• 한국해양공학회지
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• 제11권4호
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• pp.61-75
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• 1997

The social demand urges us to use some equipments and structures in high temperature environment. By this occasion, the necessity of studying the fatigue crack growth is an important aspect of new materials. However, the present situation is rarely to accumulate the fatigue data. Especially, 1Cr-1Mo-0.25V steel and 304 stainless steel have been increased to be used under the severe condition of high temperature. And so, the fatigue estimation of those materials is important and appropriate. Fatigue tests have been carried out to examine the crack initiation, growth behaviour for the small fatigue crack of 1Cr-1Mo-0.25V steel and 304 stainless steel at room temperature and 538^{\circ}C$. The remote measurement system which has many merits of checking and saving the image for detailed examination was applied to closely detect the crack length. Generally, the fatigue crack initiated in the form of multiple cracks and grew each other. And then it coalesced to become a major crack. The major crack governed the rest of the fatigue life. In the growing process, each peripheral cracks interact and grow for a certain period. After then, it coalesced and fractured. On the basis of the above experimental data for the small crack, a simulation program was developed to predict the residual life time and to estimate the integrity of machine elements and structures. At the same time, the simulation was extended to 1Cr-1Mo-0.25V steel. The simulation results have shown a good agreement to those of the experimental ones for both materials of 1Cr-1Mo-0.25V steel and 304 stainless steel with small cracks. The NASCRAC has applied to compare the fatigue life with the experimental results. And so, it can be said that the simulation program is valuable tools to the industrial fields.

• Corrosion Science and Technology
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• 제2권4호
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• pp.178-182
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• 2003

Slow Strain Rate Tests (SSRT) were carried out to investigate the effect of environmental factors on the Stress Corrosion Cracking (SCC) susceptibility of 3.5NiCrMoV steels used in discs for Low-Pressure (LP) steam turbines in electric power generating plants. The influences of dissolved oxygen on the stress corrosion cracking of turbine steel were studied, For this purpose, specimens were strained at variously oxygenated conditions at $150^{\circ}C$ in pure water. When the specimen was strained with $1{\times}10^{-7}s^{-1}$ at $150^{\circ}C$ in pure water, increasing concentration of dissolved oxygen decreased the elongation and the UTS. The corrosion potential and the corrosion rare increased as the amounts of dissolved oxygen increased. The increase of the SCC susceptibility of the turbine steel in a highly dissolved oxygen environment is due to the non protectiveness of the oxide layer on the turbine steel surface and the increase of the corrosion current. These results clearly indicate that oxygen concentration increases Stress Corrosion Cracking susceptibility in turbine steel at $150^{\circ}C$.

• 대한금속재료학회지
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• 제46권12호
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• pp.788-799
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• 2008

This study is concerned with the effects of Mo, Cr, and V addition on tensile and Charpy impact properties of API X80 linepipe steels. Four kinds of steels were processed by varying Mo, Cr, and V additions, and their microstructures and tensile and Charpy impact properties were investigated. Since the addition of Mo and V promoted to form fine acicular ferrite and granular bainite, while prohibiting the coarsening of granular bainite, it increased the strength and upper shelf energy, and decreased the energy transition temperature. The Cr addition promoted the formation of coarse granular bainite and secondary phases such as martensite-austenite constituents, thereby leading to the increased effective grain size, energy transition temperature, and strength and to the decreased upper shelf energy. The steel containing 0.3wt.% Mo and 0.06wt.% V without Cr had the highest upper shelf energy and the lowest energy transition temperature because its microstructure was composed of fine acicular ferrite and granular bainite, together with a small amount of hard secondary phases, while its tensile properties maintained excellent.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2004년도 추계학술발표회 발표논문집
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• pp.1023-1023
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• 2004

• 한국가스학회:학술대회논문집
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• 한국가스학회 1998년도 추계학술발표회 논문집
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• pp.334-339
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• 1998

• 한국재료학회:학술대회논문집
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• 한국재료학회 2004년도 춘계학술발표대회 및 제6회 신소재 심포지엄
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• pp.36-36
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• 2004

• 비파괴검사학회지
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• 제21권5호
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• pp.532-541
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• 2001

재료의 노화에 의한 강도 및 인성 감소치를 정량적으로 평가하는 비파괴적 기법의 개발이 시도되고 있으며 전기비저항법도 그 중의 하나이다. 본 연구에서는 전기비저항법의 열화도 평가에의 적용성을 살펴보기 위해 먼저 6종 10 가지의 비자성 금속을 측정시료로 선택하여 직류 two-point probe법(혹은 2탐침법)으로 전기비저항을 계산한 결과와 비접촉식 와전류 방법으로 전기비저항을 측정한 결과를 서로 비교하였다. 또한, 1Cr-1Mo-0.25V강에 대하여 비파괴적 측정 방법인 four-point probe 방법(혹은 4탐침법)을 사용하여 얻은 결과와 기존의 2탐침법을 사용하여 얻은 결과를 서로 비교하여 현장에서의 4탐침 기술의 적용 가능성을 살펴보았다. 1Cr-1Mo-0.25V강에 대하여 2탐침법으로 구한 비저항 값과 4탐침법으로 구한 비저항 측정값의 차이는 0.6%였다. 따라서 4탐침법을 사용하여 현장에서 금속소재의 열화도의 비파괴적 평가가 가능하다고 사료된다.

• 열처리공학회지
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• 제31권4호
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• pp.180-186
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• 2018

High temperature flow behaviors of Fe-Cr-Mo-V-W-C tool steel were investigated using isothermal compression tests on a Gleeble simulator. The compressive test temperature was varied from 850 to $1,150^{\circ}C$ with the strain rate ranges of 0.05 and $10s^{-1}$. The maximum height reduction was 45%. The dynamic softening related to the dynamic recrystallization was observed during hot deformation. The constitutive model based on Arrhenius-typed equation with the Zener-Hollomon parameter was proposed to simulate the hot deformation behavior of Fe-Cr-Mo-V-W-C steel. An artificial neural network (ANN) model was also developed to compare with the constitutive model. It was concluded that the ANN model showed more accurate prediction compared with the constitutive model for describing the hot compressive behavior of Fe-Cr-Mo-V-W-C steel.

• 한국안전학회지
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• 제21권4호
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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.

• 대한기계학회논문집A
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• 제26권7호
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• pp.1262-1269
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• 2002

It is inevitable to evaluate the life of turbine rotor because the operating periods of power plants need to be extended. For the test, seven kinds of specimens with different degradation levels were prepared by the isothermal heat treatment at $630^{\circ}C$. Magnetic methods utilizing Barkhausen noise coercive force($BN_c$) were applied to detect the degradation caused by thermal aging. Magnetic property of material is related with domain dynamics and that is affected by the microstructure of material. Therefore $BN_c$ is very sensitive to the microstructure change of the material. With the increase of degradation, $BN_c$ was decreased and this phenomenon is considered due to precipitations and grain size. The result was compared with Vickers hardness($H_v$) and coercive force($H_c$) to detect the relative variation, and was related with $H_v$ and YS to estimate the change of the mechanical properties with the degradation.