발간논문

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The effects of aluminum on the strength of cast iron has been investigated with the variation of the amount of aluminum added and of the degree of carbon saturation. All the specimens with various compositions were prepared on a laboratory scale from the original raw materials. It was found that addition of aluminum, about 0.7%, to the ordinary cast iron showed the most strong influence on the tensile strength of this material giving a value of 33㎏/㎟ at the degree of carbon saturation of 0.85, which is about 15% higher value than the tensile strength for those not containing aluminum at all. The tendency of such increase was more distinct in the hypoeutectic composition range. On the contrary, in case with the spheroidal graphite cast iron the tensile strength was decreased with the addition of aluminum. Microscopic examination and EPMA revealed that the type of graphite effected by the addition of aluminum was the major to make the above results come out.

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For the purpose of quality improvements of tool steels, tool steels were remelted by the electroslag remelting process. The remelted ingot did not show any visible defects. A considerable removal of non-metallic inclusions and efficient desulfurization were observed. Tempered structure of SKD 11 remelted showed more finer and discontinuous eutectic carbide bands in comparision with those of the conventionally melted. Furthermore, precipitated carbides in the matrix exhibited fine, dense and uniform distributions. Both carbides were identified to be M_7C₃type. The toughness, in particular, transversal toughness was remarkably improved, resulting in the reduction of anisotropy.

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Cyclic oxidation of iron sheet has been studied in the temperature range of 850℃∼1050℃ at various partial pressures of O₂ and H₂. Rates of the first and second stage oxidation could be described in terms of a solid state diffusion of iron ion through wustite layer, and expressed by the equation of t/Δw=Δw/k_p+1/k₁ where t, Δw, k_p and k₁ are reaction time, weight change, and parabolic and linear rate constant, respectively. The activation energy for oxidation was found to be about 27㎉/㏖. Results revealed that rate of oxidation follows simply the flat surface geometry, whereas at the later stage oxidation was characterized by the oxidation of residual iron island in product layer. Such an effect became more significant by repeated oxidation and reduction. Reduction rate of iron oxide was faster in temperature range of 900℃∼950℃ at which the product layer had proper pore structure. At the later stage of reduction the rate was also characterized by the reduction of residual wustite island in the outer product layer.

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The extraction of Th(IV) from an aqueous solution of constant ionic strength with Versatic Acid-10 dissolved in kerosene was studied as a function of pH and the concentration of Th(IV) and organic acid. The effects of sulphate and chloride ions on the extraction of the Th(IV) were briefly examined. It was revealed that (ThR₄·2RH) is the predominant species of extracted Th(IV) in the Versatic Acid-10. The chemical equation and the apparent equilibrium constants thereof have been determined as follows: Th^(4+)+3(R₂H₂)_(org) = (ThR₄·2RH)_(org)+4H^+ K_(Th)=(ThR₄·2RH)_(org)·(H^+)⁴_(aq)/(Th^(4+)_(aq)·(R₂H₂)³_(org)=1.1×10^(-11)

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Fe-25%Ni-0.2%C alloy, Ms temperature of which was controlled to -16℃ so that strain-induced martensite could be transformed at room temperature, was prepared by high frequency induction furnace and plastically deformed to various deformation degrees by rolling at room temperature. Then the aspects of strain-induced martensite transformation, athermal martensite transformation and austenite stabilization were studied. Strain-induced martensite was not produced up to 20% deformation degree. However, in the range of higher deformation degree than 20%, the amount of the martensite increased with increasing deformation degree. The morphology of strain-induced martensite appeared to be massive type, regardless of deformation degree. Athermal martensite, transformed by subzero cooling the deformed austenite, was produced preferentially near the prior strain-induced martensite and the athermal martensite plate became smaller as the deformation degree was increased. The austenite stabilization was increased with deformation degree in transformation of austenite to athermal martensite.

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