Abstract
Ni-based superalloy, which has excellent high-temperature strength and corrosion resistance, is mainly used in aviation materials, high-performance internal combustion engines, and turbines for thermal and nuclear power generation. For this reason, refining the impurities in Ni-based superalloys is a very important technical task. Nevertheless, the original technology for the melting and refining of Ni-based superalloys is still insufficient. Therefore, in this study, the effect of the CaO-Al2O3-MgO-TiO2 slag on the removal efficiency of an impurity element sulfur in Incoloy® 825 superalloy, one of the representative Ni-based superalloys, was investigated. The desulfurization behavior according to the change of TiO2 content and CaO/Al2O3 (=C/A, basicity) ratio as experimental variables was observed at 1773 K (1500 °C). Although the TiO2 content in the slag increases to 15 mass pct, the mass transfer coefficient of sulfur in molten alloy showed a constant value. Alternatively, under the condition of C/A > 1.0 of slag, the mass transfer coefficient of sulfur showed a constant value, whereas under the condition of C/A < 1.0, the mass transfer coefficient of sulfur greatly decreased as CaO decreased. Hence, in the desulfurization of Incoloy® 825 superalloy using the CaO-Al2O3-MgO-TiO2 slag, the TiO2 content in the slag does not have a considerable effect on the desulfurization rate and desulfurization mechanism (metal phase mass transfer controlled regime), but the basicity of the slag has a significant effect on desulfurization mechanism. When the slag basicity decreases below the critical level, i.e., C/A < 1.0, which is corresponding to sulfur distribution ratio, Ls < 200, it was confirmed that the desulfurization mechanism shifts from the metal phase mass transfer-controlled regime to the slag phase mass transfer-controlled regime due to the variation in the physicochemical properties of the slag such as viscosity and sulfide capacity. In addition, the different desulfurization rates between steel and Ni alloy melts were discussed by employing the diffusivity of sulfur in both systems.
Highlights
IntroductionNICKEL-BASED superalloys usually have high strength and corrosion resistance at severe environment with high temperatures
NICKEL-BASED superalloys usually have high strength and corrosion resistance at severe environment with high temperatures. They have been used as aviation material, high-performance internal combustion engine, thermal power, and nuclear
Degawa and Ototani [8] investigated desulfurization behavior of Ni-based superalloy, InconelÒ 738 using calcia (CaO) refractory. They concluded that calcia refractory is very effective in desulfurization and in deoxidation as well as nitrogen removal process
Summary
NICKEL-BASED superalloys usually have high strength and corrosion resistance at severe environment with high temperatures Due to this property, they have been used as aviation material, high-performance internal combustion engine, thermal power, and nuclear. Kishimoto et al.[9] reported the desulfurization model in molten Ni-based superalloy by using solid CaO rod and calculated the effective sulfur diffusivity in molten Ni-based superalloy They found that CaO and Al2O3 generate solid calcium aluminates, which capture CaS to remove S from the melt at 1673 K (1400 °C), while CaO and Al2O3 form the solid–liquid coexisting calcium aluminates, which capture CaS and penetrate the particle boundaries of the CaO rod to remove S from the melt at 1773 K and 1873 K (1500 °C and 1600 °C).[10] they suggested that the rate-determining process of the desulfurization reaction is sulfur diffusion in the generated calcium aluminates layer at any temperature
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