발간논문

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Considerable increases in the initial permeability as well as the maximum permeability are known to be observed by giving magnetic annealing in 50-50 Ni-Fe alloys. In order to investigate the increases in the initial permeability, variation in Ni composition and the cooling rate from high temperature annealing are given to obtain the variable magnetocrystalline anistropy, and the relationship between these and the induced uniaxial anisotropy were investigated. From these results it is concluded that the high initial permeability was obtained when two anisotropy values are similar. Hypothesis to explain the mechanism are offered.

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The crystal structure of the Ni₃Nb phase which appears in an overaged Inconel 718 is examined by selected area electron diffraction. It is confirmed that the phase has an ordered close packed layer structure with a periodic stacking modulation of the stacking order which can be specified as 2H structure. The specific stacking order of the 2H structure can be expressed as AB`AB`…… The atomic arrangement on the close packed layer is identical with that of one dimensional long period superlattice of M=1 type superstructure.

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As a study of low temperature thermally activated deformation mechanism in polycrystalline copper, flow stress vs temperature curves of various prestrain hardened samples were determined for two different strain rates. Flow stress vs temperature curves showed linear relationship. Rapid change of the flow stress near 200°K reported by Adams and Cottrell, and Seeger, could not be observed. Assuming the deformation is controlled by thermally activated vacancy formation, experimental results were analysed according to Seeger`s theory, and activation volumes, length of dislocation segments, dislocation densities and athermal stresses for various prestressed states were determined.

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The crystallographic system, lattice parameter and possible space group of intermetallic compound Y₃Cd₄Zn_7 were determined through oscillat`on and Weissenberg single crystal photographs and density measurements. This compound is orthorhombic with a=29.19±1, b=29.56±2. c=51.86±2A˚. and possible space group of Fdd 2.

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Kinetics and mechanism of direct reduction of sintered ferric oxide and of calcium ferrites containing various amount of lime placed in carbon powders at the temperature range of 800℃ to 1,200℃ were studied. The findings from the experimental work are summarized as follow: (1) The reaction of the sintered ferric oxide pellet was clearly controlled by chemical reaction at the metal-oxide interface and the reduction rates followed the [1-(1-R)^⅓] relationship. (2) The reduction rates of calcium ferrites except for dicalcium ferrite did not follow the above relationship and a general form [1-(1-R)^((1-n))] for the rate calculation of the calcium ferrites was introduced from the experimental data. (3) Dicalcium ferrite has an accelerating or inhibiting effect on the reduction of calcium ferrites. An empirical equation was shown to describe the experimental data. (4) The reaction at the iron-ferrite interface was not similar for the calcium ferrites. An adaptable reaction model was proposed to explain the calcium ferrites reduction. 1. Direct reduction of sintered ferric oxide pellets with carbon was characterized by reduction rates that were essentially independent of time during reduction over the temperature range of 900℃ to 1100℃. The rates followed the [1-(1-R)^⅓] relationship which developed by McKewan. 2. The reduction rates of sintered calcium ferrite pellets with various lime additions did not follow the [1-(1-R)^⅓] relationship with the exception of dicalcium ferrite (2CaO·Fe₂O₃). A general form adaptable for the rate calculations [1-(1-R)^((1-n))], was introduced from the experimental data. 3. At the temperature range of 900℃ to 1100℃ in carbon the reduction rate of dicalcium ferrite is nearly one-third that of ferric oxide, a half that of mono-calcium ferrite, and nearly one-fourth that of calcium diferrite. 4. Reduction rates for calcium ferrites are not a linear combination of those for each calcium ferrite, since dicalcium ferrite has an accelerating or inhibiting effect on the reduction rate of calcium ferrites. The accelerating effect increases with increasing lime content up to the lime percent identical with CaO·2Fe₂O₃ and decreases rapidly with increasing lime content up to about 30 percent. Inhibiting effect was occurred in the calcium ferrites containing lime more than 30 percent. 5. An empirical equation, K=[K₁N_(Fe2O3)/(1+K₃N_(2CaO·Fe2O3)+K₂N_(2CaO·Fe2O3), that described the experimental data was shown to be equivalent in form to a derived equation based on the theory of activated adsorption. 6. The rate-controlling reduction reaction, that is, reaction at the iron-ferrite interface, is not similar for the calcium ferrites. Two macographs and a concentric striped reaction model were proposed to verify the fact and to give interpretation for the mechanism of calcium ferrites reduction.

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Forge-quenching heat treatment of medium carbon steel mechanical properties, such as toughness and impact value, is widely used for quality improvement of hot working parts. In the present study, oxidation-decarburization phenomena were investigated in relation to heating temperature for closed die forging, and forge-quenching conditions were discussed in order to improve mechanical properties and to obtain uniform micro-structure. Decarburized depth was measured to be 0.5m/m at 1,200℃, 1-3m/m at 1,300℃, and 0.5m/m at 1,400℃. Excellent hardenability was obtained at high quenching temperature(950-1,050℃) by application of its self-heat in closed die forging of one heat system. Also uniform tempered structure and improved toughness were obtained at high tempering temperature(550-600℃). Thus, impact value of tempered medium carbon steel that was forge-quenched was doubled as compared with that of ordinary quenched steel. In view of the above results, it may be concluded that application of the forge quenching method makes it possible to replace alloy steel parts with carbon steel or to reduce the weight of carbon steel parts.

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Slag-metal reactions for iron-tungsten melts under CO·CO₂ gas atmosphere are investigated at 1,650℃ and 1,750℃. When Pco/Pco₂ is more than 5.0, slag-metal distributions of tungsten, [W]/(WO₃), can be determined by the following reaction equilibriclm. (WO₃)+3[Fe]=[W]+3(FeO) log K_f=-22,300/T+10.66 The equilibrium constant, K_f, for the slag-metal reaction slightly decreases when Pco/Pco₂ is less than 3.3 at 1,650℃, and suddenly decreases below Pco/Pco₂=5.0 at 1,750℃. (FeO) contents of slag phase increase, as Pco/Pco₂ decrease according to the following reaction, [Fe]+CO₂=(FeO)+CO, An estimation for slag-metal reactions on open melting is as follows; (WO₃)+2.5[Fe]=[W]+2.5(FeO_(1.2)) log K_i=-14,288/T+6.31

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Synthetic zinc ferrite was reduced with hydrogen by the use of thermal balance, in order to understand the course of the reduction. At the initiation of the reaction, zinc ferrite is reduced to Fe₃O₄ and Zn(g) as following equation at 700-900℃ and under 30∼55×10^(-3)mmHg of hydrogen pressure. 3ZnFe₂O₄+4H₂=2Fe₃O₄+3Zn+4H₂O. Activation energy associated with the reaction was determined to be 17.4㎉/㏖. Zinc ferrite decomposes to Fe₃O₄ and Zn(g) at 700∼950℃ under the pressure of 10^(-4)mmHg. And this decomposition process is considered to be a predominant factor in the reduction of zinc ferrite.

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