• Journal of Advanced Marine Engineering and Technology
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• 제36권8호
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• pp.1076-1082
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• 2012

오늘날 LNG선의 용접에는 아크와 플라즈마가 사용되고 있으나 아크용접은 에너지 밀도가 낮아 후판에 대해서 다층용접이 불가피하며, 고밀도 열원인 레이저 용접에 비하여 용접속도에도 한계가 있다. 따라서 후판 용접시 다층용접에 의한 용접부의 조직적 결함이나 과대 입열로 형성된 열영향부 등의 문제를 해소하기 위하여 키홀용접에 의한 원패스 용접이 고려되고 있다. 키홀용접이 가능한 열원은 레이저, 전자빔, 플라즈마가 있으며, 현재 플라즈마 용접이 아크를 대체하여 LNG선 카고탱크의 멤브레인 용접에 적용되고 있다. 최근에는 멤브레인의 용접에 레이저를 적용하기 위한 많은 연구가 진행 중에 있다. 본 연구에서는 LNG선용 스테인리스강에 대한 파이버 레이저 및 플라즈마 아크 용접의 용접성, 기계적 성질 및 미세조직을 비교하였다. 그 결과 레이저 용접이 더 빠른 용접속도에서 좁은 용접부와 열영향부를 얻을 수 있었다. 따라서 LNG선의 용접에서는 파이버 레이저가 보다 우수한 용접법이라는 것을 알 수 있었다.

• Journal of Welding and Joining
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• 제25권1호
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• pp.42-48
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• 2007

For high performance and structural stability, application of high strength steel has continuously increased. However, the change of the base metal gives rise to problems with the accuracy management of the welded structure. It is attributed to the martensite phase transformation of the high strength low alloy steel weldment. The purpose of this study is to establish the predictive equation of transverse shrinkage and residual stress for the HY-100 weldment. In order to do it, high speed quenching dilatometer tests were performed to define a coefficient of thermal expansion(CTE) at the heating and cooling stage of HY-100 with various cooling rates. Uncoupled thermal-mechanical finite element(FE) models with CTE were proposed to evaluate the effect of the martensite phase transformation on transverse shrinkage and residual stresses at the weldment. FEA results were verified by comparing with experimental results. Based on the results of extensive FEA and experiments, the predictive equation of transverse shrinkage and longitudinal shrinkage force at the HY-100 weldment were formulated as the function of welding heat input/in-plane rigidity and welding heat input respectively.

• Journal of Welding and Joining
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• 제26권5호
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• pp.60-65
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• 2008

Ferritic stainless steels of the 400 series have been available for automotive exhaust system, heat exchanger, radiater etc. in various industrial because heat resistance, corrosion resistance and strength are excellent. Especially, automotive exhaust system is required good heat resistance because typical temperature of exhaust system exposed during operation of engine is reach up to $800^{\circ}C$. However, research for effect of high temperature in ferritic stainless steels is not enough. In this study, high temperature tensile properties of lap weld of ferritic stainless steels(STS 429) were investigated. In accordance with heat input, lap welds had been produced and were evaluated at high temperature($800^{\circ}C$) to compare high temperature tensile properties. In addition, room temperature tensile tests were carried out for non-aging and aging specimens. As a result of R.T tensile test, non-aging specimens were fractured in base metal except for low heat input specimen and aging specimens were fractured in weld metal. Also high temperature tensile test were carried out by aging specimen. After high temperature tensile test, fracture of aged specimen was occurred in base metal except for low heat input specimen. Fracture surface of low heat input specimen in weld metal was confirmed as brittle fracture with observation using scanning electron microscope(SEM). Significant decrease in ultimate tensile strength (between 82 and 85%) was observed for aged ferritic stainless steels(STS 429) when tested at high temperature.

• Journal of Welding and Joining
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• 제20권3호
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• pp.98-104
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• 2002

A study on microstructures of welds for Zircaloy-4 sheath end closure by the resistance upset welding methods was carried out. Two upset welding process variations such as magnetic farce and multi-impulse resistance welding were used. Grain size and microhardness across welds were analysed in terms of welding parameters. Magnetic farce resistance weld with one cycle of unbalanced mode has smaller upset length and $\alpha-grain$ size in heat affected zone than those of multi-impulse resistance weld because of lower heat input and shorter welding time. Heat affected zone formed by two upset resistance welding variations revealed fine Widmanstatten structure or martensitic ${\alpha}'$ structure due to the high heating rate and foster cooling rate. Magnetic force resistance welds showed recrystallized grains before grain growth, whereas multi-impulse resistance welds showed full grain growth.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2001년도 추계학술대회 논문집(Proceeding of the KOSME 2001 Autumn Annual Meeting)
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• pp.88-91
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• 2001

New type of vehicles made from thin aluminum alloy are under rapid development and some product are already on the market, but when welding sheet metal joint of aluminum alloy, the main problem are burn-through by the high heat input. The purpose of this study, it is to prevent the burn-through and to shallow penetration that AC pulse welding has diminish the penetration and to lower the droplet temperature at Al sheet metal welding. In this study, we observed the bead appearance and whether it was appeared burn through or not , as the variation of the root gap in the lap and butt-joint welding process.

• Journal of Welding and Joining
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• 제17권3호
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• pp.96-101
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• 1999

Microstructures and mechanical properties of weldments were studied for EH36 TMCP higher-strength hull steel with electrogas welding jprocess. In case of a newly designed EH36 TMCP steel for large heat input welding process, the Microstructures of HAZ shows more narrow width of grain coarsed region than that of conventional EH36 TMCP weldments, the amount of acicular ferrite, which is beneficial to impact toughness, increased while the amount of grain-boundary ferrite decreased. Charpy V-notched impact tests show that a newly designed EH36 TMCP steel weldment satisfies all the requirement of specifications, especially at the fusion line +2mm where the conventional EH36 TMCP steel fails to exceed the requirement.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 1997년도 특별강연 및 춘계학술발표 개요집
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• pp.151-153
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• 1997

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 1998년도 특별강연 및 춘계학술발표 개요집
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• pp.147-149
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• 1998

• International Journal of Korean Welding Society
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• 제2권1호
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• pp.21-24
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• 2002

The main purpose of this study was to investigate the formation mechanisms of imperfections such as irregular humps, outer cavity and inner cavity in the laser fusion zone of diamond saw blade. Laser beam welding was conducted to join two parts of blade; mild steel shank and Fe-Co-Ni sintered tip. The variables were beam power and travel speed. The microstructure and elements distributions of specimens were analyzed with SEM, AES, EPMA and so on. It was found that these imperfections were responded to heat input. Irregular humps were reduced in 10.4∼l7.6kJ/m heat input range. However there were no clear evidences, which could explain the relations between humps formation and heat input. The number of outer cavity and inner cavity decreased as heat input was increased. Considering both possible defects formations mechanisms, it could be thought that outer cavity was caused by insufficient refill of keyhole, which was from rapid solidification of molten metal and fast molten metal flow to the rear keyhole wall at low heat input. More inner cavities were found near the interface of the fusion zone and sintered segment and in the bottom of the fusion zone. Inner cavity was mainly formed in the upper fusion zone at high heat input whereas was in the bottom at low heat input. Inner cavity was from trapping of coarsened preexist pores in the sintered tip and metal vapor due to rapid solidification of molten metal before the bubbles escaped.

• 한국재료학회지
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• 제21권1호
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• pp.50-55
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• 2011

Energy resistance welding (ERW) is a pipe-producing process that has high productivity and low manufacturing cost. However, the high heat input of ERW degrades the mechanical property of the pipe. This study investigates the effect of heat input and alloying elements on microstructure and mechanical properties of ERW pipes. As the heat input increased, the ferrite amount increased. The ferrite amount in the weld centerline was larger than t at in the weld boundary. Medium carbon steels (S45C and K55) having 0.3~0.4wt.% carbon yielded a significant difference of ferrite amount in the weld centerline and weld boundary. High alloyed steels (DP780 and K55) having 1.5~1.6wt.% Mn showed a ferrite rich zone in the weld centerline. These phenomena are probably due to decarburization and demanganisation in the weld centerline. As the ferrite fraction increased, the hardness decreased a little for the S45C steels. In addition, DP780 steels and K55 steels showed that the hardness drops when those steels have a ferrite rich zone. But we demonstrated the good tensile property of the DP780 steels and K55 steels in which Mn is included.