Quantitative thermal investigation of a fayalite particle and a Si-rich layer in oxide scale formed on steel

Fayalite (Fe2SiO4) is a major component of olivine and is often formed in the surface oxide layer (oxide scale) on steel plates owing to its high-temperature oxidation. The thermal conductivity of the oxide scale and its constituents, including fayalite, is essential for controlling the cooling rate of hot steel plates. Therefore, this study uses modulated thermoreflectance microscopy to determine the thermal effusivity/conductivity of fayalite particles with diameters smaller than 180 μm because the sample commercially available is such a small size. This thermal optical microscopy enables the measurement of thermal effusivity for small areas, such as 10 μm. The thermal effusivity and conductivity were found to be 4.1 ± 0.2 kJs-0.5K-1m-2 and 6.0 ± 0.5 Wm-1K-1, respectively. These values are representative of the bulk value. Additionally, the thermal conductivity of fayalite is shown to be higher than that of wüstite (Fe1-xO), which is the main component of the oxide scale. The oxide scale formed on the thick steel plate comprises a Si-rich layer, a wüstite layer, and a magnetite layer. Furthermore, the Si-rich layer comprises fayalite, wüstite, and pores. The effective thermal conductivity of the Si-rich layer was calculated by observing the oxide scale formed on the steel plate. The low thermal conductivity of the Si-rich layer indicates that the thermal conductivity of the layer is strongly affected by the porosity of the oxide scale. Furthermore, although the Si-rich layer is thin, it significantly contributes to the heat resistance of the oxide scale.