발간논문

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The diffusion- controlled coarsening behavior of the second phase particles situated along the grain boundary in the ferrite matrix has been investigated theoretically and experimentally. The effect of volume fraction Φ on the growth rate of precipitate particles has been included in the equation obtained for 0.63 carbon steel. The average particle size has been found to increase as t¼ after a long elapse of heat treating time, irrespective of the value of Φ. The value for the limit-of volume fraction of cementite particles in ferrite matrix has been calculated to be 0.4233. In general, it has been shown that the growth rate of the grain boundary particles increases with increasing volume fraction. The theoretical growth rate equation is give as follows:(수식)

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Several new test procedures were tried to measure flow behavior of sheet metals under combined stress stales utilizing a normalized aluminum-killed steel which was nearly isotropic. Equibiaxial stress condition was obtained From a through-thickness compression test on a disk-slacked circular cylinder. Plane strain and pure shear conditions were obtained by plane strain compression tests, in which one of in-plane strains or the thickness strain is zero. Experimental Flow stresses of the steel were slightly higher in the equibiaxial loading path, and slightly lower in the plane-strain path than the expected values from the Mises theory. Experimental flow stresses in the pure shear path fitted well with the theoretical values.

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TiN was deposited onto phosphorous bronze, NaCl single crystal and p-Si (100) by Biased Activated Reactive Evaporation (ARE). The influence of plasma conditions during deposition on TiN film`s preferred orientation and resistivity was investigated using electrostatic plasma probe. Electron temperature and electron density were measured at different substrate biases and ARE discharge currents. Films deposited by negative bias, therefore high electron temperature, showed a strong (200) preferred orientation and a low electrical resistivity with high hardness. High ARE discharge current, therefore high electron density, on the other hand, enhanced a (111) texture and showed higher resistivity. The cause of (200) preferred orientation by high negative bias is considered to be that titanium ions of upper critical energy can bombard growing film surface to transform (111) texture into (200) preferred orientation by deep impact insertion of N between Ti atoms.

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Hydrodynamic approaches are developed to find optimum operational conditions of rectangular continuous casting tundishes thorough application of water model experiments and three-dimensional computer simulations. Water model experiments via volume analysis in a continuously flowing vessel are made for various physical flow control devices such as ladle submerged nozzle, weir and dam. RTD curves are obtained by means of conductivity method. The three-dimensional computer simulations are compared with the above experimental results. The three-dimensional computer simulations are compared with the above experimental results. It is found that the weir and the dam can play an important role in obtaining desirable flow patterns; they efficiently increase the minimum retention time, plug flow volume ratio and the mean residence time, and confine the turbulent mixing effect near fluid inlet regions. Tracer dispersion patterns simulating turbulent mixing effects show good agreement between the experiments and computed results. The present results could be used as basic data for designing a new tundish system in a real steel processing.

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Experimental and theoretical studies were made in order to discuss the characteristics of recirculating flow in the ladle injection refining process. A mathematical prediction for the recirculating flow rate of liquid in a gas stirred ladle is developed on the basis of the turbulent clavier-Stokes equation using the k-ε model and momentum balance. Mean residence time of a tracer particle is measured in order to evaluate the recirculating flow rate of the liquid in the water model. The calculated valve of the recirculating flow rate is well agreed with the experimental result in the water model. The volumetric entrainment ratio is found to be very large with a magnitude of 30-120 under a normal stirring condition.

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The effects of δ-ferrite on crack propagation were studied on CF-type cast duplex stainless steel at room temperature under constant load and under alternating load. The crack propagation rate of CF-8 decreased as the amount of δ-ferrite increased. The crack propagation rate of CF-3 with 9.3% δ-ferrite was lower than that of CF-8 with 7.7% δ-ferrite, but higher than that of 8% δ-ferrite. In case of CF-8 with 12% δ-ferrite. the role of δ-ferrite as a crack arrester in non-heat-treated material was small compared to that in heat-treated one. Wrought 304 stainless steel had a slower crack propagation rate than CF-8 with 0% δ-ferrite.

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The effects of boron on hydrogen embrittlement of pure nickel have been studied by electrolytic charging with hydrogen at room temperature. The degree of hydrogen embrittlement of pure nickel decreased and the fracture mode chanced from intergranular fracture to transgranular fracture as a small amount of boron was added. The break-through-time of permeation current decreased and the hydrogen solubility decreased with boron addition. Boron was considered to be a hydrogen trap since less embrittlement was observed in nickel with boron.

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Ultrafine carbon powders were prepared under the following conditions in Cl₂C = CH₂+ H₂system; reaction temperature of 650 - 1,450℃, total was pressure of one atmosphere, 6.2-38.6 vol.% of Cl₂C = CH₂ and total gas flow-rate of 140-690㎖/min. Yield, particle sire and specific surface area of the pyrolytic carbon powders were investigated as functions of the reaction variables. The yield increases and decreases with increase of the reaction temperature and the gas flow-rate, respectively. The particle sire increases with increase of temperature and shows a maximum value of 0.5㎜ at 950℃, and then decreases at higher temperatures. The particle size keeps an almost constant value (i.e., 0.2㎛) within the range of 6.2 to 38.6 vol.% of Cl₂C = CH₂, while decreases with increase of the total gas flow-rate. The specific surface area and the diameter of carbon powders prepared at 1,450℃ are 30㎡/gr and below 0.1㎛, respectively.

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