2024 年 64 巻 Extra-edition 号 p. ex118-ex122
Nitrocarburizing is a thermochemical process used to enhance wear-, corrosion- and fatigue resistance in steels in wide variety of applications. Due to the complex atmosphere and relatively high temperature, 540 – 580°C, many challenges are presented when attempting to monitor it and control for a specific outcome. Monitoring ammonia (NH3) is of particular interest due to its strong correlation to the nitriding potential, which in turn can be related to the metallurgical outcome. Current methods to monitor NH3 rely on two main methods: 1) extracting the furnace gas to a separate measuring station outside the furnace where NH3 is measured, or 2) estimating NH3 indirectly through measurements of N2, H2 and CO/CO2. Risks associated with extractive methods pertains to the formation of salts that clog the extraction pipes, owning to water condensation. While estimating NH3 from other gases avoid the risk of salt formation, there are concerns regarding the accuracy of these estimates. The present study evaluated an alternative measuring setup, using tunable diode laser absorption spectroscopy (TDLAS), which allowed for direct measuring of NH3 content without extractive analysis. The study was conducted in an industrial environment, and the results obtained from TDLAS were compared to measurements done by Fourier transform infrared technique (FTIR) and estimates based on the furnace's hydrogen sensor. Overall, TDLAS showed good agreement with FTIR measurements, indicating its accuracy in directly measuring NH3 content. Additionally, it was found that directly measured NH3 deviated significantly from estimated NH3. It is thus concluded that measured NH3 should be preferred when controlling the furnace using nitriding potential. To this end, TDLAS can provide a cost-efficient method for measuring NH3 directly in industrial environments.
