발간논문

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The moving-heat source method was applied to investigate the radial temperature gradient and the cooling rate on metal surface by the irradiation of a high-energy pulsed laser. In order to calculate those values the data obtained by irradiation of a Q-switched ruby laser on pure Sn in vacuum were used. The results showed that the temperature gradient near the crater was approximately 2.3×10^6 ℃/㎝ and the cooling rate was about 7×10^9 ℃/sec. When they were compared with other kind of investigation, similar results were obtained.

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Dual phase steels were produced by quenched from intercritical temperature (780℃) after 0%-68% cold working at room temperature. A study has been made on the change of austenite morphology, the annealing time for dissolving cementite into austenite at intercritical temperature, and the mechanical prorerties of the dual phase steels. High strength and good ductility was acquired by annealing highly cold worked steel for a short time. At this time the martensite structure was fine. The annealing time for dissolving cementite into austenite was shorter in ferrite-pearlite structure than in spheroidized structure. The ductility of dual phase steel was improved when the undissolved cementite was little. The mechanical properties of dual phase steel were influenced by the volume fraction, the size, and the shape of martensite and undissolved cementites. The best combination of strength and ductility was obtained when the specimen with the reduction rete 68% was annealing for 15 min and then quenched.

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The thermal decomposition process of ammonium paratungstate (A.P.T.), 5(NH₄)₂O ·5H₂O, was studied by differential thermal analysis, thermogravimetry, and X-Ray power diffraction. A small quantity of water and ammonia was released below 250℃ and most of water and ammonia was driven off rapidly about 305℃. At 320℃, the ammonia remained 1 mole and the compound changed into amorphous state and γoxide, followed by the formation of αoxide at about 440℃. The shape of tungstic oxide particles was similar to that of APT and tungstic oxide particles were smaller than APT particles because of fracture of the latter during decomposition. Tungstic oxide particles were annealed for various periods of time (1 hr. to 10 hrs.) over the temperature range of 500 to 1000℃. The particles became finer due to further fracture at higher temperatures. The particles were composed of fine grains, the growth of which increased with increasing time and temperature. The color of the oxide powder changed from yellow to green with increasing temperature due to the formation of WO_(2.9).

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The effect of dissimilar metal (1100-0 Al, 6061-T6, 5083-H32, SM15C, SCM2, SCM21, SNCM8, SNCM21, STS304, DIN x10CrNiMoTi1810, Cu, brass, bronze) on the galvanic corrosion of 5086-H116 Al alloy in the synthetic sea water was studied by the galvanic current method and corrosion product morphologies on the 5086-H116 Al alloy exposed in the salt spray test chamber were observed by the metallurgical microscope and scanning electron microscope. Results show that the potential difference of uncoupled dissimilar metals is a poor indicator of the extent(rate) of galvanic corrosion of coupled dissimilar metals. Galvanic series for 5086-H116 Al alloy in synthetic sea water based on the galvanic current density was Cu≥brass>SM15C≥SNCM21≥bronze>SNCM8>SCM2≥SCM21>STS304≥DIN x10CrNiMoTi1810≥6061-T6>1100-0≥5083-H32. Corrosion product morphologies on the 5086-H116 Al alloy exposed in the salt spray test chamber agreed with the galvanic current data. When coupled with other Al alloy there was no galvanic effect but when coupled with Cu, brass, SM15C, pitting and intergranular corrosion occurred on the surface of the 5086-H116 Al alloy near the interface of the dissimilar metal.

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Spheroidal graphite cast iron specimens of 2.5-5.8 carbon equivalent were casted into CO₂sand mold, 2-10㎜ thickness Y-block. Both, the microstructures and abrasion characteristics of the samples are studied. The results are as follows. 1) Even though the carbon equivalent is the same in the spheroidal graphite cast iron, the amounts of cementite and ferrite are different whenever the silicon and the carbon contents are different. 2) Under the present expermental conditions, the relationship between the abrasion speed and the abraded amount are: 0.5 m/sec for abrasive wear, 1 m/sec for corrosive wear and 4 m/sec for adhesive wear. 3) The amount of abraded material during corrosion wear increases as the cooling rate decreases for the same carbon equivalent and increases as the carbon equivalent. 4) The amount of abraded material during adhesive wear increase with the decreasing carbon equivalent and with the faster cooling rates for castings of 2-4㎜ in thickness.

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The effects of recrystallized and rolling textures obtained by two kinds of double step cold rolling schedules with intermediate annealing on the formability of commercial 17% Cr ferritic stainless steel sheets were examined by the plastic strain ratios and earing behavior in deep-drawing. The relation between average plastic strain ratio and textures was qualitatively interpreted. The optimum condition of high average plastic strain ratio, low anisotropy, and high yield strength was found at 50-50% reduction for 86% of total reduction process ( r￣＝1.24, Δr＝0.88, Y.S.＝38.4kg/㎟, U.T.S.＝52.1kg/㎟), and 63%-76% reduction for non-specified total reduction process ( r￣＝1.36, Δr＝0.97, Y.S.＝40㎏/㎟). Qualitative agreement in r￣ values between experimental and calculated values were satisfactory. With increasing total cold reduction of double step cold rolling, yield strength and ultimate tensile strength increased because of texture component, and the longitudinal strength was superior to the transverse strength due to (001) [110] texture. The hardness and work hardening exponent were not influenced by the rolling reduction and texture.

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Iron is probably the most important metal on earth, and the ironmaking capability of any region often closely resembles the cultural, scientific and technological development of that region. This study examines the technological and metallurgical development of the Korean Peninsula exhibited immediately prior to the Three Kingdoms period. Initially, the cultural characteristics of areas situated close to shell mound sites on the south coast are reviewed with special reference to excavated iron artifacts. Chemical analyses and metallographic examinations have been carried out on both these artifacts and their related ironmaking residues. It is shown that 1) Shell mound sites containing ironmaking residues are situated close to rich iron ore deposits. 2) Forged steel and cast iron artifacts were produced on the same sites in the same period. 3) Internal carburization of low carbon steel was produced by multiple piling and forging operations. 4) Water quenching was employed to acheive high hardness for cutting tools.

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