Aluminum deoxidation equilibrium in liquid Fe-16 pct Cr alloy
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Fe-16 pct Cr alloys, the representative ferritic stainless steel, are corrosion-resistant and heat-resistant materials with applications such as heat-resistant instrument, burner, top plate of sink, bolt, nut, cold-drawn bar, and mesh. The surface quality of the high chromium alloy steels is of primary importance since the frequency of defect generation in the surface is still high. In order to avoid the formation of inclusions having harmful effects such as solid alumina, this material is often deoxidized with silicon. However, inclusions containing solid alumina can be easily found in products due to extremely small amounts of Al present in the alloy. Numerous studies have reported on the thermodynamics of the Fe-Cr-O system and Fe-Al-O system, but studies on aluminum deoxidation of iron alloy containing high chromium are limited.[1,2] The present study was undertaken in order to obtain the thermodynamic data for aluminum deoxidation equilibrium over a wide range of aluminum content covering up to 1 mass pct Al in liquid Fe-16 pct Cr alloy. The electromagnetic levitation-melting technique using a cold crucible was employed in this study. The merits of this technique are no contamination of sample from crucible materials, complete separation of deoxidation products from molten steel by gravity, and faster time to reach reaction equilibrium. The schematic diagram of the experimental apparatus used in the present study is shown in Figure 1(a) and the crosssectional diagram of a cold crucible in Figure 1(b). Fe-16 pct Cr alloy and Fe-Cr-Al alloy were prepared with a cold crucible in order to avoid contamination from crucible materials, using electrolytic iron and metallic Al and Cr (purity
SANG-BEOM LEE, Technical Researcher, is with the Stainless Steel Research Group, Technical Research Lab., POSCO, Pohang, 790-785, Republic of Korea. JU-HAN CHOI, Ph.D. Course Student, and HAE-GEON LEE and PETER CHANG-HEE RHEE, Professors, are with the Department of Materials Science and Engineering, POSTECH, Pohang, 790-784, Republic of Korea. Contact e-mail: [email protected] SUNG-MO JUNG, Research Professor, is with the Graduate School of Iron and Steel Technology, POSTECH. Manuscript submitted August 12, 2004. 414—VOLUME 36B, JUNE 2005
99.9 pct). Thirty-five to forty grams of Fe-16 pct Cr alloy and an appropriate amount of Fe-16 pct Cr-Al alloy were placed in the copper crucible and purged under a purified Ar-1 vol pct H2 gas mixture for 15 minutes. The sample was heated by applying high-frequency current (20 kW power and 170 kHz frequency) to the coil surrounding the cold crucible. After the temperature reached 1923 K, the atmosphere was changed to Ar, which was purified by passing a Mg chip column at 723 K. From preliminary experiments, there was no change of [O] and [Al] after 10 minutes. After holding at 1923 K for 15 minutes for the definite attainment of reaction equilibrium, the melt was rapidly quenched by turning off the power and blowing He gas in the chamber. The temperature was measured with a two-color pyrom
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