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The 460-MPa-class steel was developed by thermomechanical control process for shipbuilding, and the maximum plate thickness was 100 mm, which has the fine grain size as 5–20 µm. The surfaces were studied in terms of micro and nano structures, surface roughness, and surface energy to evaluate the effect of fracture toughness in large steel structure. The thick steel plate has possibility to occur unstable fracture because the fracture toughness will be decrease with increase of thickness. The increase in the temperature in thermomechanical control process accelerated the surface energy and created both micro and nano structures on the surfaces more effectively. It was effective to avoid brittle fracture in the base metal when the brittle crack was deviated into base metal. The developed 460-MPa-class steel plate improves the brittle fracture safety despite being a thick steel plate through the fine grain size. They had to be designed in such a manner as to avoid crack initiation, especially in welded joints. In this study, brittle crack arrest designs were developed for large weld construction using arrest design concept and micro and nano structures in high strength steel plate.

Recently, there have been the increase of ship size and the development of oil and gas in arctic region. These trends have led to the requirements such as high strength, good toughness at low temperature and good weldability for prevent of brittle fracture at service temperature. There has been the key issue of crack arrestability in large size structure such as container ship. In this report for the first time, crack arrest toughness of thick steel plate welds was evaluated by large scale ESSO test for estimate of brittle crack arrestability in thick steel plate. For large structures using thick steel plates, fracture toughness of welded joint is an important factor to obtain structural integrity. In general, there are two kinds of design concepts based on fracture toughness: crack initiation and crack arrest. So far, when steel structures such as buildings, bridges and ships were manufactured using thick steel plates (max. 80~100mm in thickness), they had to be designed in order to avoid crack initiation, especially in welded joint. However, crack arrest design has been considered as a second line of defense and applied to limited industries like pipelines and nuclear power plants. Although welded joint is the weakest part to brittle fracture, there are few results to investigate crack arrest toughness of welded joint. In this study, brittle crack arrest designs were developed for hatch side coaming of large container ships using arrest weld, hole, and insert technology.

This paper investigates effects of variables related to 3-D finite element welding residual stress analyses of penetration nozzles on residual stresses. Five variables are considered; (i) number of elements in the circumferential direction, (ii) number of beads in the circumferential direction, (iii) kinematic boundary conditions, (iv) circumferential modelling angle in 3-D finite element models and (v) detailed weld shape. Based on sensitivity analysis results, guidelines for 3-D finite element welding residual stress analyses of penetration nozzles are provided.

This study evaluated the brittle Crack Arrest Temperature (CAT) through an isothermal crack arrest test. Generally, the embrittled zone is required in a CAT test. According to conventional studies, the test method consists of arresting a double tension–type brittle crack with an Electron Beam melt-run, which acts as a crack initiator and crack runway. However, conventional CAT test methods have disadvantages in that they need additional processes to create an embrittled zone with Electron Beam Welding (EBW). A Local Temperature Gradient (LTG) system is developed to overcome the limitations of the conventional EBW embrittlement CAT test method. LTG systems do not require additional EBW to develop a brittle zone to initiate a brittle crack. In a LTG system, the LTG method is proposed instead of EBW to create the embrittled zone. The LTG system is free of flaws. In EBW, it is impossible to perform accurate CAT tests because the presence of defects affects the brittle crack arrest. In this study, a new CAT test method is introduced to evaluate brittle CATs more easily. Furthermore, both test methods were compared for the same specimens to verify the validity of the test method.

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.

Recently, several circumferential cracks were found in the control rod drive mechanism (CRDM) nozzles of U.S. nuclear power plants. According to the accident analyses, coolant leaks were caused by primary water stress corrosion cracking (PWSCC). The tensile residual stresses caused by welding, corrosion sensitive materials, and boric acid solution cause PWSCC. Therefore, an exact estimation of the residual stress is important for reliable operation. In this study, finite element simulations were conducted to investigate the effects of the tube geometry (thickness and radius) on the residual stresses in a J-groove weld for different CRDM tube locations. Two different tube locations were considered (center-hole and steepest side hill tube), and the tube radius and thickness variables (ro/t=2, 3, 4) included two different reference values (ro=51.6, t=16.9mm).

Primary water stress corrosion cracking (PWSCC) has been observed around the weld region of control rod drive mechanism (CRDM) nozzles in nuclear power plants overseas. The weld has a J-shaped groove and it connects the CRDM nozzle with the reactor vessel upper head (RVUH). It is a dissimilar metal weld (DMW), because the CRDM is made of alloy 600 and the RVUH is made of carbon steel. In this study, finite element analysis (FEA) was performed to estimate the stress condition around the weld region. Generally, it is known that a high tensile region is more susceptible to PWSCC. FEA was performed as for the condition of welding, hydrostatic test and normal operation successively to observe how the residual stress changes due to plant condition. The FEA results show that a high tensile stress region is formed around the weld starting point on the inner surface and around the weld stop point on the outer surface.

In nuclear power plants, the reactor pressure vessel (RPV) upper head control rod drive mechanism (CRDM) penetration nozzles are fabricated using J-groove weld geometry. Recently, the incidences of cracking in Alloy 600 CRDM nozzles and their associated welds have increased significantly. The cracking mechanism has been attributed to primary water stress corrosion cracking (PWSCC), and it has been shown to be driven by welding residual stresses and operational stresses in the weld region. The weld-induced residual stress is the main factor contributing to crack growth. Therefore, an exact estimation of the residual stress is important for ensuring reliable operation. This study presents the residual stress computation performed for an RPV CRDM penetration nozzle in Korea. Based on two and three dimensional finite element analyses, the effect of welding variables on the residual stress variation is estimated for sensitivity analysis.

In pressurized water nuclear reactors, dissimilar metal welds are susceptible to primary water stress corrosion cracking. To access this problem, accurate estimation of welding residual stresses is important. This paper provides general welding residual stress profiles in dissimilar metal nozzle butt welds using finite element analysis. By introducing a simplified shape for dissimilar metal nozzle butt welds, changes in the welding residual stress distribution can be seen using a geometry variable. Based on the results, a welding residual stress profile for dissimilar metal nozzle butt welds is proposed that modifies the existing welding residual stress profile for austenitic pipe butt welds.

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/t, geometry of fillet, and adjacent nozzle.

This paper reports tensile properties and residual stresses of Ni-Co thin films. To measure elastic (and plastic) properties, direct tensile tests using dog-bone type specimens are performed first. Assuming that residual stresses vary linearly through the film thickness, bending and membrane residual stress components are measured using cantilever beam and T-structure beam specimens, respectively. Averaged values of Young’s modulus, yield strength and tensile strength are found to be about 163GPa, 1,700MPa and 2,000MPa, respectively. The membrane and bending residual stress components are found to be about 825MPa and 47MPa, respectively.

현대 사회는 다양한 구조물과 다양한 운송 수단에 묻혀서 삶을 살아가고 있다. 이러한 다양성 속에서 금속 구조물은 현대 사회를 이루는 근간이 됨은 의심할 나위가 없으며, 이러한 금속 구조물들은 다양한 방식으로 접합함 으로서 그 형태를 이루게 된다. 그 다양한 접합 방법 중 하나가 용접으로 이는 금속과 금속을 서로 맞붙여 금속 사이를 용접재를 이용하여 강한 열로 접합시키는 맞대기(butt weld) 접합이 대표적인 경우이다. 다양한 구조물에서 이러한 용접방식이 사용되어지고 있는데, 이러한 접합 구조물의 안전성을 위해서는 용접부에 대한 안전성이 우선 확보되어야 하며, 이를 위해서 다양한 방법으로 용접부에 대한 신뢰성 검증이 진행되어 지고 있다. 이러한 상황에서 최근에 선박과 같은 구조물에서 연료의 효율성 및 속도 증가를 위해서 선박의 전체적인 무게를 감소시키려고 하고 있으며, 이러한 요구는 사용되어지는 철판(panel)의 두께를 점점 얇아지도록 만들고 있는 상황이다. 이러한 현실에서 잘못된 설계로 인해서 얇은 철판(panel)들의 맞대기 용접을 진행하여 접합함에 있어서 용접 진행간 발생하는 열 응력에 의해서 과도한 변형이 일어나며, 이를 보정하기 위해서 막대한 수선비용을 초래하는 경우가 발생하고 있다. 이러한 문제들을 사전에 예방하기 위해서는 다양한 용접재, 용접방법에 대한 실험을 통해서 올바른 설계가 진행되어야만 한다. 그러나, 이러한 실험을 진행하기 위해서는 많은 시간과 비용이 소모되는 바 이러한 단점을 보완하기 위해서 다양한 방법의 시뮬레이션이 진행되고 있다. 본 연구에서는 실제 실험을 진행하지 않고도 실제 용접에서 발생할 수 있는 잔류응력을 예측할 수 있는 박판용접(thin section weld)을 모사할 수 있는 모델링 기법을 제시하고자 한다. 이를 위해서 2D 와 3D

Nozzle in nuclear power plant is connected to pipe using safe end. Dissimilar metal weld between nozzle and safe end is followed by similar metal weld between safe end and pipe. And thus residual stress in dissimilar metal weld can be affected by similar metal weld. Similar metal weld impose bending stress on dissimilar metal weld, which is according to the length of safe end. In this study, simple nozzle model which covers various radius to thickness ratios was proposed to quantify residual stress in dissimilar metal weld based on finite element analyses. As a result, short length of safe end was proved to be more effective to mitigate residual stress in dissimilar metal weld and critical effective length of safe end is provided according to the radius to thickness ratio.

This paper provides simulational round robin test results for welding residual stress prediction of safety/relief nozzle. To quantify the welding variables and define the recommendation for prediction and determination of welding residual stress, 6 partners in 5 institutes participated in round robin test. It is concluded that compressive axial and hoop residual stress occurs in dissimilar metal weld and pre-existing residual stress distribution in dissimilar metal weld was affected by similar metal weld due to short length of safe end. Although the reason for the deviation among the results was not pursued further, the effect of several key elements of FE analyses on welding residual stress was investigated in this paper.

Weld overlay is one of the residual stress mitigation methods which arrest crack initiation and crack growth. Therefore weld overlay can be applied to the region where cracking is likely to be. An overlay weld used in this manner is termed a preemptive weld overlay(PWOL). In pressurized water reactor(PWR) dissimilar metal weld is susceptible region for primary water stress corrosion cracking(PWSCC). In order to examine the effect of PWOL on residual stress mitigation, PWOL was applied to a specific dissimilar metal weld of Kori nuclear power plant by finite element analysis method. As a result, strong compressive residual stress was made in PWSCC susceptible region and PWOL was proved effective preemptive repair method for weldment.

In nuclear power plants, ferritic low alloy steel components were connected with austenitic stainless steel piping system through alloy 82/182 butt weld. There have been incidents recently where cracking has been observed in the dissimilar metal weld. Alloy 82/182 is susceptible to primary water stress corrosion cracking. Weld-induced residual stress is main factor for crack growth. Therefore exact estimation of residual stress is important for reliable operating. This paper presents residual stress computation performed by 6” safety & relief nozzle. Based on 2 dimensional and 3 dimensional finite element analyses, effect of welding variables on residual stress variation is estimated for sensitivity analysis. 1. 서 론 최근 V.C Summer 원전 등에서 이종금속용접부 (Dissimilar Metal Weld) 균열이 관찰되었다. (1~3) 균열은 Inconel 계열 용접 금속에서 발생하였으며 일차수응력부식균열(PWSCC)이 균열의 주요 발생 원인이었다. 일차수응력부식균열은 재료의 민감도, 용접부 인장 잔류 응력 및 사용 중 하중(In-service load), 부식 환경과 같은 3가지 인자의 상호작용에 의해 발생한다. (2,4,5)

In nuclear power plants, ferritic low alloy steel nozzle was connected with austenitic stainless steel piping system through alloy 82/182 butt weld. Accurate estimation of residual stress for weldment is important in the sense that alloy 82/182 is susceptible to stress corrosion cracking. There are many results which predict residual stress distribution for alloy 82/182 weld between nozzle and pipe. However, nozzle and piping system usually connected through safe end which has short length. In this paper, residual stress distribution for pressurizer nozzle of Kori nuclear power plant was predicted using FE analysis, which consideded safe end. As a result, existing residual stress profile was redistributed and residual stress of inner surface was decreased specially. It means that safe end should be considered to reduce conservatism when estimating the piping system. 1. 서 론 최근 들어 V.C Summer, Ringhals 및 Tsuruga 2 등의 원전 이종 금속 용접부(DMW, Dissimilar Metal Weld)에서 (1,2)균열이 발생되었다. 균열의 위치는 상대적으로 운전 압력 및 운전 온도가 높은 원자로(reactor) 및 가압기(pressurizer)의 노즐 용접부로서 균열의 원인은 용접부 금속인 니켈 합금(Alloy 82/182) 재료의 일차수 응력부식균열(PWSCC)로 판명되었다. Alloy 82/182 재료는 응력 부식 균열에 민감하다고 알려져 있으며 주로 이종 금속 용접부 용접 재료로 사용되었다. 응력부식균열은 재료의 민감도, 수화학 환경 및 용접 잔류응력(welding residual stress)의 상호 작용에 의해 발생한다.

In nuclear power plants, RPV (Reactor Pressure Vessel) upper head CRDM (Control Rod Drive Mechanism) penetration tubes has been fabricated J-groove weld geometry. Recently, the incidences of cracking in Alloy 600 CRDM tubes and their associated welds have increased significantly. The cracking mechanism has been attributed to PWSCC (Pressurized Water Stress Corrosion Cracking) and has been shown to be driven by welding residual stresses and operational stresses in the weld region. Weld induced residual stress is main factor for crack growth. Therefore exact estimation of residual stress is important for reliable operating. In this point, we have been conducting detailed welding simulation analyses for Korea Nuclear Reactor Pressure Vessel to predict the magnitude of weld residual stresses in penetration tubes. In the present work, the FE (Finite Element) simulations were conducted to investigate the effects of tube geometry (location and ro/t) and material properties on the residual stresses in the J-groove weld for a different location of CRDM tubes. The variables of tube location included three (center-hole, intermediate and steepest side hill tube) inclination angles (Ψ). And this comparison was performed for different tube geometry (ro/t = 2, 3, 4), different yield strength (σo) of tube. In CRDM tube, when increases in tube inclination angle (Ψ), axial residual stress are gradually increased, but hoop residual stresses are decreased at the nearby weld root. In effect of tube radius and thickness, when the thickness of CRDM tubes increases the residual stresses are gradually decreased at the inner surface of tube. And there is no effect of CRDM tube radius (ro). In effect of plastic properties of Alloy 600 material in CRDM, when yield strength of the tube increases the axial residual stresses decreases but hoop residual stress increases.Copyright © 2012 by ASME

Laser shock peening (LSP) is an innovative surface treatment technique, which is successfully applied to improve fatigue performance of metallic components. After the treatment, the fatigue strength and fatigue life of a metallic material can be increased remarkably owing to the presence of compressive residual stresses in the material. Recently, the incidences of cracking in Alloy 600 small-caliber penetration nozzles (CRDM (control rod drive mechanism) and BMI (bottom mounted instrument)) have increased significantly. The cracking mechanism has been attributed to primary water stress corrosion cracking (PWSCC) and has been shown to be driven by welding residual stresses and operational stresses in the weld region. For this reason, to mitigating weld residual stress, preventive maintenance of BMI nozzles was considered application of laser shock peening process. The present study is to predict the residual stresses distribution along the peening surface and the interior of the target (Inconel alloy 600 steel) induced by single and multiple LSP processes using the finite element method. The simulations were accomplished using a commercial finite element package ABAQUS, employing both explicit and implicit methodologies. Effects of parameters related to finite element simulation of laser shock peening process to determine compressive residual stresses of Inconel alloy 600 steel are discussed, in particular parameters associated with the LSP process, such as the maximum pressure, pressure pulse duration, laser spot size and number of shots. It is found that about 2HEL maximum pressure and a certain range of the pulse duration can produce maximum compressive residual stresses near the surface, and thus proper choices of these parameters are important. But plastically affected depth increase with increasing maximum pressure and pulse duration. For the laser spot size, residual stresses are not affected, provided it is larger than a certain size. Magnitudes of the compressive residual stresses and plastically affected depth are found to increase with increasing number of shots, but the effect is less pronounced for more shots. Thus, the amplitude of the initial tensile residual stresses was remarkably changed by LSP. Additionally, In order to evaluate the influence of initial residual stresses in Inconel alloy 600 steel, the initial condition option was employed in the finite element code.

This paper provides the through-thickness welding residual stress profile in dissimilar metal nozzle butt welds of pressurized water reactors. For systematic investigations of the effects of geometric variables, i.e. the thickness and the radius of the nozzle and the length of the safe end, on welding residual stresses, idealized shape of nozzle is proposed and elastic-plastic thermo-mechanical finite element analyses are conducted. Through-wall welding residual stress profiles for dissimilar metal nozzle butt welds are proposed, which take a modified form of existing welding residual stress profiles developed for austenitic pipe butt weld in R6 code.Copyright © 2010 by ASME