• Journal of the Korean Society of Propulsion Engineers
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• v.23 no.2
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• pp.111-117
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• 2019

An experimental study on the welding part of a heat transfer plate that constitutes the lightweight and high efficiency recuperator is presented in this paper. In particular, to find out the service life of the welded part, fatigue characteristics were determined through experiments. Experiments were carried out on two materials (STS347, AL20-25 + nb), which are selected as the material of the recuperator; further, the specimens were manufactured through the methods used for actual fabrication and the standards recommended by ASTM. To evaluate the mechanical properties of the specimens at room and high temperature, MTS-810 was used in a high-temperature furnace. The tensile test was carried out at room and high temperatures for each specimen. The fatigue test was carried out by setting the load ratio corresponding to 50%, 40%, 30%, 20%, and 10% of the tensile strength at the stress ratio of 0.1. Finally, the fatigue life characteristics obtained by the experiment were compared with the stresses owing to the load generated in the operating conditions of the recuperator, and the lifetime of the welds was evaluated to prepare for the operation time required by the UAV.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.3
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• pp.117-126
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• 2017

The variation in microstructure and mechanical properties during heat treatment was examined in a series of 0.27% C-1.5% Mn-1.0% Cr steels with silicon contents in the range of 0 to 1.0 wt%. It was found that addition of 0.5%~1.0% silicon increased both tensile strength and impact toughness through solid solution strengthening and microstructural refinement. 0.27% C-1.0% Si-1.5% Mn-1.0% Cr steel showed tensile strength of 1,700 MPa in the as-quenched condition and the steel revealed a full martensitic structure even after air cooling from $900^{\circ}C$ to room temperature, showing air hardening characteristics. Tempering at $150^{\circ}C$ which corresponds to the typical paint-baking temperature after painting of body in white, slightly decreased the tensile strength and increased elongation, but substantially increased the impact toughness compared to the as-quenched steel.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.11 no.4
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• pp.173-179
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• 2012

In recent years, Magnesium(Mg) and its alloys have become a center of special interest in the automotive industry. Due to their high specific mechanical properties, they offer a significant weight saving potential in modern vehicle constructions. Most Mg alloys show very good machinability and processability, and even the most complicated die casting parts can be easily produced. In this study, Microstructure, Vickers hardness and tensile tests were examined and performed for each specimen to verify effects of forming conditions. Also to verify upsettability and forming limit of the specimen at room temperature and elevated temperature, upsetting experiments were performed. For comparison, experiments at elevated temperature were performed for various Mg alloy, such as AZ31, AZ91, and AM50. The experimental results were compared with those of CAE analysis to propose forming limit of Magnesium alloys.

• Korean Journal of Metals and Materials
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• v.50 no.6
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• pp.427-432
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• 2012

This paper presents the material properties of carbon steels for ships, and offshore structures (ASTM A131) are tested under a series of arctic and cryogenic temperature conditions. For material tension tests, among the ASTM 131 steels, Grades A and B of mild steel and Grade AH of high tensile steel have been used. The obtained mechanical properties of the materials from the material tension tests were applied in a 13,000TEU class container ship to define the effect of low temperature on the ultimate longitudinal strength of the target structure by using the ALPS/HULL intelligent supersize finite element method. The tensile coupon test results showed increased strength and nonuniform fracture strain behaviors within different grades and temperatures. Increasing the material strength resulted in increasing the ultimate longitudinal strength of the ship.

• Journal of Welding and Joining
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• v.31 no.1
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• pp.58-64
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• 2013

The following results were obtained, microstructures and tensile properties in arc brazed joints of DP(dual phase) steel using Cu-5.3wt%Sn insert metal was investigated as function of brazing current. 1) The Fusion Zone was composed of ${\alpha}Fe+{\gamma}Cu$ and Cu23Sn2. The reason for the formation of these solid solutions. Despite, Fe & Cu were impossible to solid solution at room temperature. It's melting & reaction to something of insert metal & Base Metal (DP Steel) by Arc. Brazing Process has faster cooling rate then Cast Process, Supersaturated solid solution at room temperature. 2) The increase Hardness of Fusion Zone was directly proportional to the rise of welding current. Because, ${\alpha}Fe+{\gamma}Cu$ phase (higher hardness than the Cu23Sn2.(104.1Hv < 271.9Hv)) Volume fraction was Growth, due to increasing the amount of base metal melting by High current. 3) The results of tensile shear test by Brazing, All specimens happen to fracture in Fusion Zone. On the other hand, when Brazing Current increasing tend to rise tensile load. but it was very small, about 26-30% of the base metal. 4) The result of fracture analysis, The crack initiate at Triple Point for meet to Upper B.M/Under B.M/Fusion Zone. This Crack propagated to Fusion zone. So ruptured by tensile strength. The Reason to in the fusion zone fracture, Fusion zone by Brazing of hardness (strength) was very lower then the base metal (DP steel). In addition the Fusion Zone's thickness in triple point was thin than the base metal's thickness in triple point.

• Proceedings of the KSME Conference
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• 2001.06c
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• pp.525-530
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• 2001

As a part of efforts to examine feasibility of warm forging near-net-shape process for non-heat-treated steel to replace quenched and tempered S45C steel, the optimized process condition has been determined to be $820^{\circ}C$ for heating, 10/sec for strain rate of forging and approximately 250MPa for flow stress from observed results such as the $A_{3}$ transformation temperature of about $790^{\circ}C$, the fully dynamic recrystallized behavior between $800^{\circ}C\;and\;850^{\circ}C$ when compressed up to 63% engineering strain at 10/sec strain rate, and the high temperature microsturctural stability. Also, controlled cooling rate of $6.3^{\circ}C/sec$ by water-spraying at a rate of $0.10cc/sec-cm^{2}$ for 60seconds followed by air-cooling right after forging process has been considered in this study as a feasible approach based on examination of the microsturcture of mixed ${\alpha}-ferrite$ and pearlite, the hardness and tensile properties meeting specification, and the reduced total cooling time to room temperature. Successive works would be carried out for the impact strength, machinalility, and forgeability at this process in the near future.

• Journal of the Korean Society of Safety
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• v.21 no.4 s.76
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• pp.33-41
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• 2006

Material degradation should be considered to assess integrity and residual life of high temperature equipments. However, the property data reflecting degradation are not sufficient for practical use. In this study measuring properties for 1Cr-1Mo-0.25V forging steel generally used for turbine rotor was carried out. Degradation was simulated by isothermal ageing. heat treatment and variation of microstructure was observed. Mechanical properties such as tensile strength, impact energy, hardness and fracture toughness were measured. Assuming a semi-elliptical surface crack at the bore hole in a turbine rotor, material risk was estimated by using the aged material property data obtained in this study. Safety margin was decreased and life of the rotor was exhausted. This procedure can be used in assessing the residual life of a turbine rotor due to material degradation.

• Journal of the Korean Society for Heat Treatment
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• v.21 no.6
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• pp.293-299
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• 2008

In order to provide with fundamental data of the wrought Mg alloy for press forging, the effect of annealing temperature on the microstructure, texture development and tensile properties is studied in a commercial AZ31B Mg alloy sheet. Basal texture i.e. $(0001){\pm}5^{\circ}$[21$\bar{3}$0] is developed in a commercial AZ31B Mg sheet, and the texture is not changed considerably by annealing over $400^{\circ}C{\times}30min$, while (10$\bar{3}$0) component with high intensity can be observed due to abnormal grain growth. When the sheet is tensile-deformed with RD, $45^{\circ}$ and TD directions at room temperature, fracture strains are given by 25.8, 21.4 and 11.9% in the order of RD, $45^{\circ}$ and TD directions, respectively. With increasing annealing temperature up to $450^{\circ}C{\times}30min$, little change in mean grain size can be revealed by annealing below $300^{\circ}C{\times}30min$ but an abnormal grain growth, where some grains become significantly coarser than the rest, occurs by annealing above $400^{\circ}C{\times}30min$. The maximum tensile strain of around 25% is obtained by annealing below $300^{\circ}C{\times}30min$, but it is abruptly decreased to 16% by annealing above $400^{\circ}C{\times}30min$ owing to intergranular fracture of abnormal grown grains.

• Composites Research
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• v.22 no.3
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• pp.35-43
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• 2009

In this study, for a radiator hose made of carbon black filled EPDM(ethylene-propylene diene monomer) rubber, a measuring method of crosslink density was established to analyze the aging behaviors under thermo-oxidative stresses. At $125^{\circ}C$, the crosslink density of the rubber specimens decreased slightly in the initial stage, but increased with increasing the aging time. Such variation in crosslink density was similar to that of tensile strength. This might be due to the formation of sulphoxide crosslinks as well as to additional crosslinks made by the reaction of unvalcunized sulfurs. A high temperature aging of rubber specimens at $180^{\circ}C$ caused a slight increase in crosslink density while it did a large decrease in tensile strength and elongation. With aging at high temperature, the formation of carbonyl groups in EPDM molecule chain and formation of sulphoxide crosslink, rather than the crosslink density variation itself, had a large influence on such changes in mechanical property.

• 연구논문집
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• s.33
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• pp.167-174
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• 2003

Alloy 713LC was developed to improve the tensile strength and ductility by reducing the carbon content of Alloy 713C. As Alloy 713LC was designed to minimize the mechanical property change with process conditions, it is generally utilized in the parts which have thick and thin sections simultaneously. In the thick and the thin sections, quite different properties are required. Consequently it is essential to crucially control the local mechanical properties of a parts by optimizing the process condition and heat treatment. In this research, high temperature mechanical properties including creep-rupture and strain-control low cycle fatigue were investigated together with the microstructural variations with heat treatment. Failure mechanism was also analyzed by observing the fracture surface to correlate the variation of mechanical properties with the microstructural change.