2024 年 64 巻 Extra-edition 号 p. ex193-ex198
In gas nitriding of steel, a compound layer is formed on the surface of the steel, which is composed of ε-phase and γ'-phase of iron nitrides. It is known that the mechanical properties improved depend on which of these phases is formed more. The compound layer obtained by gas nitriding of iron can be predicted by the Lehrer diagram, which shows the relationship between temperature, nitriding potential KN, and the stable phase in the Fe-N system. However, in industrial steels, ε-phase and γ'-phase may be mixed in a complex phase composition due to the influence of alloying elements such as C and Cr. The result is not always consistent with the phase composition shown in the Lehrer diagram. Therefore, further study is needed on the stability of the phase composition of the compound layer of industrial steels.
In this study, the effect of the temperature history following nitriding treatment on the phase composition of the generated compound layer was investigated for industrial steel SCM. In the experiments, specimens were gas nitrided at KN for a predetermined time, moved to the low temperature zone in a nitriding atmosphere, held for a predetermined time, and then quenched to room temperature. The phase composition of the compound layer at each temperature history was analyzed by EBSD. As a result, it was confirmed that the phase composition in the compound layer changed by holding at low temperatures, specifically, the ratio of ε-phase decreased and γ'-phase increased. This change in phase composition occurred in a relatively short period of time, from 10 to 30 minutes. In addition, the C and N distribution of the ε-phase and γ'-phase in the compound layer before and after low temperature holding was analyzed by EPMA. The C enrichment in the ε-phase after low-temperature holding was particularly pronounced compared to that before holding. The phase stability of the ε-phase in the Fe-C-N system was estimated by the computational phase diagram. The results show that for the ε-phase to be stable at low temperatures, C must be more enriched than when held at high temperatures. This implies that the change in phase composition of the compound layer due to holding at low temperatures is caused by the redistribution of C and N, driven by the change in phase stability of the ε-phase and γ'-phase due to the temperature change.
