• Journal of Welding and Joining
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• 제15권1호
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• pp.92-100
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• 1997

손상된 ASTM A-470 class 8 고압 증기 터어빈 로터강의 수명 연장을 위해 보수 용접이 행하여졌다. SAW 용접부의 미세경도 측정결과 모재의 경도는 VHN 253이었으나 모재에 근접한 열영향부의 경도는 VHN 227로 떨어졌다. 이와 같이 경도가 떨어진 영역을 "연화층"이라 정의하였으며 약 0.5-0.6mm의 크기를 나타내었다. 한편 크립 파단 시험시 파괴는 연화층 부근에서 일어났으며 593.deg. C와 19Ksi(132 Mpa)에서 파단시간은 772.4hr이었다. 연화층의 크기를 줄이기 위해서 MIG 및 TIG 용접이 행하여졌는데 연화층 크기는 MIG의 경우 0.3-0.4mm이고, TIG의 경우에는 입열량의 크기에 따라 0부터 0.4mm의 크기를 나타내었다. 그러나 크립 파단시간은 연화층 크기가 작아질수록 감소하였다. 특히 TIG 용접부의 경우 크립 파단시간은 입열량과 밀접한 관계를 나타내었는데 입열량이 클수록 파단시간은 길어졌다. 대부분의 파괴는 ICHAZ에서 발생되었으나 입열량이 감소함에 따라 파단부는 CGHAZ으로 이동하였다. 파단면은 tearing과 dimple을 갖는 입내파괴를 나타내었다.파괴를 나타내었다.

• 대한기계학회논문집A
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• 제24권9호
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• pp.2326-2333
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• 2000

Creep tests for type 316L(N) stainless steel were carried out using constant-load creep machines at 55$0^{\circ}C$, 575$^{\circ}C$ and $600^{\circ}C$. Material constants necessary to predict creep rupture time were obtained from the experimental creep data. And the applicability of Monkman-Grant(M-G) and modified M-G relationships was discussed. The log-log plot of M-G relationship between the rupture time($t_r$,) and the minimum creep rate ($ $\varepsilon$ _m$) was dependent on test temperatures. The slope of m was 1,05 at 55$0^{\circ}C$ and m was 1.30 at $600^{\circ}C$. On the other hand, the log-log plot of modified M-G relationship between $t_r/$\varepsilon$_r$, and $ $\varepsilon$ _m$ was independent on stresses and temperatures. That is, the slope of m＇ was approximately 1.35 in all the data. Thus, modified M-G relationship for creep life prediction could be utilized more reasonably than that of M-G relationship for type 316L(N) stainless steel. It was analyzed that the constant slopes regardless of temperatures or applied stresses in the modified relationship were due to an intergranular fracture grown by wedge-type cavities.

• Journal of Welding and Joining
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• 제18권6호
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• pp.62-67
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• 2000

Metallurgical characteristics of bonded region and high temperature mechanical properties of heat resistant alloy, Fe-35Ni-26Cr during liquid phase diffusion bonding were investigated employing AM17 insert metal. The insert metal for bonding, AM17 was newly developed Ni-base metal using interpolation method. Bonding of specimens were carried out at 1,403~1,463K for 600s in vacuum. The microconstituents in the bonded interlayer disappeared in the bonding temperature over 1,423K. The microstructures, alloying elements and hardness distribution in the base metal. The tensile strength and elongation of the joints at elevated temperatures were the same level as one of the base metal in the bonding temperature over 1,423K. The creep rupture strength and rupture lives of joints were almost identical to those of base metal.

• 대한기계학회논문집A
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• 제25권12호
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• pp.1995-2001
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• 2001

In this study, a small punch creep(SP-Creep) test using miniaturized specimen(10${\times}$10${\times}$0.5mm) is described to develop the new creep test method for high temperature structural materials. The SP-Creep test is applied to 2.25Cr-lMo(STBA24) steel which is widely used as boiler tube material. The test temperatures applied for the creep deformation of miniaturized specimens are between 550∼600$^{\circ}C$. The SP-Creep curves depend definitely on applied load and creep temperature, and show the three stages of creep behavior like in conventional uniaxial tensile creep curves. The load exponent of miniaturized specimen decreases with increasing test temperature, and its behavior is similar to stress exponent behavior of uniaxial creep test. The creep activation energy obtained from the relationship between SP-Creep rate and test temperature decreases as the applied load increases. A predicting equation or SP-Creep rate for 2.25Cr-lMo steel is suggested. and a good agreement between experimental and calculated data has been found.

• 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 : $Q_{SP-P}\;{\risingdotseq}\;1.37 \;Q_{TEN},\; Q_{SP-{\sigma}}{\risingdotseq}1.53\; Q_{TEN}$.

• 대한기계학회논문집A
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• 제32권11호
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• pp.951-956
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• 2008

The high temperature creep properties of the generating plant's high temperature tube, pipe and header and such are very significant in accordance with long-time exposure to the high temperature and pressure environment. Not only this, but as the welding procedure is compulsory for the cohesion of components, the creep properties regarding the local microstructures of steel weldment are very important. In order to understand the creep properties regarding the local microstructures of steel weldment, the SP-Creep test which is easy to get sample from the field component was conducted. The local microstructure of steel weldment, that is, W.M. and B.M.'s microstructures were observed using the SEM. The rupture time of W.M. was longer as 110 % averagely in a same condition, which is the consequence of the difference of the microstructure. Each lethargy coefficient of B.M. and W.M. is evaluated by the relation among the temperature, load and the rupture time from SP-Creep Test. The life estimation equation can be induced by the transformation of Power-law. B.M. and W.M. for each $550\;^{\circ}C$ and $575\;^{\circ}C$, the very similar to normal temperature of the domestic thermal power generation in working, are estimated.

• 한국자동차공학회논문집
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• 제13권3호
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• pp.117-122
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• 2005

Titanium alloy has widely been used as material for glasses frame parts because it has high specific strength. It is also light and harmless to human body. However, we have little design data about the mechanical properties such as the creep behaviors of the alloy. Therefore, in this study, creep tests under four constant stress conditions have been conducted with four different temperature conditions. A series of creep tests had been performed to get the basic design data and life prediction of titanium products and we have gotten the following results. First, the stress exponents decrease as the test temperatures increased. Secondly, the creep activation energy gradually decrease as the stresses became bigger. Thirdly, the constant of Larson-Miller parameter on this alloy was estimated as about 2.5. Finally, the fractographs at the creep rupture showed the ductile fracture due to the intergranullar rupture and some dimples.

• 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.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1995년도 춘계학술대회논문집
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• pp.238-251
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• 1995

Creep of metals has been explained conventionally by dislocation climb and grain boundary sliding indiffusion controlled process. The reorienations of the atoms in the grain by three dimensional crystallizing $\pi$-bondings are visualized as grain rotatins during slow deformation, fold formatin at triple point, increased crevice dspace between grains. grain boundary sliding, grain boundary micration and formation of cracks at the grain boundaries . And also the rupture time and average creep strain rate are explained by the three-dimensional crystallizing $\pi$- bondings and they can be determined by uniaxial tensile test.

• 한국자동차공학회논문집
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• 제14권2호
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• pp.16-22
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• 2006

Stainless steel has widely been used in various industrial fields because it has high corrosion resistance. But, we have little design data about the creep life prediction of SUS316L stainless steel. Therefore, in this study, a series of creep tests and study on them under 16 constant stress and temperature combined conditions have been performed to get the creep design data and life prediction of SUS316L stainless steels and we have gotten the following results. First, the stress exponents decrease as the test temperatures increase. Secondly, the creep activation energy gradually decreases as the stresses become bigger. Thirdly, the constant of Larson-Miller parameters on this alloy is estimated about 10. And last, the creep rupture fractographs show the intergranular ductile fracture with many dimples.