• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.222-227
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• 2001

Nonlinear acoustic effect has been considered as an effective tool for the evaluation of material degradation. In this paper, the applicability of nonlinear acoustic effect to the evaluation of degradation of 2.25Cr-1Mo steel is investigated. Firstly, a number of 2.25Cr-1Mo steel samples were heat-treated, and their damage mechanism was examined. Secondly, Ultrasonic nonlinear parameter was measured. Nonlinear acoustic parameter was found to be clearly sensitive to the material degradation.

• 한국정밀공학회지
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• 제15권8호
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• pp.135-144
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• 1998

In this research, the fatigue behaviors of Cr-Mo(SCM415) steel and carbonitrided CrMo(SCM415) steel are examined. The description of experiments in detail as follows. For SCM415 and carbonitried Cr-Mo(SCM415) steel, smooth specimens and hole defected specimens are fabricated and tested under a rotary bending tester, respectively. By performing the rotary bending fatigue test for smooth and hole defected specimens, the characteristics of the fatigue strength are examined for parameters such as tensile strength, hardness at surface, and residual stresses.

• 한국안전학회지
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• 제17권4호
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• pp.71-79
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• 2002

It was attempted to evaluate the degree of degradation of thermally aged 2.25-1Mo steek by ultrasonic monlinear parameter(UNP) measurement and X-ray diffraction analysis of extracted carbide. Artificial aging was performed to simulate the microstructural degradation in 2.25Cr-1Mo steel arising from long time exposure at $540{\circ}C$. Microstructural analysis (number of carbides per unit area) and measurements of mechanical properties(Vickers hardness, DBTT) and degradation evaluation parameters(UNP and intensity ration of X-ray diffraction peak of electrolytically extracted carbide) were performed. Both of UNP and intensity ratio of X-ray diffraction peak for M6C carbide to that of M23C6 carbide(IR) increased abruptly in the initial 1000 hour of aging and then changed little. UNP and IR were proposed as potential parameters to evaluate the degree of aging degradation of 2.25Cr-1Mo steel.

• 대한기계학회논문집A
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• 제25권12호
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• pp.2116-2124
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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 studdy the four classes of the thermally aged 1Cr-lMo-0.25V specimens were prepared using an artificially accelerated aging method at 630$\^{C}$. 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 derived from the harmonic generation level is sensitive and will be a good parameter to evaluate the material degradation.

• 열처리공학회지
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• 제14권2호
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• pp.110-117
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• 2001

Artificial aging was performed to simulate the microstructural degradation in 2.25CrMo steel arising from long time exposure at $540^{\circ}C$. It was found that the carbides became coarser and spheroidized as aging time increased. An attempt was made to evaluate the microstructural degradation in artificially aging heat treated 2.25CrMo steel by the ultrasonic attenuation and velocity measurements. Ultrasonic velocity was found essentially insensitive to the microstructural changes resulting from aging heat treatment. However, the ultrasonic attenuation was observed to increase with increasing aging time. Also, it was noticed that the change of ultrasonic attenuation with aging time was more sensitive at high frequency regions.

• 대한기계학회논문집
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• 제18권11호
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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.

• 열처리공학회지
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• 제15권2호
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• pp.70-75
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• 2002

The depletion of solid solution elements from matrix and the change of carbides during artificial aging of 2.25CrMo steel at $630^{\circ}C$ were investigated. The Mo and Cr elements were found to be depleted drastically in the early stage of aging. The change of carbides was confirmed by analyzing the XRD patterns of electrolytically extracted carbides. Four type of carbides, $M_{23}C_6$, $M_3C$, $M_2C$ and $M_6C$, were found to exist in the specimen before aging. The amount of $M_6C$ carbides increased with aging time, while that of $M_3C$ carbides diminished after short aging time.

• Journal of Welding and Joining
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• 제14권4호
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• pp.89-98
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• 1996

This study was performed to establish the characteristics of the heat affected zones from view point of the repair weldability for a damaged CrMoV steam turbine rotor steel. Characterization of the heat affected zones of the weldment was conducted with respect to various of postweld heat treatment temperatures, $566^{\circ}C$, $621^{\circ}C$ and $677^{\circ}C$. The evaluations of the heat affected zones were carried out in terms of microstructural characterization, microhardness measurements, Charpy v-notch impact, tensile and stress-rupture tests. The results indicated that the effect of the postweld heat treatment at $677^{\circ}C$ exhibited the favorable microstructure and mechanical properties for the stability of the heat affected zones. While the heat affected zone of the weldment, produced without postweld heat treatment, displayed the inferior toughness and microstructure indicating localized carbide precipitations on the grain boundary. It was also indicated that the stability of the heat affected zones were deteriorated by the formation of the cavitation on the grain boundaries.

• 한국재료학회지
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• 제14권2호
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• pp.101-109
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• 2004

The influences of solidification rates and carbon contents on the formation of the $\delta$-ferrite were studied by directional solidification in modified 12%Cr-l %Mo steels. Directional solidification experimental results showed that solidification microstructure depended on solidification rate and carbon content and chromium equivalent. The length of the mushy zone increased and the dendrite arm spacings decreased as the solidification rate increased. The volume fraction of the 8-ferrite decreased with increasing the solidification rate and carbon content. The volume fraction of the ferrite showed much higher at low solidification rates with planar and cellular interfaces than that at high solidification rates with dendritic interface. It is expected that macro-segregation of C causes lower C content at the lower solidification fraction in the directionally solidified sample, where lower C results in higher volume fraction of the ferrite. In order to estimate solidification microstructure in modified 12Cr-l%Mo steels, various solidification conditions, such as solidification rate, cooling rate, segregation, alloy composition, should be considered.

• 비파괴검사학회지
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• 제22권5호
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• pp.516-522
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• 2002

발전소의 터빈 로터강의 수명을 평가하기 위하여 $630^{\circ}C$에서 등온 열처리된 7가지의 서로 다른 시편을 준비하였다. 탄소강, 스테인레스강 등의 금속재료의 유동특성을 평가하기 위하여 개발된 구형압입 시험을 통하여 서로 다르게 열화된 1Cr-1Mo-0.25V강의 기계적 물성을 측정하고 이를 인장시험에서의 결과와 비교하였다. 유동특성을 결정짓는 요소로 재료항복기울기와 구속인자지수를 사용하였다. 이는 가공경화지수의 변화가 매우 작고 불규칙적으로 나타났기 때문이다. 또한 재료항복기울기의 영향을 고려하였을 때 구속인자지수의 영향은 적은 것으로 나타났다.