• Proceedings of the KSME Conference
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• 2001.06a
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• pp.78-83
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• 2001

It's required mechanical properties of in-service facilities to maintain safety operation in power plants as well as chemical plants. In this study the four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared using an artificially accelerated aging method at $630^{\circ}C$. Ultrasonic tests, tensile tests, $K_{IC}$ tests and hardness tests were performed in order to evaluate the degree of degradation of the material. The mechanical properties were decreased as degraded, but the attenuation coefficient and the harmonic generation level of a ultrasonic signal were increased. Expecially the nonlinear parameter of the signal is sensitive and will be a good parameter to evaluate the material degradation.

• Proceedings of the KSME Conference
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• 2001.06a
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• pp.268-273
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• 2001

Fossil power plants operated in high temperature condition are composed of components such as turbine, boiler, and piping system. Among these components, turbine blades made with 12%Cr steel operate at a temperature above $500^{\circ}C$. Due to the long term service, turbine blades experience material degradation manifested by change in mechanical and microstructural properties. The need to make life assessment and to evaluate material degradation of turbine blade is strongly required but in reality, there is a lack of knowledge in defining failure mechanism and fundamental data for this component. Therefore, in making life assessment of turbine blade, evaluation of material degradation must be a priority. For this purpose, evaluation of toughness degradation is very important. The major cause of toughness degradation in 12Cr turbine blade is reported to be critical corrosion pitting induced by segregation of impurity elements(P etc.), coarsening of carbide, and corrosion, but the of materials for in-service application. In this study, the purpose of research is focused on evaluating toughness degradation with respect to operation time for 12%Cr steel turbine blade under high temperature steam environment and quantitatively detecting the degradation properties which is the cause of toughness degradation by means of non-destructive method, electrochemical polarization.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.129-136
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• 2001

The remaining life estimation for the aged components in power plants as well as chemical plants are very important beacuse mechanical properties of the components are degraded with time of service exposure in high temperature. Since it is difficult to take specimens from the operating components to evaluate mechanical properties of components, nondestructive techniques are needed to estimate the degradation. In this study, test materials with 4 different degradation levels were prepared by isothermal aging heat treatment at $630^{\circ}C$. And the DC potential drop method and destructive methods such as tensile, $K_{IC}$ and hardness tests were used in order to evaluate the degradation of 1Cr-1Mo-0.25V steels. The objective of this study is to investigate the possibility of the application of DCPD method to estimate the material degradation, and to analyse the relationship between the electrical resistivity and the degree of material degradation.

• Journal of the Korean Society of Safety
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• v.21 no.4 s.76
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• pp.33-41
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• 2006

Material degradation should be considered to assess integrity and residual life of high temperature equipments. However, the property data reflecting degradation are not sufficient for practical use. In this study measuring properties for 1Cr-1Mo-0.25V forging steel generally used for turbine rotor was carried out. Degradation was simulated by isothermal ageing. heat treatment and variation of microstructure was observed. Mechanical properties such as tensile strength, impact energy, hardness and fracture toughness were measured. Assuming a semi-elliptical surface crack at the bore hole in a turbine rotor, material risk was estimated by using the aged material property data obtained in this study. Safety margin was decreased and life of the rotor was exhausted. This procedure can be used in assessing the residual life of a turbine rotor due to material degradation.

• Journal of Welding and Joining
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• v.16 no.3
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• pp.121-128
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• 1998

The degradation of mechanical properties in the high nitrogen steel HN3 developed for nuclear fusion reactor has been evaluated quantitatively using the small punch(SP) test, X-ray diffraction (XRD) analysis has also been conducted to identify carbides or nitrides precipitated on grain boundaries of the heat treated samples. Mechanical properties of the steel HN3 significantly decreased with increasing heat treatment time and temperature or with decreasing testing temperature. Combination of XRD and metallurgical observation, revealed that the material degradation in the thermally aged steel was caused by precipitation of carbides on the grain boundaries. While the weld metal showed the lowest mechanical properties among various microstructures in GTA weldments. By combining SP test and XRD analysis, cryogenic fracture behaviors and aging degradation for high nitrogen steel could be successfully evaluated in nondestructive manner.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.30 no.11
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• pp.693-698
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• 2017

In this study, the thermal degradation properties of polyethylene terephthalate film has been examined by the capacitance, Tan ${\delta}$, thermography, FTIR, and SEM results at temperatures of $90{\sim}170^{\circ}C$ and frequencies of 0.3~3,000 kHz. It was found that the capacitance decreased with increasing thermal imaging temperature, probably caused by weakening of chemical bond with increasing temperature. Tan ${\delta}$ decreased upon increasing temperature from $90^{\circ}C$ to $170^{\circ}C$, probably due to the molecular motion of COOH radical or OH radical. The FT-IR measurement reveals that no structural change of the material occurs upon thermal radiation. The SEM measurement shows that the material is stabilized by thermal decomposition with increasing temperature; however, excessive thermal degradation obstructs the stabilization of the material.

• Journal of the Korean Society for Heat Treatment
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• v.34 no.6
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• pp.315-322
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• 2021

In this study, we investigate the variation in microstruture and mechanical property of austenitic 316L stainless steel during long-term material degradation. To simulate the material degradation, the AISI 316 steel was exposed to accelerate under a temperature of 600℃ for up to 10000 hours at each predetermined heat treatment time. As the long-term material degradation time increase, the grain shape was changed from polygonal grains with annealing twins to circular grains. Most twins distributed uniformly interior of grains are recovered and disappered with long-term material degradation. Also, the δ ferrite along grain boundaries decomposed and transformed into the σ phase resulting in decrease of elongation of austenitic 316L stainless steel.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1998.06a
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• pp.199-202
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• 1998

In examining application of polymer as electrical insulators, it is very important to perform accelerated aging test substituted the process which polymer insulator is degraded for long-time by the process of short-time. The purpose of this paper is to examine the properties of water degradation which affect on the efficiency of epoxy insulator. To do this, the surface properties on epoxy insulating material have been investigated after long-time accelerated degradation in boiling water condition. The experimental results showed the contact angle and surface resistivity after treatment to decrease the sample of water treatment. In dielectric properties, dielectric constant was increased by the aging development with water treatments.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.09a
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• pp.47-51
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• 2001

The electrical degradation phenomena of epoxy composites to be used as a molding material for transformers were studied. The electrets were first manufactured by applying high voltages to five kinds of specimens given a mixing rate, and then TSC(Thermally Stimulated Current) values at the temperature range of $-160\sim200[^{\circ}C]$ were measured from a series of experiments. The behaviour of carrier and its origin in epoxy composites were examined, respectively, And various effects of electrical degradation on epoxy composites were also discussed in this study.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.13 no.5
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• pp.383-389
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• 2000

The relationship between the DC degradation characteristics of the ZnO varistor and the ambient sintering-process is investigated in this study. ZnO varistors made o matsuoka’s composition were fabricated by standard ceramic techniques. The ambient sintering-process is performed at the extraordinary electrical-furnace which is equipped with the vacuum system. Gases used in sintering process were oxygen nitrogen argon and air. Using XRD and SEM the phase and microstructure of samples were analyzed respectively. The conditions of DC degradation tests were conducted at 115$\pm$2$^{\circ}C$ for 13 h. Current-voltage analysis is used to determine nonlinear coefficients($\alpha$). Frequency analysis are performed to understand electrical properties as DC degradation test. From above analysis it is found that the ZnO varistor sintered in oxygen atmosphere showed superior properties at the DC degradation test and degradation phenomenon of ZnO varistor is caused by the change of electrical properties in grain boundary. These results are in accordance with Gupta’s degradation model.