발간논문

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The different starting structures of the present study were ferrite + coarse pearlite (F+ P₁), ferrit + fine pearlite(F+P₂) and martensite(M). True stress vs. true strain curves were analysed before and after tempering. When compared the work hardening rate before tempering, the DP steel with the M-starting structure has shown the highest value. Uniform true strain was decreased with increasing tempering temperatures up to 400℃, and then increased above 400℃. Void formation behavior beneath the fracture surfaces of the tensile specimens has been studied. Preferred sites of void formation are the ferrite-martensite interfaces, the elongated martensite ends and the precipitate particles. Voids are also formed by splitting of martensite phases in the case of tempering.

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Low cycle fatigue properties of polycrystalline and directionally solidified CM247LC Superalloys were studied at ambient, 760℃, 871℃ and 982℃. At constant total strain, fatigue life of directionally solidified specimens were much longer than that of polycrystalline specimens. The lower Youngs modulus, and so the lower applied stress, of directionally solidified alloys was ascribed as a major source of longer fatigue life of these alloys. When evaluated at constant plastic strain, fatigue life was lowest at 760℃ in both polycrystalline and directionally solidified specime Inhomogeneous shearing of γ`particles by extended dislocations and/or superdislocations at lower temperatures was attributed to be responsible for this behavior.

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The combustion synthesis of titanium carbide from elemental powders has been experimentally studied as following. 1) All the products synthesized by SHS method were nearly pure TiC powders. The lattice constant was increased with increasing C/Ti atomic ratio, however, at about 0.8 C/Ti atomic ratio the lattice constant reached a plateau of 4.325Å. 2) The combustion of titanium and carbon powders was accompanied by the evolution of gases having steady state combustion mode with propagation rate of 10㎜/sec. at 2600℃. 3) The size distribution of the product was relatively wide in the range of 2㎛ and 40㎛.

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A hot tool steel, SKD61, nitrided in DC plasma and gas environments was investigated to see the properties of the nitrided layers and the effect of reheating on them. It was observed that as nitriding time increases, the thickness of the nitrided layers (compound and diffusion layers) increases, while surface hardness decreases. X-ray diffraction analyses showed that the compound layers obtainded from the both methods are composed of r`-Fe₄N and ε-Fe_(2-3)N phases which are completely decomposed by reheating at and above 550℃ As reheating temperature went up, surface hardness became low and the hardness difference between surface and bulk matrix became small. At and above 660℃ which was higher than the tempering temperature of the steel, the surface and bulk matrix hardnesses decreased very sharply. It was suggested that to improve the life time of dies, their service temperatures be kept below the tempering temperatures.

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Ta obtain fundamental informations for fine tungsten carbide powders, the preparation of fine tungsten carbide by vapor phase reaction of WCI_6-CH₄,-H₂ system has been carried out in the temperature range of 1000-1400℃. The effects of reaction conditions on particle size and morphology of tungsten carbide powders have been investigated by utilizing X-ray diffraction, SEM and TEM technique . The synthesis temperature of only WC powders is above 1350℃ in the condition of the flow rate of argon and Hz gas is 220㎖/min respectively and CH₄, gas 40㎖/min. `The particle size of the reaction products obtained at 1400℃ is about 0.02㎛ and the shape of WC is spherical. Temperature dependence of the particle size is weak and the lattice parameters a and c decrease gradually with increasing reaction temperature. Also the crystallite size increases gradually with increasing reaction temperature.

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