발간논문

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High temperature oxidation of Fe-Cr stainless steels at 1250℃ in reheating environment containing various excess oxygen contents has been studied. In reheating environment, the oxidation rate was drastically increased as compared to the one in dry oxygen environment due to the presence of water vapor. The dissociation process plays a major role on the scale growth of both Fe-13%Cr and Fe17%Cr stainless steels. A finger-type oxide was developed at the scale/metal interface of Fe-17%Cr stainless steel. As an excess oxygen content increased from 0.8% to 3.0%, the oxidation rate was increased about 50% and the penetration depth of finger-type oxide was also increased. To minimize metallic yield losses and surface defects, the excess oxygen contents in reheating furnace should be decreased for Fe-Cr stainless steels.

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High temperature oxidation of 316L stainless steel at the temperatures between 1200℃ and 1300℃ in dry oxygen environments has been studied. The oxidation rate of 316L steel was 2∼6 times higher than that of 18%Cr-8%Ni steel and even higher than that of 13%Cr steel. The catastrophic oxidation of 316L was caused by the evaporation of MoO₃. The oxidation was proceeded through repeating process which is scale breakaway followed by formation of Cr₂O₃ layer at the scale/metal interface. This oxidation process results in a formation of very porous multi-layered oxide scale.

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Vacuum distillation at 10^(-5) Torr was carried out to refine indium and gallium metal under various conditions. Recovery of both metals decreased with the increase of temperature and reaction time. Activation energies of indium and gallium for vacuum distillation were found to be 32.9㎉/mole and 83.2㎉/mole, respectively, and distillation rates of both metals were controlled by evaporation rate at the interface. It was possible to produce 99.999% indium metal from the starting material of 99.97% at 1000℃ within 1 hour. When gallium of 99.9% purity was refined at 1050℃, Bi, Pb, Zn and Mg were eliminated to less than 1ppm but only 25% of Fe and 80% of Ca were eliminated to produce 99.99% gallium metal within 2 hours.

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The effects of indium on the phase transformation of 2090 Al-Li alloys has been studied with a differential scanning calorimeter (DSC), a transmission electron microscope(TEM), and a micro-hardness tester at the temperature ranges of 130℃∼220℃. The results show that indium addition to the 2090 Al-Li alloy retards the formation of prior precipitate(precursor) and δ´phase, whereas it accelerates the formation of T₁ and θ´(especially θ´phase). Above 190℃, the formation of δ´phases is retarted than those at 130℃ and at 160℃. The addition of 0.15wt% indium increases the hardness at the early stage of aging. The cause of this effect is attributed to the formation of GP zone. The main phases, in peak aged condition of indium free 2090 alloys are δ´and T₁; indium bearing 2090 alloys are θ´and T₁ (especially θ´Phases).

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The effects of low temperature aging on the impact toughness and mechanical properties of Al-1.9%Li-3.0%Cu-0.15%Zr alloy have been investigated. It shows that the impact absorption energy and elongation are increased in the order of over-aged, peak-aged and under-aged specimen at the various aging temperature, and these values in the specimen peak-aged at lower aging temperature are increased. In the specimens aged at 120℃ and 140℃, dimples are observed in the matrix and around the grain boundary. Intergranular fracture has been observed noticeably in the 160℃ peak aged specimen. These results are attributed to the microstructural changes by the low temperature aging treatment that forms more dense and homogeneous distribution of T₁ phases on the dislocations within the matrix and more narrow precipitate free zone in the grain boundary at 120℃ and 140℃ than at 160℃.

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The growth of fatigue cracks was investigated in near-eutectic solder joints of ceramic leadless chip carriers (LLCC`s) packages subjected to thermal displacement-induced fatigue at temperature ranges of from -35℃ to 125℃ with 1hr to 4hr time period. Under these conditions, it was observed that the coarsened region of the eutectic phases developed at 45℃ to the printed circuit board during the early stage of cyclic loading. It was discussed that microstructural coarsening could lead to premature transgranular failure, which propagated up from beneath the LLCC into the fillet, resulting in deteriorated reliability of electronic systems.

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The effects of atomization variables such as protrusion, primary and secondary atomizer gas pressure, and gap between primary and secondary atomizer on the spray condition were studied by two stage atomizer with water as spraying material. Spray condition was improved with decreasing protrusion value and with increasing gas pressure of primary atomizer, due it is thought to countervail of the scattering and upward component of secondary atomizing gas by primary one directed downwards. With increasing the gas pressure of secondary atomizer, the breaking point rised to atomizer and splash-back occured intensely. Good spray condition was achieved in case of the gas pressure ratio of primary and secondary atomizer got the value of 0.5-1.

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An Analysis of heat transfer has been carried out for continuous galvanizing system of steel strip using analytical method and finite difference method. This study takes into account temperature dependent material properties, thickness and width of strip, line speed and cooling and heating conditions. The calculated results using ISIJ model fit well with experimental ones. The effective cooling length is the sum of the length of nozzle zone in continuous cooling system mixed with natural cooling zone and nozzle cooling zone.

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In this study, TiC, TiN and TiC/TiN coatings were deposited on the tool steel(SKH9) by PECVD to evaluate the performance of coatings. So, TiC, TiN and TiC/TiN coatings have been characterized using XRD, AES and EDS. Microhardness mearsurement, indentation, scratch adhesion and wear test were carried out to investigate mechanical properties. As a result, the hardness of TiN, TiC and TiC/TiN coatings were Hv1700, Hv4000 and Hv2400, and critical loads(L_c) in scratch adhesion test were 14, 10 and 28N respectively. TiC/TiN double layers have shown excellent properties in indentation, scratch adhesion test and wear resistance. This is thougtht due to high hardness of TiC innerlayer, toughness of TiN outerlayer, and the inhibition of crack propagation by its deflection at the TiC/TiN interface.

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Coal injection effects on the behavior of a blast furnace raceway were discussed based on analyses of operating parameters and coke sampling experiments through the tuyere of the blast furnace as well as one-dimensional mathematical simulation of coal combustion characteristics in the raceway. Coke sampling lance was inserted through the tuyere during the shut-down of blast furnace to measure the raceway depth. It was revealed that the raceway depth decreases with the increase of coal injection rate, which could result in the increase of wall temperature and gas permeability resistance at the lower part of the furnace. The one-dimensional mathematical model considering the kinetics of coal and coke combustions was developed in order to simulate the combustion characteristics, such as gas temperatures and compositions in the raceway. It was found that the calculated peak temperature of gas way similar to the adiabatic flame temperature which is one of the operating parameters, and that the calculated effects of blast temperature, moisture addition and oxygen enrichment on the gas temperature except the coal injection rate were well matched with the changes of adiabatic flame temperature.

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