• 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.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 1993년도 특별강연 및 추계학술발표 개요집
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• pp.129-132
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• 1993

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집A
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• pp.322-327
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• 2000

This paper investigates the time-dependent crack growth in X20CrMoV 12 1 steel weld joints. Crack growth test are carried out $545^{\circ}C$ on side-grooved 1/2T CT specimens under static loads. A simulated material is produced for the intercritical HAZ, where fracture normally occurs. Constitutive properties are obtained for the simulated HAZ material as well as for the base metal and weld metal. Finite element analyses of crack growth are performed on the models with and without a HAZ layer, using the experimental crack length-time history. The inclusion of HAZ layer increase the load line velocities significantly. The crack growth rates are correlated reasonably well with $C^*$. The smallest crack size for the validity of $C^*$ is found much smaller than the ASTM crack initiation size for 1T CT specimen of creep ductile materials.

• 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.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.250-255
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• 2001

It is inevitable to evaluate the life of turbine rotor because the operating periods of power plants need to be extended. The magnetic methods utilizing Magnetic Barkhausen noise curve were applied to detect the degradation caused by thermal aging. The Magnetic property of material depends on the domain dynamics and it is affected by the microstructure of material. Therefore the magnetic property is very sensitive to the microstructure change of the material. It is, thus, very useful to detect the state of degradation of varying materials. The test specimen made of 1Cr-1Mo-0.25V steel was used widely for turbine rotor material, and seven kinds of specimens with different degradation levels were prepared by the isothermal heat treatment at $630^{\circ}C$. With the increase of degradation, BHN was decreased. The result was compared with coercive force and vickers hardness.

• Journal of Welding and Joining
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• 제18권2호
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• pp.232-232
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• 2000

The hydrogen attack characteristics of 3Cr-1Mo-V steel as simulated weld heat affected state were studied in this paper. The hydrogen attack susceptibility was evaluated by the ratios of Charpy impact absorbed energy at 0℃($vE_{0HA}$/$vE_{0}$) and reduction of area by tensile test ($RA_{HA}$/RA) before and after exposure to hydrogen at 600℃ under 450kgf/㎠ for 300hr. The values of $vE_{0HA}$/$vE_{0}$ and $RA_{HA}$/RA were aggravated as the peak temperature of the simulated heat affected zone(HAZ) raised. These results were due to the increase of the possession of bubbles along the grain boundaries, which were resulted in the reduction of grain boundary area to be precipitated carbides due to grain coarsening and the carbon dissolved in the martensite-austenite constituent near by the prior austenite grain boundary. The possession ratio of methane bubbles formed along prior austenite grain boundaries were increased with raising the peak temperature. (Received February 22, 2000)

• Journal of Welding and Joining
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• 제18권2호
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• pp.105-111
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• 2000

The hydrogen attack characteristics of 3Cr-1Mo-V steel as simulated weld heat affected state were studied in this paper. The hydrogen attack susceptibility was evaluated by the ratios of Charpy impact absorbed energy at 0℃({TEX}$vE_{0} {HA}_/vE_{0}${/TEX}) and reduction of area by tensile test({TEX}$RA_{HA}/RA${/TEX}) before and after exposure to hydrogen at 600℃ under 450kgf/㎠ for 300hr. The values of {TEX}$vE_{0} {HA}_/vE_{0}${/TEX} and {TEX}$RA_{HA}/RA${/TEX} were aggravated as the peak temperature of the simulated heat affected zone(HAZ) raised. These results were due to the increase of the possession of bubbles along the grain boundaries, which were resulted in the reduction of grain boundary area to be precipitated carbides due to grain coarsening and the carbon dissolved in the martensite-austenite constituent near by the prior austenite grain boundary. The possession ratio of methane bubbles formed along prior austenite grain boundaries were increased with raising the peak temperature.

• 열처리공학회지
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• 제22권6호
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• pp.361-367
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• 2009

The two-step gas nitriding was adopted to increase the depth of surface hardening in 3%Cr-Mo-V steel. The two-step gas nitriding consisted of Step I; $520^{\circ}C{\times}20\;hrs$ and Step II; $550^{\circ}{\times}70\;hrs$. The layer of two-step gas nitriding showed better uniformity and deeper nitriding layer than one-step gas-nitriding layer. The maximum surface hardness showed the value of 850 Hv. The maximum depth of nitrogen permeation showed $750\;{\mu}m$ (350 Hv). X-ray diffraction analysis showed that compound layer was mainly consisted of CrN and $\varepsilon-Fe_3N$ phases. These phases were presumed contributing to surface hardening and wear resistance. However, the corrosion resistance of gas-nitrided Cr-Mo-V steel were not improved in the solution of 1 N HCl and NaOH. Therefore, it is necessary that the continuous study for improvement of corrosion resistance hereafter.

• 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.

• Journal of Mechanical Science and Technology
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• 제16권2호
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• pp.147-154
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• 2002

The objective of this paper is to develop a nondestructive method for estimating the fracture toughness (K$\_$IC/) of CrMoV steels used as the rotor material of steam turbines in power plants. To achieve this objective, a number of CrMoV steel samples were heat-treated, and the fracture appearance transition temperature (FATT) was determined as a function of aging time. Nonlinear ultrasonics was employed as the theoretical basis to explain the harmonic generation in a damaged material, and the nonlinearity parameter of the second harmonic wave was the experimental measure used to be correlated to the fracture toughness of the rotor steel. The nondestructive procedure for estimating the 7c consists of two steps. First, the correlations between the nonlinearity parameter and the FATT are sought. The FATT values are then used to estimate K$\_$IC/, using the K$\_$IC/ versus excess temperature (i.e., T-FATT) correlation that is available in the literature for CrMoV rotor steel.