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The autoclave corrosion test was performed on the zirconium alloys in various aqueous solutions of LiOH, NaOH, KOH, RbOH, and CsOH, The oxide growth mechanism at metal-oxide interface was evaluated using SEM on the samples which were prepared to have an equal oxide thickness at pre-transition regime and post transition regime. At a low concentration (4.3 mmol), the corrosion rate was gradually decreased as the ionic radius of cation increased. However. at a high concentration (32.5 mmol). the corrosion rate was significantly accelerated in LiOH solution and slightly in NaOH solution. The morphologies and the growth rates of the oxides at metal-oxide interface were dependent on the ionic radius of cation even if the samples have an equal oxide thickness in LiOH, NaOH, and KOH, The grain orientation in Zr matrix also controlled the growth rates and the morphologies of the oxides. Therefore. It is considered that the oxide growth mechanism of Zr alloys can be controlled by the cation incorporation into zirconium oxide.

keyword : Zirconium alloy, Oxide, Nuclear Fuel cladding, LiOH, Corrosion, Oxide microstructure

Recently, the finite element analysis has been coupled with recrystallization modeling equations in the prediction of recrystallization behavior during forming processes at elevated temperatures. In this study, a forging process, converting a cylindrical billet into a gear blank through buster, blocker and finisher operations was analyzed by a rigid thermoviscoplastic finite element method. The Sellars equation was applied to the thermo-mechanical histories obtained from the analysis to predict the recrystallization behavior occurring during the forging process. As a result, it was found that the dynamic recrystallization would occur 3-4 times, 1-2 times and none in the buster, blocker and finisher operations, respectively, and that the static and metadynamic recrystallizations would occur between operations. It was also found that austenite grain sizes measured at several moments during the forging process were in good agreement with predicted ones.

keyword : Finite element method, Sellars equation, Recrystallization, Hot forging, Austenite grain

The effects of Bi2O3 addition to PbO/B2O3 solvent on the growth kinetics of (TbBi)3(FeAlGa)5O12 single crystal thick films grown by liquid phase epitaxy(LPE) were investigated. The growth of garnet films from PbO/B2O3/Bi2O3 solvent was higher than that from PbO/B2O3. Decreasing molar ratio R1(Fe2O3/Y2O3) and increasing R3 (PbO+BiO3/B2O3), the growth rate of garnet thick films becomes large. The growth rate of single crystal thick film can be expressed in terms of the driving force for growth of garnet thick film In(Cl/Ce) and the resistances to growth δ/D + Cl/K. The growth rate of single crystal thick film depends mostly upon the resistances to growth rather than the driving force for growth. There was a large decrease in the crystal growth resistance δ/D + Cl/K as low as 103sec/cm, and the bulk supersaturation In(Cl/Ce) is decreased by 101~102 if Bi2O3 is added to PbO/B2O3 solvent.

keyword : LPE, Liquid phase Epitaxy, Garnet, Single crystal, Growth kinetics driving force, Bulk supersaturation, (TbBi)3(FeAlGa)5O12 single crystal thick filims

During fabrication and operation of the industrial steel structures, many defects and/or cracking occurred mainly in the weldments. Various causes can exist but the initiation and propagation of HAC(Hydrogen Assisted Cracking) in the weld HAZ(Heat Affected Zone) must be prevented because of its characteristics of catastrophic failure. This study described an investigation of a possible mechanism of HAC in the conventional structural steel weld HAZ. Proposed mechanism was based on simulation of PMHAZ(Partially Melted HAZ), cracking test of HAZ, diffusible hydrogen content. thermal history of welding, preheat treatment and the extensive microstructural and fractographic examinations. The fracture surface had mixture of intergranular and/or quasi-cleavage and/or microvoid coalescence morphologies which are typical in the hydrogen related fracture surface. The fracture mode was related to the stress intensity and hydrogen content at crack tip. HAC occured by microscopic plasticity. HAC initiated in the PMHAZ of martensite/bainite structures with concentrated hydrogen content along the melted and resolidified grain boundary, and propagated along the fusion line adjacent to the PMHAZ.

keyword : HAC, PMHAZ, Microscopic plasticity, Stress intensity factor, Weld thermal cycle. Fracture morphology, Gleeble simulation, Diffusible hydrogen concent, Implant cracking test

This study was performed to evaluate basic characteristics of electron beam weldability for high strength Al alloys. The Al alloys used ere non-heat treatable A5083-O and heat treatable A6061-T6, and A7075-T6. The principal welding process parameters, such as accelerating voltage, beam current, welding speed and chamber pressure were investigated. The dimension and microstructure of welds were evaluated with OLM, and SEM (EDAX). In addition, weldability variation (cracking and porosity formation) due to process parameters was also evaluated, The EBW machine used for the welding development has a maximum operating voltage of 150kV with a beam current of 100mA, giving a maximum output power of 15kW. Welds had many discontinuities such as cracks, cold shut, root porosity, and spikes. The tendency to form weld discontinuities was strongly dependent upon the EB process parameters and chemistry. Although three Al alloys were welded under the same conditions, alloying elements had an important effect on the dimension of the weld and weldability. A6061-T6 shoed lower depth than the others, The variation of the weld depth was found to be sensitive to the vaporization tendency of the alloying elements. Si, which is the major element of A6061-T6, was more difficult to vaporize than other elements (such as Al, Mg and Zn).

keyword : Aluminum alloy, Electron beam welding, Aspect ratio, Beam active parameter, Softening, Porosity, Spiking, Crack

The smelting reduction behaviors of chrome ore by Fe-C melt and graphite in the molten slag of CaO-MgO-Al2O3-SiO2, system in which a dissolution as a rate-controlling process was excluded have been studied to determine rate-controlling steps in the temperature range of 1475 to 1650℃. Samples were investigated using ICP-AES and EDS. The results were as follows: In the smelting reduction experiments by Fe-C melt, the apparent activation energy was found to be 104kJ/mol and independent of initial chromium content in the Fe-C melt. Also, mass transfer coefficient was constant regardless of the variation of slag volume. Therefore, it was concluded that the reaction rate was controlled by mass transfer of chromium oxide through the slag boundary layer. In the reduction experiments by graphite, the apparent activation energy was found to be 185kJ/mol. The reaction rate under these conditions was found to be mixed control of chemical reaction involved in breaking the C-C bonds in the carbon lattice and a mass transfer of chromium oxide in the slag phase. Dissolution of chrome ore was interrupted by protection of a crust of high-melting MgAl2O4 euhedral crystals which had precipitated from the molten slag when the content of (MgO+Al2O3) was higher than about 3lwt% at 1500℃, 38wt% at 1550℃, 45wt% at 1600℃ and 5lwt% at 1650℃, respectively.

keyword : Smelting reduction, Chrome ore, Rate-controlling step, Apparent activation energy, Mass transfer

Two cylindrical models for the reduction of iron ores are proposed. One model is for the long cylindrical particle and the other for disc-shape particle. The determination method of the rate parameters included in these models are illustrated. The models are applied to the reduction data for a long-cylindrical pellet of which the height is 2.3 times longer than the diameter and a disc-shaped pellet of which thickness is as 0.34 time thick as the diameter. According to the application results, the fractional reductions calculated by the proposed models are on the good agreement of the observed ones and the reasonable values of the rate parameters are obtained.

keyword : Iron ore, shape, Gaseous reduction, Unreacted-core model, Long cylindrical model, Disc model, Non-spherical particle

Trial tests were made to examine the effects of casting pattern on the surface quality of SCM 440 billets in horizontal continuous casting. The withdrawal cycle and the casting speed were varied 40~80 cycle/min and 0.7~1.0 m/min respectively. The main results obtained could be summarized as follows: When the withdrawal cycle is low, the withdrawal mark is irregular and the surface of billet is rough due to the formation of the lapped skin. The increase of withdrawal cycle results in the uniform withdrawal mark and the fine surface of cast products. The very low or the very high withdrawal cycle is essential for the prevention of transverse cracks on surface and corner cracks of the billet. For the case of the withdrawal cycle of 50 cycle/min, the effects of casting speed on the formation of transverse cracks and on the surface roughness could not be justified accurately due to the formation of lapped skin. The casting speed does not affect, however, the formation of the corner crack.

keyword : Horizontal continuous casting, Billet, Withdrawal cycle, Casting speed, Surface quality, Transverse crack, Corner crack