발간논문

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Nitrogen adsorption phenomena of intermetallic compounds FeTi, which has execllent hydrogen-storage characteristics, were studied before and after activation. The specific surface areas of the unactivated and activated FeTi are 0.05 and 0.28m²g^(-1), respectively. The specific surface area of the activated FeTi is 5.6 times as large as that of the unactivated sample. Adsorption and desorption isotherms of the activated FeTi were obtained. Adsorption isotherm is similar to type B among the five types of isotherms classified by BDDT. Its hysteresis curve has the form of type B among de Boer`s five types of hysteresis. Desorption pore size analysis shows Chat the activated FeTi has very small quantity of mesopore whose radius is 20-80Å. The shape of adsorption isotherm and hysteresis curve, desorption pore size distribution analysis and SEM micrographs show that, the activated FeTi is nonporous powders with very small quantity of mesopore. The average adsorption rates of the activated FeTi show linear dependence on the nitrogen pressure at 77K.

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A study has been made on the fracture behavior of a two-phase (α₂+β) Ti₃Al-Nb alloy, with particular emphasis on the effect of morphology. The coarse colony structure obtained by continuously annealing in the two-phase region directly from the β region results in poor elongation ductility. The fine colony structure obtained by two-phase annealing th β-quenched specimen also results in poor elongation ductility. However, upon obtaining a fine equiaxed or basket-weave structures by following different transformation paths, improvements occur in elongation ductility without any sacrifice in strength. For the fine equiaxed structure, maximum elongation is obtained at the intermediate volume fraction of α₂, while for the basket-weave structure, increase in the volume fraction of α₂ increases the elongation. Detailed fracture analysis shows that different fracture mechanism operates for different morphology and the critical microstructural unit for arresting fracture also varies depending on the morphology.

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For the development of a smelting reduction process, the kinetic approach to the reduction reaction is needed to evaluate the productivity of the process. In the present study, a theoretical kinetic model was established for the smelting reduction reactions of molten iron oxide in liquid slag. The caluculated results using this model were well agreed with the experimental results reported. The overall reduction rate was increased with the increases of seed slag volume, residual coal amount in slag, and iron oxide concentration in slag. The input coal size and bath stirring intensity also can give an effect to the reduction rate. From the present kinetic model, it was estimated that the annual iron productivity of one million can be achieved with the optimum operating conditions in a smelting furnace with capacity of 300 tonnes per a charge when the prereduced iron ore of 60% PRD be used for the smelting reduction.

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Prior particle boundaries are often observed in Rene 95 powders consolidated by hot isostatic pressing(HIP). The PPB and grain boundary morphology are well known to impart significant effects on mechanical properties at service conditions. The intent of this work is to improve PPB boundary and γ` morphology by changing HIP and post heat-treatment conditions. HIP temperature was controlled within 1050-1150℃, and post heat-treatment was done on the HIP` ed Rene 95. Thickness of the PPB is decreasing with increasing temperature. Post heat-treatment after HIP` ing results in serrated grain boundaries and PPB formed inside grains. The PPB formed in the HIP` ed and post heat-treated Rene 95 is composed of Al and Zr rich oxides, Nb and Ti rich carbides, and large γ` phases. Low temperature HIP followed by post heat-treatment is appropriate to obtain serrated grain boundaries and fine grains where PPB is enclosed. However γ` denuded zones are formed along the grain boundaries during long time exposure at high temperature.

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Melt flow rate is considered to be an important process variable in gas atomization because it greatly affects the droplet sizes, microstructures and mechanical properties of the final products. It is, however, difficult to predict the melt flow rate accuratly because it is also influenced by the aspiration pressure under the melt orifice in confined type gas atomization. It seems to be impossible to measure the orifice pressure directly during gas atomization. Hence upto now the orifice pressure has been measured statically with the pressure gage connected to the upper side of melt orifice without flow. But it is different from the real atomizing conditions, in which melt flows through the orifice. In this study, the dynamic orifice pressure was calculated by measuring the flow-out time of the melt. The dynamic aspiration effect varies with the protrusion height of melt nozzle, increases with the increasing gas jet velocity and decreases as melt flow rate increases.

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The shape change of deposited rod with processing time has been numerically calculated by mass balance on the deposit surface between sprayed and deposited material. Local growth velocity of deposit surface is function of the surface geometry at a given position and processing time. The shape of the deposit calculated by assuming parallel spray density and normal distribution shows a reasonalbly good rod form. The radius of the rod is propotional to square root of melt flow rate, and inverse to square root of substrate withdrawal velocity.

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A steady state model on the shape of rod during spray forming has been developed. Steady shape of the rod has been calculated numerically under the assumption of parallel spray density distribution and normal distribution. Time for reaching steady shape of rod during spray forming has been calculated numerically. The calculated steady shape of rod agrees well with the result of transient model, reported previously. The radius of the rod is proportional to square root of melt flow rate, and inverse to square root of substrate withdrawal velocity. Steady shape of the rod can be obtained at following condition : V$lt;a·cosφ, where V, a and φ is substrate withdrawal velocity, spray density constant and spray angle respectively.

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An investigation has been performed on the manufacturing of dispersion strengthened Cu5vol.% W materials for resistance welding electrodes and contact tips in gas-metal arc welding guns by mechanical alloying. Composite Cu-5 vol. % W powders were produced by high energy ball milling of Cu and W powders in an attritor in argon atmosphere for various milling times, 2, 8, 16, and 24 hrs. The green compacts were sintered at 1000℃ in a flowing hydrogen atmosphere for 2 hrs. The experimental results of powder characteristics revealed that during mechanical alloying of Cu-W powders the particle size was decreased and the shape changed from dendritic- to irregular- and finally to flaky type and also the W particles were uniformly distributed within the Cu matrix. The densification and the properties of the Cu-5 % W composites were closely related to the powder characteristics and discussed in terms of microstructural development.

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As an alternative way to investigate how iron ore sinter in a blast furnace behaves in the process of reduction and mineral formation during softening and melting, tests under loading of iron ore sinter in temperature ranges of 800℃ and 1515℃ have been made. It was found that iron ore sinter at basicity of 1.8 behaved fairly good, while that of 1.6 shrinked rapidly as compared with that at higher basicity. It was also found that metallic iron was readily reduced from Fe0 in temperatures below 1100℃ and over 1400℃, whereas the reduction was retarded in temperature ranges between 1100℃ and 1400℃, whereas the reduction was retarded in temperature ranges between 1100℃ and 1400℃. A mechanism on the formation of mineral phase and its transfer into slag were confirmed and analyzed from the phase diagram of CaO-SiO₂-MgO-10%Al₂O₃. The magnesio-wustite phase was found to be melted into a slag in temperature ranges of 1300℃ and 1400℃, thereby raising a dripping start temperature of the melt and enhancing a blast furance operation.

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The experimental results for the fabrication of(W·Ti)C powder by self-combustion reaction are summerized as follows : 1) The reaction parameter to influence greatly in synthesis of (W·Ti)C powder is the mole ratio of each elements. Excess carbon must be added to control the final composition because of the carbon deficiency during the reaction. Experimental results show that the optimum mole ratio of W, Ti and C is 0.2, 0.7, and 1.2 respectively. Self-combustion is difficult with increasing the carbon mole ratio compared with the stoichiometry and the steady state combustion is not occurred. 2) In case that the mole ratio of Ti to W is large, combustion is impossible due to decreasing of adiabatic reaction temperature. Adiabatic reaction temperature of(W·Ti)C is about 2900 to 3000°K and the steady state combustion at this temperature is performed. 3) The particle size and the compression density of W powder are constant regradless of the reaction products. However, the reaction rate decreases with increasing the particle size. The reaction rate increases with the compression density of W powder and is constant above that of 2.77 g/㎤.

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