• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2006년도 춘계 학술대회 개요집
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• pp.137-139
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• 2006

In order to establish the optimum hot forming temperature and solution heat treatment temperature for 25% chromium super duplex stainless steel weld, a commercial 25%Cr-10%Ni-4%Mo weld metal for super duplex stainless steel(UNS S32750) with different solution heat treatment conditions at $1100^{\circ}C,\;1050^{\circ}C,\;1025^{\circ}C\;and\;1000^{\circ}C$ for 1.5 hours has been investigated by means of optical metallography, and estimated mechanical properties. It is found that exposure to elevated temperatures at $1050^{\circ}C,\;1025^{\circ}C\;and\;1000^{\circ}C$ except $1100^{\circ}C$ brings partial decomposition of ferrite to austenite and sigma phase, which deteriorates their properties and heat treatment at $1100^{\circ}C$ shows acceptable mechanical properties.

• 소성∙가공
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• 제21권8호
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• pp.506-511
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• 2012

Aluminum brazing normally requires a careful control of temperature due to the small interval between brazing and melting temperatures for base materials. Unsuitable processing conditions, including brazing temperature outside admissible range, gap between brazed materials or inadequate flux feeding, can lead to joining defects. In this study, A357 was used as a filler metal for the brazing of pure aluminum base materials and brazed at temperatures in the semi-solid state. Interface microstructures with base materials were observed using optical metallography(OM) and scanning electron microcopy(SEM) with energy dispersive spectroscopy(EDS), and compared to conventional aluminum brazing.

• 한국주조공학회지
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• 제34권1호
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• pp.1-5
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• 2014

The austempering transformation behavior in Fe-0.7wt.%C-2.3wt.%Si-0.3wt.%Mn steel is investigated. Each specimen was austenitized for 60 min at $900^{\circ}C$, and austempered at $380^{\circ}C$ for different time periods varying from 2 min to 256 min. After the austempering heat treatment, the Stage I and II evolutions are performed using optical metallography, X-ray diffraction and image analyses. Variations in the X-ray diffraction patterns and lattice parameters of the ferrite and austenite demonstrate that the residual austenite decomposes into ferrite and carbide during the Stage II evolution; moreover the amount of ferrite increases during the Stage I evolution. While the amount of austenite increases during Stage I, it dicreases during Stage II. Overall, the variations in the volume fractions of the microstructure and carbide formation in stages I and II meet high temperature austempering reaction of the ausferrite microstructure.

• 한국주조공학회지
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• 제32권6호
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• pp.284-288
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• 2012

The effects of Si content on the variation of microstructure and processing window of austempered ductile cast iron were investigated. Four different Si contents between 2.42 and 3.37 wt.% were used. The influence of silicon on the microstructure and processing window of these materials were studied. Austenitizing was performed at $900^{\circ}C$ for 60min and austempering temperature were both $340^{\circ}C$ and $360^{\circ}C$ and austempering time were for 4min upto 119min and for 5min upto 160min respectively. After heat treatment, the evolution of stage I and stage II were performed by optical metallography, XRD, hardness test. The results showed that $t_2$ was delayed as Si contents was increased due to the fact that Si retarded the formation of cementite ($Fe_3C$). The high silicon content promoted the stability of the metastable two-phase combination of austenite and ausferrite.

• 한국주조공학회지
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• 제22권2호
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• pp.75-81
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• 2002

The main objectives were to investigate the suitability of CaO crucible for melting TiAl alloys and to develop investment mold for investment casting of TiAl alloys. TiAl alloy specimen were prepared by plasma arc furnace under argon atmosphere. After melting of TiAl alloy using CaO crucible, the results showed that there is little contamination of oxygen in the TiAl bulk. Conventional vacuum induction furnaces can be readily adaptable to produce cast parts of TiAl without high skilled techniques. The determination of optical metallography and microhardness profiles in investment cast TiAl alloy rods has allowed the gradation of the relative thermal stability of the oxides examined. The molds used for the present study were $ZrO_2$, $Al_2O_3$, CaO stabilized $ZrO_2$ and $ZrSiO_4$. Even although high temperature of mold preheating, $Al_2O_3$ mold is a promising mold material for investment casting of TiAl alloys in terms of thermal stability, cost and handling strength. It is important to take thermal stability and preheating temperature of mold into consideration for investment casting of TiAl alloys.

• 한국해양공학회지
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• 제23권2호
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• pp.69-73
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• 2009

The objective of this study was to obtain a better understanding of the delayed hydride cracking (DHC) of Zr-2.5Nb alloy. The DHC model has some defects: first, it cannot explain why the DHC velocity (DHCV) becomes constant regardless of an applied stress intensity factor, even though the stress gradient is affected by the applied stress intensity factor at the notch tip. Second, it cannot explain why the DHCV has a strong dependence on the method of approaching the test temperature by a cool-down or a heating-up, even under the same stress gradient, and third, it cannot predict any hydride size effect on the DHC velocity. The DHC tests were conducted on Zr-2.5Nb compact tension specimens with the test temperatures reached by a heating-up method and a cool-down method. Crack velocities were measured in hydrided specimens, which were cooled from solution-treatment temperatures at different rates by being furnace-cooled, water-quenched, and liquid nitrogen-quenched. The resulting hydride size, morphology, and distributions were examined by optical metallography. It was found that fast cooling rates, which produce very finely dispersed hydrides, result in higher crack growth rates. This different DHC behavior of the Zr-2.5Nb tube with the cooling rate after a homogenization treatment is due to the precipitation of the $\gamma$-hydrides only in the water-quenched Zr-2.5Nb tube. This experiment will provide supporting evidence that the terminal solid solubility of a dissolution (TSSD) of $\gamma$-hydrides is higher than that of $\delta$-hydrides.

• 한국재료학회지
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• 제26권12호
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• pp.709-713
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• 2016

Grain morphology, phase stability and mechanical properties in binary Ti-Al alloys containing 43-52 mo1% Al have been investigated. Isothermal forging was used to control the grain sizes of these alloys in the range of 5 to $350{\mu}m$. Grain morphology and volume fraction of ${\alpha}_2$ phase were observed by optical metallography and scanning electron microscopy. Compressive properties were evaluated at room temperature, 1070 K, and 1270 K in an argon atmosphere. Work hardening is significant at room temperature, but it hardly took place at 1070 K and 1270 K because of dynamical recrystallization. The grain morphologies were determined as functions of aluminum content and processing conditions. The transus curve of ${\alpha}$ and ${\alpha}+{\gamma}$ shifted more to the aluminum-rich side than was the case in McCullough's phase diagram. Flow stress at room temperature depends strongly on the volume fraction of the ${\alpha}_2$ phase and the grain size, whereas flow stress at 1070 K is insensitive to the alloy composition or the grain size, and flow stress at 1270 K depends mainly on the grain size. The ${\alpha}_2$ phase in the alloys does not increase the proof stress at high temperatures. These observations indicate that improvement of both the proof stress at high temperature and the room temperature ductility should be achieved to obtain slightly Ti-rich TiAl base alloys.

• 보존과학회지
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• 제17권
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• pp.57-64
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• 2005

청평사 강선루 회전문 부근에서 출토된 조선시대 $16\~17$세기 철물과 철촉에 대한 금속조직학적으로 조사하기 위하여 유물에서 부식되지 않은 시편을 채취하여 마운트와 연마 및 부식을 시켜 금속조직을 관찰하고 비금속개재물부분은 SEM으로 관찰한 뒤 EDS로 분석하였다. 금속학적 조직조사와 SEM-EDS분석을 통하여 철물과 철촉은 고체저온환원법에 의한 순철에 가까운 괴련철 혹은 해면철을 이용하여 철을 생산하고, 단야로 같은 곳에서 침탄을 하여 탄소량을 높인 것으로 밝혀졌다. 또한 철물은 탄소량을 어느 정도 높인 다음 담금질과 열처리과정을 반복하여 철기의 강도를 인위적으로 많이 높이는 작업을 한 것으로 밝혀졌다. 이번 조사연구를 통하여 조선시대에도 괴련철을 소재로 하여 철을 생산하였다는 것을 알 수 있어 의미 있는 결과를 얻었다.

• 한국주조공학회지
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• 제30권2호
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• pp.60-65
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• 2010

In this study, we investigate the effects of austempering heat treatment on the processing window and mechanical properties in cast and hot-rolled Fe-0.7 C-2.3 Si-0.3 Mn steel. Each specimens were austenitised at $900^{\circ}C$ for 7 min, and austempered at $260^{\circ}C,\;320^{\circ}C$, and $380^{\circ}C$ for the various periods of time from 2 min to 240 min. After heat treatment, the evaluation of stage I and stage II as performed by optical metallography, XRD, hardness test. Both cast and hot rolled specimens had similar processing window. So grain size effect is not important to the austempered high carbon high silicon cast steel. When the austempering temperature was $260^{\circ}C$, the microstructure consisted of the lower ausferrite while the upper ausferrite structure was formed at $380^{\circ}C$. As the austempering temperature increases from 260 to $380^{\circ}C$, the strength and hardness decreased, elongaton and volume fraction of austenite increased. In addition, there was no change of mechanical properties between cast and hot-rolled specimens.

• 보존과학회지
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• 제28권3호
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• pp.257-264
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• 2012

고대의 거울은 청동으로 만들어지는 것이 일반적이며, 철제거울의 경우 국내에서 출토된 예를 찾아보기는 매우 드물다. 본 연구에서는 불명철기의 보존처리과정에서 원형을 찾아 그 대상이 철제거울임을 밝혀냈으며, 그 과정에서 부식되지 않은 시편을 채취하여 광학현미경과 미세경도시험기, SEM-EDS를 이용하여 미세조직을 분석하고 이를 통해 제작기법을 연구하였다. 그 결과 철제거울의 조직은 비금속개재물이 거의 없고 시편 일부에서 망상 시멘타이트 조직이 관찰되는 것으로 미루어 주철을 부어 만든 거울을 고체상태에서 탈탄시켜 탄소량을 낮춘 고체탈탄강으로 판단된다. 주조품의 백주철 조직은 높은 탄소 함량으로 우수한 경도를 가져 절삭이나 연마가 어렵다. 때문에 주조된 철제거울의 거친 경면을 연마할 수 있도록 CO 또는 $CO_2$를 차단하여 $850^{\circ}C$이하의 온도에서 탈탄시켜 주조품의 경도를 낮춘 것으로 추정된다. 따라서 탄소 함량에 따른 조직의 변화가 관찰된 것으로 판단된다.