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The hot salt corrosion of 99.995% pure Ni. IN-100 and IN-713LC alloys by Na₂So₄ melt has been studied at 910℃ in Pt catalyzed 0.2% SO₂-O₂ gas atmosphere. The values of corrosion current density Icorr., measured by the Tafel method increased in IN-100 and IN-713LC alloys after galvanostatic treatment, but decreased in Ni. These effects were interpreted in terms of the change n the basicity of the melt and the protectiveness of the oxide scale during polarization. The values of Icorr. measured after galvanostatic treatment were found to have a correlation with the parameters such as the activation potential and the stabilized potential after the cathodic treatment. However, these parameters and the Icorr. values measured either prior to or after galvanostatic treatment did not correlate well with the long term corrosion resistance of the alloys.

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The Fe_(99)Cu₁ alloy was prepared by the arc melting and annealed at 743 and 823K during the various annealing time. The local structure change around Cu atoms was observed using the fluorescence XAFS because of dilute Cu atoms. In initial stage, the Fourier transform of Cu K-edge XAFS spectra still retain the bcc structure because Cu is substantial in the bcc α-Fe matrix. At the intermediate annealing at 743 and 823K the Fourier transform of Cu K-edge XAFS spectra indicates a distribution of bcc and fcc Cu precipitate dominated by bcc structure. Continued annealing to 700 and 550h at 743 and 823K results in a further decrease in the amplitude of all the peaks in the Fourier transforms of the Cu K-edge spectra. The Fourier transforms more closely resemble those typical of a fcc structure than a bcc structure. This paper proves that fluorescence X-ray absorption spectroscopy can be successfully used to probe the local structure of highly diluted atoms in metallic alloy.

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For the purpose of investigating the applicalibity of AISI TI5 grade high speed steel for metal injection molding. The mixed elemental and prealloyed powders were sintered in a vacuum and N₂-20%H₂ atmosphere. The debinding rates of the prealloyed and the mixed elementa powder were 92.5% and 105.3%, respectively. It is considered that the fine pores existed on the surface of the prealloyed powder caused the binder to remain in the powder by a capillary action during the debinding process. The carbon contents of the prealloyed and mixed powder were 2.01wt.% and 1.79Wt.%, respectively, after debinding. When sintered at an optimum temperature in a vacuum, the rate of carbon loss after the sintering for the prealloyed and mixed elemental powder showed a little difference of 19.5% and 17.9%, respectively. When sintered in N₂-20%H₂ atmosphere the carbon loss for the mixed elemental powder was found too high, over 86%, to effectively control the carbon content. The maximum sintered densities after the vacuum sintering were above 97% for both mixed and prealloyed powder whereas the sintered densities of the mixed and prealloyed powder after N₂-20%H₂ sintering were 94% and 77.1%, respectively. Therefore it was difficult to apply the prealloyed powder into the metal injection molding process in the atmosphere of N₂-20%H₂gas.

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Microstructure and high temperature deformation behavior of austenitic Ni-Cr-Mo-Ti steel have been investigated. This steel was one of the precipitation hardenable Fe-base superalloys which is used for the high temperature applications. Composition of the steel used in this investigation was 26.5Ni-15.9Cr-1.3Mo-2.2Ti(wt.%). The ingot prepared by the vacuum induction melting and electro-slag remelted process was homogenized and hot forged followed by the solution and aging treatments. The main precipitate after aging treatment at 720℃ for 16 hrs was small spherical γ(fcc Ni Ti). Cylindrical compressive specimens of 8mm diameter and 12mm height were machined from the hot forged plates. Compressive tests were carried out in a compression testing machine equipped with a specially constructed testing apparatus. A little changes were observed in the microstructure of specimen surface after compression tests because of the low deformation due to the friction between the specimen and the specimen jig. On the contrary, large changes in the microstructure of center region were caused by the compressive deformation. This deformed structure was dynamically recrystallized at the high temperature of 1000℃ and low strain rate of 10^(-2)sec^(-1). The strain rate sensitivity and activation energy for the hot deformation were also studied.

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In order to realize hot direct rolling(HDR) process, it is prerequisite to produce high temperature slabs by the continuous casting process. For this purpose, two-dimensional mathematical model has been used to estimate the change in slab temperature from the meniscus to the edge heater. The secondary cooling patterns have been modified to obtain slabs with high temperature via slab surface temperature measurements and thermal analysis of the slabs. The effect of the thermal insulator on slab temperature has been studied to find out the optimum condition for obtaining slabs with higher temperature. As means to investigate the temperature profile of slab in the edge heater and to validate mathematical model, temperature of slab with embedded thermocouples was measured in the edge heater. Based on the above temperature controlling methods, the HDR has been operated successfully.

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Damping capacity and mechanical properties were investigated with respect to the amount of cold rolling for an Fe-23%Mn alloy. Maximum damping capacity was observed around 10% reduction at strain amplitude range of 2×10^(-4)∼4×10^(-4). The internal stacking faults in ε martensite plates are considered the most important damping source in this strain range. However, the strain amplitude range of 5×10^(-4)_∼6×10^(-4) higher than that showed the maximum damping capacity around 2% reduction. This means that γ/ε interface is the principal damping source in this strain range. Hardness increased continuously with the amount of deformation. Especially, when deformed at the range of 10%∼20%, the hardness increased steeply due to the work hardening as well as the stress-induced martensitic transformation. Mechanical properties such as yield strength and tensile strength were increased with the increase of cold work, but elongation showed reverse tendency. The 10% cold rolling showed the best combination of damping capacity and tensile strength.

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The corrosion characteristics of AZ91 Mg alloy were studied in various anion-containing solutions. Through weight loss test, changes of corrosion potential under open circuit condition, changes of corrosion current density under potentiostatic condition and optical microscopy, effects of various anions on corrosion behaviours of AZ91 Mg alloy were investgated. It was found that chloride ion had two different effects on corrosion behaviours of AZ91 Mg alloy. When only chloride ions exist in water solution without any other anions, those make pitting corrosion, accelerate more rapidly. But when small amount of chloride ions exist with sulfate or nitrate ions, those make pitting corrosion occur slowly rather than in only sulfate or nitrate ions containing solution. These results were interpreted as competitive adsorption between chloride ion and sulfate or nitrate ion.

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The application of electromagnetics to the material processes has aroused keen interests. Especially EMC(Electromagnetic Moldless Casting) is one of the most important technologies. EMC is a kind of electromagnetic confining system which includes fluid dynamic, solidification phenomena in addition to electromagnetic phenomena. In electromagnetic process of material, the numerical simulation is very important than any other common process. In the case of electromagnetic confining process, numerical simulation still has many difficulties. This study deals with the method of numerical analysis for electromagnetic confining system. Selected system is EML(Electromagnetic Levitation), which shares many similar properties with EMC. A seires of parametric studies conducted with this system provided understanding about the effects of main parameters. For example, the role of dynamic pressure on the determination of the shape was found almost negligible in EML.

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Emulsification process caused by mechanical agitation of the molten magnesium and NaCl(50 ㏖.%)-KCl salt was employed to prepare salt coated magnesium powders (SCMPs) used as a hot metal desulphurizer. Spherical magnesium particles were observed to be entrapped in the salt when the two liquids kept at 700-820℃ were agitated at 1,000-2,500 rpm and quenched. The friable salt was then easily removed by pulverizing and sieving, so that spherical SCMPs could be prepared. This indicates that molten magnesium droplets are dispersed in the molten salt. The degree of dispersion increases with increasing temperature and agitation time. Process efficiency was evaluated by recovery. Recovery means the portion of the magnesium in SCMPs with size of 300-2,500 ㎛ to the input magnesium, because SCMPs in this size range can be directly used as a hot metal desulphurizer. The recovery decreases with increasing temperature and agitation speed. The maximum recovery of 93.4% was achieved when the two liquids kept at 750℃ were agitated at 1,000 rpm for 15 minutes. An novel phenomenon was also observed that the average particle size of SCMPs remained unchanged for about 25 minutes after cease of agitation. Strong stability of a magnesium-salt emulsion makes the emulsification process suitable for practical production of SCMPs.

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The activities of iron oxides in CaO-MgO-Fe₁O-SiO₂-MnO-∑M_xO_y slags which are saturated with MgO and in equilibrium with molten iron were discussed in terms of the concept of the regular solution model as well as an ionic solution model suggested by Ishiguro & Okubo to quantify the oxygen potential of the BOF steelmaking slag. The results obtained are as follows; 1) By the regression analysis of the FeO activity with slag composition based on the Ishiguro and Okubo`s ionic model, the following equation was obtained, logγ_(FeO) = -9.609N_(O^(2-)) + 12.58(N_(O^(2-))² - 6.062(N_(O²))³ + 2.935 2) From the analysis of the activities of FeO and Fe₂O₃ by introducing the regular solution model, it was revealed that the model is not adequate to quantify the activities of FeO and Fe₂O₃ of BOF steelmaking slag yet.

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