• Advances in materials Research
• /
• v.4 no.2
• /
• pp.77-85
• /
• 2015

The influence of rare earth lanthanum (La) on solidification cooling range, microstructure of aluminum-magnesium (Al-Mg) alloy and mechanical properties were investigated. Five kinds of Al-Mg alloys with rare earth content of La (i.e., 0, 0.5, 1.0, 1.5 and 2.0 wt.%) were prepared. Samples were either slowly cooled in furnace or water cooled. Results indicate that the addition of the rare earth (RE) La can significantly influence the solidification range, the resultant microstructure, and tensile strength. RE La can extend the alloy solidification range, increase the solidification time, and also greatly improve the flow performance. The addition of La takes a metamorphism effect on Al-Mg alloy, resulting in that the finer the grain is obtained, the rounder the morphology becomes. RE La can significantly increase the mechanical properties for its metamorphism and reinforcement. When the La content is about 1.5 wt.%, the tensile strength of Al-Mg alloy reaches its maximum value of 314 MPa.

• Nuclear Engineering and Technology
• /
• v.54 no.6
• /
• pp.2188-2197
• /
• 2022

To ensure the safety margin of a reactor pressure vessel (RPV) under normal operating conditions, it is regulated through the pressure-temperature (P-T) limit curve. The stress intensity factor (SIF) obtained by the internal pressure and thermal load should be obtained through crack analysis of the nozzle corner crack in advance to generate the P-T limit curve for the nozzle. In the ASME code Section XI, Appendix G, the SIF via the internal pressure for the nozzle corner crack is expressed as a function of the cooling or heating rate, and the wall thickness, however, the SIF via the thermal load is presented as a polynomial format based on the stress linearization analysis results. Inevitably, the SIF can only be obtained through finite element (FE) analysis. In this paper, simple prediction equations of the SIF via the thermal load under, cool-down and heat-up conditions are presented. For the Korean standard nuclear power plant, three geometric variables were set and 72 cases of RPV models were made, and then the heat transfer analysis and thermal stress analysis were performed sequentially. Based on the FE results, simple engineering solutions predicting the value of thermal SIF under cool-down and heat-up conditions are suggested.

• Computers and Concrete
• /
• v.21 no.3
• /
• pp.279-289
• /
• 2018

Hydration heat and thermal induced cracking have always been a fatal problem for massive concrete structures. In order to study a massive reinforced concrete wall of a storage tank for liquefied natural gas (LNG) during its construction, two mock-ups of $0.8m{\times}0.8m{\times}0.8m$ without and with metal corrugated pipes were designed based on the actual wall construction plan. Temperature distribution and strain development of both mock-ups were measured and compared inside and on the surface of them. Meanwhile, time-dependent thermal and mechanical properties of the concrete were tested standardly and introduced into the finite-element (FE) software with a proposed hydration degree model. According to the comparison results, the FE simulation of temperature field agreed well with the measured data. Besides, the maximum temperature rise was slightly higher and the shrinkage was generally larger in the mock-up without pipes, indicating that corrugated pipes could reduce concrete temperature and decrease shrinkage of surrounding concrete. In addition, the cooling rate decreased approximately linearly with the reduction of heat transfer coefficient h, implying that a target cooling curve can be achieved by calculating a desired coefficient h. Moreover, the maximum cooling rate did not necessarily decrease with the extension of demoulding time. It is better to remove the formwork at least after 116 hours after concrete casting, which promises lower risk of thermal cracking of early-age concrete.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.12 no.8
• /
• pp.3366-3373
• /
• 2011

In this study, performance characteristics of a water source heat pump system using $CO_2$ as a refrigerant are investigated experimentally. Cooling and heating capacities and COP of the system are analyzed for various system performance variables such as refrigerant charge, expansion valve opening, compressor frequency and internal heat exchanger. Results show that optimum amount of refrigerant charge and expansion valve opening exists at maximum point of COP curve, and cooling capacity increases but COP decreases with the increase of compressor frequency. When the internal heat exchanger is installed, cooling capacity increases about 4.0% whereas heating capacity decreases about 0.89% compared to the case without internal heat exchanger.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.14 no.1
• /
• pp.12-16
• /
• 2004

In this study, we have investigated the development of the crystal growth stability and reproducibility for large and high-quality DAST. DAST crystal were grown from a saturated methanol solution by a slow cooling method and DAST was synthesized by the condensation of 4-methyl-n-methyl pyridinum tosylate, which was prepared from 4-pocoline and methyl toluenesulponate and 4-N-dimethyl amino-bezaldehyde in the presence of piperidine. We had synthesized DAST crystals in dry Argon atmosphere in order to avoid the formation of hydride organge co-crystals, DAST$.$$H_2O$. Since DAST molecules crystallize in a humid atmosphere, crystal structure become centrosymmetric, and then second order NLO (nonlinear optical) properties would be disappeared. We fixed the growth orientation of DAST crystal (001) surface. The crystal growth was proceeded at a cooling rate of $H_2O$/day and the cooling period is for 4 days. The dimensions of seed crystal was $2.5\times 3.6\times0.4\textrm{mm}^3$ and we have obtained a DAST crystal with the dimension of $10\times 10.5\times3.0\textrm{mm}^3$. The color of grown DAST crystal is red and it's surface appears to be metallic green.

• Steel and Composite Structures
• /
• v.8 no.1
• /
• pp.1-18
• /
• 2008

Observations from experiments and real fire indicate that restrained steel beams have better fire-resistant capability than isolated beams. Due to the effects of restraints, a steel beam in fire condition can undergo very large deflections and the run away damage may be avoided. In addition, axial forces will be induced with temperature increasing and play an important role on the behaviour of the restrained beam. The factors influencing the behavior of a restrained beam subjected to fire include the stiffness of axial and rotational restraints, the load type on the beam and the distribution of temperature in the cross-section of the beam, etc. In this paper, a simplified model is proposed to analyze the performance of restrained steel beams in fire condition. Based on an assumption of the deflection curve of the beam, the axial force, together with the strain and stress distributions in the beam, can be determined. By integrating the stress, the combined moment and force in the cross-section of the beam can be obtained. Then, through substituting the moment and axial force into the equilibrium equation, the behavior of the restrained beam in fire condition can be worked out. Furthermore, for the safety evaluation and repair after a fire, the behaviour of restrained beams during cooling should be understood. For a restrained beam experiencing very high temperatures, the strength of the steel will recover when temperature decreases, but the contraction force, which is produced by thermal contraction, will aggravate the tensile stresses in the beam. In this paper, the behaviour of the restrained beam in cooling phase is analyzed, and the effect of the contraction force is discussed.

• Nuclear Engineering and Technology
• /
• v.25 no.1
• /
• pp.8-19
• /
• 1993

The natural convection model of the consolidated system has been developed to make sure the removal of decay heat generated in the spent fuel for the loss of forced cooling accident. The numerical technique employed was based on the ADI scheme. The calculation of heat generation rate in the spent fuel was peformed by the ANS-79 decay heat model, and the nonuniform surface heat flux is assumed with a chopped sine curve for the conservative decay heat generation input. The sensitivity study was performed to examine the possibility of the pool bulk boiling by varying the various parameters, i.e. inter-fuel spacing ratio, heat generation power, and radius of the fuel rod. The application results of this model show that the natural circulation flow through compacted spent fuel bundles enables the pool temperature to control in a safe and effective manner, after the required cooling time. The corresponding acceptance criteria of the cooling time for rearranging the spent fuel rods were also found.

• Journal of the Korean Society for Heat Treatment
• /
• v.36 no.3
• /
• pp.121-127
• /
• 2023

When high carbon steel is heated in an appropriate austenizing temperature range and subjected to austempering, the size and shape of lamellar structure can be controlled. The high carbon steel sheet having the pearlite structure has excellent elastic characteristics because it has strong restoring force when properly rolled, and is applied in a process known as patenting-process using lead bath. In the case of isothermal treatment using lead-medium, it is possible to quickly reach a uniform temperature due to high heat transfer characteristics, but it is difficult to replace it with process technology that requires treatment to remove harmfulness lead. In this study, we intend to develop fluidization technology using garnet powder to replace the lead medium. After heating the high-carbon steel, the cooling rate was changed by compressed air to the vicinity of the nose of the continuous cooling curve, and then maintained for 90 s and then exposed to room temperature. The microstructure of the treated specimens were analyzed and compared with the existing products treated with lead bath. The higher the flow rate of compressed air, the faster the cooling rate to the pearlite transformation temperature, so lamellar spacing decreases and the hardness tends to increase.

• The Journal of the Petrological Society of Korea
• /
• v.21 no.3
• /
• pp.335-365
• /
• 2012

Fission-track (FT) thermochronological records from SE Korean Cretaceous-Paleogene granitic plutons in different fault-bounded blocks reveal contrasting cooling and later thermal histories. Overall cooling patterns are represented by a monotonous (J-shaped) curve in most plutons except some Cretaceous granites retaining a complicated (N-shaped) path due to post-reset re-cooling. Discriminative cooling rates over different temperature ranges can be explained for individual plutons with respect to relative pluton sizes, differences in initial heat loss depending on country rocks, and the presence and proximity of later igneous activity. Even within a single batholith, cooling times for different isotherms were roughly contemporaneous with respect to positions. Insignificant deviations in cooling ages from two different plutons in succession across the Yangsan fault may suggest their contemporaneity before major horizontal fault movement. The extent of later thermal rise recorded locally along the Yangsan and Dongnae fault zones were reached the Apatite Partial Stability Zone ($70-125^{\circ}C$), but did not exceed $200^{\circ}C$. Thermal alteration from fractured zones in the Yangsan-Ulsan fault junction may suggest a thermal reset above $290^{\circ}C$ resulting a complete reset in FT sphene age (31 Ma), caused by a tectonic subsidence in Early Oligocene. A consistency in FT zircon/apatite ages (24 Ma) may imply a sudden rapid cooling over $200-105^{\circ}C$, plausibly related to the abrupt tectonic uplift of the Pohang-Gampo Block including the fault junction in Late Oligocene. A remarkable trend of lower cooling ages for $300-200-100^{\circ}C$ isotherms (i.e., 19% for FT sphene and K-Ar biotite; 20% for FT zircon; 27% for FT apatite) from the east of the Ulsan fault (Pohang-Gampo Block) comparing to the west of the fault may be attributed to retarded cooling times from the Paleogene granites and also reflected by their partially-reduced apatite ages due to later thermal effects.

• Journal of Korea Foundry Society
• /
• v.7 no.4
• /
• pp.358-365
• /
• 1987

To investigate the grain refining mechanism of Al by the addition of Ti-B, the unidirectional solidifications of 99.9%Al and 99.7%Al were performed under the condition of varing the pouring temperature. The solidification modes of Al were studied by the cooling curve analyses, metallographic and microprobe examinations. The results were as follows: 1) Grains were most refined with an addition of 0.15wt.%Ti-0.021wt.%B but the grain size with 0.2wt.%Ti-0.028wt.%B was increased. 2) The grain size of 99.7wt.%Al was even more refined than that of 99.9wt.%Al with the same amount of Ti-B. 3) As the pouring temperature increased, the grain size of pure Al and an alloy with 0.lwt.%Ti-0.014wt.%B was increased. However, an alloy with 0.2wt.%Ti-0.028wt.%B did not show any effects of temperature. 4) TiC(Al-Ti) and (Al-Ti-C) were identified as nucleants for Al.