발간논문

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The effect of aluminium addition into the molten borax on the boronized depth in a mild steel has been studied quantitatively. Experiments were made with the molten borax, which was previously dehydrated at 450℃ and held at 900℃. The amounts of aluminium added ranged from 0 to 20% for the predetermined holding time. Although the boronized layer was not detected in case of the only borax-molten solution, the boron-penetrated depth was increased as increasing the amounts of aluminium added and the holding time. The micro-hardness of the boronized layer was about 10 times as hard as the mild steel.

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In order to show the connection between microstructure and magnetic properties, studies have been made of the effect of composition and aging treatment on the magnetic properties of the alloy system SmCo_5-SmCu_5. A number of ingots of the ternary composition SmCo_(5-x)Cu_x (x=1 to 4) were prepared by arc-melting under a protective atmosphere, resulting in a rapid freezing from the molten state. The samples were then annealed at various temperatures between 400℃ and 1000℃. The magnetic properties of these compounds were the highest in the vicinity of the composition SmCo_(3.5)Cu_(1.5) and SmCo_(3.0)Cu_(2.0). When specimens of the composition SmCo_(3.5) Cu_(1.5) annealed at 1000℃ and 400℃ were examined by optical microscope, X-ray diffraction, and electron-probe techniques, there were observed fine needle-shaped precipitates which may be responsible for the magnetic hardening of this compound. Their hexagonal cell constants (a=8.402A, c=12.172A) were obtained from FeKα powder diffraction data. The results indicate that precipitates occurring at the low temperature annealing are the same kind of phase as Sm₂CO_(17) precipitates at the high temperature annealing, and they appear to overage rapidly during the annealing between 400℃ and 600℃. Therefore, annealing at the high temperature over 600℃ does not lead to a further increase in the magnetic properties of SmCo_(5-x)Cu_x alloys.

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Through reviewing the development of the full mold process with unbonded sand molds, it is emphasized that this process has great advantages in comparision with the conventional cavity mold process. To find the optimum pouring rate range for gray iron castings in full molds with foamed polystyrene patterns, the relations of sprue area vs. pouring rate and of pouring temperature vs. pouring rate were investigated through foundry experiment. The patterns used in this experiment were designed according to Wallace`s report for bar, cylinder and plate castings which was authorized by AFS in order to compare quantitatively the experimental results with the formula for the conventional cavity mold process. The sprue choke sections were varied in six steps, namely, from 0.75 to 2.5 times of Wallace`s formula. It was cleared by this study that sprue area, shape of casting, and pouring temperature were the dominant factors to determine the pouring rate for a sound casting.

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Nodular graphite cast irons which are consisting of three kinds of matrix, i.e. pearlite, mixture of pearlite and ferrite, and ferrite, have been prepared, then the plastic deformations and fracture behaviors have been studied by the methods of observing the stress-strain curves in tension test and load-time curves in impact test respectively. The results obtained are summarized as follows: (1) The stress-strain curves for the nodular graphite cast iron of ferrite matrix have shown discontinuous yielding behaviors as observed in those of mild steel. (2) The tensile and yield strength for the three kinds of nodular graphite cast iron are almost independent of the strain rate between 1.5×10^(-5) and 1.5×10^(-3)sec^(-1). (3) The equation σ_t=Kε^n_t seems to be correct in the plastic regions of the stress-strain curves for all kinds of nodular graphite cast irons we prepared. (4) The nodular graphite cast iron of ferrite matrix shows better performance is combination of strength and ductility compared with other two cast irons. (5) The nodular graphite cast irons of two types of matrix, ferrite, and mixture of pearlite and ferrite, show the quite similar fracture behaviors in impact tests. (6) It is concluded that the impact energies of the nodular graphite cast irons of three types of matrix are not more than the crack initiation energies.

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The effects of magnesium diffusion on the spheroidization of flake graphite with Fe-C and Fe-C-Si alloys have been investigated by means of microscopy. In this study, solid cast iron was immerged in molten magnesium bath to diffuse magnesium into the cast iron. The spheroidization of the flake graphite was accomplished by the magnesium diffusion. The results obtained from this experimental work are summarized as follows: 1) The flake graphite near the surface of the cast iron was changed to the fine nodular or lump shape by the magnesium diffusion. 2) The thickness of diffusion layer formed by the magnesium diffusion, which is dependent on the diffusion temperature, time and carbon equivalent, was about 0.2 ㎜ to 0.4 ㎜. 3) Graphite content was decreased in the surface layer where magnesium diffused and the decreased graphite was accumulated under the diffusion layer. 4) In the Fe-C-Si alloy, the shape of graphite formed on the surface became more spherical with the increase in silicon content. 5) With the X-ray diffraction patterns, it was considered that certain intermetallic compound was formed in the diffusion layer.

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In the reduction process of metallic tungsten powder from demestic APT, the thermal decomposition of APT and the reducibility of WO₃ were investigated. APT was completely decomposed to angular plane-shaped skeleton particles of WO₃ at 450℃ an 30 minute, which were easily crushed to primary fine particles. When 6 gr of WO₃ was packed into nickel boat 6㎜ in thickness with high packing density of 1.25g/㏄, it was completely reduced to metallic tungsten at 800℃ with hydrogen flow rate of 1,000㏄/min in 40 minutes and also at 900℃ with hydrogen flow rate of 1,500㏄/min in 20 minutes. It is expected that in industrial process WO₃, which has lower packing density, will be reduced to metallic tungsten with lower hydrogen flow rate and shorter reduction time than those limited conditions. Reducibility was retarded with decrease of reduction temperature and hydrogen flow rate and with increase of sample thickness and water content of hydrogen.

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Effects of tuyere angles range in 5°at 35° on melting velocity, tapping temperature and erosion of acidic fire brick had been studied in the Griffin type cupola operation using lump anthracite. The results were as follows: 1) Optimum tuyere angle was 20°. 2) Melting velocity was decreased as tuyere angle increase. 3) Tapping temperature was highest at 20° tuyere angle.

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Effects of reduction temperature, hydrogen flow rate, water content of hydrogen, and amount of the charge on the particle size of the reduced tungsten powder from APT or WO₃ were investigated. The size distribution of the reduced powder was measured with sedimentation balance and the shape and particle size of the powder was observed with electron microscope. It is considered that the growth of the reduced metallic tungsten particle was occurred the evaporation and condensation reaction of WO₃ in wet hydrogen atmosphere and the reaction was accelerated with the increase of reducing temperature, amount of charge, and water vapor content of hydrogen. The evaporation and condensation reaction, however, was retarded by two step reduction of APT with hydrogen since preliminary reduction at 600℃ transforms APT to fine WO₂ which has the highest sublimation point among the tungsten oxides. And the subsequent final reduction of WO₂ at 800℃ provided the fine metallic tungsten powder. The portion of the fine metallic tungsten powder under 2μ obtained by the two step reduction process was amounted to 78%.

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