Characterization of Oxide Scales Formed on Plain Carbon Steels in Dry and Wet Atmospheres and Their Eutectoid Transformation from FeO in Inert Atmosphere

Abstract

Thermal gravimetric analysis (TGA) was used to investigate plain carbon steel exposed to dry air and wet air containing 4 vol% water vapor at 750 °C to simulate the formation of tertiary scale during hot-strip rolling. After the exposure to oxidation condition, the specimens were isothermally held at temperature in the range of 300–500 °C in inert argon (Ar) atmosphere for 10–240 min, and the isothermal phase transformation behavior of FeO was investigated. The morphology, element distribution, and phase composition of oxide scales after isothermal transformation were characterized by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction, respectively. The experimental results indicate that wet air accelerated the growth rate of the oxide scale. Moreover, the time–temperature-transformation (TTT) diagrams of oxide scales grown in dry and wet air conditions were constructed to elucidate the isothermal phase transformation behavior of the FeO layer. The rate of isothermal phase transformation in the scale formed in wet air was significantly delayed compared to that in dry air, which caused the C-type TTT diagram to shift to the right. Two mechanisms elucidating the effects of initial oxidation atmosphere on the subsequent isothermal phase transformation behavior of FeO during inert atmosphere annealing were proposed.