• 열처리공학회지
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• 제8권1호
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• pp.53-64
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• 1995

A study was made to characterize the microstructures and mechanical properties of the base metal and the heat-affected zone(HAZ) in Cu-bearing HSLA-100 steel. The Gleeble thermal/mechanical simulator was used to simulated the weld HAZ. The relationship between microstructure and toughness of HAZ was studied by impact test, O. M, SEM, TEM, and DSC. The toughness requirement of military specification value was met in all test temperatures for the base metal. The decrease of HAZ toughness comparing to base plate is ascribed to the coarsed-grain and the formation of bainite. Obliquely sectioned Charpy specimens show that secondary crack propagate easily along bainite lath. Improved toughness(240J) at HAZ of $Tp_2=950^{\circ}C$ is due to the fine grain, and reasonable toughness(160~00J) in the intercritical reheated HZA is achieved by the addition of small amount of carbon which affects the formation of "M-A". Cu precipitated during ageing for increasing the strength of base metal is dissolved during single thermal cycle to $1,350^{\circ}C$ and is precipitated little on cooling and heating during subsequent weld thermal cycle. Thus, the decrease of toughness does not occur owing to the precipitation of Cu.

• Journal of Welding and Joining
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• 제29권1호
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• pp.65-73
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• 2011

Microstructural characteristics of two high strength (600 MPa & 800 MPa) weld metals produced by flux-cored arc welding process (FCAW) were evaluated. The 600 MPa grade weld metal was consisted of 75% acicular ferrite and 25% ferrite which was formed at relatively high temperature (grain boundary ferrite, widmanstatten ferrite, polygonal ferrite). However, the 800 MPa grade weld metal was composed of about 85% acicular ferrite and 15% low temperature forming phases (bainite, martensite). The prior austenite grain size of 800 MPa grade weld metal was decreased by solute drag force. The compositions and sizes of inclusions which are the dominant factors for the formation of acicular ferrite were analyzed by a transmission electron microscopy (TEM). In both 600 MPa and 800MPa grade weld metals, the inclusions were mainly consisted of Ti-oxide and Mn-oxide, and the average size of inclusions was $0.7{\mu}m$. The 800 MPa grade weld metal exhibited higher tensile strength and similar toughness compared with the 600 MPa grade weld metal. This result is mainly due to a higher fraction of low temperature products and a lower fraction of grain boundary ferrite in the 800 MPa grade weld metal.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2010년도 춘계학술발표대회 초록집
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• pp.70-70
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• 2010

In the present study, to estimate the mechanical properties of 800 MPa grade weld metal, welding was carried out using 800 and 600 MPa grade flux cored arc welding (FCAW) consumable and characteristics of the weld metals were investigated. The chemical composition of weld metals was investigated by an optical emission spectroscopy (OES) method. The microstructure of weld metals was analyzed by optical microscopy (OM) and secondary electron microscopy (SEM). The compositions and sizes of inclusions which are the dominant factors for the nuclei of acicular ferrite were analyzed by an transmission electron microscopy (TEM). In addition, mechanical properties of the weld metals were evaluated through tensile tests and charpy impact tests. Mostly the acicular ferrite phase which has high strength and toughness was observed. The 600 MPa grade weld metal was consisted of 75% acicular ferrite and 25% ferrite which was formed at high temperature (grain boundary ferrite, widmanstatten ferrite, polygonal ferrite). However, the 800 MPa grade weld metal was composed of about 73% acicular ferrite and 27% low temperature phase (bainite, martensite). Toughness was considerably decreased due to the increase of tensile strength (from 600 MPa to 800 MPa). The sizes of inclusions which were observed in both weld metal were $0.4{\sim}0.8\;{\mu}m$, it is effective size to form acicular ferrite.

• 열처리공학회지
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• 제36권1호
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• pp.1-6
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• 2023

In order to process plastic with similar mechanical performance to metal materials, it is necessary to improve the strength and hardness of core parts of the injection equipment in extrusion system. The tempering process is a heat treatment performed to reduce brittleness and improve elongation along with improvement of dimensional defects of martensite formed after quenching. In this study, changes in microstructure and mechanical properties according to temperature were evaluated after quenching and tempering of SM30C material. As a result, the strength and hardness were gradually decreased by tempering at 250~400℃, and the decrease was greatly increased under the tempering condition at 450℃. Under the tempering condition of 200~400℃, the main structure was lath martensite, and the precipitation amount and size of needle-shaped cementite increased along the lath with the increase of the tempering temperature. Most of the shape of cementite has a needle-like structure, and the formation of some spherical cementite is observed. Under the tempering condition of 450℃, a mixed structure of ferrite and martensite was formed according to the decomposition of martensite.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2006년도 춘계학술발표회 논문집(I)
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• pp.610-613
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• 2006

This paper is the fundamental study for manufacture of spalling resistance concrete and also analyses the mechanism and spalling resistance method with materials, mixture proportion and lateral confinement. The present work with the basic experiment achieved successful method for spalling resistance using both proper amounts of fiber contents and lateral confinement using metal lath. Moreover, the developed spalling resistance method was applied for full sized column construction in the Doosan We've Poseidon I field, located in Busan city. Authors investigated the physical properties examining workability, placeability and pumpability. These studies are continuously processing to develop new technology expecting remarkable impact on the spalling resistance and fire resistance performance of high-raise building construction in the future.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2005년도 추계 학술논문 발표대회
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• pp.7-10
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• 2005

This paper is about manufacture of spalling resistance concrete and also investigates the spalling mechanism and spalling resistance method with diverse materials, mixture proportion and lateral confinement. The present work with the basic experiment achieved successful method for spatting resistance using both proper amounts of fiber contents and lateral confinement using metal lath. Moreover, the developed spatting resistance method was applied for full sized column construction in the Doosan We've Poseidon I field, located in Busan city. The author investigated the physical properties examining workability, placeability and pumpability. These studies are continuously processing to develop new technology expecting remarkable impact on the spatting resistance and fire resistance performance of high-raise building construction in the future.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2005년도 봄학술 발표회 논문집(II)
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• pp.57-60
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• 2005

This study discusses spalling and fire enduring performance of high performance concrete (HPC) RC column subjected to loading under heating for 3 hours. According to the test, both the plain concrete and the concrete attached with fire enduring PC panel exceed allowable temperature after 60 minutes due to the exposure of steel bar and falling off of concrete resulting from severe spalling failure. It leads to buckling of main bar and at the same time, occurrence of collapse of plain HPC column member is observed after 2 hours and 1 hour 40 minutes's exposure to fire, respectively. On the other hand, HPC applying both PP fiber of 0.1$\%$ by mass of concrete and PP fiber+lateral confinement by metal lath maintains their original cross section, which is satisfied with the 3 hours fire endurance criteria, by discharging internal vapour pressure and enhanced lateral confinement force.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2006년도 춘계학술논문 발표대회 제6권1호
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• pp.5-8
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• 2006

High strength concrete has been increasingly used in high rue building and it is very obvious re consider fire resistance performance of that. Unlike the normal strength concrete, high strength concrete in sudden elevating temperature at fire is susceptible to spalling with severe explosion and surface split, due to high density of concrete. In order to endure the spalling, inner space temperature of concrete should be control less than certain point. Therefore this study investigated the influence of covering materials on high strength concrete finishing spray-on materials of fiber composite spray mortar(FCSM). Both polypropylene(PP) and polyvinyl alcohol(PVA) fiber were used in this test. Test showed that concrete, covering 18mm mortar containing PVA fiber and confining metal lath 2.3mm thickness, decreased 50% of main bar ambient temperature. compared with control concrete. In addition, concrete covering 18mm mortar without fiber caused falling of covering materials and then it was exposed in elevating temperature. As a result, spatting of the concrete occurred same as control concrete. However, concrete covering spray-on mortar containing PVA or PP fiber resisted spatting occurrence.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2008년도 춘계 학술발표회 제20권1호
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• pp.865-868
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• 2008

본 연구에서는 최근 사용이 증대되고 있는 고성능 콘크리트의 폭렬현상을 방지하기 위해 일련의 연구 중 메탈라스 횡구속을 병행한 비탈형 거푸집을 사용한 고성능 콘크리트 RC기둥부재의 내화특성을 검토하였는데, 그 결과를 요약하면 다음과 같다. 유동성 및 공기량은 모두 목표 범위를 만족하였고, 압축강도는 재령 28일에서 80 MPa이상으로 고강도범위를 나타내었다. 폭렬 특성으로는 섬유를 혼입하지 않은 플레인의 경우는 심한 폭렬이 발생하였고, 나일론(이하 NY)+폴리프로필렌(이하 PP) 섬유를 0.05%를 혼입한 경우에는 표면부에만 부분적인 박리폭렬이 발생하였다. 비탈형 거푸집 변화에 따라서는 비탈형 25-20, 비탈형 50-20의 경우에는 모두 마 감재부분에 폭렬이 발생하였는데, 비탈형 50-20의 경우가 비탈형 25-20보다 양호한 폭렬방지 성능을 나타내었다. 비탈형 50-40의 경우는 폭렬이 방지되었는데 질량감소율은 10%이하로 나타났고, 온도이력은 모재표면부 최고온도가 $376.1^{\circ}C$로 가장 우수한 내화 성능을 나타내었다.

• 한국화재소방학회논문지
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• 제23권2호
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• pp.27-35
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• 2009

본 연구는 질석계 내화피복재를 뿜칠한 단순지지 TSC 합성보에 대해 하중 재하의 영향과 합성보의 형상, 웨브에 설치한 메탈라스의 영향 등을 변수로 내화성능 평가를 하였다. 내화피복재는 3시간 내화성능의 요구치가 40mm 이나 25mm만 뿜칠 후 내화실험을 실시하였다. 또한 강재 표면이 부착강도에 미치는 영향도 확인하였다. 실험결과 TSC 합성보에 25mm의 내화피복재를 뿜칠한 경우 재하와 비재하시험 모두 3시간의 내화성능을 확보하는 것으로 나타났다. 재하와 비재하에 대한 내화성능 기준으로 온도를 비교하면, 평균온도는 $250^{\circ}C$ 이하, 최고온도는 $310^{\circ}C$ 이하가 되었다. 또한 보의 형상 및 메탈라스의 부착 유소(昭)' 등에 의한 온도의 영향은 없었다.