발간논문

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Prediction of weld penetration for high power laser beam welding with hot rolled steel plates was carried out based on real test results. Low and high carbon bearing steels with the C_(CQ) range of 0.34-0.87 and tensile strength range of 450 MPa-1,020 MPa were used for base plates. Several important welding parameters i.e laser output power, travel speed, position of focus and shield gas flow rate were taken into consideration. With metallograpic technique, weld penetration was measured for the bead-on-plate weld on the l0mm thick test materials. It was clear that analytical approach with various material constants showed poor reliability because the conventional method was originally based on the arc processes, where some specific phenomena such as energy reflection on the solid surface, key holing and laser induced plasma formation were neglected. Statistical process, on the other hand, agreed well with the test results. The prediction equation derived in this study was considered to be able to utilize as a system software or design parameter.

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The sensitization behaviors to intergranular attack of Type 409 stainless steel have determined by chemical etching and double loop EPR test. The results of chemical etching test and double loop EPR test agreed well with each other. The degree of sensitizaton for specimens after heat-treatment at 1100℃ and 600℃ was significantly high. Then, T-T-S diagram of these annealed alloys is represented by two step "C" shape curve. Precipitates of severly sensitized specimens in grain boundries were identified as Ti(C, N) by EDS analysis. It is suggested that sensitization behavior of Type 409 stainless steel is influenced by micro-galvanic corrosion due to concentrated Ti(C, N) precipitates in grain boundaries, besides by chromium depletion. Especially, it was found that high temperature sensitization produced more Ti(C, N) precipitates than low temperature sensitization

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The creep-rupture properties of a Ni-Cr-W-Al-Ti superalloy(KM 1557) have been investigated in the temperature range of 750℃ and 1000℃. In the stress and temperature ranges studied, the inverse of the rupture life(strain rate) exhibited power law dependence on the applied stress. The stress exponents for this alloy were between 5 and 12, and the activation energy for the creep was determined to be approximately 115 ㎉/mole. Precipitate free zones formed by grain boundary migration or sliding and the accommodation of sliding by the dislocation motion, were observed from the creep-ruptured specimens. Cracks were nucleated at the grain boundary or triple point, and propagated along the grain boundary containing PFZs. The shape of grains remained unchanged for the samples tested at low temperatures of 750 and 800℃. On the contrary, highly elongated grain shape was observed along the direction of applied stress when tested at high temperatures of 850, 900, 950, and 1,000℃.

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The purpose of this study is to design and fabricate a new zirconium alloy with improved higher mechanical properties and to characterize it. The new experimental zirconium allay was Zr-1Nb-0.5Sn-xMo with addition of 0, 0.2, 0.5 and 0.7wt.% Mo, respectively. In order to characterize the experimental alloys, age hardening treatment was done at 500℃ for 1h, 4h, and 8h. Then tensile strength at room temperature and Vickers micro-hardness have been measured, and micro-structural observation have been performed to identify the distribution and the shape of the precipitates. Results showed that hardness and tensile strength increased with increasing aging time up to about 4 hours, while ductility decreased. Especially, the alloy with addition of 0.5% Mo(Zr-1Nb-0.5Sn-0.5Mo) showed highest hardness and tensile strength among the four experimental alloys. Transmission electron microscopic examination revealed formation of fine spheroidal and cylindrical precipitates in the alloys with Mo addition, and SADP analysis revealed that the precipitates were mainly β-Nb and ZrMo₂. From these experimental alloys, it was observed that the micro-hardness and tensile strength considerably increased with the age hardening treatment, due to the formation of very fine precipitates which were evenly distributed through the matrix and particularly at twin boundaries.

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The objective of this study is to investigate microstructural characteristics of a two-layer hardfaced armor plate after ballistic impact. In order to deaelop the newly-designed armor plate, high hardness, hypereutectic high Cr white iron and ductile SUS 309 stainless steel were hardfaced nn an HY l00 steel plate using self-shielding flux cored arc welding method. This hardfaced plate was ballistically impacted, and metallurgical observations of the area near the perforated region were conducted. The high Cr white iron hardfaced layer was obseved to block effectively a fast traveling prajectile, although microcracks were initiated at coarse primary carbides of the high Cr hardfaced layer, and propagated to form longer cracks However, very few cracks were found in the stainless steel hardfaced layer, which tended to hinder the propagating crack. These foundings suggest that ballistic performance of the multi-layered armor plates fabricated by hardfacing technique might be improved.

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The detailed transmission electron microscopy(TEM) was conducted to study the growth mechanism of pearlite. The experiments were performed using vanadium carbonitrides, which precipitate during the pearlite growth in the Fe-C-V and Fe-C-V-Mn alloys, in order to trace the sequences of the pearlite growth. The growth mechanism based on the present experimental results are presented in the following two papers. In this first paper, the interphase boundary precipitation(IBP) accompanied with the pearlite growth is presented. The main features of IBP are : (a) IBP arrays are observed not only in the normal pearlite area but also in the lamellae fault area. In addition, IBP arrays are also found at the lamellar holes; (b) The characteristics of IBP, such as spacing, direction and the crystallographic orientation with the pearlitic ferrite matrix, remain unchanged during the pearlite growth. It was also found that the perturbation of the lamellar structure did not alter the characteristics of IBP; (c) The rows of IBP arrays are not parallel but inclined with respect to the nodule boundaries. Based on the present experimental observations, the integrated growth mechanism of pearlite will is presented in the following paper.

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Based on the observation of interphase boundary precipitation(IBP) discussed in the first paper, the effort was devoted to characterize the sidewise growth of pearlite in terms of the ledge migration mechanism, which has been proposed especially for the edgewise growth of pearlite. Entire pearlite lamellae are parallel each other and their length is linearly changed within a colony. In additon, lamellar holes were observed at the starting points of the branching and the rows of IBP arrays were observed to be extended to the next lamellar plate through this lamellar holes. From these observations, it is proposed that the sidewise growth of pearlite does not occur through a random branching, but through a time-sequential branching via the laterai movement of growth ledges on pearlite-austenite interfaces. In contrast to the Hackeny and Shiflet`s edgewise growth model, this newly proposed growth model opearlite explains that the simultaneous growth of pearlite in sidewise and edgewise directions is achieved by the operation of the same growth Ledge migration. (Received January 6, 1995)

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As a model experiment, Cu powders were ball-milled in a tumbiling mill in order to investigate the morphological evolution of ductile metal powder during mechanical alloying. The change of particle morphology was studied by microscopic observations. The particle morphology of Cu powders changed from dendritic particles of feed metals to disk-shaped particles, and then to globular purticles, which was similar to the observation during mechanical alloying of the premix of ductile intemetalic systems. It was found that the morphological change was due to the pure mechanical effect rising from collsions between the tools and powders. The globular particles were formed by the characteristic agglomeration of disik-shaped particles, that is, enclosure, folding, infolding and file-up process. It was also found that the globular particies coarsened by agglomeration and/or by continuous wrapping of the disk-shaped particles.

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This study aims at investigating the effect of matrix microstructure of high Cr white iron hardfacing alloys on fracture toughness and abrasion resistance. The hardfacing alloys were deposited twice on a mild steel plate using self-shielding flux cored arc welding method. In order to examine the matrix effect, different matrices such as pearlite, austenite, and mixture of pearlite and austenite were employed by changing the ratio of Mn/Si, while the total volume fraction of carbides was fixed. As the volume fraction of austenite in the matrix was increased, hardness and abrasion resistance were nearly constant, but fracture toughness was increased. In-situ observation of fracture process showed that cracks initiated at large primary M_7C₃ carbides tended to be blocked at the austenitic matrix, whereas they propagated through the pearlitic matrix. It could be concluded that abrasion resistance was controlled mainly by the volume fraction of primary carbides, while fracture toughness was controlled mainly by the amount of austenite in the matrix. Considering both abrasion resistance and fracture toughness, thus, the austenitic matrix was preferred for the high Cr white iron hardfacing alloys.

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Mechanical alloying behavior of Bi₂Te₃ alloy, which is the basic composition of thermoelectric materials used for thermoelectric cooling applications, has been investigated with variations of milling time and ball-to-powder weight ratio. Formation of Bi₂Te₃ intermetallic compound was completed by mechanical alloying of elemental Bi and Te powders for 1 hour at ball-to-powder weight ratio of 2 : 1 Processing time for completion of Bi₂Te₃ formation was found to be inversely proportional to the ball-to-powder weight ratio. Formation of Bi₂Te₃ was completed by mechanical alloying for 30 minutes and 10 minutes at ball-to-powder weight ratio of 4:1 and 10:1, respectively. Lattice constants of Bi₂Te₃ processed by mechanical alloying, a=0.4387㎚ and c=3.0481㎚, were not varied with ball-to-powder weight ratio and milling time. Lattice parameters of mechanically alloyed Bi₂Te₃ are in excellent agreement with values of a=0.4385㎚ and c=3.0483㎚, which were reported for Bi₂Te₃ powders processed by melting and grinding.

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