Papers by Author: Takashi Onishi

Abstract: The boundary constants between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were calculated in order to clarify the formation mechanism of fayalite scale (Fe₂SiO₄) or chromite scale (FeCr₂O₄), which can form as a “sub-scale” in Si or Cr containing steels. The diffusion coefficient of oxygen in the alloy, Do, and the oxygen concentration at the specimen surface, N_O^(s), which are constituents of the internal oxidation rate constant, (2D_ON_O^(s)/N_B^(O)n), were calculated for various oxidation conditions, and the rate equation for internal oxidation was derived. By comparing the calculated and measured values of (2D_ON_O^(s)/N_B^(O)n), we confirmed that the rate equation determined for internal oxidation was reasonable. The boundary condition between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were also calculated by substituting the calculated values of D_O and N_O^(s) into the rate equation.

Abstract: The adhesion and micro-structure of the primary scale formed in LNG combustion gas on steels containing Si, up to 3.0mass%, have been studied using a hot-compression test at 1273K, Raman spectroscopy, and X-Ray absorption fine structure analysis (XAFS). The Fe2SiO4 at the scale/steel interface is increased as the Si content increases, suppressing the diffusion of Fe ions from the steel and forming dense and highly-adhesive subscale, when oxidized below the temperature at which the Fe2SiO4 melts. The behavior of the secondary scale growth in air up to 1173K on these has been studied using in-situ XRD. It is apparent that the formation behavior of Fe2SiO4 ,FeO and Fe2O3 is influenced by the Si content.

Abstract: Hardness of oxide scales on Fe-(0, 0.5, 1.5, 3.0)Si alloys was studied at room temperature after oxidation at 1273 K for 18 ks in oxygen, and at 1073 and 1273 K for 180 and 1080 ks in dry air, by micro-Vickers hardness measurements. After oxidation at 1273 K for 18 ks, high-temperature hardness of oxide scales on Fe-(0, 1.5, 3.0)Si alloys was also measured at 1273 K. Oxide scales on Fe-Si alloys were mainly Fe2O3, Fe3O4, FeO and Fe2SiO4. Hardness of Fe2O3, Fe3O4 and FeO on Fe was 6.7, 4.0 and 3.5 (GPa), respectively, and hardness of Fe2O3 on Fe-Si alloys slightly increased with increasing silicon content at room temperature. At 1273 K, hardness of Fe3O4 and FeO on Fe was 0.08 and 0.05 (GPa), respectively, and hardness of Fe2O3 on Fe-1.5Si alloy was 0.32 (GPa), and that of Fe2O3 and Fe2SiO4 on Fe-3.0Si alloy was 0.53 and 0.63 (GPa), respectively.