발간논문

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This theoretical study was carried out on the determination of the morphological stability criterion of planar solid-liquid interface. The system of a directional solidification under the nonequilibrium steady-state was defined as a continuous open system and the basic assumptions for the perturbation on the planar interface were introduced in order to analyze the pattern formation. The pattern formation was also interpreted by applying the marginal stability hypothesis. The results obtained can be summarized as follows. 1. The morphological stability criterion was T_mΓω²+ G_L-mG_c $gt; 0 and this could be reduced to the CS criterion. 2. The critical wavelength of perturbation, λ_c, was calculated by applying the marginal stability. From this result, it could be predicted that the shapes of the interface were changed with the wave length of perturbation. 3. It was suggested that the cell size, ℓ, can be limited by the critical wavelength of perturbation.

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The effects of nonmetallic infusions and microstructures on the sulfide stress-corrosion cracking (SSCC) of welded line-pipe steels have been investigated by using electrical potential method. SSCC tests were conducted in H₂S-saturated aqueous solution containing 5 wt.% NaCl and 0.5 wt.% CH₃ COOH at temperature of 35±2℃ by using smooth U-bend specimens. Sulfide stress-corrosion (SSC) crack initiation and propagation took place in the weld fusion zone (WFZ) of two different kinds of American Petroleum Institute (API) grade X-65 due to the presence of elongated nonmetallic inclusions such as manganese sulfides. However, SSC crack of API grade J-55 initiated and also propagated along the pearlite band nearly perpendicular to the specimen surface of heat affected zone (HAZ) of both upper and lower half regions of the full thickness due to the globular morphology of nonmetallic inclusions such as calcium sulfides in WFZ produced by Ca treatment. And the unstable crack propagation was arrested in the midthickness region due to the orientation change of pearlite band nearly by 90˚, which gives rise to the occurrence of the stripes in the SSCC fracture surface. The SSCC kinetics of line-pipe steel weldments have been divided into the three distinct stages, namely crack incubation, slow crack propagation and unstable crack propagation stages, and were discussed in terms of nonmetallic inclusions and microstructures. It is suggested that the susceptibility to SSCC was rather affected by nonmetallic inclusions than microstructures.

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Influences of carbon and magnesia additions to CaO-base flux on the desulfurization behavior of liquid iron were investigated by using a Tamman electric furnace under N₂ gas atmosphere at 1350℃. The effects of either graphite or powdered coke on the behavior of desulfurization were found to be the same in this condition. The degree of desulfurization was improved with the addition of 10% MaO at the hard gas blowing, whereas decreased at the soft gas blowing, and also decreased with the addition of MgO over 10% under the gas flowrate of 1ℓ/min. On the other hand, the rate of desulfurization under this experimental conditions was found to be a first order reaction.

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For the purpose of investigating the graphitization mechanism of Al alloyed cast iron, Fe-Al-C system alloy was melted in high frequency induction furnace by changing the aluminum content, and crystallographically discussed the structure of Fe-Al-C cast iron. The results obtained in this work are summarised as follows. 1. With increasing the aluminum contents, in low aluminum range (0-4 wt.%), the mean and maximum graphite length were increased, and perovskite type carbide dud not appear. 2. Graphitization mechanism at the low aluminum range is thought to be controlled not only by decrement of iron-carbon bond energy and increment of carbon-carbon bond energy owing to the ionization energy difference between aluminum and carbon, but also by the rise of austenite-graphite eutectic temperature- 3. At the medium aluminum range (6-16 wt.%), the amount of carbide was increased with increasing the aluminum content, which is thought to be deduced from the fact that Al atom can not be the second nearest neighbor atoms each other in Fe₃ Al ordering. 4. As the aluminum content came to the vicinity of stoichiometric number of Fe₃ Al, carbide took the form of dendrite. 5. Graphitization ability at the high aluminum range (over 20 wt.%) is thought to be due to the decrement of the number of octahedral sites for carbon which is followed by FeAl ordering.

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Arsenic and antimony are not readily removed from crude copper by simple oxidation, as evident in coventional fire refining practice, and from thermodynamic considerations. Removal of various impurites from the liquid crude copper using flux has been studied thermodynamically and experimetally. 1. In the oxidzing atomsphere of oxidzed serlimetals, there exists a great stability for M₂O_5 than fort M₂O₃. 2 In the experiment of heating mixture of CaO-CuO-Sb systems, the compounts of Ca₂Sb₂O_7 were firmed at 1200℃. 3. Calcium choride is especially interesting because arsenic and antiominy have also been elimnated probably in the forms of calcium arsenate and antimonate.

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In the flux of CaO-CaF₂ system, effects of additives on the desulphurization of molten iron have been investigated by the flux injection method. The experiment was carried out in MgO crucible of induction furnace at 1350℃. The results are summarized as follows : 1. The degree of desulphurization and the fluidization of flux increased with increasing carbon contents. 2. The addition of Al promoted the desulphurization reaction and the η_(ca), but reduced the Si-loss in comparison with the addition of carbon. 3. The sulphur distribution ratio increased with increasing basic components and decreasing the manganese distribution ratio.

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Three sheet steels were uniaxially prestrained and tension tested in 15˚intervals to the prestrain axis In the subsequent tests after prestrains higher than about 7%, the residual uniform strains were very small in orientations beyond 30˚ to the prestrain axis. Total strains to failure also decreased in these noncoaxial orientations but the tendency was not as marked as in the uniform strain. The subsequent flow curves in the noncoaxial directions skewed no sharp yield points. The work-hardening rate was initially very high but decreased rapidly, which resulted in a large decrease in uniform strain.

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