• Journal of Ocean Engineering and Technology
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• v.13 no.2 s.32
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• pp.99-107
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• 1999

Boiler high-temperature pipelines such as main steam pipe, header and steam drum in fossil power plants are degraded by creep damage due to severe operationg conditions which are high temperature and high pressure for an extended period time. Such material degradation leads to various component failures causing serious accidents at the plants. Conventional measurement techniques such as replica method, electric resistance method, and hardness test method have such disadvantages as complex preparation and measurement procedures, too many control parameters, and therefore, low practicality and they were applied only to component surfaces with good accessibility. In this paper, artificial creep degradation test and ultrasonic measurement for their creep degraded specimens have been carried out for the purpose of evaluation for creep damage which can occur in high-temperature pipeline of fossil power plant. Absolute measuring method of quantitative ultrasonic measurement for material degradation was established, and long term creep degradationtests using life prediction formula were carried out. As a result of ultrasonic tests for crept specimens, we confirmed that the sound velocity decreased and the attenuation coefficient linearly increased in proportion to the increase of creep fractiin(${\phi}$c).

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.9
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• pp.1265-1272
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• 2010

In view of the frequent start-ups and load changes in the recent past, there is a need for fossil-fuel power plants to be more efficient and reliable for long-term operation. Under high-temperature and high-pressure conditions, severe creep and fatigue damages can occur in major plant facilities, particularly, turbines and boilers. For highly stable operation and better maintenance, various techniques that facilitate a systematic assessment of the service life of critical facilities have been developed. However, to date, in Korea, to evaluate the remaining life of major facilities of fossil power plant, qualitative or semiquantitative analyses are carried out without following any standard guidelines or procedure. In this study, a standard code for assessing the remaining life of major plant facilities is proposed. This code takes into account creep and fatigue damage, which are generally accepted as dominant causes of damage to facilities. KEPIC (Korea Electric Power Industry code) is scheduled to include this guideline in 2010.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.123-128
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• 2001

The creep constants which are used to the reference stress equations of creep damage were obtained to type 316LN stainless steel, and their determining methods were described in detail. Typical Kachanov and Rabotnov(K-R) creep damage model was modified into the damage equations with reference stress concepts, and the modified equations were applied practically to type 316LN stainless steel. In order to determine the reference stress value, a series of high-temperature tensile tests and creep tests were accomplished at $550^{\circ}C$ and $600^{\circ}C$. By using the experimental creep data, the creep constants used in reference stress equations could be obtained to type 316LN stainless steel, and a creep curve on rupture strain was predicted. The reference stress concept on creep damage can be utilized easily as a design tool to predict creep life because the process, which is quantified by the measurement of voids or micro cracks during creep, is omitted.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.11
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• pp.2837-2845
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• 1994

Several methods have been developed to predict on the remaining life of the old power plants. However, Larson-Miller parameter, one of existing creep life prediction methods, has not reflected the effect of material degradatioin and grain size. So this study has been carried out to research the effects of material degradation and austenite grain coarsening on the life prediction of 3.5Ni-Cr-Mo-V steel. An experimental result shows that carbide coarsening has no significant effects on the creep rupture life and the Larson-Miller parameter, but grain coarsening has an important influence on the creep ruptrure life and the Larson-Miller parameter. Therefore Larson-Miller constant, K should be determined to consider on the chemical composition and the grain size of materials.

• Journal of the Korean Society for Nondestructive Testing
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• v.19 no.2
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• pp.110-117
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• 1999

In order to ultrasonically evaluate creep cavities pure copper samples were subjected to creep test and their microstructures were examined. Ultrasonic velocities. frequency-dependent magnitude spectra and attenuations were measured on a series of copper samples obtained from the different stages of creep test. Velocities measured in three directions with respect to the loading axis decreased and their anisotropy increased as a function of the creep-induced porosity. The anisotropic behavior could be attributed to the progressive change of pore shape and preferred orientation as the creep advanced. The 2% porosity by volume decreased the longitudinal and shear wave velocities by 11% and 4%, respectively. Furthermore, both velocities decreased nonlinearly with the porosity. As the creep damage developed, the magnitude spectra lost high frequency components and their central frequencies shifted to lower values. The attenuation showed almost linear behavior in the frequency range used. Normalized velocity, central frequency shift and attenuation slope were selected as nondestructive evaluation parameters. These results were presented and showed good relations with the porosity content.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.105-110
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• 2001

It has recently been raised main issue how solve the problem of insufficient energy. One of the solution is to increase the thermal efficiency of power generation system. For the purpose of high efficiency, it is necessary to increase the steam temperature and pressure. So, the use of modified $9{\sim}12%Cr$ steel having superior creep rupture strength and oxidation resistance is required to endure such severe environment. The evaluation of creep properties of those heat resistance material is very important to secure the reliability of high temperature and pressure structural components. Since creep properties are determined by microstructural change such as carbide precipitation and coarsening, It is certain that there are some relationship between creep properties and hardness affected by microstructure. In this study, SP-Creep ruptured test for newly developed 9Cr steel being used as boiler valve material was performed, and creep properties of the material were evaluated. Also, hardness test were performed and hardness results were related to the creep properties such as LMP and creep strength to verify the availability of SP-Creep test as creep test method.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.2
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• pp.177-183
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• 2008

The typical heat-resisting rotor steels such as 2.25CrMo, 9CrMo and 12CrW steel were experimentally studied in order to understand their materials degradation under high temperature and pressure during the long-term service, and then use the basic studies for the structural health monitoring. In order to monitor the materials degradation, it was conducted by the isothermal aging for 2.25CrMo steel, creep-fatigue for 9CrMo steel and creep for 12Cr steel with the incremental step test. The ultrasonic wave properties, electrical resistivity and coercivity were interpreted in relation to microstructural changes at each material and showed strong sensitivity to the specific microstructural evolution.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.8
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• pp.831-836
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• 2014

In this study, high temperature integrity evaluation on a pressure vessel of the expansion tank operating at elevated temperature of $510^{\circ}C$ in the sodium test facility of the SEFLA(Sodium Thermal-hydraulic Experiment Loop for Finned-tube Sodium-to-Air heat exchanger) to be constructed at KAERI has been performed. Evaluations of creep-fatigue damage based on a full 3D finite element analyses were conducted for the expansion tank according to the recent elevated temperature design codes of ASME Section III Subsection NH and French RCC-MRx. It was shown that the expansion tank maintains its integrity under the intended creep-fatigue loads. Quantitative code comparisons were conducted for the pressure vessel of austenitic stainless steel 316L.

• International Journal of Highway Engineering
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• v.7 no.2 s.24
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• pp.69-76
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• 2005

While the repeated traffic loading accumulates the damage of asphalt pavement, the damage has being healed during rest periods. And then, this healing enhances the fatigue life of asphalt pavement. A method was developed to determine the healing rate of asphalt mixture in terms of recovered dissipated creep strain energy (DCSE) per unit time, and the healing properties of four different asphalt mixtures were evaluated. The test procedure consists of repeated loading test and periodical resilient modulus tests. A normalized healing rate in terms of $DCSE/DCSE_{applied}$ was defined to evaluate the healing properties independently of the amount of damage incurred in the mixture. From the test results, it was concluded that the healing rates of asphalt mixtures were increased exponentially as the temperature was increased and more affected by the structural characteristics of mixture such as asphalt content than the binder characteristics such as the polymer modification.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2009.06a
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• pp.899-900
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• 2009