Eutectoid Transformation Kinetics of FeO under N2 and Air Atmospheres

The effect of different oxygen content on eutectoid transformation kinetics in FeO were studied in this paper. Thermogravimetric analysis was employed to investigate the eutectoid reaction in the oxide formed on pure Fe after being exposed to air at 900 °C for 10 min. The oxidized specimens were held isothermally in N2 and air from 100 s to 10,000 s in the temperature range of 350 to 550 °C, and the morphologies in FeO were observed by electron probe microanalysis. The results of the eutectoid transformation are statistically analyzed, and the dynamic model of the FeO eutectoid transformation is established based on the Johnson-Mehl-Avrami-Kolmogorov equation. Combined with the measured values and the calculation results, the time of eutectoid reaction in air is earlier than that in N2. Under the experimental conditions, the formation of Fe3O4 seams can occur at the interface of the FeO-substrate after the eutectoid reaction has begun, which means the eutectoid reaction is more determined by local ion concentration changes. At the Fe3O4-FeO interface, there is a high concentration enrichment of Fe ions, giving priority to the formation of Fe-rich FeO, which makes the eutectoid phase transition time earlier than in N2 conditions.