발간논문

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Critical crack tip opening displacement(CTOD) obtained from Dugdale Model for small scale yielding case is modified by taking into account for the effects of multiaxial stresses and work hardening in the plastic zone in the front of crack tips. By assuming this modified critical CTOD to be linear with plane strain tensile fracture strain, the equation for plane strain fracture toughness(K_(IC)) to be calculated from the tensile properties of a material, is suggested. The comparison between calculated and measured K_(IC) is made.

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The Bauschinger effect was measured by static tension-compression test at room temperature for Cu, Cu-20% Zn, Cu-30% Zn, Cu-40% Zn, and Cu-45% Zn alloys. In this study, Bauschinger stress, strain, energy and back stress were used to explain the Bauschinger effect as parameters. As stacking fault energy (SFE) is lower, slip mainly tend to change from cross slip to planer slip in fcc. In pure Cu and single phase alloys (∼30% Zn) SFE is decreased with increasing of Zn content and the Bauschinger effect strongly relied on slip mode by SFE. Because Bauschinger stress parameter is increased with prestrain, the most important factor affecting the magnitude of the Bauschinger effect seems to be back stress. In (α＋β) two phase alloys (40% Zn and 45% Zn), when prestrain is small, the Bauschinger effect relied on not only macroscopic residual stresses developed as a result of inhomogeneous deformation of the (α＋β) phases but also back stress. But if prestrain is raised, the Bauschinger effect relied on back stress only. To evaluate the Bauschinger effect precisely, we should use more than two parameters, because Bauschinger parameters showed the different results each other.

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The effects oP Sn contents and sintering temperature on dimensional change, tensile strength and hardness of sintered Fe-6 (Cu-Sn) alloy were investigated. Due to liquid phase occuring during sintering, all the specimens showed dimensional expansion after sintering and the sintering temperature at which a relatively large expansion occured decreased with Sn content. Thus the addition of small amount of Sn in place of Cu aids the sintering operation because it lowers the liquid phase forming temperature. From mechanical property tests, it has been shown that the properties obtained from compacts of Fe-4Cu-2Sn composition sintered at 900℃ were similar to those obtained from Fe-6Cu compacts sintered at 1100℃. For the specimens with larger amounts of Sn, a significant decrease in tensile strength was observed. Therefore, it is concluded that the optimum ratio of Cu : Sn is 2 : 1. The microstructures of the sintered specimens are presented and an attempt is made to relate these to the mechanical properties and dimensional change after sintering.

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The growth kinetics and yield strength characteristics of complex carbides precipitated in Cr steels spheroidized after standardizing initial particle distributions were investigated by using various quantitative metallographic techniques. Most of the grain boundaries of ferrite matrix are outlined by a string of complex carbide particles which are determined to be (Fe, Cr)₃C. The average values of activation energy have shown that the coarsening of complex carbide precipitates involves the coupled chromium-iron-carbon diffusion. The number of second phase particles pinning ferrite grain boundaries increases with increasing the chromium content, which results in a more decreasing growth rate of ferrite grains. During spheroidization hardness changes with the Hall-Perch type relation Hv＝Ho＋K_Hl^(-½). Yield strength is affected by parameters concerning both precipitate particles and ferrite grain boundaries and varies with the following Hall-Perch type equation σ_y= σ_o+k_yλ_p.g^(-½).

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Mechanical properties and microstructure tests were carried out in order to investigate the effect of inoculation of the gray cast iron (3.3-3.5%C, 2.5-2.7%Si) casted in the metal mold. Specimens were annealed at 900℃ in 2hrs and again studied the changes of their structure and mechanical properties. From the tested results, least chill depth was obtained by adding 0.2% inoculant in the metal mold at 300℃. Hardness was sharply decreased with adding inoculant but UTS was maintained constant. With increased mold temperature, hardness and UTS were decreased. Hardness was decreased drastically by annealing but UTS was slightly increased under same mold temperature.

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