발간논문

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After obtaining two different distribution of austenite, fibrous and dispersed structure, in the ferrite matrix by changing the heat treatment paths, the amount of austenite, growth process of austenite, tensile properties and stress corrosion cracking(SCC) resistance of Fe-22Cr-7Ni-3Mo duplex stainless steel were investigated. The procedures of austenite formation from ferrite single phase appeared to the following sequence : 1) formation of austenite film along the grain boundaries of ferrite 2) growth of spike-like austenite from the ferrite grain boundaries to the grain interiors 3) formation of rod-like austenite at the ferrite grain interiors. The strength and elongation of the duplex stainless steel with fibrous structure were superior to those with dispersed structure. With increasing ferrite content, the increase in yield strength for the specimen of fibrous structure was remarkable. The SCC resistance of the duplex stainless steel was closely related to ferrite content. The SCC resistance of the dispersed structure in the duplex stainless steel appeared to superior compared to that of fibrous structure. With decreasing pH in 3.5% NaCl Solution, the SCC resistance decreased down to pH 5. On the other hand, the SCC resistances were nearly unchanged below pH 5.

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Joining of 304 stainless steel to alumina ceramics using Cu-48%Ag-3.8%Ti brazing alloy was made in vacuum furnace. Effects of Sn and Al addition in Cu-Ag-Ti alloy, brazing temperature, and brazing time on interfacial reactions and bonding strength of alumina-304 joints were investigated. Bonding strength was determined by fracuture shear loading. The strong bonding is at tributable to the formation of Cu₂Ti₄O by redox reaction between Al₂O₃ and Ti at alumina interface. Thickness of interfacial reaction layer was increased and bonding strength was decreased as brazing temperature and brazing time were increased. Interfacial reaction was accelerated by Sn addition and was restrained by Al addition in brazing alloy.

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A numerical methodology, which can analyze the two-dimensional phase change process of a metal with natural convection effect in the liquid, is developed and applied to the solidification process of pure aluminum in a confined rectangular mold. Stable numerical solutions are obtained for the solidification processes of a pure aluminum in a confined rectangular mold, which proves the applicability of the present numerical analysis for the phase change processes of a metal.

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The effects of bascity and MgO in iron ore sinter on softening and melting down behaviors under load were investigated. The optmum range of basicity in iron ore sinter for softening and melting down properties under load was considered 1.7∼1.8, which was confirmed on the phase diagram of CaO-SiO₂-MgO-10% Al₂O₃ system. Mineral phases melting at high temperature were formed with the increase of MgO in iron ore sinter, which affected the enhancement of starting temperature for softening and melting down, and the optimum concentration of MgO was confirmed at the vicinity of 1.8wt.% The melting down phenomena of iron ore sinter of homogeneous slagmelt processes during softening and melting down were able to be analyzed on the phase diagram of CaO-SiO₂-MgO-10% Al₂O₃ system.

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In order to investigate the influence of cold-working degree and thermomechanical treatment cycle on the critical current density(Jc), magnetic field dependence of Jc and microstructure of Ag sheathed Bi-Pb-Sr-Ca-Cu-O ceramic superconducting tapes, various types of specimens were prepared by powder-in-tube method. By means of XRD and EPMA, major 2223 phase, minor 2212phase and impurity phases such as Ca₂PbO₄, CuO and (Sr, Ca)₃Cu_5O_x were confirmed to exist in oxide layer. It was made clear that the critical current density strongly depends on the degree of cold-working and the number of TMT cycle. This is attributed to the refining of microstructure such as densification and preferred orientation of c plane in 2223 superconducting phase. Magnetic field dependence of critical current density could be improved for BPSCCO tapes prepared with the higher degree of cold-working.

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The temperature profile of Czochralski single crystal system is simulated considering the fluid flow and surface radiation heat transfer. For radiation heat transfer, view factors of surface elements are considered. Two phases(solid and liquid) are treated as a continuous phase by assigning artificial large viscosity to the solid phase and latent heat is considered by heat capacity method. All results are calculated under the quasi-steady state assumption.

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Effect of molybdenum on the combination of strength and ductility in age-hardenable Fe-8Ni-4Mn-4.5Cr-1Al-1Cu alloys has been studied by investigating age hardening, tensile properties, fractography and precipitation behaviors by scanning transmission electron microscope and energy dispersive X-ray spectroscopy. The molybdenum free alloy was surveyed severe embrittlement with aging due to preferred precipitation of large precipitates at the prior austenite grain boundary and martensite lath boundary, on the other hand, the alloy containing 2wt.% Mo showed a good combination of strength and ductility by preventing formation of large precipitates at these place, and then precipitating finely dispersive precipitates at the intragranular. But considerable drop in ductility was due to precipitation of large Laves Fe₂Mo phase at inter-and intragranular in alloys containing 4-6wt.% Mo.

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A study has been performed to investigate the reactions of artificial ilmenite with Na₂CO₃ in the mixed gas atmosphere of CO₂and O₂ and by means of T. G. and D. T. A. The results obtained from the experiment were as follows ; 1) Chemical analysis has shown that Na-Ti compounds and Fe-oxides could be separated from ilmenite by Na₂CO₃. 2) When the reaction for FeTiO₃-Na₂CO₃ systems is carried out in the CO₂ atmosphere under specific stoichiometric proportions, it has been found that the products were Na₂Ti₃O_7 and Na₂Ti₃O_7 with their melting points of 10801℃ and 11451℃, respectively. 3) It is satisfactory to consider as 3FeTiO₃ system comes into being Fe₃O₄ for separation in magnetic method separated Fe-oxides from ilmenite.

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The solid state diffusion joining of ZrO₂ to Al 2024-SiC_w composite produced using Al 2024 interlayer was studied between 5101℃ and 570℃ for times range from 15min to 120min. Reaction bond consisting of ZrO₂/Al 2024 was dominated by the surface reaction with a very thin oxygen diffusion layer. On the composite side, Al 2024-SiC_w/Al 2024 joining was dependent on the volume diffusion of component elements. SiC_w. had not reaction to ZrO₂ and what was worse, prevented the Al 2024-SiCw/ZrO₂ joining. SiC_w. exhibited a high degree of preferred orientation due to extrusion processing showed random distribution after joining. And it could be explained that SIC_w in this composite moved freely in the solid plus liquid region of the phase diagram under the joining pressure. Fracture initiations of this composite could be detected from Sic. site. The maximum shear strength about 43㎫ was obtained in the joint bonded at 5401℃ for 60min under pressure 50㎫.

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The 7475 aluminum alloy sheets were subjected to a thermornechanical treatment to have superplastic characteristics. The superplastic specimens were tested in an Instron testing machine at constant cross head speeds to obtain a constitutive equation for an optimum superplasticity. The constitutive equation which offers the reference data for a constant-apex-strain-rate blow forming was established at 516℃ in the form of 0=K·ε˙^m. The equation was applied to the analyses of blow forming into cylinderical and conical dies. The theoretical models mainly followed Ragab`s analytic model about a constant-pressure blow forming but were modified into the models relevant to a constant-apex-strain-rate blow forming. A number of experiment have been conducted, involving the blow forming of superplastic 7475 Al alloy sheets into cylinderical and conical dies at 516℃ with the apex-strain-rate, 1.15 × 10^(-3)/sec. The theoretical results and experiments were compared and the problems such as cavity occurrence and dynamic grain growth were discussed.

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