발간논문

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A simultaneous chromizing-aluminizing coating process was developed using pure iron, and Cr-Mo alloy as substrate. In order to deposit Cr and Al simultaneously, the activities of Cr and Al should be controlled which means the composition of the Cr-Al masteralloy should be adjusted. The optimum composition of the Cr-Al masteralloy was calculated to be 90 wt%∼95wt% Cr, 5wt%∼10wt% Al using the SOLGASMIX computer program. Simultaneous chromizing and aluminizing of the pure iron and the Cr-Mo steel was performed using the master alloy powder of 95Cr-5Al wt% and NH₄Cl as an activator at the temperature range of 1223K-1323K under Ar atomosphere. NH₄Cl was found to be a better activator than NaCl and the optimum composition was 3wt% NH₄Cl.

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The Effect of Ti addition (0.016-0.283%) on high temperature oxidation behaviour of type 304 stainless steel was investigated at the temperature range of 1230 to 1300℃. Oxidation rate of Ti added Type 304 stainless steels did not depend on Ti content at 1230℃, but decreased by half compared to that of Type 304 stainless steel without Ti addition. The oxidation rate decreased with Ti content at 1260℃. The beneficial effect of reducing the oxidation rate in Ti added Type 304 stainless steels was greatly reduced with increasing temperature. it was expected that 0.068% Ti addition to Type 304 stainless steel was desirable at the temperature range of 1230-1260℃.

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The mechanical alloying behavior of elemetal powders in the V-Al and V-Al-Zr systems was examined. The effects of milling time on the phase and particle size of powder has been thoroughly studied. For mechanical alloying, SPEX mixed/mill has been used. The milling time of V-Al and additional Zr are varied to find the optimum condition of forming amorphous phase. The X-ray diffraction pattern, microstructure detection and microhardness test are carried out to analyze the MA products. The procedure of mechanical alloying is as follows ; initial state-welding-equiaxed particle formation-random welding orientation-steady state. When the final step is reached, no lamellar-structures detected. The steady state condition is observed after 10 hours and 12 hours milling for V-Al and V-Al-Zr alloy, respectively. The microhardness continuosly increase up to 10 hours and 12 hours after then it remains constant. At 48 hours milling for V-Al alloy, the forms of amorphous phase is observed from X-ray diffraction pattern and transmission electron micrograph.

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In order to analyse iron ore reduction rate and heat balance for the in-bath type smelting reduction processing operations, a series of experiments was carried out using a bench scale smelting reduction furnace of 500㎏ in capacity. Total iron concentration in slag (%T.Fe) reached to the steady state at about 30 minutes after the start of continuous iron ore feeding into the smelter. (%T.Fe) in slag at the steady state increased with the increase of iron ore feeding rate. In the present study, maximum post combustion ratio was observed to be approximately 60 %, and it changed with respect to the oxygen injection point. When the oxygen lance was submerged into slag, heat transfer efficiency was improved. Above 80% of heat transfer effieiency was obtained with 60% of post combustion ratio. The iron productivity of smelting reduction process was calculated to be about 7 ton-Fe/day/㎥ based on the present experimental results, this being approximately three times of the capacity compared to the blast furnace at same inner volume.

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Highly localized deformation, frequently referred to as adiabatic shear banding, is a phenomenon observed in metals subjected to a high rate of deformation. The purpose of the present study was to establish temperature rise by determining temperature distribution within an adiabatic shear band during its formation in an HY-100 steel. Thin walled tubular specimens were deformed at dynamic rates in a torsional Kolsky bar, and temperature measurements were conducted using an array of sixteen infrared radiation detectors. The optical system allowed each detector to focus on a band 17㎛ wide. The infra-red detector system indicated that the average temperature rise in the narrow bands attained a maximum of 600℃. It was also suggested to measure temperature rise more precisely that the width of the observed area on the specimen`s surface should be reduced further. In addition to the improvement of the temperature measuring technique, transmission electron microscopy would be very helpful to the understanding of possible phase transformations occurring during the adiabatic shear band formation.

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The plasma rotating electrode process was applied to the production of type 304 and 316 stainless steel powders in order to determine the effect of operating conditions on the particle size and its distribution, and to analyze their microstructures, cooling rate and relative amount of ferrite phase for the as-prepared powders. For the operating conditions of plasma arc current : 140∼240 A and anode rotating speed : 17,500∼25,000rpm, stainless steel powders produced showed the typical bimodal size distribution ranging 50∼425㎛ and mean volume-surface diameter ranging 143∼238㎛. The appearances and microstructures of powders were found to be solidified with cellular or dendrite structures which have being depended on cooling rate(particte size). The estimated cooling rate of powders ranging 150 to 50㎛ in size was about 10³∼10^5K/s. The weight traction of ferrite phase in type 316 powders was found to be increased with decreasing particle size, due to the difference in the solidification behavior. For type 304 powders, however, the ferrite phase was not detected by X.R.D.

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