발간논문

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Aqueous corrosion characteristics of iron aluminides in thiosulfate-chloride solution were studied as a function of chromium content. Four kinds of iron aluminides, FA-61, FA-77, FA-72 and FA-78, were prepared by arc melting followed by the thermomechanical treatment. The corrosion behavior in thiosulfate-chloride solution for the prepared alloys was investigated by electrochemical tests(potentiodynamic test, potentiostatic test and electrochemical impedance spectroscopy(EIS) measurement) and surface analyses(SEM, AES). The results of potentiodynamic test indicated that the breakdown potential increased with increasing Cr content. Cr additions were found to prevent passive film from undergoing pitting corrosion. EIS measurements showed that semicircle was less depressed when the Cr content increased. Furthermore, the Cr-bearing alloys present higher pitting resistance than a Mo-free alloy at breakdown state. The SEM observations of the metal surfaces revealed that the Cr additions changed pit morphology from hemispherical to tunneling attack. The AES results gave the evidence that the thiosulfate was reduced on the metallic surface, which inhibited the repassivation process.

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The formation, growth and disappearance behaviors of microconstituents in the bonded region of Ni-base single superalloy, CMSX-2 during transient liquid phase (TLP) bonding was investigated employing MBF-80 insert metal. TLP-bonding was carried out at 1373∼1548 K for 0∼19.6 ks in vacuum of 4 mPa. The (100) orientation of each test specimen was always aligned perpendicular to the bonding interface. Borides were formed in the bonded layer during TLP-bonding operation. In the early stage of bonding, base metal dissolution process, small and lots of microconstituents come into being, and the microconstituents combine and grew as the bonding temperature increased and the holding time was increased. The amount of microconstituents in the bonded layer after isothermal solidification was decreased with the holding time. The microconstituents in the bonded layer disappeared at the bonding condition of 1523 K×1.8 ks.

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It was constructed a mathematical model for burden distribution in the blast furnace that was consisted of the calculation of burden trajectory, optimization of burden descending velocity suitable for estimating the real base line and adoption of plural inclination angle of coke layer. It was confirmed that this mathematical model could represented the real ratio of ore to coke layer thickness. Although the particle size distribution in the radial direction was assumed to be constant because it depend mainly on the charging system, the calculated gas velocity distribution based on Ergun`s equation corresponded well on the measured one. Moreover it was ascertained that the temperature distribution measured by a shaft sonde could represent the gas velocity distribution at the shaft part of blast furnace.

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It were investigated on the analytical solution based on Coulomb-Mohr circle and the solid flow in the blast furnace using a scale model. According to the experimental result, solid flow in the blast furnace could be divided into two regions that is mass flow over the belly, funnel flow under the upper belly. The analytical solutions that contained the average vertical stress, horizontal stress and wall stress were obtained by the plastic yield condition used in soil mechanics. The analytical calculation results corresponded well to the measured one over the belly. According to the analytical solution, the ratio of horizontal stress to average vertical stress turned over at the belly part, and such abrupt change of stress distribution caused mass flow to shift funnel flow.

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A study on the preferred physico-chemical and metallurgical properties of MgO based pellets have been undertaken under high PCR BF operation condition, assuming higher pellet ratio in a ferrous burden in order to secure a higher productivity of sinter plants. Various experimental condition simulating a high PCR operation were aimed at evaluating commercial pellets under a high production rate operation. Higher burden weight resulted in a lowering of the softening temperature, whereas an increase of hydrogen content in the reducing gas raised the softening temperature. As a result of various changes inside the furnace with increased PCR, the size-consist of all charge materials must be controlled and maintained aside from monitoring metallurgical properties of them.

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The leaching behavior of tin from pure Sn and Sn-Fe couple in caustic solutions has been investigated using a rotating disc. Oxygen was used as an oxidant. The effect of concentration of NaOH, rotation speed, temperature and oxygen potential has been studied. The leaching rate of tin from Sn-Fe couple was reduced than that from pure tin at relatively low oxygen potentials due to galvanic effect. On the other hand, when the oxygen potential was high, the galvanic effect of the leaching rate became less pronounced. The rates of leaching of tin from pure Sn and Sn-Fe couple in oxygenated caustic solution were controlled by mass transfer of oxygen through liquid-solid boundary layer.

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The spheroidization behavior of low carbon steel processed by the equal channel angular pressing technique was investigated by imposing the static annealing treatment for 1 hour at various temperature of 693-973 K. By the repetitive pressings up to the accumulated effective strain of 4, the morphological change of cementite lamellar plates in pearlite was represented by the existence of either severely necked fragments parallel to each other or curled wavy fragments. Under the present annealing conditions, the microstructure of pearlite in pressed samples was characterized by the enhanced spheroidization of cementite compared to unpressed samples. The enhanced spheroidization behavior of the pearlite was discussed in terms of easy carbon dissolution from cementites into pearlitic ferrite due to the severe plastic deformation.

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