2024 年 64 巻 Extra-edition 号 p. ex123-ex128
Global attention is focused on carbon neutrality. In the future, new carburizing methods are required for sustainable and environmentally friendly heat treatment. To address this need, ultra-rapid carburization has been developed. This method is significantly faster than conventional carburization owing to the rapid temperature rise by induction heating and carburization above the eutectic temperature. This development considerably enhances the efficiency of the process. Previous studies have clarified the carburization reaction mechanism for ultra-rapid carburization, and verified that the reaction is rate-controlled by the decomposition of methane gas, the raw material. However, some problems must be solved for the industrial application of this technology. Therefore, we investigated a countermeasure against the coarsening of prior austenite grains caused by high-temperature treatment, and then considered and verified the method of setting the heat-treatment conditions. Consequently, we found that prior austenite grain size could be refined to approximately #9 by cooling after ultra-rapid carburization to form a single layer of martensite, and subsequent re-quenching. In addition, we calculated the optimal conditions that yielded a surface carbon concentration of 0.6 mass% and an effective hardened layer depth of 0.8 mm based on the carburization reaction mechanism and Fick's second law, and then calculated the amount of carbon diffusion in the steel. Finally, we verified our results experimentally.
