• Korean Journal of Metals and Materials
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• v.48 no.12
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• pp.1090-1096
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• 2010

This pitting corrosion study of welded joints of austenitic stainless steels (AISI 304L and 316L) has addressed the differentiating solidification mode using three newly introduced filler wires with a flux-cored arc welding process (FCAW). The delta ferrite (${\delta}$-ferrite) content in the welded metals increased with an increasing equivalent weight ratio of chromium/nickel ($Cr_{eq}/Ni_{eq}$). Ductility dip cracking (DDC) was observed in the welded metal containing ferrite with none of AISI 304L and 0.1% of AISI 316L. The potentiodynamic anodic polarization results revealed that the $Cr_{eq}/Ni_{eq}$ ratio in a 3.5% NaCl solution didn't much affect the pitting potential ($E_{pit}$). The AISI 316L welded metals with ${\ddot{a}}$-ferrite content of over 10% had a superior $E_{pit}$ value. Though the AISI 316L welded metal with 0.1% ferrite had larger molybdenum contents than AISI 304L specimens, it showed a similar $E_{pit}$ value because the concentration of chloride ions and the corrosion product induced severe damage near the DDC.

• Journal of the Korean Society of Industry Convergence
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• v.25 no.6_2
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• pp.1055-1062
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• 2022

In this study, the mechanical properties of 80% cold-rolled austenitic 316L stainless steel were evaluated using specimens subjected to reverse transformation at 500-750℃ for 20 minutes and reverse transformation at 700℃ for 2-60 minutes. Also, for the elastic wave obtained from the tensile test, the dominant frequency according to the reverse transformation condition was investigated by time-frequency analysis. The SEM image of the 80% cold-rolled material was transformed into martensite and showed line and cross shapes. The TEM image showed that line shapes were shown at the grain, and grain boundary of martensite. The higher the heat treatment temperature and the longer time, the larger the grain. Tensile strength decreased as the heat treatment temperature and time increased, but elongation increased. Hardness was proportional to tensile strength. This is because the grain with different directions showed the same direction due to reverse transformation. The dominant frequency was decreased and then increased as the temperature and time increased. This is because the direction of the grain is different at a low temperature and the same direction is shown at a high temperature.

• Journal of Korea Foundry Society
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• v.43 no.5
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• pp.230-240
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• 2023

The pitting corrosion behavior of 329LD cast lean duplex stainless steel and CF3M cast austenitic stainless steel was investigated in chloride environments. The pitting corrosion resistance of the 329LD alloy was superior to that of the CF3M alloy because the pitting potential, passive region, and critical pitting temperature of the low Ni-low Mo 329LD alloy were higher than those of the high Ni-medium Mo commercial CF3M alloy. There are two main reasons for the enhancement of the pitting corrosion resistance of high Cr-low Momedium N 329LD alloy compared to the low Cr-medium Mo CF3M alloy: First, the pitting resistance equivalent number (PREN_δ+γ) value of the 329LD alloy is higher than that of the CF3M alloy. Second, the passive region of the 329LD alloy is larger than that of the CF3M alloy. It indicates that the synergistic effect of the three elements by adding high Cr and low Mo-medium N to the 329LD alloy enhances the passivity of the passive film, thereby increasing the pitting corrosion resistance. It was verified that based on the PREN_γ of austenite (γ) and PREN_δ of ferrite (δ) values calculated using an N-factor of 16, the pitting corrosion of the 329LD alloy was selectively initiated at the γ-phases because PREN_γ value of austenite (γ) was smaller than that of ferrite (δ), and finally propagated from the γ-phase to the δ-phase.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1421-1426
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• 2014

To improve the inherent safety of the sodium-cooled fast reactor (SFR), the supercritical $CO_2$ ($S-CO_2$) Brayton cycle is being considered as an alternative power conversion system to steam the Rankine cycle. In the $S-CO_2$ system, a PCHE (printed circuit heat exchanger) is being considered. In this type of heat exchangers, diffusion bonding is used for joining the thin plates. In this study, the diffusion bonding characteristics of various austenitic alloys were evaluated. The tensile properties were measured at temperatures starting from the room temperature up to $650^{\circ}C$. For the 316H and 347H types of stainless steel, the tensile ductility was well maintained up to $550^{\circ}C$. However, the Incoloy 800HT showed lower strength and ductility at all temperatures. The microstructure near the bond line was examined to understand the reason for the loss of ductility at high temperatures.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.10
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• pp.125-130
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• 2017

Automotive materials and plant modules need to be prepared for freezing parts to operate in extreme areas such as Eastern Europe, Russia, and Canada. However, the only thing that has been done for ultra-qualifying materials for extremely low operating materials is that only the effects at low temperatures are conducted at room temperature, and the effects at low temperatures are only identified at low speeds. Therefore, this study examined the low-temperature characteristics of materials by conducting comparative tests on the mechanical properties of the room at the temperature and temperature of TP304 and TP316 materials, which are the most common materials.

• Advances in concrete construction
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• v.11 no.1
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• pp.33-43
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• 2021

A new type of composite insulated concrete sandwich wall (ICS-wall), which is composed of a triangle truss steel wire network, an insulating layer, and internal and external concrete layers, is proposed. To study the mechanical properties of this new ICS-wall, tensile, compression, and shearing tests were performed on 22 specimens and tensile strength and corrosion resistance tests on 6 triangle truss joints. The variables in these tests mainly include the insulating plate material, the thickness of the insulating plate, the vertical distance of the triangle truss framework, the triangle truss layout, and the connecting mode between the triangle truss and wall and the material of the triangle truss. Moreover, the failure mode, mechanical properties, and bearing capacity of the wall under tensile, shearing, and compression conditions were analyzed. Research results demonstrate that the concrete and insulating layer of the ICS-wall are pulling out, which is the main failure mode under tensile conditions. The ICS-wall, which uses a graphite polystyrene plate as the insulating layer, shows better tensile properties than the wall with an ordinary polystyrene plate. The tensile strength and bearing capacity of the wall can be improved effectively by strengthening the triangle truss connection and shortening the vertical distances of the triangle truss. The compression capacity of the wall is mainly determined by the compression capacity of concrete, and the bonding strength between the wall and the insulating plate is the main influencing factor of the shearing capacity of the wall. According to the tensile strength and corrosion resistance tests of Austenitic stainless steel, the bearing capacity of the triangle truss does not decrease after corrosion, indicating good corrosion resistance.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2003.10a
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• pp.100-105
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• 2003

25Cr-20Ni series strainless steel have an excellent high temperature strength, high oxidation and high corrosion resistance. However, further improvement can be creep strength by work hardening prior creep. In the present study, the effect of prestraining at room temperature on the creep behavior of a Class M(STS310S) and a Class A(STS310J1TB) alloy containing precipitates have been examined. Prestraining was carried out at room temperature and range of prestrain was $0.5{\sim}2.5$ % at STS310J1TB and $2.0{\sim}7.0$ % at STS310S. Creep behavior and creep rate of pre-strained specimens were compared with that of virgin specimens. Room temperature prestraining produced the creep life that is longer than that of a virgin specimen both for STS310J1TB and STS310S when creep test carried out at the temperature lower than recrystallization temperature. The reason for this improvement of creep life was ascribable to the interaction between dislocations and precipitates in addition to the dislocation-dislocation interaction in STS310J1TB and the dislocation-dislocation interaction in STS310S. The beneficial effect of prestraining in STS310J1TB was larger than that of STS310S.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.3
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• pp.73-78
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• 2016

In this study, GTAW was carried out on austenitic STS316 stainless steel. Overlay welding with the stellite-base filler metal was implemented using a double torch. The response variable was calculated on the measured Vickers hardness for process optimization using the Taguchi method and its response variable was then analyzed about effect on overlay welding characteristics. The optimal process design by the Taguchi method is extremely effective in the overlay welding process for the multiple response variables. In addition, the effects of contribution rate about each response variable was analyzed easily. The conditions of the optimal process were 105A, 18V, pre-heat treatment at $200^{\circ}C$, and post weld heat treatment at $100^{\circ}C$. The Vickers hardness of the specimens produced under the optimal condition of GTAW by the double torch was 8.19% higher than that by a single torch.

• Transactions of Materials Processing
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• v.29 no.2
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• pp.103-112
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• 2020

This study explores the effects of different pre-machining conditions on the deposition characteristics and mechanical properties of austenitic stainless steel samples repaired using direct energy deposition (DED). In the DED repair process, defects such as pores and cracks can occur at the interface between the substrate and deposited material. In this study, we varied the shape of the pre-machined zone for repair in order to prevent cracks from occurring at the slope surface. After repairs by the DED process, macro-scale cracks were observed in samples that had been pre-machined with elliptic and trapezoidal grooves. In addition, it was not possible to completely prevent micro-crack generation on the sloped interfaces, even in the capsule-type grooved sample. From observation of the fracture surfaces, it was found that the cracks around the inclined interface were due to a lack of fusion between the substrate and the powder material, which led to low tensile properties. The specimen with the capsule-type groove provided the highest tensile strength and elongation (respective of 46% and 571% compared to the trapezoidal grooved specimen). However, the tensile properties were degraded compared to the non-repaired specimen (as-hot rolled material). The fracture characteristics of the repaired specimens were determined by the cracks at the sloped interfaces. These cracks grew and coalesced with each other to form macro-cracks, they then coalesced with other cracks and propagated to the substrate, causing final fracture.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.8
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• pp.831-836
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• 2014

In this study, high temperature integrity evaluation on a pressure vessel of the expansion tank operating at elevated temperature of $510^{\circ}C$ in the sodium test facility of the SEFLA(Sodium Thermal-hydraulic Experiment Loop for Finned-tube Sodium-to-Air heat exchanger) to be constructed at KAERI has been performed. Evaluations of creep-fatigue damage based on a full 3D finite element analyses were conducted for the expansion tank according to the recent elevated temperature design codes of ASME Section III Subsection NH and French RCC-MRx. It was shown that the expansion tank maintains its integrity under the intended creep-fatigue loads. Quantitative code comparisons were conducted for the pressure vessel of austenitic stainless steel 316L.