• Nuclear Engineering and Technology
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• v.54 no.7
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• pp.2550-2563
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• 2022

This study performed a precise dynamic finite element time history elastic-plastic seismic analysis considering the welds, which have been not considered in design stage, on the nuclear components subjected to severe seismic loadings such as beyond-design basis earthquakes for sustainable nuclear power plants. First, the dynamic finite element elastic-plastic seismic analysis was performed for a general design practice that does not take into account the welds of the pressurizer surge line system, one of safety class I components in nuclear power plants, and then the reference values for the accumulated equivalent plastic strain, equivalent plastic strain, and von Mises effective stress were set. Second, the dynamic finite element elastic-plastic seismic analyses were performed for the case of considering only the mechanical strength over-mismatch of the welds as well as for the case of considering both the strength over-mismatch and welding residual strain. Third, the effects of the strength over-mismatch and welding residual strain were analyzed by comparing the finite element analysis results with the reference values. As a result of the comparison, it was found that not considering the strength over-mismatch may lead to conservative assessment results, whereas not considering the welding residual strain may be non-conservative.

• Nuclear Engineering and Technology
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• v.53 no.5
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• pp.1580-1586
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• 2021

The application of induction bending processes to industrial pipe production is increasing. The induction bending process has the effect of reducing the number of inspections and preventing leaks by reducing the weld of the pipe. For these reasons, efforts have been made to apply an induction bending process to the pipe of the PGSFR under development in Korea and this is the first attempt in the SFR design. Since the PGSFR pipe has a relatively large diameter-to-thickness ratio, it is difficult to fabricate an induction bending pipe that meets the requirements. In addition, the material properties may change because the pipe heats to a very high temperature during the induction bending process. In this study, P91 pipes were fabricated by induction bending, and the results from analyzing the induction bending process' applicability to the P91 pipe of the PGSFR are examined. The various dimensional measurements of the pipes fabricated by the induction bending process were surveyed to determine whether the requirements of the ASME Code were met. The minimum thickness, ovality, and wall buckling measured in the fabricated pipe met all the requirements. Tensile, impact, and hardness tests at various locations of the fabricated pipe also satisfied the requirements.

• International Journal of Korean Welding Society
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• v.1 no.2
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• pp.12-17
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• 2001

With the advance of the robotic welding process, procedure optimization that selects the welding procedure and predicts bead width that will be deposited is increased. A major concern involving procedure optimization should define a welding procedure that can be shown to be the best with respect to some standard and chosen combination of process parameters, which give an acceptable balance between production rate and the scope of defects for a given situation. This paper presents a new algorithm to establish a mathematical model f3r predicting bead width through a neural network and multiple regression methods, to understand relationships between process parameters and bead width, and to predict process parameters on bead width for GMA welding process. Using a series of robotic arc welding, additional multi-pass butt welds were carried out in order to verify the performance of the neural network estimator and multiple regression methods as well as to select the most suitable model. The results show that not only the proposed models can predict the bead width with reasonable accuracy and guarantee the uniform weld quality, but also a neural network model could be better than the empirical models.

• International Journal of Korean Welding Society
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• v.1 no.2
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• pp.51-60
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• 2001

A prediction method for determining the welding residual stress by artificial neural network is proposed. A three-dimensional transient thermo-mechanical analysis has been performed for the $CO_2$ arc welding using the finite element method. The first part of numerical analysis performs a three-dimensional transient heat transfer analysis, and the second part then uses the results of the first part and performs a three-dimensional transient thermo-elastic-plastic analysis to compute transient and residual stresses in the weld. Data from the finite element method are used to train a back propagation neural network to predict the residual stress. Architecturally, the fully interconnected network consists of an input layer for the voltage and current, a hidden layer to accommodate the failure mechanism mapping, and an output layer for the residual stress. The trained network is then applied to the prediction of residual stress in the four specimens. It is concluded that the accuracy of the neural network predicting method is fully comparable with the accuracy achieved by the traditional predicting method.

• Transactions of Materials Processing
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• v.33 no.3
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• pp.193-199
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• 2024

The friction welding characteristics of stainless steels, mainly used in energy and chemical plant industries due to its excellent corrosion resistance and high strength, was evaluated in this study. Friction welding was introduced and conducted at a rotation speed of 2,000 RPM, friction pressure of 30 MPa, burn-off length of 5 mm and upset pressure of 110 ~ 200 MPa on rod typed specimens. The grain boundary characteristics distributions such a grain size, shape, misorientation angle and kernel average misorientation of the welds were clarified by electron backscattering diffraction method. The application of friction welding on SUS304 alloy resulted in a significant refinement of the grain size in the weld zone (5.11 mm) compared to that of the base material (48.09 mm). The mechanical properties of the welds, on the other hand, appeared to be relatively low or similar to those of the base material, which were mainly caused by dislocation density in the initial material and grain refinement in the welds.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.12
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• pp.686-691
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• 2016

In welded steel structures, there are many stress concentration sites such as weld beads, and welding defects are likely to occur at the welded parts. When a repeated fatigue load acts on a stress concentration site, fatigue crack occurs and propagates, leading to fatigue fracture. Therefore, it is necessary to understand fatigue life, crack initiation life, and crack propagation life in order to prevent fatigue failure. In this study, a compliance method was derived for use in the study of fatigue crack propagation characteristics. This compliance can be used for automated measurement of fatigue cracks. The compliance was calculated using an in-house FEM program for a CT specimen. The results of this calculation are presented in relation to a/W and compared with calculation results using the J integral and a program from a previous study. In addition, the strain distribution in the upward and downward directions was calculated from the center of the back face of the CT specimen. In this distribution, the strain tended to decrease from the center to the top and bottom. The compliance method was achieved from these calculations and can be used for automatic execution of crack propagation tests.

• Nuclear Engineering and Technology
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• v.52 no.8
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• pp.1732-1741
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• 2020

The structural integrity of welded joints in the reactor pressure vessel (RPV) is directly related to the safety of nuclear power plants. The RPV is made from SA508-III steel in a pressurized water reactor. In this study, we investigated the effects of temperature on the tensile and fracture toughness properties of Chinese SA508-III welded joint in different sampling areas in order to provide reference data for structural integrity assessments of RPVs. The specimens used in tensile and fracture toughness tests were fabricated from the base metal (BM), weld metal (WM), and the heat-affected zone (HAZ) in the welded joint. The representative testing temperatures included the ambient temperature (20 ℃), upper shelf temperature (100 ℃), and service temperature (320 ℃). The results showed that temperature greatly affected the fracture toughness (JIC) values for the SA508-III welded joint. The JIC values for BM and HAZ both decreased remarkably from 20 ℃ to 320 ℃. The fracture morphologies showed that the BM and HAZ in the welded joint exhibited fully ductile fracture at 20 ℃, whereas partial cleavage fracture was mixed in ductile fracture mode at 100 ℃ and 320 ℃. The WM exhibited the ductile and cleavage fracture mixed mode at various temperatures, and the JIC values showed slight changes.

• Journal of the Korean Society for Heat Treatment
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• v.13 no.1
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• pp.1-9
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• 2000

The microstructural characteristics and low temperature tensile properties between $25^{\circ}C$ and $-196^{\circ}C$ for as-welded and age hardened specimen by using Al 5083-H321 for base metal, 5083-5356 and 5083-4043 weldments have been investigated. The hardness of 5083-5356 weldment decreases with aging treatment, whereas the weld region of 5083-4043 weldment shows remarkable increase in hardness after aging due to the precipitation of fine Si particle at the grain boundaries and interiors. Low temperature tensile properties of 5083 AI base metal, 5083-5356 and 5083-4043 weldments appear to be the increment of tensile strengths and elongations at the room temperature and $-196^{\circ}C$, while the decrement of tensile properties around $-50^{\circ}C$ is shown. Through the observation of fine serration to fracture in the stress-strain curve and tensile fractography, the increment of localized deformation leading to promote the neck initiation and the increment of the dimple size cause to decrease in tensile strengths and elongations around $-50^{\circ}C$. For the tensile specimen of the 5083 base metal, 5083-5356 and 5083-4043 weldments, the reason to increase in elongation after solution and aging treatment is the diminishment of fine pit, the resolution of Mg into the matrix and the spheridization of the eutectic Si.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.56-56
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• 2009

AISI 316L 스테인리스강에 새롭게 디자인한 서로 다른 3가지 응고모드를 가진 와이어로 FCAW(Flux Cored Arc Welding)을 하였다. 각각의 3가지 와이어는 Pseudobinary phase diagram에 따라 AF, FA, F모드를 가졌다. 미세조직은 $Cr_{eq}/Ni_{eq}$이 증가할수록 델타 페라이트 함량이 증가하였으며, 초정 상의 경우 초정 오스테나이트에서 초정 페라이트로 변태하였고, 연성저하균열의 민감도가 감소하였다. 연성저하균열은 이동결정립계의 형상에 따라 좌우되며, 미량의 페라이트를 함유한 오스테나이트에서는 페라이트가 핀(Pin) 역할을 제대로 하지 못하여 직선형태의 이동 결정립계 따라 입계 미끄러짐의 메커니즘을 통해 전파되었으며, 곡선형태의 이동 결정립계에서는 델타 페라이트가 핀 역할을 하여 역할을 하여 구속 상태에서 응력집중을 막고 응력을 분산시켜 균열이 전파되는 것을 방해하여 균열이 발생되지 않았다.

• Journal of the Society of Naval Architects of Korea
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• v.38 no.3
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• pp.93-106
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• 2001

T-joint and hopper knuckle joint models are typical welded joints in ship structure, which are very susceptible to fatigue damage under service condition. Fatigue test and fracture mechanical analysis were performed on these joints to find out characteristics of fatigue behavior. Unified S-N curve was developed from the test results of these two types of joint using hot spot stress concept, and also propagation life was also estimated using Paris' crack propagation law. Residual stress effect on propagation life was considered in calculating propagation life, as was done with thermo-elasto-plastic FE analysis and residual stress intensity factor calculation. Fatigue life of similar kinds of welded joint could be predicted with this unified S-N curve and fracture mechanical analysis technique.