발간논문

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Strength and impact toughness of tempered Fe-Ni-C martensites with different substructures were studied. One (Fe-4.4%Ni-0.23%C alloy) was lath type martensite with dislocation substructure and the other (Fe-25.3%Ni-0.23%C alloy) was plate type martensite with internal twins at midrib. The impact toughness of Fe-Ni-C lath martensite showed a gradual and continued increase with tempering above about 100℃, while that of twinned martensite, after revealing a rapid increase at an early stage of tempering (up to about 160℃), showed no further improvement with increasing tempering temperature. The poor impact toughness of twinned Fe-Nice tempered martensite is considered to be due to the carbides preferentially precipitated at twin boundaries.

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In joining spacers and pads on the cladding tube of the Candu type nuclear fuel pin, brazing is employed with a filler metal of Be. It is well known that the mechanical properties of the joint depends significantly on the thickness of the filler metal as well as on the brazing time and temperature. In the present study, it was attempted to find the optimum thickness of the filler metal which could give a good joint between thin Zr-4 tube and the spacer. Furthermore, the corrosion rate of the brazed area was examined in conjunction with the microstructures of the brazed zone. Brazing was carried out by induction heating for 20 seconds at 1000℃-1040℃ which is just above the eutectic point of α-Zr-ZrBe₂, the best result was obtained with the thickness of the filler metal in the range of 8-10 ㎛, where the joint strength was 20㎏/㎟. The basket weave structure was observed in the heat affected zone in brazing. The corrosion rate of this area was lower than that of the annealed structure.

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The effect of prior plastic deformation of austenite on the subsequent martensitic transformation temperature(Ms) was studied in an Fe-32.7%Ni alloy. The Ms temperature was higher, at all deformation degree up to 80%, than that of undeformed austenite and showed a maximum between 30% and 50% deformation. The changes in free energy of austenite and martensite, and the energy required for martensitic transformation, associated with the austenite deformation, were discussed, and a graphical interpretation of the observed result was made on the basis of driving force for martensitic transformation.

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Mechanical properties and microstructures in the ausextruded AISI H13 steel have been studied. Carbon extraction replicas were examined to find the evidence for a fine carbide dispersion after ausfoming. The carbides in as-ausformed specimens were observed to be about 0.05㎛ of average size, whereas those in conventionally as-quenched ones were 0.22㎛. The ausformed specimens showed higher hardness and bending strength than the conventional ones. Upon tempering, the ausformed specimens maintained the same strength level with increasing temperature up to 500℃, in spite of the marked increase in carbide size. This result is interpreted as being attributed to the formation of cell substructures, which has been verified by thin film transmission electron microscope.

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In this work all experimental methods were summarized for the determination of Carbon solubility in Mo and Mo-Va alloys. First of all the Carbon - doping technique with the subsequent chemical analysis was employed regardless of partial or complete precipitation of Carbon on the matrix during the cooling.

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20% Ni managing steels with and without Nb (0.5 w/o) were prepared in vacuum induction melting furnace, followed by hot working and heat treatments. The yield strength of the Nb-containing alloy was about 4% higher than that of Nb-free alloy under managed conditions. However, the charpy impact energy of Nb-containing alloy tested at room temperature was lower by 25%, compared with that of Nb-free alloy, under the managed condition. Precipitates phases formed in the Nb-free alloy were HCP Ni₃Ti(η) and FCC Ni₃(Ti, Al)(γ`), while in the Nb-containing alloy orthorhombic Ni₃Nb appeared in addition to the ηand γ`phases.

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