• Journal of Ocean Engineering and Technology
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• v.12 no.2 s.28
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• pp.65-70
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• 1998

In this report, the creep properties and creep life estimation by Larson-Miller Parameter(LMP) Method for 2.25Cr1Mo steel to be used as power plant tubes or other components were presented at the high temperatures of 500, 550, and $600^{\circ}C$. It was confirmed experimentally and quantitatively that a creep life estimation equation at such various high temperatures was well derived by LMP and could be used very effectively within the creep life of 10$^3$ hours, but very unreliable and even dangerous for design in a long term of creep life such as 10$^4$ or $10^5$ hours.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.10a
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• pp.818-823
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• 2005

Creep damage is one of the mosl important characteristics for the stability of high temperature structures such as huge energy converting facilities. Creep failure of Type 316LN stainless steel is highly correlated to generation and growth of the voids. In this paper, in order to investigate the correlation of creep rupture time and ultrasonic parameters (group velocity, angular velocity), creep-damaged Type 316LN specimens and measurements for the ultrasonic parameters were made. However, bi-directional measurements were applied along the load direction and the perpendicular direction to the load line by means of the contact type probe of which the central frequencies are 10MHz, 15MHz and 20MHz. Analyzing the angular velocities of the ultrasonic signals obtained from the load direction, it was confirmed that the angular velocities were declined as the creep time passed when 15MHz and 20MHz probes were used. Also, the group velocities were declined for all three frequencies as the creep time increased. Thus, positive feasibility for the creep damage evaluation by means of the angular and group velocities was confirmed. Moreover, result of analysis for the ultrasonic signal which was obtained from the perpendicular direction upon the angular and group velocities indicated little variation for both of the angular and group velocities. Therefore, the creep damage is likely to represent anisotropic itself.

• Korean Journal of Metals and Materials
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• v.47 no.10
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• pp.613-620
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• 2009

The Kachanov-Rabotnov (K-R) creep model was proposed to accurately model the long-term creep curves above $10^5$ hours of Alloy 617. To this end, a series of creep data was obtained from creep tests conducted under different stress levels at $950^{\circ}C$. Using these data, the creep constants used in the K-R model and the modified K-R model were determined by a nonlinear least square fitting (NLSF) method, respectively. The K-R model yielded poor correspondence with the experimental curves, but the modified K-R model provided good agreement with the curves. Log-log plots of ${\varepsilon}^{\ast}$-stress and ${\varepsilon}^{\ast}$-time to rupture showed good linear relationships. Constants in the modified K-R model were obtained as ${\lambda}$=2.78, and $k=1.24$, and they showed behavior close to stress independency. Using these constants, long-term creep curves above $10^5$ hours obtained from short-term creep data can be modeled by implementing the modified K-R model.

• Journal of Welding and Joining
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• v.20 no.6
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• pp.30-30
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen(l0×10×0.5mm). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at 600℃. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• Journal of Welding and Joining
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• v.20 no.6
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• pp.754-761
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen($10{\times}10{\times}0.5mm$). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at $600^{\circ}C$. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.352-357
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• 2003

Creep tests for Titan were carned out using constant-load at $600^{\circ}C$, $650^{\circ}C$ and $700^{\circ}C$. Material constants necessary to predict creep life were acquired from the experimental creep data. And the applicability of Monkman-Grant(M-G) and modified M-G relationships was discussed. It was discovered the log-log plot of M-G relationships between the rupure time(tr) and he minimum creep rate(${\varepsilon}_m$) was conditional on test temperatures. The slop of m was 2.75 at $600^{\circ}C$ and m was 1.92 at $700^{\circ}C$. However; the log-log plot of modified M-G relationships between $t_r/\varepsilon_r$ and $\varepsilon_m$ was indpendent on stresses and temperatures. That is the slop of m' was almost 3.90 in all the data. Thus, change M-G relationships to creep life prediction could be vtilized more reasonably than that of M-G relationships for type Titan. It was divided that the constant slopes never theless of temperatures of practical stresses in the modified relationship were due to an intergranular break grown by wedge-type cauities.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.1
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• pp.171-178
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• 2002

An effect is made in this study to deepen understanding of small punch(SP) creep testing which has been a round for about 10 years as a substitute for the conventional uniaxial creep testing. Even though considerable numbers of SP creep test program have been performed, most of the tests were aimed at measuring creep rupture lives only. Very flew studies showed interest on the meaning of what we were really measuring during the SP creep tests. In this paper meanings of the parameters measured during the SP creep testing, such as punch load and punch displacement rate are investigated using finite element analysis. It was shown that the measured parameters must represent the stress and strain rates of the material at the annular region located at about 0.65 mm from the center of the SP specimen. The material in this location would go through constant maximum stress and strain rate during the testing. Experimental verification is also discussed.

• Korean Journal of Metals and Materials
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• v.49 no.7
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• pp.515-521
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• 2011

Small punch creep testing has received attention due to the convenience of using smaller specimens than those of conventional uniaxial creep tests, which enables creep testing on developing or currently operational components. However, precedent studies have shown that it is necessary to consider friction between the punch and specimen when computing uniaxial equivalent stress from a finite element model. In this study, small punch creep behaviors of AISI 316L stainless steel, which is widely used in high temperature-high pressure machineries, have been compared for the two different ceramic balls such as $Si_3N_4$ and $Al_2O_3$. The optimal range of the friction coefficient is 0.4~0.5 at $650^{\circ}C$ for the best fit between experimental and simulation data of AISI 316 L stainless steel. The higher the friction coefficient, the longer the creep rupture time is. Therefore, the type of ceramic ball used must be specified for standardization of small punch creep testing.

• Journal of the Korean Society of Safety
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• v.13 no.4
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• pp.131-136
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• 1998

The creep behavior and failure mechanism of the 30 vol% hot-pressed $SiC_t/Si_3N_4$ ceramic composite was experimentally investigated at $1200^{\circ}C$ and at various stress levels in air. The creep threshold stress for zero creep rate after 100 hr was found to be approximately 60 MPa. The stress exponent was estimated to be n~1, which suggests that fiber-reinforcement reduced the stress sensitivity of the HPSN matrix with the stress exponent of 2. The tertiary stage leading to creep rupture was found at 250 MPa but was very short. The microstructure of the crept specimen showed random fiber fracture and no matrix cracking. Interfacial debonding was absent.

• Journal of Ocean Engineering and Technology
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• v.12 no.2 s.28
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• pp.71-78
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• 1998

In this report for the assessment of creep properties of high-temperature tube materials in power plants, the long-time($10^4$~105h) creep life prediction by ISM for 2.25Cr1Mo steel was studied. It was clarified experimentally and quantitatively that the newly developed long-time creep life prediction equation was very coincident with the actual experimental data with high confidence, and the model was $t_r=\alpha\varepsilon_0^{\beta}\sigma^{-1}$.