• Transactions of the Korean Society of Pressure Vessels and Piping
• /
• v.14 no.1
• /
• pp.58-65
• /
• 2018

The purpose of this study is to develop a Half Nozzle Repair(HNR) process to prevent the leakage from welds on small bore piping in Reactor Coolant System. The Codes & Standards of tempered bead and design requirements of J-Groove welds are reviewed. Automatic machine GTAW welding and machining equipments are developed to perform HNR process. Single pass welding and overlay welding equipments are conducted in order to obtain the optimal temper bead welding process parameters with Alloy 52M filler wire. Coarse grain heat affected zone(CGHAZ) is formed by rapid cooling rate in heat affected zone after welding. Accordingly, a proper temper bead technique is required to reduce CGHAZ in 1-Layer of welds by 2- and 3-Layers. Mock-up tests show that the developed HNR process is possible to meet ASME Code & Standard requirements without any defect.

• International Journal of Korean Welding Society
• /
• v.1 no.1
• /
• pp.58-62
• /
• 2001

The surface of AC8A Ah alloy was modified by adding the Cu powder using a Plasma Transferred Arc (PTA) welding process. Under the optimum fabricating conditions, the modified surface of AC8A Ah alloy was observed to possess the sound microstructure with a minimum porosity. Hardness and wear resistance properties of the as-fabricated alloy were compared with those of the 76 heat-treated one. In case of the as-fabricated alloy, the hardness of the modified layer was twice that of the matrix region. Although significant increase in the hardness of the matrix region was observed after T6 heat treatment, the hardness of the modified layer was not observed to change. The wear resistance of the modified layer was significantly increased compared to that of the matrix region. The microstructure of a weld zone and the matrix region were investigated using the optical microscope, scanning electron microscope (SEM), electron probe microanalysis (EPMA), and transmission electron microscope (TEM). The primary and eutectic silicon in the weld zone were finer and more curved than in the matrix region, while some precipitates has had been found therein. According to the TEM observation, the predominant precipitate present in the weld zone was the $\theta$'phase, which is precipitated during cooling by rapid solidification in PTA welding process. Improvement of hardness and wear properties in the weld zone in the as-fabricated condition can be explained based on the presence of $\theta$’precipitates and fine primary and eutectic silicon distribution.

• Journal of Welding and Joining
• /
• v.34 no.5
• /
• pp.13-18
• /
• 2016

Toshiba has developed a laser peening system for PWRs(pressurized water reactors) as well after the one for BWRs(boiling water reactors), and applied it for BMI(bottom-mounted instrumentation) nozzles, core deluge line nozzles and primary water inlet nozzles of Ikata Unit 1 and 2 of Shikoku Electric Power Company since 2004, which are Japanese operating PWR power plants. Laser pulses were delivered through twin optical fibers and irradiated on two portions in parallel to reduce operation time. For BMI nozzles, we developed a tiny irradiation head for small tubes and we peened the inner surface around J-groove welds after laser ultrasonic testing (LUT) as the remote inspection, and we peened the outer surface and the weld for Ikata Unit 2 supplementary. For core deluge line nozzles and primary water inlet nozzles, we peened the inner surface of the dissimilar metal welding, which is of nickel base alloy, joining a safe end and a low alloy metal nozzle. In this paper, the development and the actual application of the laser peening system for PWR power plants will be described.

• Proceedings of the KSME Conference
• /
• 2004.04a
• /
• pp.90-95
• /
• 2004

In this study, we proposed new method to mitigate tensile welding residual stress for preventing PWSCC in CRDM nozzle. Residual stress analysis using finite element method is performed to confirm benefit of the new method. In case of applying existing method, tensile axial residual stress decrease by about 28% and tensile hoop residual stress decrease by about 33%. In case of applying the new method, tensile axial residual stress decrease by about 32% and tensile hoop residual stress decrease by about 43%. Therefore, we conclude the new proposed method is more effective to prevent PWSCC than existing method.

• Proceedings of the KWS Conference
• /
• 2001.05a
• /
• pp.228-231
• /
• 2001

Elastic-plastic fracture toughness $J_{1c}$ can be used as an effective design criterion in elastic plastic fracture mechanics. Most of these systems are operated at high temperature and $J_{1c}$ values are affected by temperature. therefore, the $J_{1c}$ valuse at high temperature must be determined for use of integrity evaluation and designing of such systems. Elastic-plastic fracture toughness $J_{1c}$ tests were performed on SA516 carbon steel plate and test results were analyzed according to ASTM E 813-8, ASTM 1813-89. Safety and integrity are required for reactor pressure vessels vecause pthey are operated in high temperature. there are single specimen method, which used as evaluation of safety and integrity for reactor pressure vessels. In this study, elastic-plastic fracture toughness$(J_{1c})$ and $J-\Delta{a}$ of SA 516/70 steel used as reactor pressure vessel steel are measured and evaluated at room Temperature, $150^{\circ}C$, $250^{\circ}C$ and $370^{\circ}C$ according to unloading compliance method.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.35 no.10
• /
• pp.1337-1345
• /
• 2011

In pressurized water reactors (PWRs), the reactor pressure vessel (RPV) upper head contains numerous control rod drive mechanism (CRDM) nozzles. In the last 10 years, the incidences of cracking in alloy 600 CRDM nozzles and their associated welds has increased significantly. Several axial and circumferential cracks have been found in CRDM nozzles in European PWRs and U.S. nuclear power plants. These cracks are caused by primary water stress corrosion cracking (PWSCC) and have been shown to be driven by welding residual stresses and operational stresses in the weld region. Therefore, detailed finite-element (FE) simulations for the Korea Nuclear Reactor Pressure Vessel have been conducted in order to predict the magnitudes of the weld residual stresses in the tube materials. In particular, the weld residual stress results are compared in terms for nozzle location, geometry factor$r_o$/t, geometry of fillet, and adjacent nozzle.

• Proceedings of the KWS Conference
• /
• 2009.11a
• /
• pp.63-63
• /
• 2009

최근 연비 향상 및 배기가스 저감을 위한 친환경 경량 굴삭기에 대한 연구가 활발히 진행되고 있다. 이러한 시도는 굴삭기의 소재의 강도를 490MPa급에서 700MPa급으로 고강도화를 통하여 작업장치의 경량화를 도모하고 있다. 본 연구에서는 중장비용 고강도 열연강재로 재발중인 ATOS70강재의 기본 용접성 및 GMAW 용접부 특성을 검토하였다. 사용한 시험재는 현장시험재인 14~16mmt두께의 ATOS70강재를 사용하였고, 용접경화성 및 저온균열감수성을 평가하였다. 또한 GMAW 용접을 실시하여 용접부의 이음부 특성을 검토하였다. 14mmt 두께의 ATOS70강재의 탄소당량은 약 0.44수준이고, 모재 인장강도는 약 760MPa급 수준을 보였다. 한편 최고경도시험에 의한 용접부 최도경도는 약 300Hv 수준을 보였으며, 경사 y-groove구속시험에 의한 14mmt두께의 한계예열온도는 상온이었다. 한편 GMAW 용접부 인장시험결과 740MPa급 이상의 인장강도를 확보하였고, $-5^{\circ}C$ 용접부 Charpy 충격시험결과 48J 이상의 충격인성을 나타내었다.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.37 no.9
• /
• pp.1159-1168
• /
• 2013

In pressurized water nuclear reactors (PWRs), the reactor pressure vessel (RPV) upper head contains penetration nozzles that use a control rod drive mechanism (CRDM). The penetration nozzle uses J-groove weld geometry. Recently, the occurrence of cracking in alloy 600 CRDM penetration nozzle has increased. This is attributable to primary water stress corrosion cracking (PWSCC). PWSCC is known to be susceptible to the welding residual stress and operational stress. Generally, the tensile residual stress is the main factor contributing to crack growth. Therefore, this study investigates the effect on weld residual stress through different analysis methods for normal operating conditions using finite element analysis. In addition, this study also considers the effect of repeated normal operating condition cycles on the weld residual stress. Based on the analysis result, this paper presents a normal operating condition analysis method.

• Proceedings of the KWS Conference
• /
• 2005.06a
• /
• pp.185-187
• /
• 2005

The reactor pressure vessel head of PWR has penetrations for control rod drive mechanism and instrumentation systems. The Primary coolant water and operating temperature can cause the stress-corrosion cracking of these nickel-based alloy penetrations. It is difficult to detect and size flaws such as SCC in the reactor head penetrations using conventional W methods because of complex geometry, Therefore, the utilities are using the TOFD technique for the detection and sizing of the flaw. This study shows the correlation between the ultrasonic wave direction and the orientation of the flaw and the range of flaw depth which can be detected by the TOFD techniques.

• Proceedings of the KWS Conference
• /
• 2009.11a
• /
• pp.46-46
• /
• 2009

과거 고강도강 용접부에서 발생하는 저온균열은 주로 용접열영향부에서 발생하였는데, 이러한 문제점을 해결하기 위하여 강재 메이커들은 고강도강의 용접성을 향상시키고자 노력하였다. 이러한 노력의 결과로 TMCP, HSLA 강 등이 개발되었고 이들 강재는 예열온도를 저하시킬 수 있다는 장점 때문에 보편화되어 사용되었다. 이러한 강재는 모재 예열온도를 기준으로 적용하게 되면 용착금속에서 저온균열이 발생하는 경우가 있다. 따라서 이제는 용접재료의 용접성, 즉 용접재료의 저온균열 저항성을 평가 할 수 있는 기법이 요구된다. 본 연구의 목적은 용착금속의 저온균열 저항성을 평가하는 것인데, 저온균열 저항성은 용착금속의 미세조직에 따라 다르게 나타날 수 있다. 용착금속의 합금조성은 기본적으로 용착금속에 요구되는 최저 강도와 충격인성을 만족할 수 있도록 설계한다. 하지만 유사한 강도의 유사한 합금조성이더라도 일부 합금 성분에 의해 용착금속의 미세조직들은 상이하게 나타날 수 있는데, 미세조직 특성에 의하여 용착금속의 강도와 저온인성이 결정된다. 용착금속의 저온균열 저항성을 평가하기위하여 Gapped Bead-on-Groove(G-BOG) 시험에 사용된 모재는 50mm 두께의 mild steel을 사용하였으며, 모재의 희석을 방지하기위해 15mm 깊이로 V-groove 가공 후 buttering 용접 하였다. 용접된 시편은 다시 5mm 깊이로 V-groove로 2차 가공 후 Ar + 20% $Co_2$ gas를 사용하여 용접하였다. 용접재료는 ER-100S-G grade로 비슷한 합금조성을 갖는 2 종류를 사용하였다. A용접재료는 Ti 이 0.1% 함유 되었으며, B용접재료는 Ti 함유되지 않은 것을 사용하였다. 또한 예열 온도에 따라 저온균열 감수성을 평가하기위하여 모재의 예열온도를 각각 상온, $50^{\circ}C,\;75^{\circ}C,\;100^{\circ}C$로 하여 실험을 진행하였다. 용착금속의 미세조직을 확인해본 결과 Ti 함유된 A 용착금속 미세조직은 대부분 침상형페라이트로 나타났으며, Ti 함유되지 않은 B 용착금속 미세조직은 대부분 베이나이트로 나타났다. G-BOG 시험 결과 Ti 함유된 A 시편이 Ti 함유되지 않은 B 시편보다 저온균열 발생량이 적었다. 이는 용착금속의 미세조직분포 및 특성에 따라 저온균열감수성이 다르다는 것을 나타낸다.