발간논문

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Effects of oxide impurities on the reduction rate of wu¨stite were studied in H₂-atmosphere at temperatures above 900℃. Oxides such as Li₂O, Al₂O₃, and SiO₂were added to the wu¨stite phase as promoter. With addition of promoters the reduction rate of wu¨stite were increased significantly than that of wu¨stite only. Above 1.2 wt. % promoter the reduction rate were considerably decreased regardless of the promoters. This may be due to suppressing the diffusion of reducing gas by forming stable refractories or slags in the wu¨stite-reduced iron interface. However the increments of reduction rates below 1.2 wt. % of promoters could be well explained by the Vol`kenshtein`s lattice disturbance theory rather than the electronic defect theory. Results revealed that the initial reduction rate could be expressed in the form of 1-(1-X)^⅓= kt which suggests the reaction is surface control. An activation energy for the reduction of wu¨stite in the presence of Al₂O₃was found to be about 6.0 ㎉/㏖.

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The rate of reduction of Eu(III) ions with zinc under an inert atmosphere was investigated by use of a rotating disc geometry. Among the rate controlling steps of this heterogeneous reaction, diffusion of Eu(III) onto the zinc rotating disc surface through the solution is explained by using the well-known Levich equation, k＝0.62 D^(⅔) ν^(-1/16) ω^(½). For temperatures above 20℃, the rate of reduction is limited by the rate at which Eu(III) ions in the solution diffuse onto the zinc surface and the activation energy of this mass transfer-controlled reaction is 3.4 ㎉/㏖. For temperatures below 20℃, the rate of reduction decreased rapidly and hence a change in the rate controlling step would be occurred.

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As-rolled dual phase steels were produced in a laboratory scale by a controlled rolling process. The relationship between mechanical properties and microstructures were investigated on the tensile specimens which were produced by varying finish-rolling temperatures, cooling methods and coiling temperatures. The Fe, 0.036-0.06％ C, 1.1-1.7％ Si, 1.3-1.7％ Mn compositions showed the ferrite-martensite dual phase structures and the tensile properties of strength 55-80㎏/㎟, total elongation 25-39％ and yield ratio 553％ after finish rolling 30％ at 780-840℃ followed by oil quenching. Air cooling down to A₁temperature (720℃) before oil quenching was found to decrease the optimum finish rolling temperatures by 30-50℃ through annihilation of hot-worked microstructures. Addition of 0.4％ Cr could raise the coiling temperature to 350℃ without deteriorating the dual phase steel properties.

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The pore structure of iron reduced from wustite by hydrogen gas is characterised by measurement of pore volume and critical pore radius using mercury penetration porosimeter. By utilizing partial reduction and one hour annealing, morphology of the reduced iron is thoroughly investigated by both optical and scanning electron microscopy. Summerized results are as follows: 1) The morphological changes in reduced iron layer are closely related to the length of sintering time. 2) Swelling of reduced iron is basically due to the crystal growth of γ-iron reduced from wustite. 3) In the temperature range of 700-900℃, the critical pore radius is described by the relationship. Critical pore radius＝A·exp(-9300/RT). 4) The growth rates of the critical pore radius vary with crystal structure. In the α-iron range, the rate of growth is extremely slow compared to that occurring in the γ-iron range.

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The effect of the graphite change and the structure of matrix makes a great difference in the properties of cast iron. Therefore the method to improve the properties of cast iron by inoculation treatment which adds Fe-Si, Ca-Si alloy just before the injection, has been studied and is being studied in these days. In this study, the increase of tensile strength by the effect of inoculation and solid solution strengthening, when the small amount of Al and Cu known as the element of graphitization was added, was studied. That is, the author increased the strength through refinement and homogenization of graphite which is caused by the addition of the graphitization elements, and through the change of the structure of matrix. The results obtained in this study were as follows; 1) The tensile strength showed an increase when the small amount of Al, Cu and Al-Cu alloy known as the graphitization elements were added just before the pouring. 2) In the case of Al addition, the cast iron with 0.75% Al showed 22% more increase in tensile strength in comparison with that without addition. 3) In the case of Cu addition, the cast iron with 1.2% Cu showed a maximum tensile strength, 37㎏/㎟, and 36% more increase in tensile strength in comparison with that without addition. 4) In the case of Al-Cu (1:1) alloy addition, the cast iron with 2.0% Al-Cu alloy showed a maximum tensile strength, and 48% more increase in tensile strength in comparison with that without addition.

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Changes in mechanical properties of W-NrFe heavy alloys were observed in accordance with a variation of composition and annealing condition Alloys of three compositions, 90W-3Ni-7Fe, 90W-5Ni-5Fe and 90W-7Ni-3Fe in wt% were liquid phase sintered at 1470℃ for 1 hr in H₂atmosphere and then annealed at 700, 900, 1000 and 1400℃ for 60 hr. All specimens of 90W-3Ni-7Fe composition showed a formation of an intermetallic phase, μ-phase (W_6Fe_7), between spherical tungsten grains and the ductile γ-phase matrix in microstructure which increased its amount with increasing annealing temperatures. The intermetallic phase drastically decreased tensile strength and elongation of the alloy. Alloys of 90W-5Ni-5Fe and 90W-7Ni-3Fe compositions were consisted of tungsten grains and a ductile γ-phase matrix and showed relatively good mechanical properties throughout the examining conditions. Fracture surfaces exhibited an intercrystalline or a transcrystalline fracture behavior depending on the annealing condition while the matrix phase always showed a really good ductility.

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