| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 95 === The main goal of this study is to investigate the high-temperature oxidation of the three hot-rolling steels containing 0.01, 0.51, and 1.91 wt.% Si over the temperature range of 850~1200oC at various O2-containing environments. The thermo-mechanical simulator (Gleeble machine) was employed for oxidation tests in various conditions. The effect of 8% applied tensile strain on the scale constitution and phases of these steels was also investigated through XRD, SEM, EPMA, and TEM analyses.
The results showed that an exclusive layer of Fe2O3 formed on the three steels after oxidation at 850oC for 2 min, however, the scales formed at 1000oC for 2 min were strongly dependent on Si content, being composed of FeO, Fe3O4, and Fe2O3 for the 0.01%Si steel. However, the scales formed on the 1.91%Si steel consisted of SiO2 or Fe2SiO4 in the inner-portion and of Fe2O3 formed in the outer-portion. The amounts of Si-rich oxides gradually increased with increasing Si-content, which cause a reduction of outward iron-diffusion and inward oxygen-diffusion, thereby reducing the amounts of FeO and oxidation rates. The scale thickness of the alloys significantly increased when exposed at 1200oC for 2 min. The formation of partial melting fayalite for the steels containing 0.51% Si or higher enhanced outward diffusion of iron, which increased the oxidation rates and in turn resulted in the formation of iron oxides (FeO, Fe3O4, and Fe2O3).
When 8% tensile strain was applied during oxidation at 850oC, FeO, Fe3O4, and Fe2O3 formed on the 0.01%Si steel, and the scale thickness was much thicker than that of the strain-free oxidation. The Si-rich oxide grown in the inner-portion of the scales can provide its passivity when oxidized at 1000oC although iron oxides formed in the outer-portion of the scales were nearly identical to those of the strain-free condition. At 1200oC, however, the fayalite layer became much thinner under the applied-strain condition, indicating that the protective fayalite layer was difficult to form, which in turn enhanced outward iron diffusion along the defects created under tensile strain, thereby causing faster oxidation rates.
In addition, the effect of water vapor on the oxidation behavior of the steels was also investigated. It was concluded that the scale thickness of the three steels was much thicker when they were exposed in wet air.
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