• 한국해양공학회지
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• 제25권6호
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• pp.66-71
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• 2011

Recently, the production of shipbuilding and offshore plant industries, with a trend toward large structures, has led to an increased use of high strength ultra-thick plates. The use of ultra-thick plates increases the welding tasks, and the welding process generates distortion and residual stress in the weldment because of the rapid heating and cooling. Welding distortion and residual stress in the welded structure resulte in many troubles such as deformation and life deterioration. In particular, the welding residual stress has an important effect on welding deformation, fatigue, buckling strength, brittleness, etc. The purpose of this study was to evaluate the residual stress at a multi-pass weldment using an experimental method for EH36 high-tension steel. In this experimental method, AIS3000 was used to measure the residual stress of a welded part, HAZ, and base metal; EPMA and XRD were used to study the material properties.

• Journal of Advanced Marine Engineering and Technology
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• 제37권1호
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• pp.29-34
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• 2013

용접과정 중에 용접부가 공기에 노출되면 용접성능을 약화시킨다. 불활성가스 텅스텐 아크용접(GTAW)법을 사용하여 티타늄 용접을 하는 경우 용접부가 자주/적색, 청색, 노란색, 회색, 흰색 등의 빛깔을 보이면 이는 용접 시 공기 중의 산소와 질소에 오염된 것을 의미한다. 따라서 용접부를 대기로부터 보호하기 위한 방법으로 실딩 가스를 사용한다. 이것은 대기에 의한 오염을 방지할 뿐만 아니라 용착부와 열영향부도 상온까지 냉각될 때까지 대기로부터 차단시킨다. 본 연구에서는 Trailing Shielding Jig 의 모양과 형태 그리고 토치보호($13{\sim}20{\ell}/min$), 후방보호($22{\sim}30{\ell}/min$), 이면보호($25{\sim}30{\ell}/min$)등 건전한 용접부에 적합한 실딩 가스에 대한 결과를 도출하였다.

• 연구논문집
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• 통권23호
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• pp.127-140
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• 1993

The drive pulley, which is employed for loading and unloading raw materials in a steel mill, is usually manufactured by use of various welding processes. In this study the weldment in the pulley, in which TIG and $CO_2$ welding processes are used, has been analyzed from view point of fracture mechanics. Fracture toughness tests have been performed according to ASTM E813. A servo-hydraulic testing machine (10kN) has been employed. Also the crack propagation tests (Mode I) have been performed with compact tension specimen in compliance with ASTM E647. To predict the critical crack size in the weldment, finite element stress analysis for the drive pulley under real operating conditions have been performed. In addition, the residual stresses at the weldment and in heat-affected zone have been obtained by hole drilling method. The planar critical crack size have been predicted for the drive pulley by considering the stress analysis results and the residual stresses due to welding process. For the drive pulley considered in this study, it has been concluded that the most important factor in determining the critical crack size is the welding residual stress in the transverse direction. Also the effect of stress concentration at the root of the weldment have been noticeable. For the planar crack, the fatigue crack growth life from an initial crack size of 2mm to the critical crack size obtained as in the above have been predicted. The predicted lives were between 55, 900 and 72, 000 cycles depending on the shape of the elliptical crack. The predicted lives were in fairly good agreement for the drive pulley considered in this study.

• 한국재료학회지
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• 제30권4호
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• pp.184-196
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• 2020

In this study, effects of carbon and nickel on microstructure and low temperature Charpy impact properties of HSLA (high strength low alloy) steels are investigated. To understand the complex phase transformation behavior of HSLA steels with high strength and toughness before and after welding processes, three kinds of HSLA steels are fabricated by varying the carbon and nickel content. Microstructure analysis, low temperature Charpy impact test, and Vickers hardness test are performed for the base metals and CGHAZ (coarse-grain heat affected zone) specimens. The specimens with the lowest carbon and nickel content have the highest volume fraction of AF, the lowest volume fraction of GB, and the smallest GB packet size. So, the low temperature Charpy absorbed energy of the CGHAZ specimen is the highest. The specimens with increased carbon and nickel content have the lowest volume fraction of AF, the highest volume fraction of GB, and the largest GB packet size. So, the low temperature Charpy absorbed energy of the CGHAZ specimen is the lowest.

• Journal of Welding and Joining
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• 제27권3호
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• pp.73-79
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• 2009

This present paper investigated the mechanical properties and the microstructures of each penetration shapes classifying the conduction shape area and the keyhole shape area about electron beam welded 120(T)mm thick plated aluminum 5052 112H. As a result the penetration depth is increased linearly according to the output power, but the aspect ratio is decreased after the regular output power. In the conduction shape area, the Heat affected zone is observed relatively wider than the keyhole shape area. In the material front surface of the welded specimen, the width is decreased but the width in the material rear surface is increased. After the measuring the Micro Vikers Hardness, it showed almost similar hardness range in all parts, and after testing the tensile strength, the ultimate tensile strength is similar to the ultimate tensile strength of the base material in all the specimens, also the fracture point was generated in the base materials of all the samples. In the result of the impact test, impact absorbed energy of the Keyhole shape area is turned up very high, and also shown up the effect about four times of fracture toughness comparing the base material. In the last result of observing the fractographs, typical ductile fraction is shown in each weld metal, and in the basic material, the dimple fraction is shown. The weld metals are shown that there are no other developments of any new chemical compound during the fastness melting and solidification.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.180-185
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• 2001

In the life assessment for plant structural component, the research on deterioration of toughness and material properties occurred in weldments has been considered as very important problems. In general, the microstructures composed in weldments are hugely classified with weld metal(W.M), fusion line(F.L), heat affected zone(HAZ), and base metal(B.M). It has been reported that the creep characteristics on weldments having variable microstructures could be unpredictably changed. Furthermore, it is also known that HAZ adjacent to F.L exhibits the decreased creep strength compared to those in base or weld metals, and promotes the occurrence of Type III and Type IV cracking due to the growth of grains and the coarsening carbides precipitated in ferritic matrix by welding and PWHT processes. However, the lots of works reported up to date on creep damage in power plant components have been mostly conducted on B.M and the creep properties on a localized microstructures in weldments have not as yet been throughly investigated. In this paper, for various microstructures such as coarse grain HAZ(CGHAZ), W.M and B.M in X20CrMoV121 steel weldment, the small punch-creep(SP-Creep) test using miniaturized specimen(t=0.5mm, 0.25mm) is performed to investigate a possibility for creep characteristics evaluation.

• 한국레이저가공학회지
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• 제10권4호
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• pp.13-17
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• 2007

Electron beam weldability was investigated for 1mm thick cold rolled sheets of commercially pure grade titanium and Ti-6Al-4V alloy. Accelerating voltage of 40kV, beam current of 6mA, and weld speed of 0.8m/min was used and focal position of focused electron beam was just on the surface of workpiece. Microstructure of weld metal, the heat affected zone and base metal was observed using optical microscope. Vickers hardness was measured across the welds and the transverse tensile test was carried out. Hydroformability test was also carried out for the butt welded coupons of commercially pure grade titanium. For the electron beam welded C P Ti, the average grain size was equiaxed $\alpha(15{\sim}25{\mu}m)$ for base metal, coarse equiaxed $\alpha(80{\sim}200{\mu}m)$ for weld metal and annealed and enlarged grain($40{\sim}120{\mu}m$) for the HAZ. The vickers hardness of C P Ti was $180{\sim}200Hv$ for base metal, and $160{\sim}180Hv$ for the weld metal and the HAZ. For the electron beam welded Ti-6Al-4V alloy, the vickers hardness was 360Hv for the base metal, abd $400{\sim}425Hv$ for the weld metal and the HAZ. All the failure occurred at the base metal, when the transverse weld tensile test was carried out for both electron beam welded C P Ti and Ti-6Al-4V alloy. The formability of electron beam welded C P Ti was decreased compared with that of C P Ti base alloy.

• 한국해양공학회지
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• 제24권2호
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• pp.57-61
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• 2010

At present, welding technology is not only emphasized in the development of manufacturing technology but its application is also expanding. In these systems, attempts have been made to use SUS as a high-temperature material for special applications, and in improvements in manufacturing technologies bear watching, together with an increase in the use rate. Specifically, three-dimensional wings are often used for fasteners with the purpose of decreasing the weight (정해용, 2007). However, due to developments in welding technology, there has been a recent tendency to replace existing assembly methods with welding. Specifically, if laser welding techniques are applied, the heat-affected zone can be minimized compared toother welding techniques. However, in the case of these special welding techniques, there is an increase in residual stress, which fatally affects the fatigue life. In order to remove the residual stress and its effect on fatigue life, shot-peening is executed. The intention of this study was to obtain the optimal conditions for shot-peening.

• 대한토목학회논문집
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• 제4권3호
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• pp.19-25
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• 1984

용접구조에 있어서 가장 위험한 단면은 용접부이며 대부분의 파괴가 용접부로부터 발생하였다. 따라서 용접부의 파괴거동을 확인하는 것은 파괴방지를 위하여 중요한 요소라 할 수 있다. 본 연구는 용접부의 파괴거동에 관한 기초적인 연구로서 용접부의 국부적인 인성분포와 다층용접의 효과 및 잔류응력제거풀림의 효과를 조사하였다. 연구에 사용된 기본소재로서는 피로이력을 갖고 있는 강재와 피로이력이 없는 강재를 선택하였다. 용접부에서 미사용강재(피로이력이 없는 강재)모재, 열영향부 용착금속의 파괴인성이 각각 차이가 있었고 용착금속은 균열저지능력으로서 파괴인성이 가장 작아서 파괴발생원이 될수 있음을 확인하였다. 특히 본연구의 대상강재인 기사용강재(피로이력을 갖고 있는 강재)는 파괴인성이 용착금속 보다 작아서 균열이 존재하는 경우 가장 취약한 단면이 될수 있다는 것을 확인하였다. 다충용접은 용접부에 결합이 존재하지 않는다면 안정개선에 효과가 있음을 확인하였고 또한 잔류응력제거 풀림은 본연구에 관한 한은 효과가 없는 것으로 밝혀졌다.

• 한국기계가공학회지
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• 제19권7호
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• pp.89-97
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• 2020

Container vessels continue to grow in size, led by global shipowner. Large ships can be loaded more cargo at a time, reducing the cost of transportation per teu. this eventually leads to economies of sale, in which the production cost per unit decreases with increasing output. in accordance with the 70th Convention of the Marine Environment Protection Committee of the International Maritime Organization, as of January 1, 2020, MARPOL Annex VI Regulation 14.1.3 will be effective. All vessels must be meet these criteria to reduce Sox emissions and reduce NOx emissions by reducing the content of manned sulfur oxides from 3.5% to less than 0.5%, otherwise IACS Member States Entry to the port is denied. in order to do that need to LNG storage tank. in this study characteristic of the material after line heating (600℃,700℃,800℃,900℃) of 9% Ni steel used in the manufacture of LNG fuel tank of ship were verified using by mechanical test. In the heating method by line heating. The initial properties of steel are changed by variables such as temperature, time, speed. The experimental data of line heating presented in this paper confirmed that the initial change of 9% Ni steel could be minimized.