발간논문

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In order to improve wear and erosion-resistances of a structural alloy steel (SNCM 8) during heat-cycling, chromium-allayed layers were produced on the surface by irradiating CO₂ laser. Specimens were prepared either by electroplating of hard-chromium or coating of chromium powders on the steel followed by the laser treatment. Index values, which related the depth and the width of the alloyed layers to the scanning speed of laser, for both samples are experimentally measured. At a fixed scanning speed, while both samples resulted in a similar depth of the alloyed layers, the chromium powder coated specimen showed larger width of the alloyed layer than the chromium electroplated one. The hardness values of the alloyed layers in both samples were slightly lower than that of the martensitic region beneath the alloyed layers. But they are considerably higher than those of steel matrices. Regardless of the prior treatments before laser irradiation, distributions of chromium ware fairly uniform throughout the alloyed layers.

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Dynamic fracture toughness of simulated HAZ of a structural steel (equivalent to KS D3503) was investigated by instrumented impact testing technique. According to the validity criteria of this testing method, the measured fracture toughness values of this steel were evaluated to be valid when tested at temperatures lower than -40℃ for fine grained HAZ and lower than -l0℃ for coarse grained HAZ, respectively Even though the coarse grained HAZ fails in brittle manner at fairly low temperatures, the specimens must be fatigue precracked to obtained valid results.

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The dependence of the magneto- and electrical resistance on the composition and growth rates of two phase eutectic was investigated in the unidirectionally solidified Ge-TiGe₂ alloy system. High aligned structure was produced at the growth rate of 6 ㎝/h in the Ge-TiGe₂ eutectic alloys and this structure showed high magnetoresistance at room temperature and at liquid nitrogen temperature. The electrical anisotropic properties due to the isolated alignment of triangular Sb rods were obtained at the growth rates of 1 ㎝/h and 1.5 ㎝/h in the Insb-Sb eutectic alloys. At the low growth rate, structural and electrical anisotropies were found in the Ge-Sb eutectic alloys, and magnetoresistance was not detected in the InSb-Sb and Ge-Sb eutectic systems.

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Hot workability of aluminum alloys, 2024, 6061 and 7075, has been studied by hot torsion tests at temperatures from 320 to 515℃ and at strain rates from 1.26×10^(-3) to 5.71×10^(-3) sec^(-1). Hot working condition of these aluminum alloys was determined quantitatively from the constitutive equations obtained from flow stress curves in torsion. Experimental data of the logarith of the Zener-Hollomonn parameter showed good linear relationships to the logarith of sinh (aδ(-)). The constitutive equations of aluminum alloys, 2024, 6061 and 7075 were expressed as ε=2.71×10^(12) (sinh 0.0142δ(-)^(5.54) exp (-212/RT), ε=6.91×10^(10) (sinh 0.0108δ(-))^(5.47) exp (-163/RT) and ε=4.8l×10^(10) (sinh 0.0088δ(-))^(5.00) exp (-165/RT), respectively. The flow data of the aluminum alloys could also be well fitted to the power law δ(-)=Bε^(x)｜e,T-

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The kinetics of metallic indium leaching from sulfuric acid solution has been studied by the use of a rotating disc method. The leaching rate was found to be zeroth order with respect to indium ion concentration in the bulk solution. The activation energy for the reaction was 2.9 Kcal/㏖. at above 45℃ while it was 5.9 ㎉/㏖ at below 45℃. It means the reaction is controlled by mass transfer at above 45℃ but the reaction mechanism is considered to be changed at lower temperatures. When the concentration of hydrogen ion was high compared to the oxygen concentration in solution, the leaching rate was linearly proportional to the partial pressure of oxygen. When the solution pH is over 3, leaching rate was practically null due to the formation of In₂O₃. When foreign ions such as Fe^(2+) or Zn^(2+) exist in the bulk solution, the leaching rate was decreased, because diffusion coefficients decreases with increased ionic strength.

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Understanding of the flow pattern of a molten steel through tundish is an essential precondition for estimating and avoiding negative effects on steel cleanliness. To study removal of inclusion in the tundish, experiments were carried out using a one third scale water model of the tundish with a dam and a weir. The flow patterns were determined by a flow visualization technique and by a theoretical analysis of the tundish system. The velocity field was predicted by solving the turbulent Navier-Stokes equations using the K-ε model for turbulent viscosity. There was a good agreement between the experimental results and the predictions. A plant test revealed significant improvements on slab quality such as reduction of surface defects of the cold-rolled sheets.

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For binary liquid aluminium alloys at infinite dilution, it is demonstrated that there exists a certain correlation between the partial enthalpies of mixing, ◎, and the partial excess entropies of mixing, ◎ This correlation is resulted from the fact that both ◎ and ◎ have similar contributing terms which are related with atomic interaction. With very few exceptions, the ◎ values are in accordance with the values calculated on the basis of the Miedema model. Furthermore, the ◎ values are observed to systematically depend on the atomic number. With the help of the Samsonov`s consideration, this systematic dependence can be explained with the electronic structure of the elements.

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