발간논문

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Amorphous or nanocrystalline solid states are gaining increased importance for hydrogen absorbing materials. Especially, in the case of Mg alloys, great improvements of hydrogenation properties can be achieved by nanocrystallization. In this study, Amor-/Nanocrystalline Mg₂NiH_X was fabricated from Mg and Ni chips by mechanical alloying for 96h under 2 MPa hydrogen atmosphere. The homogeneous Mg₂NiH_X was formed in 66 : 1 ball to chips weight ratio(BCR). The results of XRD and TEM revealed that the phases were composed of the amorphous and the nanocrystalline less than 10 ㎚. From the results of pressure-composition-isotherm(PCI) test, the hydrogenation properties were improved compared with the polycrystalline counterparts. But the hydrogen capacity was reduced from 3.6 mass% to 2.25 mass%(at 483K) in the BCR 66 : 1 specimen.

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Characterization of the microstructure and its effect on rolling contact fatigue of high frequency induction-hardened bearing steels produced by double hot forging have been studied using a 0.55 wt.% C-1.68 wt.% Mn steel. Rolling contact fatigue tests were conducted under a maximum Hertzian contact stress of 492 ㎏/㎟ with a rotating speed of 8,000 rev/min and a constant supply of lubricant. Very fine martensite which was superior resistance to rolling contact fatigue, was formed in the induction-hardened layer of double hot forged steels. It was also found that the less decrease of half value breadth of martensite peak in X-ray diffraction pattern during rolling contact fatigue, the longer rolling contact fatigue life of the specimens.

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Thin slab-direct hot rolling, called mini-mill process, is expected as a future process for production of low carbon steel sheets. However, there are several problems to be solved in the processing, one of which is edge cracking in low carbon steel sheet. To prevent the edge cracks of low carbon steel sheet, an isothermal preheat treatment was adopted before roughing in the mini-mill process. From the metallurgical viewpoint, the effect of Mn/S ratio on the edge cracks of low carbon steel sheet was investigated as well. It was found that edge cracking of low carbon steels contained 0.4% Mn or higher could prevented with isothermal preheat treatment at 1150℃ for 5 minutes before roughing. Reducing S content below 0.01%, 0.3% of Mn is enough to prevent edge cracks with isothermal preheat treatment at 1150℃ for 3 minutes before roughing. A concept of edge crack index is proposed for more accurate estimation of the total amount of edge cracking as a function of Mn/S ratio in this study.

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Effects of Be addition on the characteristics of oxide film in the Al-Si-Mg alloy were investigated. Small addition of Be in the molten metal were found to decrease fading of the modifier and reduce oxidation of Mg. The DSC and XRD measurements revealed superior oxidation resistance of the Be containing alloy at high temperature, and higher pitting potential of the alloy exhibited excellent corrosion resistance at room temperature. Such improvements are due to preferential formation of tight and protective BeO film rather than γ-Al₂O₃ or MgO films.

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