발간논문

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The acid ferric s1fate leaching of the natural concentrated chalcopyrite was studied as a function of leaching time, temperature, particle size, H₂SO₂ and Fe＾(+++) concentration in order to obtain leaching data and analyse eaction kinetics. It has been confirmed that the rate limiting step of the reaction is diffusion through sulphur layer reaction product and activation energy is about 18.3 ㎉/㏖. The reaction rates were found to be nearly independent on H₂SO₄ concentration but significantly dependent on temperature and Fe＾(+++) concentration in case of very small particle size.

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The growth characteristics of a C/V graphite cast iron produced by a reduced-Mg method have been studied. The linear growth was 11.3-13.2% in C/V graphite cast iron, 17.7% in flake graphite cast iron and 10.3% in nodular graphite cast iron. As the amount of growth increased, the internal structure of graphite and graphite/matrix interface became porous and graphite particles precipitated in the matrix. The amount of the precipitated graphite particles increased with repeated thermal cycles and they formed around the preexisting graphite. This region was separated from the ferritic matrix in which no graphite particle precipitated.

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The microstructure of rapidly solidified Al-Cr alloys was investigated by scanning transmission electron microscopy (STEM), X-ray diffraction and micro-Victors hardness test. The temperature mariation during rapid solidification was studied to interpret the results. From the thermal analysis, the relationship between the cooling rate and the dendritic cell size was found to correspond to a previous theory which was obtained from an extrapolation of the measurements at slow cooling rate. The relatinship between the growth rate and the dendritic ceil size, however, was shown to be lower than those of general Al alloys, because the rate of the primary phase formation was suppressed by the increased fooling rate. By detailed composition measurements, the possible solubilily range of the complete supersaturation was found to be 3.43 wt% Cr. Above this concentration, the microstructure changed to a cellular structure by recoalescence in the last stage of solidification.

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Using a twin-roller type melt-quenching technique we investigated the effect of chemical composition on the microstructure and tensile properties of austenitic stainless steels. For three austenitic stainless steels, 0.5㎜ thick sheets were produced. For Cr-eq./Ni-eq.< 1.35, solidification typically takes place by primary austenite formation whereas if the ratio is greater than 1.35, primary ferrite formation is the solidification mode. As the Cr-eq./Ni-eq. ratio increases, the cooling rate of melt-quenched austenitic stainless steels slighlly decreases. In a melt-quenched AISI 304 (Cr-eq./Ni-eq. ratio: 1.7), the ferrite to austenite transformation was suppressed, resulting in residual ferrite. Secondary dendrite arm spacing of melt-quenched austenitic stainless steels was about 2-3㎛ . After heat treatment, the tensile properties of the melt-quenched stainless steels are bettor than those of conventional austenitic stainless steels.

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The absorption coefficient for γ-ray as well as thermal expansion coefficient was determined in the Ni-activated sintered W-powder compact, in order to examine whether the low-temperature sintered W-product can be substituted for the normal sintered W-product in radiation shield and high temperature application. The measured half value layer of the activatedly sintered W-specimen was thicker than that of the normal sintered pure W-specimen by 10-14%. However, the measured thermal expansion coefficient of the former was comparable to that of the latter in the range of temperature investigated in the present work.

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The coarsening of cementite particles in a ferrite matrix has been studied in Fe-C steel. A series of spheroidizing temperatures between 650℃ and 710℃ and spherodidzing times of up to 360 hrs. has been used. The effect of ordering of particles on their size distribution function has been studied by means of normalizing method developed for studying the Ostwald ripening kinetics. Particle size distribution curves have been determined and the coarsening curves have been obtained from the mean radius value. It has been shown that the transition from a statistically uniform particle distribution of the disperse phase to an orderly distribution along grain boundary has produced all increase in the mean relative radii of precipitates and a shift of the distribution function maximum toward larger relative particle sizes.

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The ecrystallization behaviors during continuous annealing process were studied in extra low carbon Al-killed steels with varying Mn content. The less the Mn content, the higher the nucleation rate of {111} grains. The {111} recrystallization it difficult when Mn content is excessively high. There exists, an optimum range of Mn content, annealing temperature and annealing time. Cold rolled texture is {111} $lt;hkl$gt; fibre texture and recrystallization texture is mainly of {111} $lt;112$gt; component. A model for nucleation and grain growth is proposed to explain the recrystallization process from {111} $lt;hkl$gt; cold rolled texture to {111} $lt;112$gt; recrystallized texture with movement of coincidence boundary.

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Using twin-roller type melt-quenching technique, the effect of thickness on the microstructure and the tensile properties of AISI 304 stainless steel were investigated by optical microscope, X-ray diffractometer and tensile test. In AISI 304 stainless steel, sheets whose thickness was 0.3-1.0 ㎜ could be produced. As the thickness increased, the cooling rate of melt-quenched AISI 304 stainless steel decreased. The microstructure of 0.8 ㎜-thick sheet consisted of columnar grains which formed near the sheet surface and equiaxed grains which formed near the center of the sheet cross-section. As the thickness increased in melt-quenched AISI 304 stainless steel, the Vickers hardness, 0.2% proof stress and tensile strength increased whereas the elongation decreased.

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