• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.11
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• pp.2860-2870
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• 1994

When crack are detected in aged turbine rotors of power plants, information on fracture resistance of the aged material at operating temperature is needed for determination of critical loading condition and residual life of the turbine. In this study, fracture toughness (J$_lc$) and tearing modulus(T$_mat$) of virgin and thermally degraded 1Cr-1Mo-0.25V steel, which is one of the most widely used rotor steels, were measured at 538.deg. C according to ASTM E813 and ASTM E1152, respectively. Five kinds of specimen with different degradation levels were prepared by isothermal aging heat treatment at $630^{\circ}C.$ It was observed that J$_lc$ and T$_mat$ value decreased as the degradation level increased. Analysis of microstructures using a scanning electron microscope showed that the decrement of J$_lc$ is related to segregation of impurities at grain boundaries. It was also verified that the DC electric potential drop method is accurate and reliable for crack length monitoring at elevated temperature.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.12 s.255
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• pp.1526-1533
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• 2006

The aim of this research was to confirm the existence of the thermal creep degradation by hydrides of Zr-2.5Nb pressure tube materials. Small punch creep tests were performed to obtain the relationship between a creep displacement and a loading period at $300^{\circ}C$. A creep stress and a creep strain rate were also converted from the previous results. The creep material constants and the creep stress exponents at the different hydride contents were compared. Finally the hydrides of the axial and circumferential section were observed using OM, SEM and TEM. The following conclusions were made: 1) The degradation of the thermal creep by hydrides was existed and it strongly depended on the hydride contents. 2) As the hydride contents were increased, the creep stress exponents (m) were also increased. 3) Even though the hydride was not precipitated in 50 ppm materials at $300^{\circ}C$, the degradation of thermal creep was found. Therefore, it was believed that this phenomenon strongly related to the hydride precipitation at room temperature.

• Journal of the KSME
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• v.32 no.4
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• pp.351-357
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• 1992

고온 사용 설비의 잔여수명을 진단하는 비파괴적인 여러 가지 방법 중에서 현장에서 간단히 측 정을 함으로써 경년열화 정도를 비교적 쉽게 알아낼 수 있는 경도법에 대하여 살펴보았다. 이 방법은 크립손상 평가뿐만 아니라 피로 손상 평가에도 사용되고 있으며 외국의 경우 현장에서 많이 사용되고 있는, 가장 보편화된 방법 중의 하나이다. 국내의 경우에도 노후화된 고온 사용 설비가 상당수되기 때문에 설비의 안전성이나 경제적인 운용을 위해서 이들의 손상 정도를 정 기적으로 진단할 필요성이 매우 크나 이에 대한 연구 성과는 외국에 비하여 미미한 편이기 때 문에 앞으로 이에 대한 많은 노력이 이루어져야 한다고 생각되나. 특히 현장에서는 휴대용 시 험기를 사용하여야 하나 수명평가에는 비커스 경도값을 사용하고 있기 때문에 이들의 경도 환 산에 대한 D/B 확충이 시급한 실정이다. 또한 기존 기술의 습득 및 손상 평가의 정확도 향상, 새로운 방법들의 개발 그리고 이들의 현장 적용 등을 위해서는 실기 사용재의 입수, 정보교환, 전문가시스템(expert system) 개발을 위한 D/B 구축 등 산 . 학 . 연의 협동이 절실히 요구된다.

• Journal of the Korean Society for Nondestructive Testing
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• v.34 no.4
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• pp.299-304
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• 2014

In this paper, a method is tested to measure temperatures in high-temperature welds. Protective glass was installed between an infrared thermal imaging camera and a heat source, and temperature compensation was applied to the measuring instruments. When the temperature of halogen lamps was taken in real-time and measured by the thermal camera, the temperature was found to be almost invariant with the distance between the camera and heat source. The temperature range could be predicted, through correlations with the thickness of the protective glass and the measured distance. This study suggests that the temperature measurement of welds obtained by using an infrared thermal imaging camera is valid, through experimental testing of heat sources.

• Korean Journal of Crystallography
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• v.7 no.2
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• pp.165-174
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• 1996

The effects of Ba2+, Sr2+, and Ca2+-substitutions for Pb2+ at Pb(Zr0.52Ti0.48)O3 composition were studied for thier dielictric, piezoelectric, and aging properties. For relative dielectric constants and electromechanical coupling factors, the Ba2+-substituted composition was the highest and the Ca2+-substityed composition was the lowest. In the case of mechanical quality factors, the opposite behavior occurred. Time dependence of frequency for various ion-substituted compositions is similar to that of non-substituted composition. The piezoelectric properties after aging at high temperature was the worse than that after aging at room temperature.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.2
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• pp.53-59
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• 2017

In this experimental study, the characteristic of damages on GFRP rebar exposed to high temperature only and immerged in alkaline solution after the exposure to high temperature was analyzed through microscopic image analysis. The found microcrack and pores in resin matrix were quantitatively compared if there was effect of pre-exposure to high temperature. The damages, such as microcrack and pores in resin matrix, by alkali exposure were mainly found in rebar surface. On the other hand, the pores caused by high temperatures were extensively found in a section and had greater width than those caused by the alkali exposure. In results of the quantitative comparison, the accumulated length and widths of microcrack and pores in resin matrix in pre-exposed GFRP rebar to high temperature were respectively 1.5 and 1.4 times of those in the GFRP rebar only immerged in alkali solution. Therefore, the deterioration of resin matrix by the alkali exposure could be accelerated due to the pre-exposure to high temperature.

• Journal of the Korean Society for Nondestructive Testing
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• v.36 no.5
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• pp.377-383
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• 2016

The effect of long-term aging degradation on magnetic properties of ferritic 11Cr low-carbon steel was investigated. Coercivity and hysteresis loss measured from the hysteresis loops decreased with long-term aging time and showed that the relation was well fitted by a second order exponential function. Vickers hardness also decreased with aging time and resulted in mechanical softening. In addition, the microstructural evolution was observed by the scanning electron microscopy, backscattered electron image and X-ray diffraction. The $Cr_{23}C_6$ precipitates along grain boundary grew fast and Laves ($Fe_2W$) phase on martensitic lath boundaries in interior grains was developed. The solid solution atoms depleted in matrix and lath subgrains recovered owing to precipitate coarsening with long-term aging degradation. There was a close relation with softening of magnetic and mechanical properties.

• Journal of Welding and Joining
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• v.13 no.2
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• pp.12-24
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• 1995

Recently, more researches have been actively performed for the assessment of material degradation and residual-life of elevated temperature plant components, as some of domestic fossil power plants become older than 30 years. In this paper, results of on_site residual life assessment are reported for main steam pipes of Youngwol power station #2 which have operated since 1965. For critical weld locations such as butt welds branch welds, Y_sections and a T-section, replication technique and hardness measurement technique were employed for life_assessment. When cracks were detected by conventional NDT tests, crack growth life was calculated using a computer code. On the other hand, for matrix of pipes, residual life was quantitatively estimated by an analytic method and material degradation was estimated qualitatively using diameter measurement data and grain-boundary etching method. Also, direction in further improvement of on-site life assessment techniques are proposed.

• Fire Science and Engineering
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• v.18 no.3
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• pp.51-56
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• 2004

A fire outbreak in a reinforcement concrete structure looses the organism by the different contraction and expansion of hardened cement pastes and aggregate, and causes cracks by thermal stress, leading to the deterioration of the durability. So concrete reinforcement structure is damaged partial or whole structure system. Therefore diagnosis of deterioration is needed based on mechanism of fire deterioration in general concrete structures. Fundamental information and data on the properties of concrete exposed to high temperature are necessary for accurate diagnosis of deterioration. In this study, it was presented data for the accurate diagnosis and selection of repair and reinforcement system for the deteriorated concrete heated highly, various concrete such as standard design compressive strength, fine aggregate and admixture were exposed to a high temperature environment. And fundamental data were measured engineering properties such as explosive spatting, ultrasonic pulse velocity and compressive strength.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.27 no.1
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• pp.28-33
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• 2017

SiC fiber can be used up to $1800^{\circ}C$ in both inert and air atmosphere without any problems such as melting and oxidation. SiC fibers can be applied to dust filtering processes as a bag filter at a high temperature above $700^{\circ}C$, which is far beyond the temperature range of currently available industrial bag filter. However the studies for the degradation of SiC fibers were still lacked in the harsh environment of steel industries and thermoelectric power plants. In this study, SiC fibers were reacted with steel dust and thermal power plant dust at a high temperature of $500^{\circ}C$ or higher, and the degraded shape of the fiber surface was observed by SEM. Also the degree of oxygen diffusion on the surface and inside of SiC fiber was analyzed by EDS.