발간논문

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1.65V, 0.26C steel with addition of 1Ni was selected to investigate property variations according to the microstructural changes. Isothermal heat treatment was performed at 750℃ for 20 minutes and hardness was measured as Rc27. The structures were ferritic with two kinds of vanadium carbides i.e. fibrous carbide and particle-like one. The dual phase microstructures having the same Rockwell hardness were obtained by heating the vanadium steel at 900℃ for 10 minutes. The conventional quenching and tempering heat treatment was followed at 650℃ for 17 hours and the microstructures revealed Widmanstatten carbides and carbides precipitation at lath boundaries. The mechanical properties were determined from tensile tests and compared with the observed microstructures.

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The purpose of this study is to improve the tensile properties of ductile cast iron with ferritebainite duplex matrix. Ferritic ductile cast iron was heat-treated to produce the following duplex matrixes of various proportions; ferrite-upper bainite, ferrite-lower bainite. To make the second phase, each specimen was austenitized at 880℃, and then heat-treated isothermally at 300℃ or 400℃. Tensile strength, 0.2 proof stress and hardness increased with volume fraction of the second phase without observing the law of mixture. Ductility (elongation, reduction of area) and static absorbed energy data showed a peak at 25-30% regardless of the kinds of the second phases, then reduced. The reason why ductility and static absorbed energy were improved at 25-30% of bainite, in view of measurements of martensite hardness test and strain hardening exponent, is considered to result from the reduction of soluble carbon content in bainite.

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The effect of grain size on the high temperature creep behavior of AISI 304 stainless steel was investigated. The apparent activation energy for the creep was measured to be much higher than that for the self diffusion in the steel. All of the ruptured specimens showed intergranular cracking. Both of these phenomena was attributed to the microstructure change during creep.

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Effects of hot rolling temperature and reduction ratio on the crystal grain and texture of 3% Si steel were examined by structure observation, texture coefficient and pole figures. During 1 pass hot rolling, recrystallization was started at surface layer. The degree of recrystallization would rather depend on hot rolling temperature than reduction ratio. As hot rolling temperature was lower, the thickness of recrystallized layer was decreased. Crystal grains of surface layer of 1 passed hot rolled specimen were strongly (110) and that of middle layer were (200). About 1000℃, (110) intesity was especially strong at surface layer and ¼thickness layer. Texture were generally heterogeneous in the hot rolled state. But grains were textured more prominently in the case of 6 pass hot rolling than 1 pass hot rolling. (110) texture was especially developed at ¼thickness layer in 6 pass hot rolled state. (110) texture was, however, distinct in sample E, which rolled contrary to conventional hot rolling schedule.

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A study on the decomposition of zinc cobaltate was made to improve the extraction of cobalt from the cobalt cake (Cobalty-Nitroso-β-Naphthol) which is produced as by-products at the hydrometallurgical process of zinc. It has been reported that zinc cobaltate was formed during the roasting process of the cobalt cake. Zinc cobaltate was prepared from the nitrate mixtures by drying them at 200℃ for 9 hrs and heating them at 400℃ for 4 hrs. The final product was ZnCo₂O_(3.962). It was observed that zinc cobaltate was decomposed into ZnO and Co₃O₄in the temperature range of 450-810℃ and Co₃O₄was again decomposed into CoO in the temperature range of 820-890℃, thus gave final products of ZnO and CoO over 920℃. Leachability of cobalt from the decomposed products over 920℃ has been determined in various concentrations of sulfuric acid. It was observed that 97.6% leachability of cobalt in 5N-H₂SO₄was obtained. The reaction order and apparent activation energy of the decomposition of zinc cobaltate was respectively 0.7 and 63.6 ㎉.

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