• 한국해양공학회지
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• 제27권1호
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• pp.37-42
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• 2013

The presence of a crack can increase the local stress or strain, which can cause inelastic deformation and significantly reduce the life of a component or structure. Therefore, in this study, the fatigue crack growth (FCG) behaviors of friction stir welded Al 2024-T3 and Al 7075-T6 specimens were examined, with fatigue cracks growing parallel to the dynamically recrystallized zone at variable ${\Delta}K$ values and an R ratio of 0.3. In addition, the FCG values of the base metal Al 2024-T3 and Al 7075-T6 were tested under the same conditions and parameters as comparative groups. The results showed that compared with the base metal Al 2024 specimen, which had the best fatigue property, the welded specimen had only 88% of the fatigue cycles.

• 대한기계학회논문집
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• 제14권2호
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• pp.492-501
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• 1990

It was known that the fracture incidences of offshore structure were mostly originated from the surface defects. Especially, in the case of the welded structures, since the welded region has some defects and incomplete beads which are apt to behave like the surface cracks, it has been necessary to evaluate the environmental effects on crack growth at HAZ for the design crack growth behaviour at multi-pall HAZ for SWS41 steel under free corrosion and cathodic protection(-0.9V vs Ag/Agcl) conditions. The results are summarized as follows ; (1) Crack growth rate of the as weld in air was faster than that of the parent and PWHT specimens over all .DELTA.K rang. (2) In free corrosion test, surface crack growth rate of the as welded was decreased in comparison with that of the parents. (3) In fatigue test under cathodic protection, cathodic electric potential(-0.9V vs Ag/Agcl) for the SWS41 steel parent was effective, while for the as welded ineffective. (4) There was a tendency that the exponent(m) of the Paris' equation was decreased in order of microhardness magnititude in air and under cathodic protection conditions and vise versa in free corrosion. (5) Fracture surface has dimples and ductile striations in air test, but transgranular cracks and brittle striations under cathodic protection test.

• 한국공작기계학회논문집
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• 제11권3호
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• pp.65-71
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• 2002

Friction welding has may merits such as energy efficiency, simple processing, etc. but it is difficult to obtain good welding at the welded interfaces and heat affected zone. It is discovered that stress singularity exists at the interferes and heat affected zone. The computer program based on boundary element method is utilized in this study. A mathematical model is implemented based on results from several experiments performed at and around the welded interfaces and heat affected zone of disimilar metals under static and dynamic loadings. This stay is to investigate the characteristics of the deformation and fracture behavior around interfaces for friction welded materials under static tensile load. Also, the stress distribution at the tip of crack is analyzed by using BU based on Kelvin's solution of 2-dimensional binding zone. The results of BEM are identical with those in case of considering interfaces of both heat affected zone. Also, stress singularity at the tip of interfaces appears when the elastic modulus ratio is 1.07.

• 한국압력기기공학회 논문집
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• 제11권1호
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• pp.53-60
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• 2015

This study was investigated to develop process technology of laser cladding with austenite stainless steel for Weld Inlay repair of dissimilar metal weld in reactor vessel in/outlet nozzles. Weld Inlay experiments were performed by laser cladding repair system consisting of common manipulator, laser apparatus and welding process scheduler, etc. Single pass welding experiments were conducted in order to obtain the optimum welding process parameters for filler wires of ER309L and Alloy 52M before multi-layer laser cladding. Based on the above obtained results, multi-layer laser cladding experiments were carried out, and welding qualities for weld specimens were estimated by PT, OM, SEM and EDS analysis. Consequently, it was revealed that multi-layer laser cladding on austenite stainless steel using filler wires of ER309L and Alloy 52M could be possible to meet ASME Code standard without any weld defect.

• 대한조선학회논문집
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• 제33권4호
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• pp.75-86
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• 1996

최근 선박, 해양구조물, 항공기 및 원자력 플랜트 등 대형구조물의 사용조건이 극도로 가혹해짐에 따라 이들에 대한 신뢰성 및 안전성 보장이 심각한 문제로 대두되고 있다. 이들 대형구조물에 있어, 파손사례의 대부분은 어떤 형태로든 피로현상과 관련된 것으로서 수명 및 잔여수명의 예측과 함께 안정성 평가기술의 확립이 절실히 요구되고 있다. 그러나, 실제 대다수의 결함은 복수표면 균열 상태로 존재하며, 이들은 단일균열의 경우와는 달리 급속히 합체, 성장할 가능성이 높고 따라서, 관통수명 또한 짧아지며, 관통시 표면균열의 길이가 상대적으로 커져 불안정 파괴의 위험성도 그만큼 높아지게 된다. 본 연구에서는 평판 및 필릿 용접부에 존재하는 복수의 직렬표면균열을 대상으로 인접균열간 간섭효과, 합체현상을 고찰하여 피로균열 진전거동 및 피로수명 예측프로그램을 개발하고, 이를 기존의 실험결과와 비교, 평가하여 그 정도 및 실용성을 확인하였다.

• 한국해양공학회지
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• 제16권2호
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• pp.72-79
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• 2002

In this study, time-dependent distributions of temperature and stresses, in the box-column welded from ultra thick plates with center segregation, has been analyzed by the commercial finite element package SYSWELD+, for several types and angles of groove. The major points of investigation are the optimum type and angle of groove that minimize weld stress specially at the center segregation, as well as temperature distribution, residual stresses and changes in the mechanical properties. The results can be summarized as follows; 1) Generally the thermal cycle at the root of groove exhibits relatively rapid cooling pattern, however, most of the other part weldment have a slow cooling pattern in all groove types. 2) Most of the micro-structures of weldment are composed of ferrite and pearlite, meanwhile we could find martensite and bainite locally a the root of the groove. 3) Optimum groove type for high heat input welding of box-column corner is a double groove type, and the optimum angle for the groove is 30~$45^{\circ}$ that minimize deformation and weld stress at the center segregation.

• Corrosion Science and Technology
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• 제19권4호
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• pp.224-230
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• 2020

Stainless steel has excellent corrosion resistance. The drawback is that pitting occurs easily due to the concentration of chloride. In addition, corrosion of socket weld, which is structurally and chemically weaker than the other components of the pipe, occurs rapidly. Since these two phenomena overlap, pinhole leakage occurs frequently in the seawater pipe socket welds made of stainless steel at the power plants. To analyze this specific corrosion, a metallurgical analysis of the stainless steel socket welds, where the actual corrosion occurred during the power plant operation, was performed. The micro-structure and chemical composition of each socket weld were analyzed. In addition, selective corrosion of the specific micro-structure in a mixed dendrite structure comprising γ-austenite (gamma-phase iron) and δ-ferrite (iron at high temperature) was investigated based on the characteristic micro-morphology and chemical composition of the corroded area. Finally, the different corrosion stages and characteristics of socket weld corrosion are summarized.

• 한국안전학회지
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• 제23권6호
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• pp.28-33
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• 2008

This paper presents the characteristics of mechanical properties within the heat affected zones(HAZs) of dissimilar metal weld joint between SA508 Gr.1a and F3l6 stainless steel(SS) with Alloy 82/182 filler metal. Tensile tests were performed using small-size specimens taken from the heat affected regions close to both fusion lines of weld, and the micro-structures were examined using optical microscope(OM) and transmission microscope(TEM). The results showed that significant gradients of the yield stress(YS), ultimate tensile stress(UTS), and elongations were observed within the HAZ of SA508 Gr.1a. This was attributed to the different microstructures within the HAZ developed during the welding process. In the HAZ of F316 SS, however, the welding effect dominated the YS and elongation rather than UTS. TEM micrographs demonstrated these characteristics of the HAZ of F316 SS was associated with a dislocation-induced strain hardening.

• Journal of Welding and Joining
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• 제7권2호
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• pp.35-48
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• 1989

This study is concerned with a correlation of microstructure and local brittle zone (LBZ) in offshore structural steel welds. The influence of the LBZ on fracture toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in void and cleavage microcrack formation were also identified using notched round tensile tests and subsequent SEM observations. The LBZ in the HAZ of a multiphase welded joint is the interstitially reheated coarse grained HAZ, which is influenced by metallurgical factors such as effective grain size, the major matrix structure and the amount of high-carbon martensite-austenite (M-A) constituents. The experimental results indicate that Chirpy energy was found to scale monotonically with the amount of M-A constituents, confirming that the M-A constituent is the major microstructural factor controlling the HAZ toughness. In addition, voids and microcracks are observed to initiate at M-A constituents by the shear cracking process. Thus, the M-A constituent played an important role in initiating the voids and microcracks, and consequently caused brittle fracture.

• 비파괴검사학회지
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• 제14권3호
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• pp.151-156
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• 1994

액체나트륨 내부에서의 가시화, 온도 혹은 칫수 측정, 용접부의 비파괴검사 등을 위한 고온(약 $250{\sim}650^{\circ}C$) 액침 초음파센서의 개발이 시도되어 왔다. 본 연구에서는 도파띠를 이용한 고온 액침 초음파센서의 가능성을 수조에서 실험으로 확인하였다. 가장 낮은 차수의 반대칭 판파가 분산이 거의 없는 주파수 영역에서 이용되었다. 이 판파는 주파수가 2.3MHz인 빗살구조 탐촉자에 의해 두께가 1mm이고 폭이 13mm인 스테인리스강 도파띠 내에 여기되었다. 이 판파의 감쇠상수는 공기중에서 1.2dB/m이었고, 물속에서는 380dB/m이었다. $13mm{\times}39mm$ 크기의 빔방사부위로부터 270mm 떨어진 평면 반사체에 대해 25dB의 신호 대 잡음 비를 얻었다.