• 연구논문집
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• 통권33호
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• pp.137-145
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• 2003

The effects of antimony addition on the microstructures and creep behavior of AZ31 magnesium alloy have been investigated. Constant load creep tests were carried out at temperatures ranging from $150^{\circ}C$ to $200^{\circ}C$, and an initial stress of 50MPa for AZ31 alloys containing antimony up to 0.84% by weight. Results show that small additions of antimony to AZ31 effectively decreased the creep extension and steady state creep rates. The steady state creep rate of AZ31 was reduced 2.5 times by the addition of 0.84% of antimony. The steady state creep rate of AZ31-0.84Sb alloy was controlled by dislocation climb in which the activation energy for creep was 128 kJ/mole. The microstructure of as-cast AZ31-0.84%Sb alloy showed the presence of $Mg_3Sb_2$ precipitates dispersed throughout the matrix. The main reason for the higher creep resistance in AZ31-Sb alloys is due to the presence $Mg_3Sb_2$, which effectively hindered the movement of dislocations during the elevated temperature creep.

• Journal of Welding and Joining
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• 제20권6호
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• pp.30-30
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen(l0×10×0.5mm). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at 600℃. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• Journal of Welding and Joining
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• 제20권6호
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• pp.754-761
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen($10{\times}10{\times}0.5mm$). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at $600^{\circ}C$. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• 한국주조공학회지
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• 제40권2호
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• pp.7-15
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• 2020

The high-temperature stability of Mg-8.0Zn-1.6Y (wt.%) alloys upon the addition of Ca has been investigated by characterizing the ignition temperature, microstructure, tensile and creep properties. The ignition temperature increases with an increase in the Ca content, indicating that an addition of Ca enhances the ignition resistance of the Mg-Zn-Y alloy. The as-cast microstructures of all tested alloys mainly consisted of the dendritic α-Mg matrix and I-phase (Mg₃Zn₆Y) at the grain boundaries. In the Ca-added Mg-8.0Zn-1.6Y alloys, the Ca₂Mg₆Zn₃ phase forms, with this phase fraction increasing with an increase in the Ca contents. However, a high volume fraction of the Ca₂Mg₆Zn₃ phase rather deteriorates the mechanical properties. Therefore, a moderate amount of Ca element in Mg-8.0Zn-1.6Y alloys is effective for improving the tensile and creep properties of the Mg-Zn-Y alloy. The Mg-8.0Zn-1.6Y-0.3Ca alloy exhibits the highest tensile strength and the lowest creep strain among the alloys investigated in the present study. The creep resistance of Mg-Zn-Y-Ca alloys depends on the selection of the secondary solidification phase; i.e., when Ca₂Mg₆Zn₃ forms in an alloy containing a high level of Ca, the creep resistance deteriorates because Ca₂Mg₆Zn₃ is less stable than the I-phase at a high temperature.

• 한국주조공학회지
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• 제27권5호
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• pp.198-202
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• 2007

In the present study, the high temperature mechanical properties and creep resistance of Mg-Zn-Y-Ca alloys has been investigated. The Mg-4Zn-0.8Y alloy consists of ${\alpha}$-Mg matrix and icosahedral quasicrystalline phase. Calcium addition into Mg-4n-0.8Y based alloy results in the formation of ${\tau}(Ca_{2}Mg_{6}Zn_{3})$ and $Mg_{2}Ca$ as the second solidification phases. Creep properties of the Mg-Zn-Y and Mg-Zn-Ca based alloys measured at applied stresses between 65 MPa and 85 MPa are significantly improved with adding calcium and yttrium, respectively. The improved creep resistance is due to the formation of thermally stable $Mg_{2}Ca$ phase.

• 한국도로학회논문집
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• 제6권3호
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• pp.9-15
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• 2004

아스팔트 혼합물의 균열 저항성은 일반적으로 인장 강도, 스티프니스와 같은 단일 물성치를 측정함으로써 평가된다. 그러나, 아스팔트 혼합물의 균열 성능을 평가함에 있어서 단일 물성치의 이용은 의문시되어 왔다. 따라서 본 연구에서는 아스팔트 혼합물의 균열 저항성과 관련이 있는 주요 특성치를 좀 더 심도 있게 규명하고자 하였다. 이를 위해 다양한 하중 조건 하에서 파괴 시험 크리프 시험, 강도 시험이 일반 아스팔트 혼합물과 개질 아스팔트 혼합물에 대해서 수행되었다. 시험 결과, 혼합물의 균열 저항성은 주로 미세 손상 축적 속도에 영향을 받음을 알 수 있었으며, 이 값은 파괴 에너지 한계에 영향을 주지 않으면서 m값에 반영됨을 알 수 있었다. 또한, 짧은 하중 재하 시간 (탄성거동) 동안 얻어지는 스티프니스는 혼합물의 균열 저항성의 차이를 규명하는데 한계가 있음을 알 수 있었다. 따라서 아스팔트 혼합물의 균열 저항성을 보다 명확히 평가하기 위해서는 혼합물의 크리프 거동과 파괴 한계점을 동시에 고려하는 것이 필수적임을 알 수 있었다. 또한 수퍼페이브 간접 인장 강도 시험으로부터 구한 잔여 소멸 에너지는 비교적 손쉬운 실험을 통해 아스팔트 혼합물의 균열 저항성의 상대적인 차이를 보여줄 수 있는 유용한 물성치임을 알 수 있었으며, 장기 크리프 시험에서 얻어지는 파괴 변형률은 아스팔트 혼합물의 크리프 거동과 파괴 한계점을 동시에 고려함으로써 균열 저항성을 평가할 수 있는 유용한 물성치 임을 알 수 있었다.

• 한국정밀공학회지
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• 제29권10호
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• pp.1137-1143
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• 2012

Creep characteristic is an important failure mechanism when evaluating engineering materials that are soft material as polymers or used as mechanical elements at high temperatures. One of the popular thermo-plastic polymers, Acrylonitrile Butadiene Styrene (ABS) which is used broadly for machine elements material, as it has excellent mechanical properties such as impact resistance, toughness and stiffness compared to other polymers, was studied for creep characteristic at different levels of stress and temperatures. From the experimental results, the creep limit of ABS at room temperature is 80 % of tensile strength which is higher than PE and lower than PC or PMMA. Also the creep limits decreased to linearly as the temperatures increased, up to $80^{\circ}C$ which is the softening temperature of Butadiene ($82^{\circ}C$). Also the secondary stage of creep among the three creep stages for different levels of stress and temperature was non-existent which occurred for many metals by strain hardening effect.

• 한국주조공학회지
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• 제26권1호
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• pp.34-39
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• 2006

In the present study, the possibility of Mg-Zn-Y alloys as high temperature casting alloys has been investigated. The fluidity of alloys containing yttrium were better than that of commercial AZ91 alloy because the oxide layer on the surface reduced the reaction between melt, and air and mold, which would reduce the resistance during the process of filling the mold. However, this oxide film reduced the hot-tearing resistance. In the case of ZAW942, this alloy exhibited fluidity and hot-tearing resistance better than AZ91 alloy. Because of thermally stable quasicrystal and other phases obstructed the movement of grains, the creep resistance of alloys containing rare earth elements more than 2 wt% was better than that of AZ91 alloy.

• 한국정밀공학회지
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• 제20권6호
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• pp.214-221
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• 2003

Tension mask assembly is positioned right behind the glass-made front panels of CRT type display devices. The frame-supported thin metal sheet contains numerous slits, through which electron beams are focused to enhance definition. Pretension is imposed on the masks, especially for enlarged flat screens, in order to avoid vibration due to acoustic or mechanical impact. High temperature assembly process subsequent to pretensioning, however, degenerates the creep resistance of common mask materials, and if tensile stress is high enough, tension on the mask may be loosened substantially due to creep deformation. In this study, the assembly is modeled as a combined structure of beams and wire array, and a numerical simulation is attempted for pretensioning followed by high temperature process. According to a model study, small amount of creep strain is likely to be generated, but its adverse influence is not negligible. Some structural modification measures to reduce the creep-induced tension loosening are proposed and evaluated. Also, optimal configuration of frame structure is sought for, which maintains high tension of masks and minimizes the possible creep of frame simultaneously.

• KEPCO Journal on Electric Power and Energy
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• 제6권1호
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• pp.41-47
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• 2020

The contribution of damage mechanisms to failure of steam turbine casing made of Cr-Mo-V steel was investigated. Creep-fatigue interaction on the HP side corner of turbine casing was revealed as the root cause of the catastrophic failure performed by metallurgical analysis. The steady-state pressure and transient thermal stress were analyzed based on the actual operating condition of the thermal plant. Damage of creep-fatigue interaction to crack initiation was evaluated with multiaxial effects. The contribution ratio of creep and fatigue to the crack initiation was estimated to 3:1. Temporary geometrical correct action with repair weld was executed. For long-term operation, design improvement of casing equipment for creep resistance should be needed.