• 한국공작기계학회논문집
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• 제14권2호
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• pp.42-47
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• 2005

Micro duplex stainless steel is used to denote a fine scale two-phase micro structure consisting austenite and firrite. The development of this structure was done by proper thermo-mechanical processing. The objective of present investigation is to study creep characteristics of this alloy. Since we have little design data about the W behaviors of the alloy. An apparatus has been designed and built fir conducting creep tests under constant load conditions. A series of creep tests on them have been performed to get the basic design data and life prediction of micro duplex stainless steel products and we have gotten the 1311owing results. First the stress exponents decrease as the test temperatures increase. Secondly, the creep activation energy Gradually decreases as the stresses become higher. Thirdly, the constant of Larson-Miller Parameters on this alloy is estimated as about 5. Last, the fiactographs at the creep rupture show both the ductile and brittle fracture modes according to the creep conditions.

• Journal of Welding and Joining
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• 제19권3호
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• pp.278-284
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• 2001

Both creep deformation and creep crack growth experiments have been conducted on 2.25Cr-1Mo steel weldment in order to provided an information on residual life prediction of structural component weldment containing a crack. The stress exponent of creep deformation equation for the base metal and weldment at 823k were found to be 10.2 and 7.3, respectively. These two values could be assumed that dislocation climb processes are controlling the creep deformation of both materials. The creep rate of the weldment was very low, compared with that of base metal under the same applied stress. Whereas the creep crack growth rate of the weldment was almost twice higher than that of base metal under the fixed value of $C^*$. This may indicate that the weldment is stronger than the base metal in view of creep deformation and is brittle during creep crack growth due to the intrinsic microstructure of banite and relatively higher and Mn contents.

• 대한기계학회논문집A
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• 제26권12호
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• pp.2565-2573
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• 2002

A basic research was performed to ensure the usefulness of Small Punch-creep(SP-creep) test for residual life evaluation of heat resistant components effectively. This paper presents analytical results of initial stress and strain distributions in SP specimen caused by constant loading for SP-creep test and its experimental correlations with uniaxial creep(Ten-creep) test on 9Cr1MoVNb steel. It was shown that the initial maximum equivalent stress, ${\sigma}_{eq{\cdot}max}$ from FE analysis was correlated with steady-state equivalent creep strain rate, ${\epsilon}_{qf-ss'}$ rupture time, $t_r$, activation energy, Q and Larson-Miller Parameter, LMP during SP-creep deformation. The simple correlation laws, ${\sigma}_{sp}-{\sigma}_{TEN}$, $P_{sp}-{\sigma}_{TEN}\; and\; Q_{sp}-Q_{TEN}$ adopted to established a quantitative correlation between SP-creep and Ten-creep test data. Especially, the activation energy obtained from SP-creep test is linearly related to that from Ten-creep test at $650^{\circ}C$ as follows : $Q_{SP-P}\;{\risingdotseq}\;1.37 \;Q_{TEN},\; Q_{SP-{\sigma}}{\risingdotseq}1.53\; Q_{TEN}$.

• 한국재료학회지
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• 제3권6호
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• pp.575-584
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• 1993

주조용 Ni기 초내열 합금인 Rene 80의 고온 크리프타단시 균열의 생성과 전파에 대한 분위기의 영향을 조사하기 위해 $760^{\circ}C$, 675MPa및 $982^{\circ}C$, 157MPa의 크리프시험 조건에서 산화성 분위기인 공기중과 불활성 분위기인 아르곤 가스 분위기에서 크리프 파단시험을 했다. $760^{\circ}C$, 657MPa조건의 크리프 파단시험에서는 분위기에 따라 크리프 파단 양상의 큰 차이가 없었고 파단수명과 파단연신값도 비슷하였다. 반면 $982^{\circ}C$, 157MPa의 시험에서는표면산화의 영향으로 공기중의 경우 표면균열 생성과 입계를 통한 균열전파에 의하여 크리프 파단이 진행되었으나, 아르곤 분위기에서는 내부입계에서 균열이 생성되어 표면에서 발생된 균열과 합체됨으로서 파단이 진행되었다. 파단수명은 공기중의 경우가 치밀한 표면산화물의 형성에 따라 아르곤 하에서 보다 길었으며 파단연신은 아르곤 분위기의 경우가 네킹 발생에 따라 크게 나타났다.

• 한국재료학회지
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• 제14권1호
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• pp.78-82
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• 2004

In this study, the degree of creep damages on the weldment accelerated creep degradation was nondestructively evaluated by replica method. The frequency of creep cavities occurrence has been observed highly at the intercritical HAZ. The life fraction of weldment damaged by creep has shown from 0.25(damage grade: 2) to 0.75(damage grade: 4) when it reptured. The degree of creep damages is considered to be evaluated by the metallographic replica method which is one of nondestructive evaluation methods.

• 대한기계학회논문집
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• 제18권7호
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• pp.1722-1729
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• 1994

At early stage of creep-fatigue crack growth tests, initial transient behavior which implies high crack growth rate has been generally observed by some researchers. Since the influence of the initial transient crack growth behavior on the remaining life of components is significant, cause of it should be further studied. In this study, characteristics of the initial transient behavior of 1Cr-1Mo-0.25V steel is studied experimentally by performing creep-fatigue crack growth tests at $538^{\circ}C$ in air under trapezoidal waveshapes. It is verified that the cause of the initial transient behavior is not high ${(C_t)}_{avg}$ values due to the small scale creep condition at the early stage of test, but oxidation-dominated crack growth mechanism during the transient period which is different from the creep-dominated crack growth mechanism in steady crack growth period.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 1998년도 특별강연 및 추계학술발표 개요집
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• pp.11-24
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• 1998

The invention of fusion wilding technology has brought on a revolutionary change in manufacturing industry which enables the construction of large scale high temperature plants in chemical, petrochemical and power generation industries. However, among the failure cases of high temperature components, premature failures of weldments have taken a large percentage that indicates the detrimental effect of welding on structural integrity. The accurate prediction of the high temperature behaviour of welded components is thus becoming increasingly important in order to realise an optimised design and maintenance of a plant life. In the present paper, recent research activities on high temperature behaviour of welded structures are briefly summarised. A local deformation measuring technique is proposed to determine the creep properties of weldment constituents. A damage mechanics approach is introduced to study the life reduction and ductility reduction due to the presence of a weld in high temperature structures. Finally, the high temperature creep crack growth in weldments is discussed.

• 대한기계학회논문집A
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• 제24권11호
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• pp.2812-2822
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• 2000

Several methods have been developed to predict the rupture time of the boiler tubes in thermal power plant. However, existing life prediction methods give very conservative value at operating stress of power plant and rupture strain cannot be well estimated. Therefore, in this study, rupture time and strain prediction method accounting for creep, corrosion and heat transfer is newly proposed and compared with the current research results. The creep damage evolves by continuous cavity nucleation and constrained cavity growth. The corrosion damage evolves by steam side and fire side corrosion. The results showed good correlation between the theoretically predicted rupture time and the current research results. And rupture strain may be well estimated by using the proposed method.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.274-279
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• 2001

Creep-fatigue crack growth behavior was experimentally measured particularly when a crack was located in the heat affected region of 1Cr-0.5Mo steel. Load hold times of the tests for trapezoidal fatigue waveshapes were varied among 0, 30, 300 and 3,600 seconds. Time-dependent crack growth rates were characterized by the $C_t$-parameter. It was found that the crack growth rates were the highest when the crack path was located along the fine-grained heat affected zone(FGHAZ). Cracks located in other heat affected regions had a tendency to change the crack path eventually to FGHAZ. Creep-fatigue crack growth law of the studied case is suggested in terms of $(da/dt)_{avg}$ vs. $(C_t)_{avg}$ for residual life assessment.

• 대한기계학회논문집A
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• 제26권1호
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• pp.55-60
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• 2002

For the development of a new creep test technique, the availability of SP-Creep test is discussed for 1Cr-0.5Mo boiler header material. And some results are also compared with those of 2.25Cr- 1Mo steel which widely uses as boiler superheater tube. The results can be summarized as follows. The load exponents(n) obtained by SP-Creep test for 1Cr-0.5Mo steel are decreased with increasing creep temperature and the values are 15.67, 13.89, and 17.13 at 550$^{circ}C$ ,575$^{circ}C$ and 600$^{circ}C$, respectively. The temperature dependence of the load exponent is given by n = 107.19 - 0.1108T. This reason that load exponents show the extensive range of 10∼16 is attributed to the fine carbide such as M$_{23}$C$_{6}$ in lath tempered martensitic structures. At the same creep condition, the secondary creep rate of 1Cr-0.5Mo steel is lower than the 2.25Cr-1Mo steel1 due to the strengthening microstructure composed by normalizing and tempering treatments. Through a SEM observation, it can be summarized that the primary, secondary, and tertiary creep regions of SP-Creep specimen are corresponding to plastic bending, plastic membrane stretching, and plastic instability regions among the deformation behavior of four steps in SP test, respectively.y.