• Journal of Korea Foundry Society
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• v.12 no.1
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• pp.25-31
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• 1992

The effects of Si and austempering temperature on the fracture characteristics and the microstructures of austempered ductile irons were investigated. As Si content increased from 2.28% to 3.0%, the precipitation of carbides during bainitic transformation and was suppressed the amount of retained austenite increased resulting in the increase in the fracture toughness. It is believed that the high Si limited the formation of martensite in the microstructure and minimized the segregation of the other elements at cell boundaries. But in samples with too high Si content as 3.3%, the formation of islands of free ferrite in the bainitic structures was observed and the fracture toughness was measured to have degraded.

• Journal of the Korean Society for Railway
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• v.8 no.1
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• pp.57-62
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• 2005

The frost heaving pressure can be a problem for weakening of the railway roadbed material. This study was initiated to investigate the soils frost heaving pressure and physical characteristics(Liquid limit, permeability, SEM analysis) resulting from freezing and freezing-thawing cycle process. Therefore, upon freezing a saturated soil in a closed-system from the top, a considerable pressure was developed. Weathered granite soils, sandy soil were used in the laboratory freezing test which sometimes subjected to thermal gradients under closed-systems. The frost heaving pressure arising within the soil samples and the temperature of the samples inside were monitored with elapsed time. The degree of saturation versus heaving pressure curve is also presented for weathered granite soil and the maximum pressure is closely related to this curve. Based on the laboratory test results, fine-grained soils with strong attractive forces between soil grains md water molecules, and additional water is attracted into the pores leading to further volume changes and ice segregation.

• Journal of Ocean Engineering and Technology
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• v.14 no.4
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• pp.30-34
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• 2000

The concretes cast in the sea water would be likely to be rich mix and mass concrete. Therefore it is important to check out the hydration heat of concrete and to reduce it to prevent the concrete from processing the temperature crack. Recently the antiwashout agent is used on underwater concrete for preventing from the segregation of concrete in the water. The experimental studies were done for the combined cement replaced by fly ash 30%unit weight of binder to study on the characteristics of hydration heat of antiwashout underwater concrete, and its characteristic was discussed by comparing on cast in sea water with anther one in air. The present paper showed that the hydration heat concrete replaced by 30%of fly ash was more significantly reduced than the normal concrete. The hydration heat of antiwashout underwater concrete was highter than that of normal concrete, but it was reduced lower than the normal concrete by adding fly ash.

• Journal of the Korean institute of surface engineering
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• v.38 no.1
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• pp.7-13
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• 2005

Bismuth thin films were prepared on glass substrate with RF magnetron sputtering and effects of substrate temperature on surface morphology and their electrical transport properties were investigated. Grain growth of bismuth after nucleation and the onset of coalescense of grains at 393 K were observed with field emission secondary electron microscopy. Continuous thin films could not be obtained above 473 K because of grain segregation and island formation. Hall effect measurements showed that substrate heating yields the decrease of carrier density and the increase of mobility. Resistivity of bismuth film has its minimum (about 0.7 x 10/sup -3/ Ωcm) in range of 403～433 K. Annealing of bismuth films deposited at room temperature was carried out in a radiation furnace with flowing hydrogen gas. The change of resistivity was not significant due to cancellation of the decrease of carrier density and the increase of mobility. The abrupt change of electrical properties of film annealed above 523 K was found to be caused by partial oxidation of bismuth layer in x-ray diffraction analysis.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.3
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• pp.519-527
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• 1997

AISI 316 steel has been used extensively for heater and boiler tube of the structural plants such as power, chemical and petroleum plants under severe operating conditions. Usually, material degradation due to microcrack or precipitation of carbides and segregation of impurity elements, is occured by damage accumulated for long-term service at high temperature in this material. In this study, the effect of aging time on fracture toughness was investigated to evaluate the measurement of material degradation. The elastic-plastic fracture toughness behaviour of AISI 316 steel pipe aged at $550^{\circ}C$for 1h-10000h (the aged material) was characterized using the single specimen J-R curve technique and eletric potential drop method at normal loading rate(load-line displacement speed of 0.2mm/min) in room temperature and air environment. The fracture toughness data from above experiments is compared with the $J_{in}$ obtained from predicted values of crack initiation point using potential drop method.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.9
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• pp.156-164
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• 2001

Two-dimensional solidification analysis during rheology forming process of semi-solid aluminum alloy has been studied. Two-phase fluid flow model to investigate the velocity field and temperature distribution is proposed. The proposed mathematical model is applied to the die shape of the two types. To calculate the velocity and temperature fields during rheology forming process, the earth governing equation correspondent to the liquid and solid region are adapted. Therefore, each numerical models considering the solid and liquid region existing within the semi-solid material have been developed to predict the deflect of rheology forming gnarls. The Arbitrary Boundary Maker And Cell (ABMAC) method is employed to solve the two-phase flow model of the Navier-Stokes equation. Theoretical model on the basis of the two-phase flow model is the mixture rule of solid and liquid phases. This approach is based on the liquid and solid viscosity. The liquid viscosity is pure liquid state value, however solid viscosity is considered as a function of the shear rate, solid fraction and power law curves.

• Korean Journal of Materials Research
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• v.14 no.12
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• pp.829-834
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• 2004

Annealing of $Cu(B)/Ti/SiO_2$ in vacuum has been carried out to investigate the effects of Ti underlayer on microstructure in $Cu(B)/Ti/SiO_2$ structures. For comparison, $Cu(B)/Ti/SiO_2$ structures was also annealed in vacuum. Three different temperature dependence of Cu growth can be seen in $Cu(B)/Ti/SiO_2$; B precipitates- pinned grain growth, abnormal grain growth, normal grain growth. The Ti underlayer having a strong affinity for B atoms reacts with the out-diffused B to the Ti surface and forms titanium boride at the Cu-Ti interface. The formation of titanium boride acts as a sink for the out-diffusion of B atoms. The depletion of boron in grain boundaries of Cu films, as results of the rapid diffusion of B along the grain boundaries and the insufficient segregation of B to the grain boundaries, induces grain boundaries to migrate and causes the abnormal grain growth. The increased bulk diffusion coefficient of B within Cu grains can be responsible for the normal grain growth occurring in the annealed $Cu(B)/Ti/SiO_2\;at\;600^{\circ}C$. In contrast, the $Cu/SiO_2$ structures show only the abnormal growth of grains and their sizes increasing as the temperature increases above $400^{\circ}C$.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.11a
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• pp.743-746
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• 2004

This research investigates experimentally an effect on the properties of the high flowing concrete according to variations of concrete materials and site conditions. Variations of sensitivity test are selected items as followings; (1)Concrete temperature, (2)Unit water(Surface moisture of fine aggregate), (3)Fineness modulus of fine aggregate, (4)Addition ratio of high-range water reducing agent. And fresh conditions of the high flowing concrete should be satisfied with required range including slump flow$(65{\pm}5cm)$, 50cm reaching time of slump flow$(4\~10sec)$, V-box flowing time$(10\~20sec)$, U-box height(min.300mm) and air content$(4{\pm}1\%)$. As results of sensitivity test, material variations and site conditions should be satisfied with the range as followings; (1)Concrete temperature is $10\~20^{\circ}C$ (below $30^{\circ}C$), (2)Surface moisture of fine aggregate is within ${\pm}0.6\%$, (3)Fineness modulus of fine aggregate is $2.6{\pm}0.2$ and (4)addition ratio of high range water reducing agent is within $1\%$ considered flow-ability, self-compaction and segregation resistance of the high flowing concrete.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.10
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• pp.1971-1981
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• 2002

Two-dimensional solidification analysis during rheology forming process of semi-solid aluminum alloy has been studied. Two-phase flow model to investigate the velocity field and temperature distribution is proposed. The proposed mathematical model is applied to the die shape of the two types. To calculate the velocities and temperature fields during rheology forming process, the each governing equations correspondent to the liquid and solid region are adapted. Therefore, each numerical model considering the solid and liquid coexisting region within the semi-solid material have been developed to predict the defects of rheology forming parts. The Arbitrary Boundary Maker And Cell(ABMAC) method is employed to solve the two-Phase flow model of the Navier-Stokes equation. Theoretical model basis of the two-phase flow model is the mixture rule of solid and liquid phases. This approach is based on using the liquid and solid viscosity. The Liquid viscosity is pure liquid state value, however solid viscosity is considered as a function of the shear rate, solid fraction and power law curves.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.6 s.177
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• pp.1398-1407
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• 2000

The structural steels of power plant show the decrease of mechanical properties due to degradation such as temper embrittlement, creep damage and softening during long-term operation at high temper ature. The typical causes of material degradation damage are the creation and coarsening of carbides(M23C6, M6C) and the segregation of impurities(P, Sb and Sn) to grain boundary. It is also well known that material degradation induces the cleavage fracture and increases the ductile-brittle transition temperature of steels. So, it is very important to evaluate degradation damage to secure the reliable and efficient service condition and to prevent brittle failure in service. However, it would not be appropriate to sample a large test piece from in-service components. Therefore, it is necessary to develop a couple of new approaches to the non-destructive estimation technique which may be applicable to assessing the material degradation of the components with not to influence their essential strength. The purpose of this study is to propose and establish a new electrochemical technique for non-destructive evaluation of material degradation damage for Cr-Mo steels which is widely used in the high temperature structural components. And the electrochemical anodic polarization test results are compared with those of semi-nondestructive SP test.