• Journal of the Korean Society for Precision Engineering
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• v.25 no.7
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• pp.116-123
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• 2008

In the fossil power plant, the reliability of the components which consist of the many welded parts depends on the quality of welding. The residual stress is occurred by the heat flux of high temperature during weld process. This decreases the mechanical properties as the strength of fatigue and fracture or causes the stress corrosion cracking and fatigue fracture. Especially, the accidents due to the residual stress occurred at the weld parts of high-temperature and high-pressure pipes and steam headers. Also, the residual stress of the welded part in the recently constructed power plants has been brought into relief as the cause of various accidents. The aim of this study is the measurement of the residual stress using the x-ray diffraction method. The merits of this are more accurate and applicable than other methods. The materials used for the study is P92 steel for the use of high temperature pipe on super critical condition. The variables of tests are the post-weld heat treatment, the surface roughness and the depth from the original surface. The test results were analyzed by the distributed characteristics of the full width at half maximum intensity (FWHM) in x-ray diffraction intensity curve and by the relation of hardness with FWHM.

• Journal of Welding and Joining
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• v.17 no.6
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• pp.90-99
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• 1999

Significant portion of the total manufacturing time for a pipe fabrication process is spent on the welding following primary machining and fit-up processes. To achieve a reliable weld bead appearance, automatic seam tracking and adaptive control to fill the groove are urgently needed. For the seam tracking in welding processes, the vision sensors have been successfully applied. However, the adaptive filling control of the multi-torches system for the appropriate welded area has not been implemented in the area of SAW(submerged arc welding) by now. The term adaptive control is often used to describe recent advances in welding process control by strictly this only applies to a system which is able to cope with dynamic changes in system performance. In welding applications, the term adaptive control may not imply the conventional control theory definition but may be used in the more descriptive sense to explain the need for the process to adapt to the changing welding conditions. This paper proposed various types of methodologies for obtaining a good bead appearance based on multi-torches welding system with the vision system in SAW. The methodologies for adaptive filling control used welding current/voltage, arc voltage/welding current/wire feed speed combination and welding speed by using vision sensor. It was shown that the algorithm for welding current/voltage combination and welding speed revealed sound weld bead appearance compared with that of voltage/current combination.

• Journal of Welding and Joining
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• v.23 no.6
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• pp.61-66
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• 2005

A probabilistic approach for evaluating failure risk is suggested in this paper. Probabilistic fracture analyses were performed for a pressurized pipe of a Cr-Mo steel reflecting variation of material properties at high temperature. A crack was assumed to be located along the weld fusion line. Probability density functions of major variables were determined by statistical analyses of material creep and creep crack growth data measured by the previous experimental studies by authors. Distributions of these variables were implemented in Monte Carlo simulation of this study. As a fracture parameter for characterizing growth of a fusion line crack between two materials with different creep properties, $C_t$ normalized with $C^*$ was employed. And the elapsed time was also normalized with tT, Resultingly, failure probability as a function of operating time was evaluated fur various cases. Conventional deterministic life assessment result was turned out to be conservative compared with that of probabilistic result. Sensitivity analysis for each input variable was conducted to understand the most influencing variable to the analysis results. Internal pressure, creep crack growth coefficient and creep coefficient were more sensitive to failure probability than other variables.

• Journal of the Korean Society of Safety
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• v.24 no.1
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• pp.7-13
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• 2009

The objective of this study is to design a laboratory specimen for simulating residual stress of circumferential butt welding between pipes. Specimen type and method to generate residual stress were proposed based on the review of prior studies and parametric finite element analysis. To prove the proposed specimen type and loading method, the residual stress was generated using the designed specimen by applying proposed method and was measured. The measured residual stress using X-ray diffraction reasonably agreed with the results of finite element analysis considered in the specimen design. Comparison of residual strains measured at several locations on the specimen and given by finite element simulation also showed good agreement. Therefore, it is indicated that the designed specimen in this study can reasonably simulate the axial residual stress of a circumferential butt welding of pipe.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.6
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• pp.1460-1468
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• 1995

The objective of this paper is to evaluate the material properties of SA106 Gr. C carbon steel and its associated weld manufactured for main steam system of Yonggwang 3,4 nuclear generating stations. A total of 43 tensile and 35 fracture toughness tests were performed and the effects of various parameters such as pipe size, crack plane orientation, test temperature, welding on material properties were discussed. Test results show that the effects of crack plane orientation, test temperature, and welding on fracture toughness were significant while the effects of pipe size, specimen orientation and test temperature on tensile properties were negligible. Especially the dependence of J-R curves on the crack plane orientation appears to be the characteristics of carbon steel.

• Proceedings of the KWS Conference
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• v.43
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• pp.150-152
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• 2004

This study was aimed at evaluation of residual stress of steel pipe structures. The production process of pipes was complex (at first bending was done by roll forming or press forming and welding was final process of making of steel pipes). So there could be effected high residual stresses in steel pipes. In order to evaluate the changes of residual stress the locations of measurement were selected carefully. Measurements of residual stress were done for various kinds of pipes (shapes in circular and square). For the evaluation of residual stress, hole-drilling method (ASTM E837 was applied. The results showed that along the weld Eine high tensile stress were measured as effected, and high tensile stresses were measured where large plastic deformation developed. Through these efforts, experimental results could be more effectively assisted by numerical method.

• Transactions on Control, Automation and Systems Engineering
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• v.2 no.3
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• pp.182-188
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• 2000

High-frequency electric resistance welding (HERW) technique is one of the most productive manufacturing method currently available for pipe and tube production because of its high welding speed. In this process, a heat input is controlled by skilled operators observing color and shape of bead but such a manual control can not provide reliability and stability required for manufacturing pipes of high grade quality because of a variety of bead shapes and noisy environment. In this paper, in an effort to provide reliable quality inspection, we propose a neural network-based method for classification of bead shape. The proposed method utilizes the structure of Kohonen network and is designed to learn the skill of the expert operators and to provide a good solution to classify bead shapes according to their welding conditions. This proposed method is implemented on the real pipe manufacturing process, and a series of experiments are performed to show its effectiveness.

• Journal of the Korean Society of Safety
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• v.15 no.2
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• pp.51-58
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• 2000

Recently, the J/T analysis based on elastic-plastic finite element analysis is popularly used in the nuclear industry to assess the integrity of a cracked pipe. The objective of this paper is to evaluate the effect of stress-strain curve for weld metal, variation of crack incremental length(${\delta}a$), and crack face pressure on the J/T analysis result. For this purpose, a parametric analysis was performed and the results calculated from finite element analysis were compared with those from the piping experimental data(stainless steel weldment pipe with circumferential through-wall crack). The numerical result using base metal material property is in agreement with the experimental one and the maximum load is decreased as the ${\delta}a$ for J/T analysis is increased.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.299-304
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• 2008

This paper presents plastic limit loads of piping branch junctions with local wall thinning under combined pressure and in-plane bending, based on systematic three-dimensional finite element limit analyses using elastic-perfectly plastic materials. An ideal branch junction without weld or reinforcement around the intersection is considered with two locations of wall thinning; one in the run pipe, and the other in the branch pipe. Based on FE results, effects of thinning geometries on plastic limit moments are quantified and simple approximations of plastic limit loads are proposed.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.10
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• pp.568-572
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• 2013

The main components, such as a reactor vessel, in commercial nuclear power plants have been welded to pipes with dissimilar metal in which Primary Water Corrosion Cracking (PWSCC) has been occurred. PWSCC has become a worldwide issue recently. This paper addresses the results of experimental and numerical analysis to prevent PWSCC by changing the stress profile that is tensile stress to compressive stress at interesting regions with plastic deformation generated by mechanical pressure. Based on the results of experimental and numerical analysis with a 6 inch pipe and dissimilar metal welded pipes, compressive stress 68~206 Mpa is generated at all locations of inner surface in the heat affected zone.