• Journal of Korea Foundry Society
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• v.35 no.1
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• pp.1-7
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• 2015

In this study, we investigated the effect of carbon on mechanical properties with different austempering conditions of high carbon(0.7~1.3wt.%C)-2.0wt.%Si steels. The specimens were austenitized at 850, 925 and $1020^{\circ}C$, and austempered at 260, 320 and $380^{\circ}C$ for the various period of time from 3 min to 300 min. After heat treatment, the evolution of stage I and stage II was identified with optical microscope, XRD and hardness test. When the austempering temperature was $260^{\circ}C$, the microstructure consisted of the lower ausferrite while the upper ausferrite micro-structure was formed at $380^{\circ}C$. As the austempering temperature increases from 260 to $380^{\circ}C$, the tensile strength decreases and elongation increases. In addition, when carbon content increases, tensile strength and elongation decrease.

• Journal of Korea Foundry Society
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• v.32 no.6
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• pp.284-288
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• 2012

The effects of Si content on the variation of microstructure and processing window of austempered ductile cast iron were investigated. Four different Si contents between 2.42 and 3.37 wt.% were used. The influence of silicon on the microstructure and processing window of these materials were studied. Austenitizing was performed at $900^{\circ}C$ for 60min and austempering temperature were both $340^{\circ}C$ and $360^{\circ}C$ and austempering time were for 4min upto 119min and for 5min upto 160min respectively. After heat treatment, the evolution of stage I and stage II were performed by optical metallography, XRD, hardness test. The results showed that $t_2$ was delayed as Si contents was increased due to the fact that Si retarded the formation of cementite ($Fe_3C$). The high silicon content promoted the stability of the metastable two-phase combination of austenite and ausferrite.

• Journal of the Korean Ceramic Society
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• v.47 no.5
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• pp.387-393
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• 2010

To improve concrete quality one of the most widely used chemical admixtures is polycarboxylate type superplasticizer. Unlike lignosulfonate and naphthalene-sulfonate, it has high dispersion property and excellent sustainable dispersion property for cement and concrete. Thus, polycarboxylate type superplasticizer has been widely used as a high-performance water reducing admixture together with silica fume in high-performance concrete and other applications for the dispersion of high-strength concrete over 100 MPa. However, even though there have been many studied on the dispersion of concrete by the structure of polycarboxylate type superplasticizer, there have a few studied that clarified the relationships between its rheological properties and microstructure properties in the early hydration behavior of ordinary portland cement. To investigate the correlations between the rheological properties and microstructure of cementitious materials with polycarboxylate type superplasticizer, this study experimented on the rheology, pore structure, heat evolution, and consistency in early hydration as well as on the compressive strength by early dispersion characteristics.

• International Journal of Aeronautical and Space Sciences
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• v.3 no.1
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• pp.95-104
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• 2002

Numerical solution of the full Navier-Stokes equation as well as the energy equation has been obtained for the unsteady natural convection in a rectangular enclosure. One side wall was maintained at very high temperature simulating fires. Especially the effect of surface radiation was taken into account. While the enclosed air was assumed to be transparent, the internal walls directly interacted one another through the surface radiation. Due to a significant temperature difference in the flow field, the equation of state was used instead of the Boussinesq approximation. It was found that the rapid heating of the adiabatic ceiling and floor by the incoming radiation from the hot wall made the evolution at thermo-fluid field highly unstable in the initial period. Therefore, the secondary cells brought about at the floor region greatly affected the heat transfer mechanism inside the enclosure. The heat transfer rate was augmented by the radiation, resulting in requiring less time for the flow to reach the steady state. At the steady state neglecting radiation two internal hydraulic jumps were clearly observed in upper/left as well as in lower/right comer. However, the hydraulic jump in the lower/right comer could not be observed for the case including radiation due to its high momentum flow over the bottom wall. Radiation resulted in a faster establishment of the steady state phenomena.

• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.3
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• pp.225-235
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• 2010

There are many types of the geometric transitions such as dent, bulge, protrusion, expansion, etc, on the inner and outer surfaces of heat exchanger tubes, steam generator tubes, and condenser tubes of nuclear power plants. Such geometric transition causes a local residual stress in heat exchanger tubes and acts as a structural factor accelerating the evolution of defects, in particular stress corrosion cracks. In the conventional eddy current test methods, the bobbin coil profilometry can provide 2-dimensional geometric information on the variation of the average inner diameter along the tube length, but the 3-dimensional distribution and the quantitative size of a local geometric transition existing in the tube cannot be measured. In this paper, a new eddy current probe, developed for the 3-dimensional profile measurement, is introduced and its superior performance is compared with that from the conventional bobbin coil profilometry for the various types of geometric transition. Also, the accuracy of the probe for the quantitative profile measurement is verified by comparing the results with that from the laser profilometry. It is expected that the new eddy current probe and techniques can be effectively used for an optimization of the tube expansion process, and the management of tubes with geometric transitions in service.

• Journal of the Korean Society for Heat Treatment
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• v.6 no.3
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• pp.138-143
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• 1993

Effects of aging treatment on the microstructural evolution of a rapidly solidified Al-Li-Cu alloy were investigated by differential scanning calorimetry (DSC) and transmission electron microscopy(TEM). It was found that the precipitation sequence was: supersaturated solid solution ${\alpha}^{\prime}$ ${\rightarrow}$ metastable ${\delta}^{\prime}$ + stable precipitate $T_1$ ${\rightarrow}$ stable precipitates ${\delta}+T_1$. Two exothermic and two endothermic peaks are detected by DSC analysis. The two exothermic and endothermic peaks corresponded to ${\delta}^{\prime}$ and ${\delta}+T_1$ precipitation and dissolution reactions respectively. The enthalpy of ${\delta}^{\prime}$ precipitation decreased with increasing of aging temperature and time until the finishing point of precipitation. The activation energy for precipitation of ${\delta}^{\prime}$ was measured as 80KJ/mol and the energy for dissolution was 93KJ/mol. These values arc higher than those of AI-Li binary alloy. Peak hardness value (Hv 170) was obtamed at $210^{\circ}C$ for 1hr aging treatment which coincided with finishing point of ${\delta}^{\prime}$ precipitation.

• Korean Journal of Remote Sensing
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• v.37 no.4
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• pp.665-685
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• 2021

This study focused on the a 20-year evaluation of the dynamism of critical thermal anomalies in Busan metropolitan area prompted by unusual infrastructural development and demographic growth rate. Archived Landsat thermal data derived-LST was the major input for UTFVI and hot spot analysis (Getis-Ord Gi^*). Results revealed that the surface urban heat island-affected area has gradually expanded overtime from 23.32% to 32.36%; while the critical positive thermal anomalies (level-3 hotspots) have also spatially increased from 19.88% in 2000 to 23.56% in 2020, recording a net LST difference of > 5℃ between the maximum level-3 hotspot and minimum level-3 coldspot each year. It is been observed that thermal conditions of Busan have gradually deteriorated with time, which is potentially inherent in the rate of urban expansion. Thus, this work serves as an eye-opener to powers that be, to think and act constructively towards a sustainable thermal conform for city dwellers.

• Transactions of the Korean hydrogen and new energy society
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• v.34 no.6
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• pp.608-614
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• 2023

The fast filling process of high-pressure hydrogen has an important impact on the filling efficiency and safety. In this paper, a specific study is carried out on the thermophysical phenomena during the fast filling process. Starting from the gas state equation of hydrogen, the change law of the hydrogen storage temperature is obtained, and then the temperature rise prediction is constructed. The model can clarify the relationship between the filling parameters and the temperature rise during the fast filling process, thereby revealing the flow and heat transfer laws of the fast charging process. To improve the theoretical research basis for the evaluation of vehicle-mounted hydrogen fast charging capacity, temperature prediction and optimization of hydrogenation methods.

• Journal of the Korean Society for Heat Treatment
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• v.37 no.2
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• pp.58-65
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• 2024

In the present study, the effect of annealing condition on the microstructures and mechanical properties of the cold-rolled CoCrFeMnNi high entropy alloys were studied. Annealing treatment was performed under six different temperatures. Microstructural analyses confirmed that annealing below 800℃ resulted in the formation of intermetallic sigma (σ) phase within face-centered cubic (FCC) matrix, and this σ phase has beneficial effects on the formation of fine-grained structures through retardation of grain growth and recrystallization due to Zener pinning effect. This led to the enhanced yield strength and tensile strength of ~646 and ~855 MPa, respectively. The microstructures annealed above 800℃ demonstrated single FCC phase, and fully-recrystallized single FCC microstructure resulted in a slight increase in ductility with a considerable decrease in strength. The evolution of mechanical properties, such as strength, ductility, and strain hardening exponent, will be discussed.

• Korean Journal of Materials Research
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• v.14 no.1
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• pp.28-34
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• 2004

IN 738LC, the major material for gas-turbine for power generation, was heat treated at $750^{\circ}C$, $850^{\circ}C$, $950^{\circ}C$ for 1000, 2000, and 4000 hrs and the microstructural evolution and mechanical properties were examined using optical microscope, XRD, SEM/EDS. The results showed ${\gamma}$', the main strengthening elements in this alloy, was about 300 nm in size and was about 56% by area fraction in as-cast samples. The area fraction of ${\gamma}$' peaked at 2000 hours at $750^{\circ}C$. The average diameter of the ${\gamma}$' which was about 300 nm at ascast specimen increased to about 1 $\mu\textrm{m}$ after heat treatment at $950^{\circ}C$ for 4000 hrs. Carbides were formed at dendrite, cell or grain boundaries which was ascribed to the segregation caused by solute redistribution during solidification. It was found that MC type carbides formed at low temperature, whereas carbides of $M_{23}$ /$C_{ 6}$/ type formed at higher temperature or at longer degradation. The hardness and impact energy decreased as the heat treatment temperature or time of retention increased, which was inaccrodance with the area fraction of ${\gamma}$'.