발간논문

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This study has been carried out to investigate into some effects of single or multi addition of alloying elements and surface hardening treatment in order to increase the properties of wear resistance of gray cast iron during enlubricated dry sliding wear test. The results obtained from this study are as follows, 1. In the as-cast, gray cast iron alloyed with Cr, Mo, Mo-Cr, Mo-Cr-Cu, and Mo-Cr-Ni in additional sequence has shown fine pearlitic structures, decreased ferritic fraction and increased the hardness and the properties of wear resistance. 2. In the case of surface hardening treatment to gray cast iron alloyed with Cr, Mo, Mo-Cr, Mo-Cr-Cu and Mo-Cr-Ni in additional sequence has been shown to increase the surface hardness. The depth of hardened layer and the properties of wear resistance decreased the deviation of the wear rate with the addition of the alloying elements, as compared to as-cast. 3. The surface hardening treated specimens have removed the peak point toward the high speed direction and d creased the wear loss, as compared to as-cast, wear loss has decreased as the hardness values increased at the peak point.

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This study was undertaken to investigate the behavior of antimony in the Sb-Fe-Pb system. The phase relation and the antimony activity in the Sb-Fe-Pb system were determined at 1473K by using the isothermal equilihrium and isopiestic methods, respectively. The antimony activities in the Sb-Fe-Pb system determined from the present work and the phase relations obtained by Schuhmann method were utilized in calculation of the activities of iron and lead. The results showed that the concentration of iron has considerable effect on the antimony activity and that the activities of iran and lead exhibit positive deviations from Raoultian behavior in the low concentration range of iron and lead, respectively. The activity coefficients of elements for each of the binary Sb-Pb and Sb-Fe systems and the interaction parameters between solutes in antimony were calculated at 1473k. The activity coefficients of lead and iron in infinite dilution of antirnony at 1473k are as follows : ln γ_(pb) = ln 0.86 + 0.30 X_(pb) + 4.8X_(Fe), ln γ_(Fe) = ln 0.38 + 0.32 X_(Fe) + 4.8X_(pb) The solubility of lead in slag becomes larger with increasing the oxygen potential, and lead is much more soluble in iron silicate slags than in calcium ferrite slags.

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The relationships between the microstructure and dynamic fracture toughness, kid, were mainly investigated. In order to measure a fracture toughness using instrumented impact test, the necessity of low b1ow test for obtaining true energy by the compliance correction and Equivalent energy method were confirmed. It was proven that 2nd pass performance was capable of enhancing bond toughness. K_(Id) value decreased abruptly with increasing 1st pass heat input from 2.5KJ/㎜ to 9.0KJ/㎜. Bond toughness, however, improved noticeably when 2nd pass was carried, the heat input of which was between 2.5KJ/㎜ and 4.5KJ/㎜.

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The effects of slag competition and smelting temperature on the smelting reduction of chromite ore from South Africa was investigated using the high frequency induction furnace(100 Kg/Ch.). The results obtained are summarized as follows. Chromite content in molten slag were rapidly reduced when the slag`s basicity was higher than 1.0. The reduction rate of chromite in molten slag rapidly increased as the reaction temperature increased and the apparent activation energy of whole reaction was determined to be 143㎉/㏖. (MgO), and (Al₂O₃) in the slag which precipitated on the surface if chromite particle seemed to reduce the rate of reduction and dissolution of Cr oxide.

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An experimental investigation was carried out to clarify the behaviors of dephosphorization and desulfurization of carbon-saturated liquid iron by using the powdered flux of CaO-CaF₂-Fe₂O₃at 1350℃. Optimum compositions of flux were deduced from the series of investigations with variation if parameters such as the contents of Fe₂O₃ and CaF₂ and basicity index in the slag phase, and also initial silicon content in the liquid iron. It was found that the slag compositions of 40 to 50 wt% Fe₂O₃ higher basicity and lower CaF₂content were recommendable for the simultaneous removal of sulfur and phosphor from the melt. The beneficial effect of lower initial silicon content in the melt that observed in the dephosphorization was not similar1y applied to the desulfurization. It was also noticed that the presence of CaF₂ in the slag enhanced considerably both dephosphorization and also desulfurization evidently in comparison with that of CaCl₂, or Na₂CO₃. From the results of the present experiment, it is concluded that the high dephosphorization as well as desulfurization(70 % and 80 %, respectively) can be achieved by using CaO-CaF₂-Fe₂O₃ powdered flux under an Ar gas bubbling condition.

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Structures, crystallization reactions and kinetics of co-evaporated Tb-Fe and Gd-Fe alloy films were studied by transmission electron microscopy and differential thermal analysis. Amorphous films were formed for the Tb-Fe alloy with 24 to 55 at5 Tb. Alloy films with up to 24 at% Tb were composed of α-Fe microcrystals dispersed in an amorphous matrix. Alloy films with more than 55 at%Tb content some α-Tb crystals. The phenomena of preferential crystallization of α-Fe and α-RE microcrystals were interpreted using a hypothetical metastable eutectic system with of α-Fe and α-RE phases. The activation energies for the crystallization of the Tb-Fe and the Gd-Fe alloy films were the largest at near the composition of the Laves phase, TbFe₂ or GdFe₂.

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The effect of HfB₂ and ZrB₂ dispersoids on magnetic properties and microstructure of Fe-Nd-B magnetic powders as well as magnet has been investigated. Amorphous rapidly solidified powders of Fe-Nd-B alloys with 1 to 3 wt.% HfB₂(or ZnB₂) were prepared by the melt spinning process. Powders were consolidated by hot extrusion into fully densified magnet bars. The fine dispersoids of HfB₂(or ZrB₂) were formed while crystallizing of the amorphous material during hot extrusion process. The coercivity of Fe-Nd-B magnet increased as the dispersoids were formed. The microstructure of the extruded magnet indicated that the enhancement of coercivity was caused by grain refinement due to formation of fine HfB₂(or ZrB₂) dispersoids. In general, the addition of HfB₂(or ZrB₂) in crystallized powder were accompanied with the decrease in the remnant magnetization. Curie temperature of the Fe-Nd-B alloy was independent of the amount of HfB₂ in the composition range investigated.

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The effect of thermomechanical treatment(TMT) on the fatigue and wear properties of a rail steel was examined. The sample was austenitized at 1090℃ and then transformed to pearlitic structure by holding at 700℃ for 17 hr. Samples were then cold rolled to the thickness of 25 ㎜. These samples were heated at 760℃ for 5 to 50 sec. The reduction of pearlitic interlamellar spacing and refinement of the austenite gram size were achieved by the TMT. The sample heated at 760℃ for 10 sec after cold rolling showed better fatigue strength and wear resistance than both the sample without cold rolling and the one with only cold rolling treatment. The fatigue cracks tend to orient them selves parallel to pearlite lamellars. This could be explained by Miller-Smith model. The higher fatigue srtrngth the sample has, the higher wear resistance it has.

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In this study the microstructures of rapidly solidified Al-4Li-2Cu, Al-4Li-2Cu-0.3Zr alloy powders and the mechanical properties of extrudates have been investigated. The alloy powders have been prepared by means of gas atomization technique. Rapidly solidified alloy powders show the icosahedral intermetallic compounds of five-fold symmetry which are randomly distributed in cell-boundaries of α matrix. Both of these alloy powders, heat-treated at 200℃ for 1 hour have fine δ` phases homogeneousy precipitated throughout the matrix. The precipitation of δ` phase within the α matrix increases the microhardness. After the heat-treatment at 300℃ for 1 hour, all the δ` phases has been transformed to icosahedral intermetallic compounds. Room temperature tensile strength of Al-L₁-Cu and Al-Li-Cu-Zr extrudates are 325MPa, 350MPa, respectively. Under the isothermal aging condition at 180℃, Al₃Li & A1₃Zr are precipitated in the matrix. The tensile strength of Al-Li-Cu extrudate is 425 MPa and that of Al-Li-u-Zr extrudate is 430MPa after 24 hours isothermal aging at 180℃

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The effect of plastic deformation in thermomechanical treatment on the strength and stress corrosion crack propagation of overaged 7050 Al alloy was investigated by examining mechanical properties, clack growth rates and microstructure. As the amount of plastic deformation increased, the strength and the stress corrosion susceptibility decreased and the susceptibility of SCC was not influenced considerably by the amount of plastic deformation in the range of above 5% reduction ratio in area. The strength deterioration was attributed to the coarsening of precipitates in the matrix which was accelerated by the plastic deformation, while the SCC characteristics was due to the preservation of precipitates morphology in the brain boundaries in spite of increasing plastic deformation. This is thought to be resulted from the difference of dislocation densities at the matrix and the grain boundaries.

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