂ premixed flame/mixture on iron-based metal walls have been investigated experimentally. A premixed NH₃/O₂/N₂ flame or mixture formed by a quartz tube burner impinged on the disc-shaped metal test plate, of which the temperature was kept at 550 ^oC by an infrared lamp heater. As materials for the test plate, SACM645, SUS304 and SUS310S, which are widely used as industrial steels, were examined. After being exposed to either the NH3/O2/N2 flame or mixture for 5 hours, the surface hardness was measured by using a nano indenter. Distributions of nitrogen atom diffused into the metal wall were measured by means of the Electron Probe Micro Analyzer (EPMA) and Secondary Ion Mass Spectrometry (SIMS). In addition, spatial distributions of the NH₃ concentration in the NH₃/O₂/N₂ flame and mixture were measured by employing a two-photon absorption laser-induced fluorescence (TALIF) technique. It was found that the surface hardness of the test plates increased significantly after being exposed to the NH₃/O₂/N₂ flame/mixture. The distributions of surface hardness and nitrogen atom concentration show that nitriding occurred by the present NH₃/O₂/N₂ flame/mixture with the equivalent level of the conventional gas nitriding method. Moreover, it is also found that NH₃ concentration in the vicinity of the wall plays an important role in the surface hardness increase, and a wider distribution of NH₃ can result in a broader surface hardness distribution.