발간논문

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When stress amplitude was large, the best resistivity against fatigue crack growth was found in the pearlitic specimen due to high strength of the pearlitic matrix, while, when stress amplitude was small, it was found in the ferrite-pearlitic specimen due to high K_th of the specimen. The growth rate of fatigue crack increased with decreasing the cyclic frequency, while it was hardly affected by the wave form. The relation between the constant C and the exponent m in the Paris equation could be described by C = (1.1 ×10^(-4)㎜/cycle)/(20.1 Mpa√m)＾m for all specimens tested. Graphite nodules were found to act as the notches for crack initiation and to serve as the crack blunting points resisting against the fatigue crack growth.

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Accelerated N＾+ ions were implanted into iron and steel to study the change of the mechanical properties which are sensitive to the surface. The hardness of tool steel (SKD 61) was increased by about 50% with implantation at dose of 5×10＾(16) N＾+ ions/㎝² at 50 keV beam energy. It was found that both the hardness and the fatigue life ware increased with increasing dose of N＾+ ions. A dry wear test conducted on pin-on-disc showed that the wear rate of implanted samples was greater than that of unimplanted ones due to the change in oxidation behavior with implantation, while the rate of running-in-wear was decreased by implantation under lubricated condition. Evidence from TEM supported the existence of nitride precipitates of particle size of 10-100 A˚at dislocations in implanted pure iron. SIMS analysis was employed to study the chemical composition of the implanted layer.

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The Ostwald ripening behaivor of carbide particles precipitated in 1-3% W steels at a temperature range from 660℃ to 700℃ has been investigated by using various quantitative metallographic examinations. The value for the slope of each straight line obtained from the relation between the log of the average particle size and the spheroidizing time is approximately 0.27 which indicates that the growth of precipitate particles in tungsten steels is mainly governed by the mechanism of non-volume diffusion. An average activation energy of 69.6 Kcal/mole for W steels was found, which was interpreted as an indication of the interdiffusion mechanism of Fe, C and W in steel. As the spheroidizing time increases, the carbides formed in the tungsten steel with an atomic ratio of 0.12 become a mixture of Fe₃C and WC. In the carte of tungsten steels with an atomic ratio of 0.25 and 0.36 respectively, however, the final stable carbide mixtures of Fe₃C and WC and formed after an intermediate stage where the semi-stable carbides of Fe₄W₂C and Fe_(21)W₂C type are coarsened.

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Detailed examination was carried out for clarifying the strengthening mechanism of complex precipitation during aging at 100∼600℃ in Fe-1Cu, Fe-3Ni-1Al and Fe-1Cu-3Ni-1Al alloys by the measurements of hardness, impact test, differential thermal analysis and the transmission electron micrography observation. Among the alloys studied Fe-1Cu-3Ni-1Al alloy aged at 500℃ showed the largest hardness increase with time while the impact energy value decreased concurrently. The phases precipitated in the ferrite matrix during aging at 500℃ were found to be ε-Cu and NiAl intermetallic compound. It was also observed that the ε-Cu coarsened rapidly and became randomly oriented large particles tending to be overawed whereas NiAl precipitates grew very slowly and kept coherency with the ferrite matrix showing no overaging for a considerable period of prolonged aging. It was concluded that the major strengthening mechanism during aging at 500℃ in Fe-1Cu-3Ni-1Al was the coherent precipitation of NiAl phase in ferrite matrix. The role of Cu in Fe-1Cu-3Ni-1Al was interpreted as the promotion of NiAl precipitation so that the alloy could reach its maximum hardness more rapidly.

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Charpy impact testing was performed to investigate fracture behavior of AISI-4140 steel from viewpoint of variation in the intrinsic toughness of martensitic matrix with alloying additions of Ni(4140-Ni) and Si(4140-Si). In 4140-Si steel, transgranular fracture becomes the primary fracture mode as temperature deceases. When coarse carbides exist at grain boundaries, transition from intergranular fracture to transgranular fracture occurs as testing temperature decreases. In 4140-Ni steel, transgranular fracture occurs at very low temperature when there is few carbides at grain boundaries, while fracture mode is predominantly intergranular type in the presence of coarse carbides at grain boundaries. The different fracture behavior of these steels is attributed to alloying elements; since Si-addition decreases the intrinsic toughness of the matrix, transgranular fracture becomes easier to occur with decrease in temperature in 4140-Si steel; since Ni-addition increases the fracture toughness, transgranular fracture becomes more difficult to occur in 4140-Ni steel.

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A study has been made on the segregation reaction of nickel oxide with calcium chloride and carbon reductant over the temperature range from 700 to 1000℃ using a tube type furnace in an inert atmosphaere. The overall segregation reaction was found to consist of three steps: in the first step, clacium chloride is hydrolyzed generating hydrogen chloride gas, in the second step, nickel oxide is reacted with the hydrogen chloride gas and converted to nickel chloride, and in the third step, the nickel chloride is reduced by hydrogen as that is generated by the reaction of carbon reductant and water vapor. In the above reactions, the first and the third step were rapid in comparison with the second step. Consequently it is assumed that the second step reaction would be a rate determining step in the segregation reaction. The activation energy of the chlorometallization reaction of nickel oxide was determined for powder sample to be 10.5 ㎉/mold. The reaction rate was observed being approximately proportional to he partial pressure of hydrogen chloride gas and reaction temperature at constant water vapor pressure.

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The phase transformation and hardness change by annealing treatment were examined for Fe_(73)Cr_(10)Mo_5B_(10)C₂ metallic glass produced by melt spinning, and were compared with those of the rapidly solidified crystalline alloy of the same composition. On annealing, this metallic glass transformed into a microcrystalline structure consisting of fine α-Fe crystals and fine boride phases. After annealing at 850℃ for 30 min. the transformed structure became very stable and the structure remained unchanged with further annealing. In this state, samples could be 180˚-bend deformed. In the case of rapidly solidified alloy of the same composition, larger grains and larger precipitates compared to the metallic glass were formed, and they formed a network in grain boundaries. Hardness changes by annealing matched well with the microstructural changes.

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