• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.24 no.11
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• pp.777-783
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• 2012

The hardness and strength test was performed to make the manufacturing process of SA213 P92 boiler pipe steel. And the microstructure change was studied to find out the cause of room temperature property of P92 steel, ie, low hardness and strength property. The room temperature property of P92 steel depends on the improper normalizing and cooling rate. Especially, Ferrite was formed and the steel had low hardness when the temperature was decreased slowly under the cooling rate $1^{\circ}C$/min after normalizing at the temperature around $A_{c1}$ to $A_{c3}$. The critical heat treatment temperature and cooling rate was over $900^{\circ}C$ and over $10^{\circ}C$/min to satisfy the minimum yield and tensile stress which was laid down by ASME Code.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.5
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• pp.241-247
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• 2013

Hardenability and mechanical properties of boron-bearing steels containing C, Mo and Cr were investigated in this study. Using quench dilatometer, the steel specimens were cooled down to room temperature at different cooling rates to construct continuous cooling transformation diagrams and then the transformation products from austenite were examined. A critical cooling rate was introduced as an index to quantitatively evaluate the hardenability. The C addition to boron-bearing steels did not significantly affect hardenability compared to boron-free steels although it increases the hardenability. With the same content, the Mo addition largely increased the hardenability of boron-bearing steels than the Cr addition because it decreased both the transformation start and finish temperatures at low cooling rates. In particular, the Mo addition completely suppressed the formation of eutectoid ferrite even at the slow cooling rate of $0.2^{\circ}C/s$, whereas the Cr addition nearly suppressed it at the cooling rates above $3^{\circ}C/s$.

• Journal of computational fluids engineering
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• v.16 no.4
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• pp.110-115
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• 2011

A modern optical fiber manufacturing process requires the sufficient cooling of glass fibers freshly drawn from the heated and softened silica preform in the furnace, since the inadequately cooled glass fibers are known to cause improper polymer resin coating on the fiber surface and to adversely affect the product quality of optical fibers. In order to greatly enhance the fiber cooling effectiveness at increasingly high fiber drawing speed, it is necessary to use a dedicated glass fiber cooling unit with helium gas injection between glass fiber drawing and coating processes. The present numerical study features a series of three-dimensional flow and heat transfer computations on the cooling gas and the fast moving glass fiber to analyze the cooling performance of glass fiber cooling unit, in which the helium is supplied through the discretely located rectangular injection holes. The air entrainment into the cooling unit at the fiber inlet is also included in the computational model and it is found to be critical in determining the helium purity in the cooling gas and the cooling effectiveness on glass fiber. The effects of fiber drawing speed and helium injection rate on the helium purity decrease by air entrainment and the glass fiber cooling are also investigated and discussed.

• Korean Journal of Materials Research
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• v.25 no.9
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• pp.497-502
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• 2015

The hardenability of boron steel specimens with different molybdenum and chromium contents was investigated using dilatometry and microstructural observations, and then was quantitatively measured at a critical cooling rate corresponding to 90 % martensite hardness obtained from a hardness distribution plotted as a function of cooling rate. Based on the results, the effect of an austenitizing temperature on the hardenability and tensile properties was discussed in terms of segregation and precipitation behavior of boron atoms at austenite grain boundaries. The molybdenum addition completely suppressed the formation of pro-eutectoid ferrite even at the slowest cooling rate of $0.2^{\circ}C/s$, while the chromium addition did at the cooling rates above $3^{\circ}C/s$. On the other hand, the hardenability of the molybdenum-added boron steel specimens decreased with an increasing austenitizing temperature. This is associated with the preferred precipitation of boron atoms since a considerable number of boron atoms could be concentrated along austenite grain boundaries by a non-equilibrium segregation mechanism. The secondary ion mass spectroscopy results showed that boron atoms were mostly segregated at austenite grain boundaries without noticeable precipitation at higher austenitization temperatures, while they formed as precipitates at lower austenitization temperatures, particularly in the molybdenum-added boron steel specimens.

• Korean Journal of Materials Research
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• v.23 no.10
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• pp.555-561
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• 2013

The hardenability of low-carbon boron steels with different molybdenum and chromium contents was investigated using dilatometry, microstructural observations and secondary ion mass spectroscopy (SIMS), and then discussed in terms of the segregation and precipitation behaviors of boron. The hardenability was quantitatively evaluated by a critical cooling rate obtained from the hardness distribution plotted as a function of cooling rate. It was found that the molybdenum addition was more effective than the chromium addition to increase the hardenability of boron steels, in contrast to boron-free steels. The addition of 0.2 wt.% molybdenum completely suppressed the formation of eutectoid ferrite, even at the slow cooling rate of $0.2^{\circ}C/s$, while the addition of 0.5 wt.% chromium did this at cooling rates above $3^{\circ}C/s$. The SIMS analysis results to observe the boron distribution at the austenite grain boundaries confirmed that the addition of 0.2 wt.% molybdenum effectively increased the hardenability of boron steels, as the boron atoms were significantly segregated to the austenite grain boundaries without the precipitation of borocarbide, thus retarding the austenite-to-ferrite transformation compared to the addition of 0.5 wt.% chromium. On the other hand, the synergistic effect of molybdenum and boron on the hardenability of boron steels could be explained from thermodynamic and kinetic perspectives.

• Journal of Korea Foundry Society
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• v.23 no.5
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• pp.276-285
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• 2003

In the undercooled melt of $Pd_{40}Cu_{30}Ni_{10}P_{20}$ alloy, the solidification behavior including nucleation and growth of crystals at the micrometer level has been observed in-situ by use of a confocal scanning laser microscope combined with an infrared image furnace. The $Pd_{40}Cu_{30}Ni_{10}P_{20}$ alloy specimens were cooled from the liquid state to glass transition temperature. 575 K, at various cooling late under a helium gas flow. According to the cooling rate, the morphologies of the solidification front are changed among various types, irregular jog like front, columnar dendritic front, cellular grain, star like shape jog and fine grain, etc. The velocities of the solid-liquid interface are measured to be $10^{-5}{\sim}10^{-8}$ m/s which are at least two orders higher than the theoretical crystal growth rates. Combining the morphologies observed in terms of cooling rates and their solidification behaviors, we conclude that phase separation takes place in the undercooled molten $Pd_{40}Cu_{30}Ni_{10}P_{20}$ alloy. The continuous cooling transformation (CCT) diagram was constructed from solidification onset time at various linear cooling conditions with different rate. The CCT diagram suggests that the critical cooling rate for glassy solidification is about 1.5 K/s, which is in agreement with the previous calorimetric findings.

• Journal of the Korean Society for Precision Engineering
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• v.28 no.1
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• pp.63-72
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• 2011

In recent years, hot-stamped components are more increasingly used in the automotive industry in order to reduce weight and to improve the strength of vehicles. In hot stamping process, blank is hot formed and press hardened in a tool. However, in hot stamping without cooling channel, temperature of the tool increases gradually in mass production thus cannot meet the critical cooling rate to obtain high strength over 1500MPa. Warpage occurs in the hot stamped component due to non-uniform stress state caused by unbalanced cooling. Therefore, tools should be uniformly as well as rapidly cooled down by the coolant which flows through cooling channel. In this paper, optimal design method of cooling channel to obtain uniform and high strength of the component is proposed. Optimized cooling channel is applied to the hot press V-bending process. As a result of measuring strength, hardness and microstructure of the hot formed parts, it is known that the design methodology of cooling channel is effective to the hot stamping process.

• Fashion & Textile Research Journal
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• v.2 no.3
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• pp.265-271
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• 2000

Heat stress results in fatigue, a decline in strength, alertness., and mental capacity. The problem is compounded when high humidity exists. To help relieve worker heat stress, many types of cooling units are marketed. While workers may experience some cooling, critical body core temperatures often continue to elevate. This study was designed to find the effects of three kinds of cooling vest with portable frozen gel strips on thermophysiological parameters and on temperature and humidity within clothing. The heart rate, rectal, and skin temperature as well as sweat rate and clothing microclimate were measured during 80 min in 5 healthy males. Inquiries were also made into the subjective rating thermal, humidity comfort, and fatigue sensations. The main findings in our experiments are as follows: (a) Physiological parameters such as rectal temperature was the lowest in garb A1, intermediate in garb A, and the highest in garb A2 throughout the experiment. And mean skin temperature was the lowest in garb A, intermediate in garb A1, and the highest in garb A2; (b) Temperature and humidity within clothing (back) were garb in Al, intermediate in garb A, and the highest in garb A2. But the temperature and humidity within clothing (chest) were garb in A, intermediate in garb A1, and the highest in garb A2; (c) Most participants (4 out of 5 persons) answered that they felt more comfortable and fatigueless in garb A1 than in garb A and A2. It is concluded that local cooling in garb A1 of the upper torso could physiological reduce the thermal strain in participants wearing cooling vest.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.1
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• pp.17-27
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• 2009

Water jet impingement cooling is used to remove heat from high-temperature surfaces such as hot steel plates in the steel manufacturing process (thermo-mechanical cooling process; TMCP). In those processes, uniform cooling is the most critical factor to ensure high strength steel and good quality. In this study, experiments are performed to measure the heat transfer coefficient together with the inverse heat conduction problem (IHCP) analysis for a plate cooled by planar water jet. In the inverse heat transfer analysis, spatial and temporal variations of heat transfer coefficient, with no information regarding its functional form, are determined by employing the conjugate gradient method with an adjoint problem. To estimate the two dimensional distribution of heat transfer coefficient and heat flux for planar waterjet cooling, eight thermo-couple are installed inside the plate. The results show that heat transfer coefficient is approximately uniform in the span-wise direction in the early stage of cooling. In the later stage where the forced-convection effect is important, the heat transfer coefficient becomes larger in the edge region. The surface temperature vs. heat flux characteristics are also investigated for the entire boiling regimes. In addition, the heat transfer rate for the two different plate geometries are compared at the same Reynolds number.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.47 no.1
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• pp.12-17
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• 2014

The cooling of precooked skipjack Katsuwonus pelamis is a critical thermal process in tuna canning because it affects the quality and yield of the canned tuna, as well as productivity. The combined method of vacuum cooling followed by water spraying (VC-WS) was investigated to increase cooling rates, and prevent loss of yield of the precooked tuna during vacuum cooling. For VC-WS, the precooked skipjack was cooled to $30^{\circ}C$ by vacuum at 31 mmHg and then wetted by spraying water for 2 min. The effects of VC-WS on cooling times, cooling loss, color, texture and lipid oxidation of the precooked tuna were compared with conventional spray cooling (SC).The cooling times for precooked skipjack from $75^{\circ}C-30^{\circ}C$ were 11 min for VC-WS and 145 min for SC. The cooling losses were 1.7 % for VC-WS and 1.6 % for SC. Peroxide and thiobarbituric acid (TBA) values of VC-WS were lower than those of SC. The loin of the VC-WS-treated skipjack was brighter and harder than the SC loin, as indicated by higher lightness and hardness values. Based on these results, we believe that the VC-WS process could compensate for the cooling loss of vacuum cooling and minimize changes in quality that occur during cooling of precooked skipjack tuna.