발간논문

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The shape of dendrite and arm spacing are dependent on the condition of casting. The dendritic structure and variation in concentration of the solute element affect in mechanical properties. Accordingly deeper understanding of the factor affecting the solidification of casting and the resultant segregation are needed. The results obtained from this study are as follows: 1) The dendrites of Cu-4∼12% Sn alloy grow like plate within the range of the cooling rate obtained by this experiment. 2) The relationship between the primary dendrite arm spacing, the content of Sn and cooling rate was obtained as follows: dp=K√Co/Va dp: primary dendrite arm spacing(mm) 3) The secondary dendrite arm spacing increases with decreasing cooling rate, but is little affected by Sn content. This relationship is obtained as follows: log ds=-0.255 log Va＋2.35 ds: Secondary dendrite arm spacing (mm) 4) The eutectoid phase (α＋o⌒) precipitates independent or the cooling rate in Cu-10 and 12% Sn alloys, but in Cu-8% Sn alloy it precipitates at locations of low cooling rate, and is not observed in Cu-4 and 6% Sn alloy.

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Tensile properties and impact toughness of 0.25C-2.5Ni-2.5Cr-0.5Mo-0.1V steel have been studied in relation to three different transformation structures of martensite, mixed structure of 50% martensite+50% lower bainite, and lower bainite. Mixed structure has shown higher tensile and yield strength than any of the single structure of martensite or lower bainite, when they have been tempered at 230℃ and 350℃. As the tempering temperature increases, strength of the martensite and that of the mixed structure have decreased gradually. But lower bainite structure has shown the decrease of strength only by increasing the tempering temperature above about 400℃. Impact toughness of the three structures has been tested after tempering at 450℃ for 40 min to the same ultimate tensile strength level of 145㎏/㎟ . Martensite structure has shown the highest impact toughness. Test resultes of tensile properties and impact toughness are discussed in connection with electron microscopic structure.

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The hot corrosion behavior of preoxidized nickel coated with Na₂SO₄ has been studied at 1014℃, 932℃ and 853℃ in dry air. It has been observed that hot corrosion rates are remarkably retarded by means of the protective pre-oxide but, after a certain time, accelerated oxidation occurs severely because of local dissolution or mechanical fracturing by thermal stress of the protective oxide layer. The oxide layers which were formed during an accelerated oxidation stage were porous and lamellar type and sulfide phase was observed at just outer side of the matrix and grain boundaries. Based upon the above results, the hot corrosion mechanism of preoxidized nickel by Na₂SO₄ is considered as follows. In an initial stage, protective oxide layers are locally dissolved by oxide ion in fused Na₂SO₄ or mechanically fractured due to thermal stress. Fused Na₂SO₄ penetrates through the porous outer pre-oxide and forms liquid sulfide reacting with nickel matrix. In the later stage, liquid sulfide easily penetrates along the grain boundaries due to its low interfacial energy with nickel, thus grain boundary isolation occurs.

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The vanadium attack of 18-8 stainless steel was studied at temperatures between 700℃∼1100℃. In the corrosion by V₂O_5, at temperatures up to 800℃, the amount of weight loss by corrosion was very small. But at 900℃ the weight loss was very large. At 1000℃, however, after 2 hours the weight loss was less than that of at 900℃. At 1100℃, the weight loss was increased in a large amount. In the corrosion by V₂O_5-Na₂SO₄ mixtures, at temperatures up to 800℃, the weight loss showed maximum in the regions of 15∼25% Na₂SO₄. At temperatures above 900℃, the weight loss was maximum when corroded by V₂O_5 only. And as the amount of V₂O_5 in the mixtures was decreased, the amount of weight loss was decreased too. But at 1100℃ the amount of weight loss was maximum in the vicinity of 90% Na₂SO₄.

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Tensile strength, yield strength and elongation of aluminum and 2024 aluminum alloy strips were measured with various tensile directions to the rolling direction. Their preferred orientations were also determined using a pole figure device. A simple method of analysis was suggested for qualitative correlation between planar anisotropy of yield strength and preferred orientation of strip specimens. The analysis was made on the basis of an assumption that the yield stress is inversely proportional to an arithmetical average of Schmid factors for all slip systems. For cold rolled aluminum and recrystallized aluminum strip specimens, directionality of yield strength was qualitatively in agreement with the result analyzed by the method. For 2024-T4 aluminum alloy, the longitudinal tensile properties were superior to the transverse properties due to mechanical fibering.

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Sintering characteristics cf iron ore-anthracite mixture by means of shatter index, drum index and reduction degradation index have been fully studied with varying the amount of anthracite as fuel. The sinter was compared with that of coke. Results withdrawn form experiments are as follows. 1) Exposed to the property of the sinter with the use of anthracite, good results have been obtained in 5% mixed anthracite, and the results have been showed little variation when the results compared with coke. 2) The more mixing rate, the less textures with calcium ferrites and glass pool with dendrite of magnetite are well developed. 3) The sinter with the use of anthracite was favoured because of using domestic resources and using anthracite is cheaper more than coke.

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Austenite series high Mn Steel was studied on high strength-toughness and wear-resistance at room temperature, -78℃ and up to ultimate low temperature of -196℃, The test was made with the solution treated, cold worked (30% reduction of cross section area) and age hardened sample at 600℃, The testing results that considerable ductile fractures mixed with about 45% cleavage fractures and a little grain boundary fractures were generated up to -78℃, when the temperature came down to -196℃, grain boundary fractures with about 55% cleavage fractures were observed. Ultimate tensile strength was not demonstrated as yielding point owing to the unstable fracture. At the ultimate low temperature of -196℃, wear resistance is increased by work hardening. that is, owing to ε-Martensite, twin induced by contact working of wear surface.

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