• 한국세라믹학회지
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• 제53권6호
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• pp.659-664
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• 2016

Ash deposition of heat exchange boiler, caused mainly by accumulation of particulate matter, reduces heat transfer of the boiler system. Heat and mass transfer through porous media such as ash deposits mainly depend on the microstructure of deposited ash. Therefore, in this study, we investigated microstructural and thermal properties of the ash deposited on the boiler tube. Samples for this research were obtained from the fuel economizer tube in an industrial waste incinerator. To characterize microstructures of the ash deposit samples, scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD) and BET analysis were employed. The results revealed that it had a porous structure with small particles mostly of less than a few micrometers; the contents of Ca and S were 19.3, 22.6% and 18.5, 18.7%, respectively. Also, the results showed that it consisted mainly of anhydrite ($CaSO_4$) crystals. - The thermal conductivities of the ash deposit sample obtained from the economizer tube in industrial waste incinerator were measured to be 0.63 and 0.54 W/mK at $200^{\circ}C$, which were about 100 times less than the thermal conductivity (61.32 W/mK) of the boiler tube itself, indicating that ash deposition on the boiler tube was closely related to a decrease in boiler heat transfer.

• 공업화학
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• 제16권2호
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• pp.180-186
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• 2005

본 논문은 유기물과 중금속이 함께 포함되어 있는 폐수 슬러지의 시멘트 고형화를 위해 포졸란물질인 실리카흄을 고화보조재로 사용한 연구결과이다. 실리카흄을 고화보조재로 사용할 경우 고화체의 일축압축강도는 현저히 증가하는 결과를 보여주었으며, 실리카흄의 시멘트 대체가 전체 배합물의 15%까지 증가하였을 때 일축압축강도는 실리카흄을 전혀 사용하지 않았을 경우에 비하여 66.7%까지 증가하는 결과를 보여주었다. 또한 시멘트가 실리카흄으로 대체되는 양이 증가하면서 총 유기탄소와 중금속인 크롬의 용출량은 감소하는 결과를 보여주었으며, 배합물 중 실리카흄이 5%를 차지할 때 중성의 용출액에서 총 유기탄소의 85%가 용출억제되었으며, 산성의 용출액에서 0.76 mg-Cr/g-Cr의 크롬만이 용출되었다. 전자주사현미경과 X-선 회절분석, 적외선 분광분석 등을 이용한 미세구조분석 결과 실리카흄은 시멘트경화 초기에 크링커 입자를 피복하여 초결(initial setting)과 수화를 저해하는 에트링가이트(ettringite)의 형성을 감소시키는 것으로 나타났다. 본 연구결과 중금속과 유기물이 혼합된 슬러지의 시멘트 고형화에 있어서 실리카흄의 첨가는 유기물질이 시멘트 수화반응에 미치는 악영향을 억제하고 중금속 및 유기물의 용출을 감소시키는 것으로 나타났다.

• 한국레이저가공학회지
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• 제3권1호
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• pp.29-37
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• 2000

For a development purpose of thick metal / metal Graded-Boundary Materials(GBM), a basic research on the fabrication of Ni-Cr/steel GBM was carried out by a laser beam and its mechanical properties and thermal characteristics were investigated. In order to produce a compositionally graded boundary region between substrate steel and added Ni-Cr alloy, a series of surface alloying treatments was performed with a high power CO$_2$ laser beam. Ni-Cr sheet was placed on a low carbon steel plate(0.18%C), and then a CO$_2$ laser beam was irradiated on the surface to produce a homogeneous alloyed layer. On this first surface-alloyed layer, another Ni-Cr sheet was placed and then the CO$_2$ laser beam was irradiated again to produce second surface-alloyed layer. Sequential repetitions of laser surface alloying treatment 4 times resulted in a graded-boundary region with the thickness of about 1.4mm. Simultaneous concentration profiles of different kinds of alloying elements(Ni and Cr) showed from 42%Ni, 45%Cr and 13%Fe on surface region to 0%Ni, 0%Cr and 99%Fe in substrate region. Also a thermal conductivity gradient resulted in graded-region and its value changed from 0.03㎈/cm s$\^{C}$ in surface region to 0.1㎈/cm s$\^{C}$ in substrate region. Microstructural observation showed that any visible root porosities and solidification shrinkage cracks were not formed in graded region between alloyed layer and substrate region during rapid cooling.

• 열처리공학회지
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• 제19권2호
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• pp.96-102
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• 2006

Development of Rotation-Cylinder Method(RCM) is given for Mg-Al-Zn-X(X=Sr) alloys, identifying some of the key factors that need to be controlled and indicating subsequent casting processes. Effects of Sr addition on the microstructure and mechanical properties of as-cast and T6 heat-treated Mg-Al-Zn-X alloys fabricated by the RCM were investigated. The microstructural and mechanical characterizations were performed by utilizing optical microscope, scanning electron microscope, transmission electron microscope, hardness test and ultimate tensile test. The solution and aging treatment time was varied to optimize the T6 heat treatment conditions, and experimental results were discussed. The grain size of Sr containing alloys was refined by increasing Sr content, but the tensile and yield strengths were increased by addition up to 1wt% Sr.

• 대한금속재료학회지
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• 제48권12호
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• pp.1047-1055
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• 2010

The objective of this study is to investigate the ballistic properties of Zr-based amorphous alloy surface composites fabricated by high-energy electron-beam irradiation. The mixture of Zr-based amorphous powders and $LiF+MgF_2$ flux powders was deposited on a pure Ti substrate, and then an electron beam irradiated this powder mixture to fabricate a one-layer surface composite. A four-layer surface composite, in which the composite layer thickness was larger than 3 mm, was also fabricated by irradiating the deposited powder mixture by an electron beam three times on the one-layer surface composite. The microstructural analysis results indicated that a small amount of fine crystalline particles were homogeneously distributed in the amorphous matrix of the surface composite layer. According to the ballistic impact test results, the surface composite layers effectively blocked a fast traveling projectile, while many cracks were formed at the composite layers, and thus the surface composite plates were not perforated. The surface composite layer containing ductile ${\beta}$ dendritic phases showed a better ballistic performance than the one without dendrites because dendritic phases hindered the propagation of shear bands or cracks.

• Advances in concrete construction
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• 제8권1호
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• pp.21-31
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• 2019

The rich recipe of ultra high performance concrete (UHPC) offers the higher mechanical, durability and dense microstructure property. The variable like cement/sand ratio, amount of supplementary cementitious material, water/binder ratio, amount of fiber etc. alters the UHPC hardened properties to any extent. Therefore, to understand the effects of these variables on the performance of UHPC, inevitably a stage-wise development is required. In the present experimental study, the effect of sand/cement ratio, the addition of finer material (fly ash and quartz powder) and, hybrid fiber on the fresh, compressive and microstructural property of UHPC is evaluated. The experiment is conducted in three phases; the first phase evaluates the flow value and strength attainment of ingredients, the second phase evaluates the efficiency of finer materials (fly ash and quartz powder) to develop the UHPC and the third phase evaluate the effect of hybrid fiber on the flow value and strength of ultra high performance hybrid fiber reinforced concrete (UHP-HFRC). It has been seen that the addition of fly ash improves the flow value and compressive strength of UHPC as compared to quartz powder. Further, the usage of hybrid fiber in fly ash contained matrix decreases the flow value and improves the strength of the UHP-HFRC matrix. The dense interface between matrix and fiber and, a higher amount of calcium silicate hydrate (CSH) in fly ash contained UHP-HFRC is revealed by SEM and XRD respectively. The dense interface (bond between the fiber and the UHPC matrix) and the higher CSH formation are the reason for the improvement in the compressive strength of fly ash based UHP-HFRC. The differential thermal analysis (DTA/TGA) shows the similar type of mass loss pattern, however, the amount of mass loss differs in fly ash and quartz powder contained UHP-HFRC.

• 한국군사과학기술학회지
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• 제22권4호
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• pp.492-500
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• 2019

Equal channel angular pressing(ECAP) is a severe plastic deformation technique capable of introducing large shear strain in bulk metal materials. However, if an ECAPed material has an inhomogeneous microstructure and anisotropic mechanical properties, this material is difficult to apply as structural components subjected to multi-axial stress during use. In this study, extruded oxygen-free copper(OFC) rods with a large diameter of 42 mm are extruded through ECAP by route Bc up to 12 passes. The variations in the microstructure, hardness, tensile properties, and microstructural and mechanical homogeneity of the ECAPed samples are systematically analyzed. High-strength OFC rods with a homogeneous and equiaxed-ultrafine grain structure are obtained by the repeated application of ECAP up to 8 and 12 passes. ECAPed samples with 4 and 8 passes exhibit much smaller differences in terms of the average grain sizes on the cross-sectional area and the tensile strengths along the axial and circumferential directions, as compared to the samples with 1 and 2 passes. Therefore, it is considered that the OFC materials, which are fabricated via the ECAP process with pass numbers of a multiple of 4, are suitable to be applied as high-strength structural parts used under multi-axial stress conditions.

• 대한금속재료학회지
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• 제49권12호
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• pp.924-929
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• 2011

It is well known that grain boundary cavitation is the main failure mechanism in austenitic stainless steel under tensile hold creep-fatigue interaction conditions. The cavities are nucleated at the grain boundary during cyclic loading and grow to become grain boundary cracks. The attenuation of ultrasound depends on scattering and absorption in polycrystalline materials. Scattering occurs when a propagation wave encounters microstructural discontinuities, such as internal voids or cavities. Since the density of the creep-fatigue cavities increases with the fatigue cycles, the attenuation of ultrasound will also be increased with the fatigue cycles and this attenuation can be detected nondestructively. In this study, it is found that individual grain boundary cavities are formed and grow up to about 100 cycles and then, these cavities coalesce to become cracks. The measured ultrasonic attenuation increased with the cycles up to cycle 100, where it reached a maximum value and then decreased with further cycles. These experimental measurements strongly indicate that the open pores of cavities contribute to the attenuation of ultrasonic waves. However, when the cavities develop, at the grain boundary cracks whose crack surfaces are in contact with each other, there is no longer any open space and the ultrasonic wave may propagate across the cracks. Therefore, the attenuation of ultrasonic waves will be decreased. This phenomenon of maximum attenuation is very important to judge the stage of grain boundary crack development, which is the indication of the dangerous stage of the structures.

• 지질공학
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• 제32권3호
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• pp.339-350
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• 2022

경주시 골굴사 신생대 응회암에서는 다양한 형태의 타포니가 다수 발달한다. 응회암은 풍화변질되어 있는데, 석영, 장석류, 운모, 버미큘라이트, 녹니석, 스멕타이트와 제올라이트 광물인 아날사이트 등으로 구성된다. 타포니는 초기 단계에서는 각력이 제거된 동공이나, 응회암 결정 입자의 풍화변질에 의해 형성된 미세 동공에서부터 시작하여 점차 확대된다. 타포니는 사면의 반대방향으로 약하게 경사지면서 발달한다. 타포니의 상관계수는 폭-높이 0.839, 폭-깊이 0.900, 높이-깊이 0.856인데 서로 밀접한 관련성이 있다. 타포니 벽체가 제거되면 반달형이나 신장된 타포니로 되는데, 작은 동공들이 횡적으로 병합되면서 확대된다. 커다란 타포니 동공들은 암반 사면에 비하여 내부가 비어있기 때문에 암반의 강도가 더 낮다. 따라서 타포니 사면의 강도, 동공의 비율, 체적, 합병 양상에 대한 지속적인 모니터링이 요구된다.

• 열처리공학회지
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• 제34권3호
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• pp.122-129
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• 2021

This study focuses on the microstructural development of 99% magnesium alloy sheet manufactured using twin roll casting (TRC) process. Herein, a plate with a thickness of 5 mm was manufactured using the TRC process, homogenization heat treatment was performed at 400℃ for 2-32 h, and finally, the change in microstructure was evaluated via optical microscopy and textural analysis. The results suggest that the plate manufactured using the TRC process was not destroyed and was successfully rolled into a plate. Microscopic observation suggested that the dendritic cast structure was arranged along the rolling direction. And the central layer of the rolled plate, where was present in a liquid state at the beginning of rolling, solidified later during the TRC process to form central segregation. The initial cast structure and inhomogeneous structure of the plate were recrystallized by homogenization heat treatment for only 2 h, and it was confirmed that the segregated part of the central layer became homogeneous and recrystallization occurred. Grain growth occurred as the heat treatment time increased, and secondary recrystallization occurred, wherein only some grains were grown. The textural analysis, which was conducted via X-ray diffraction, confirmed that the relatively weak basal plane texture developed using the TRC process was formed into a random texture after heat treatment.