• 열처리공학회지
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• 제26권6호
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• pp.293-299
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• 2013

A specimen of weld metal was prepared by GTA welding with weld wire of super duplex stainless steel. Aging treatment was conducted for the sample at the temperature range of 700 to $900^{\circ}C$ for 5 to 300 minutes. The effect of volume fraction of ${\sigma}$-phase to intergranular corrosion of weld metal has been investigated and the results were derived as follows. The volume fraction of ${\sigma}$ phase tends to increase with an increase of aging temperature and time and intergranular corrosion of weld metal was increased by an increase of ${\sigma}$ phase. Degree of sensitization representing intergranular corrosion was found to tend to increase with an increase of aging time at 700 to $800^{\circ}C$, while it decreased by an increase of aging time at $900^{\circ}C$.

• 한국자동차공학회논문집
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• 제10권2호
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• pp.186-192
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• 2002

The fatigue crack propagation characteristics were investigated on dissimilar friction weld of two kind of heat-resisting steels (STR3 and STR35) commonly used in valve materials for vehicles. A small circular artificial defect was machined to induce fatigue crack at bonded line, heat affected zone and base metal of the weld on the surface of the specimens. From the results of the experiment, the fatigue limits of the materials STR3 and STR35 were obtained to be 429.0MPa and 409.4MPa respectably. The STR35 base metal and 1.0mm HAZ specimens showed 190% and 82% higher fatigue life than STR3 base metal. And the fatigue life of 1.0mm HAZ specimen was shown 99% on STR3 and 29% on STR35 higher than that of their base metal. But the fatigue life of weld interface specimen was shown 18% on STR3 and 72% on STR35 lower than that of their base metal because of the weld interface separation.

• Journal of Advanced Marine Engineering and Technology
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• 제11권3호
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• pp.45-52
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• 1987

Generally the factors affected largely by the cold cracking sensitivity of the weld are the quantity of the diffusible hydrogen, the brittleness and hardness of the bond area and the tensile restraint stress. These factors have relation each other, and if we can reduce one of these factors, it becomes instrumental to the root cracks prevention of weld. This study deals with the gravity type-underwater-welding of KR Grade A-3 marine steel plate using E4303 welding electrode in order to compare wet-underwater-welding with in-air- welding, resulting in obtaining the tensile restraint characteristics, the hardness distribution, the quantity of diffusible hydrogen and the macro- and micro-crack properties in both underwater and in-air welds. The main results obtained are as follows: 1) The quantity of diffusible hydrogen measured for 48 hours is about 18cc/100g-weld-metal for the in-air-weld of one pass and about 48cc/100g-weld-metal for the underwater-weld of one pass which is about 3 times penetration of diffusible hydrogen compairing with the case of the in-air-weld. However, it was experimentally confirmed that, by the multi-pass welding of 2 to 5 passes, the diffusible hydrogen in the underwater weld metal can be reduced as much as 27 to 49%. 2) The hardness of the weld metal indicates the highest value in the heat affected zones of underwater weld for more rapid cooling rate, resulting in the higher sensitivity of cold cracking. So, it is desirable to soften the higher hardness in the HAZ by tempering effect such as the multi-pass welding in the underwater welding. 3) At the bond vicinity of the underwater weld HAZ, micro cracks were found as resulted by both more rapid cooling rate and more diffusible hydrogen and also by the stress corrosion cracking under the tensile restraint stress in the underwater. But this could be prevented by the tempering effect of the following weld bead such as the multi-pass welding.

• 대한기계학회논문집A
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• 제37권5호
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• pp.649-655
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• 2013

이종금속용접부에 대한 실제 용접 공정 중 용접부에서 결함이 발견되면 이를 제거하고 보수용접이 수행된다. 일반적으로 보수용접을 수행하면 용접부에서 인장 잔류응력이 크게 증가될 수 있는 것으로 알려져 있다. 따라서 Alloy 82/182를 사용하여 보수용접이 수행된 이종금속용접부의 일차수 응력부식 균열 현상을 평가하기 위해서는 보수용접에 의한 용접부의 응력 변화를 정확하게 평가해야 한다. 본 논문에서는 비선형 유한요소해석을 수행하여 보수용접에 의한 원자력 이종금속 맞대기 용접부의 응력 분포를 평가하였다. 특히 보수용접 공정 모사를 위한 여러 유한요소 해석방법이 이종금속용접부의 응력분포에 미치는 영향을 평가하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회A
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• pp.534-537
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• 2008

In recent years, the dissimilar metal, Alloy 82/182 welds used to connect stainless steel piping and low alloy steel or carbon steel components in nuclear reactor piping system have experienced cracking due to primary water stress corrosion(PWSCC). It is well known that one reason of the cracking is the residual stress by the weld. But, it is difficult to estimate exactly weld residual stress due to many parameters of welding. In this paper, the analysis of 3 FEM models made by ABAQUS Code is performed to estimate exactly the weld residual stress on the dissimilar metal weld. 3 FEM models are Butt model, Repair model and Overlay model and are the plane.strain 2D model. The thermal analysis and the stress analysis are performed on each model and the residual stresses on each model were calculated and compared respectively. Also, the specimen of Butt model was made and the residual stresses were measured by X-Ray method and Hole Drilling Technique. These results were compared with the FEM result of Butt model.

• 한국압력기기공학회 논문집
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• 제7권4호
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• pp.1-11
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• 2011

This paper presents the effects of preventive maintenance schemes on the residual stress distributions in dissimilar metal welds. Dissimilar metal weld is known susceptible to PWSCC and thus, effective maintenance schemes to prevent PWSCC are needed. Three preventive maintenances schemes, i.e. weld overlay, MSIP and inlay weld which are widely used in nuclear power plants, are selected and their effects on welding residual stresses are investigated via finite element analyses. As results, weld overlay and MSIP were proved effective method to mitigate residual stresses and inlay weld, on the other hand, produces strong tensile residual stresses in the inner surface. Although Alloy 690 known to be resistant to PWSCC are used in inlay weld, continuous careful observation are needed since tensile welding residual stresses are key parameter for PWSCC.

• 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.

• Journal of Welding and Joining
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• 제31권6호
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• pp.112-118
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• 2013

In this study, constant loading test (CLT) was performed to evaluate the hydrogen embrittlement resistance for multipass FCA weld metals of 600MPa tensile strength grade. The microstructures of weld metal-2 having the smallest carbon equivalent (Ceq=0.37) consisted of grain boundary ferrite and widmanstatten ferrite in the acicular ferrite matrix. The weld metal-1 having the largest Ceq=0.47, showed the microstructures of grain boundary ferrite, widmanstatten ferrite and the large amount of bainite (vol.%=19%) in the acicular ferrite matrix. The weld metal-3 having the Ceq=0.41, which was composed of grain boundary ferrite, widmanstatten ferrite, and the small amount of bainite (vol.%=9%) in the acicular ferrite matrix. Hydrogen desorption spectrometry (TDS) used to analyze the amount of diffusible hydrogen and trapping site for the hydrogen pre-charged specimens electrochemically for 24 hours. With increasing the current density of hydrogen pre-charging, the released amount of diffusible hydrogen was increased. Furthermore, as increasing carbon equivalent of weld metals, the released diffusible hydrogen was increased. The main trapping sites of diffusible hydrogen for the weld metal having a low carbon equivalent (Ceq=0.37) were grain boundaries and those of weld metals having a relatively high carbon equivalent (Ceq: 0.41~0.47) were grain boundaries and dislocation. The fracture time for the hydrogen pre-charged specimens in the constant loading test was decreased as the carbon equivalent increased from 0.37 to 0.47. This result is mainly due to the increment of bainite that is vulnerable to hydrogen embrittlement.

• 한국생산제조학회지
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• 제9권3호
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• pp.160-168
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• 2000

Hydroforming is a metal forming process that enables circular metal tubes to be formed in to the parts with the complex cross section along the curved axial direction. Recently this hydroforming process is largely used for the production of the automotive parts. This paper presents the results of tube bending and hydroforming simulations in cases of the varying weld line positions of the tube. Ten cases of prebending and hydroforming simulations are carried out to find the optiaml weld line position.

• 한국재료학회지
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• 제14권4호
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• pp.239-245
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• 2004

Resistance spot weldability of coated steels for automobile applications was investigated. Test samples were prepared from commercial products of 0.8 mm in thickness. Based on the tensile-shear test, surface quality examination and cracking behavior, it was clear that aluminized steels showed good weldability. Microstructural inspection revealed that the coated materials was piled up at the split zone in the welding of aluminized steels. It was also demonstrated that no weld crack was found in the aluminized steel weld metal even the welding was carried out with higher current. However, through thickness cracks were detected at the weld metal of zinc coated steel. Small particles were found on the crack surface of zinc coated steel weld metal. It was thought that zinc vapor played key role to form the weld crack.