Synopsis : Blast furnace (BF) slags have been utilized in cement, concrete aggregate, roadbed materials, and earthwork materials. If an appropriate control of the elution and compound formation is developed under severer environmental conditions, their usage would be more diverse. Because the chemical composition of BF slag is similar to that of Portland cement, the possibility of ettringite (3CaO·Al 2 O 3 ·3CaSO 4 ·32H 2 O) formation from BF slag following a mechanism similar to that of cement hydration might be possible under a wet alkaline environment. Therefore, the effect of an alkaline solution on ettringite formation from BF slags was investigated by slag-leaching experiments and thermodynamic calculations using PHREEQC. The formation of ettringite was observed only for the high pH solutions in the experiments, whereas its thermochemical possibility from the air-cooled BF slags was always expected by the calculation. The kinetic analysis showed that the dissolution of alumina from the slag may control the whole reaction rate. The mixing of granulated BF slag with air-cooled ones tended to enhance the ettringite formation. Furthermore, a technique for removing the ettringite formed in the slag was also developed.
Oxide scales exert important effects on the surface quality of steel products. In hot rolling processes in the steel industry, the heterogeneous structure of oxide scales, such as an uneven scale/steel interface, causes surface defects. For steels containing Ni, the structure of oxide scales at the scale/steel interface is characteristic and complicated. Consequently, this study focuses on Ni and Cu, which are nobler than Fe, and investigates their effects on the structure of oxide scales of steels. Fe-10mass%Ni and Fe-10mass%Cu alloys were oxidized at 1000 or 1200°C in an atmosphere of 10%O 2 +20%H 2 O+70%N 2 for 1800 s. In all cases, uneven scale/steel interfaces were observed because both Ni and Cu are nobler than Fe. Furthermore, different structure of oxide scales was formed at the scale/steel interface between the Fe-Ni and Fe-Cu alloys depending on the alloys' solid solubility limit to austenite.
Ni containing steel is known to form a complex oxide scale, which consists of an outer layer of Feoxides and an inner layer of FeO with complicated distribution of Ni(Fe) metal particles. Due to the complex microstructure, descaling of the oxide scale formed on Ni containing steel during a hot-rolling process is very difficult. In order to improve the descaling process, microstructural control of the inner oxide layer to eliminate its detrimental effect is necessary. In this study, the change in microstructure of the outer and inner layers formed on Fe-5 mass%Ni alloy during oxidation is investigated. In particular, the change in the microstructure of the metal particles in the inner layer with oxidation time is considered. The inner layer consisted of FeO, Ni(Fe), and voids. The concentration of Ni in the Ni(Fe) was found to increase across the inner layer from the scale/steel interface toward the outer/inner scale interface due to the equilibrium Ni concentration in the Ni(Fe) particles with FeO, which corresponded to the oxygen potential gradient in the inner layers. The number and area fraction of the Ni(Fe) metal particles decreased, whereas the size of the particles increased with oxidation time. This coarsening of the metal particles was proposed to be due to Ostwald ripening.
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