• Korean Journal of Materials Research
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• v.33 no.12
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• pp.558-564
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• 2023

In this study, we designed and manufactured a large angular contact ball bearing (LACBB) with low deformation using JIS-SUJ2 steel and analyzed changes in its structural characteristics and chemical composition upon heat treatment. The bearing was produced by hot forging and heat treatment including a quenching and tempering (Q/T) process, and its properties were analyzed using 4 mm thick specimens. A difference in the size distribution of the carbide in the outer and inner parts of the bearing was observed and it was confirmed that large and non-uniform carbide was distributed in the inner part of the bearing. After heat treatment, the hardness value of the outer part increased from 13.4 HRC to 61 HRC and the inner part increased from 8.0 HRC to 59.7 HRC. As a result of X-ray diffraction (XRD) measurements, the volume fraction of the retained austenite contained in the outer part was calculated to be 3.5~4.8 % and the inner part was calculated to be 3.6~5.0 %. The surface chemical composition and the content of chemical bonds were quantified through X-ray photoelectron spectroscopy (XPS), and a decrease in C=C bonds and an increase in Fe-C bonds were confirmed.

• Proceedings of the Plant Resources Society of Korea Conference
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• 2023.04a
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• pp.30-30
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• 2023

한국산 체꽃속(Scabiosa L.)의 외부형태형질을 바탕으로 구분한 5가지 변이 유형[솔체꽃(S. comosa) Type 1-2, 체꽃(S. tschiliensis f. pinnata) Type 3-4, 구름체꽃(S. tschiliensis f. alpina) Type 5]에 대해 잎 표피와 화분의 미세구조를 관찰하였다. 잎 표피세포의 크기는 향축면은 평균 12.4 × 6.7 ㎛로, Type 1이 가장 작고 Type 3가 가장 컸으며, 배축면은 평균 9.4 × 4.6 ㎛로 모든 유형이 비슷하게 나타났다. 모용은 단세포성 선상 모용이 일부 유형에서, 다세포성 비선상 모용이 모든 유형에서 관찰되었다. 선상 모용은 향축면에서 Type 1과 5에만 있었으며, 두 유형 중 Type 5의 모용이 길이 72.2 ㎛로 더 길고, 단위면적 (0.01 mm²) 당 개수도 5개로 더 많았다. 배축면에서는 Type 1, 3 및 5에 있고, 단위면적 당 개수가 Type 1과 5는 3~6개로 비슷하였으나, Type 3은 1개 이하로 매우 드물게 나타났다. 기공복합체는 부등형으로 양면에 분포하며, 향축면의 공변세포 크기는 평균 4.4 × 2.7 ㎛로 너비는 유사하나 길이는 Type 1과 5가 3.1 ㎛, 3.9 ㎛로 나머지에 비해 작았다. 단위면적 당 기공의 수는 향축면과 배축면이 평균 40.2개, 149.2개이고, 양면 모두 Type 1이 66.3개와 177.2개로 가장 많았다. 화분은 모두 단립으로 삼구형이고, 극축 길이(P)와 적도면 지름(E)의 비율은 0.83~1.12로 모양은 아단구형~약장구형이다. 표면무늬는 과립상 표면에 자상 돌기가 있고, Type 3과 4에서만 유공상이 확인되었다. 이상의 결과에서, 솔체꽃 유형인 Type 1과 2가 잎 표면의 선상 모용 유무 및 기공의 수에 뚜렷한 차이를 보여 Type 1은 솔체꽃과는 다른 새로운 변종 또는 품종일 가능성이 있다고 생각된다. 또한 체꽃 유형인 Type 3과 4의 경우, Type 3의 잎 배축면에 털이 드물게 있으나, 두 유형의 화분 표면무늬가 동일하고, 다른 형질들도 모두 중복되어 연속적인 변이로 보이며, 구름체꽃 유형인 Type 5는 나머지 유형과 뚜렷이 구분되었다.

• The Journal of the Convergence on Culture Technology
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• v.9 no.6
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• pp.855-859
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• 2023

Copper oxide (CuO) nanoparticles were successfully synthesized using a precursor in which industrial starch was impregnated with an aqueous solution of copper (II) nitrate trihydrate. The microstructure of the precursor impregnated with an aqueous solution of copper nitrate trihydrate was confirmed with a scanning electron microscope (SEM), and the particle size and the crystal structure of the copper oxide particles produced as the temperature of the heat treatment of the precursor increased was analyzed by X-ray diffraction (XRD) and the scanning electron microscope (SEM). As a result of the analysis, it was confirmed that the temperature at which the organic matter of the precursor is completely thermally decomposed is 450-490℃, and that the size and crystallinity of the copper oxide particles increased as the heat treatment temperature increased. The size of the copper oxide particles obtained through heat treatment at 500-800℃ during 1 hour was 100nm~2㎛. It was confirmed that the copper oxide crystalline phase is formed at a heat treatment temperature of 400℃, and only the copper oxide single phase existed up to 800℃. And it was also confirmed that the size of particles produced increased as the calcination temperature increased.

• Journal of Powder Materials
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• v.30 no.5
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• pp.431-435
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• 2023

An irradiation hardening of Inconel 718 produced by selective laser melting (SLM) was studied based on the microstructural observation and mechanical behavior. Ion irradiation for emulating neutron irradiation has been proposed owing to advantages such as low radiation emission and short experimental periods. To prevent softening caused by the dissolution of γ' and γ" precipitates due to irradiation, only solution annealing (SA) was performed. SLM SA Inconel 718 specimen was ion irradiated to demonstrate the difference in microstructure and mechanical properties between the irradiated and non-irradiated specimens. After exposing specimens to Fe³⁺ ions irradiation up to 100 dpa (displacement per atom) at an ambient temperature, the hardness of irradiated specimens was measured by nano-indentation as a function of depth. The depth distribution profile of Fe³⁺ and dpa were calculated by the Monte Carlo SRIM (Stopping and Range of Ions in Matter)-2013 code under the assumption of the displacement threshold energy of 40 eV. A transmission electron microscope was utilized to observe the formation of irradiation defects such as dislocation loops. This study reveals that the Frank partial dislocation loops induce irradiation hardening of SLM SA Inconel 718 specimens.

• Journal of the Korean Society of Industry Convergence
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• v.26 no.6_3
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• pp.1289-1295
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• 2023

Polyurethane foam insulation required for storing and transporting cryogenic liquefied gas is already widely used as a thermal insulation material for commercial LNG carriers and onshore due to its stable price and high insulation performance. These polyurethane foams are reported to have different mechanical performance depending on the density, and the density parameter is determined depending on the amount of the blowing agent. In this study, density-dependent polyurethane foam was fabricated by adjusting the amount of blowing agent. The mechanical properties of polyurethane foam were analyzed in the room temperature and cryogenic temperature range of -163℃ at 1.5 mm/min, which is a quasi-static load range, and the cells were observed through microstructure analysis. The characteristics of linear elasticity, plateau, and densification, which are quasi-static mechanical behaviors of polyurethane foam, were shown, and the correlation between density and mechanical properties in a cryogenic environment was confirmed. The correlation between mechanical behavior and cell size was also analyzed through SEM morphology analysis. Polyurethane foam with a density of 180 kg/m³ had a density about twice as high as that of a polyurethane foam with a density of 96 kg/m³, but yield strength was about 51% higher and cell size was about 9.5% smaller.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.33 no.6
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• pp.282-287
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• 2023

The effect of stress relief heat treatment temperature and duration time on the microstructure, residual stress and Vickers hardness of additively manufactured Ti-6Al-4V alloy using laser powder bed fusion process was clarified. As a result of stress relief heat treatment for 240 minutes at 823 K and 60 minutes or more at 873 K, residual stress was decreased less than 30 MPa without grain growth and phase transformation which causes dimensional distortion and deterioration of mechanical properties. In addition, hardness was increased with increasing heat treatment temperature and duration time. It was deduced that the refinement of acicular martensitic α' phase due to the increasing duration time of isothermal heat treatment at 773~873 K, which was not detected by XRD and phase map analysis using SEM-EBSD, probably increases the hardness.

• Design & Manufacturing
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• v.17 no.4
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• pp.63-71
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• 2023

In order to increase the extrusion production speed of aluminum, extrusion die cooling technology using liquid nitrogen has recently attracted a lot of attention. Increasing the extrusion speed increases the temperature of the bearing area of extrusion dies and the extrusion profile, which may cause defects on the surface of extruded profile. Extrusion die cooling technology is to directly inject liquid nitrogen through a cooling channel formed between the die and the backer inside the die-set. The liquid nitrogen removes heat from the die-set, and gaseous nitrogen at the exit of the channel, covers the extrusion profile of an inert atmosphere reducing the oxidation and the profile temperature. The aim of this study is to evaluate the cooling capacity by applying die cooling to extrusion of Al-Mn-based aluminum alloy flat tubes, and to investigate the effects of die cooling on the change in extrusion characteristics of flat tubes. Cooling capacity was confirmed by observing the temperature change of the extrusion profile depending on whether or not die cooling is applied. To observe changes in material characteristics due to die cooling, surface observation is conducted and microstructure and precipitate analysis are performed by FE-SEM on the surface and longitudinal cross section of the extruded flat tubes.

• Journal of the Korea Institute of Building Construction
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• v.24 no.2
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• pp.193-202
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• 2024

This research delves into the evaluation of the suitability of ultrasonic pulse velocity as a diagnostic tool for early detection of frost damage in concrete. The investigation involves the measurement of compressive strength and ultrasonic pulse velocity concerning the depth of freezing for individual mortar specimens, followed by an analysis of their microstructure and their interrelation. The findings indicate a consistent decrease in both compressive strength and ultrasonic pulse velocity with increasing freezing depth. Furthermore, a correlation between compressive strength and ultrasonic pulse velocity concerning the depth of early frost damage is established. Consequently, the study asserts the potential of utilizing the ultrasonic pulse velocity method for early detection of frost damage in concrete, with prospects for quantifying the depth of damage through further research endeavors.

• Journal of Powder Materials
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• v.31 no.2
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• pp.152-162
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• 2024

The directed energy deposited (DED) alloys show higher hardness values than the welded alloys due to the finer microstructure following the high cooling rate. However, defects such as microcracks, pores, and the residual stress are remained within the DED alloy. These defects deteriorate the wear behavior so post-processing such as heat treatment and hot isostatic pressing (HIP) are applied to DED alloys to reduce the defects. HIP was chosen in this study because the high pressure and temperature uniformly reduced the defects. The HIP is processed at 1150℃ under 100 MPa for 4 hours. After HIP, microcracks are disappeared and porosity is reduced by 86.9%. Carbides are spherodized due to the interdiffusion of Cr and C between the dendrite and interdendrite region. After HIP, the nanohardness (GPa) of carbides increased from 11.1 to 12, and the Co matrix decreased from 8.8 to 7.9. Vickers hardness (HV) decreased by 18.9 % after HIP. The dislocation density (10^-2/m²) decreased from 7.34 to 0.34 and the residual stress (MPa) changed from tensile 79 to a compressive -246 by HIP. This study indicates that HIP is effective in reducing defects, and the HIP DED Stellite 6 exhibits a higher HV than welded Stellite 6.

• Journal of the Korean Recycled Construction Resources Institute
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• v.12 no.1
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• pp.53-62
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• 2024

In this paper, an experimental study on the strengths and material properties of concrete manufactured by using coal gangue, as a fine aggregate was conducted. Experimental parameters included coal gangue aggregate contents as a replacement of fine aggregate by 50 % and 100 % (by volume) and fly ash contents. The water-binder ratio was fixed at 0.38. In addition, 30 % of the OPC binder was replaced with fly ash in some mixtures. Test of the unit weight, compressive, split tensile, and flexural tensile strength of concrete were performed and test results were analyzed. Unit weight, compressive strength, split tensile strength, and flexural tensile strength decreased as the coal recycled aggregates increased. In addition, TGA and SEM experiments, which are microstructure experiments, were conducted to analyze thermogravimetric analysis and ITZ by section.