• Proceedings of the KSME Conference
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• 2001.06a
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• pp.223-230
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• 2001

The Monkman-Grant (M-G) and its modified parameters were estimated for modified type 316LN and $9{\sim}12Cr-1Mo$ steels with chemical variations. Several sets of creep data were obtained by constant-load creep tests in $550-650^{\circ}C$ ranges. The relation parameters, m, $m^*$, C and $C^*$ were proposed and discussed for two alloy systems. In creep fracture mode, type 316LN steel showed domination of the intergranular fracture caused by growth and coalescence of cavities. On the other hand, the Cr-Mo steel showed transgranular fracture of the ductile type caused from softening at high temperature. In spite of the basic differences in creep fracture modes as well as creep properties, the M-G and its modified relations demonstrated linearity within the $2{\sigma}$ standard deviation. The value of the m parameter of the M-G relation was 0.90 in the 316LN steel and 0.84 in the Cr-Mo steel. The value of the $m^*$ parameter of the modified relation was 0.94 in the 316LN steel and 0.89 in Cr-Mo steel. The modified relation was superior to the M-G relation because the $m^*$ slopes almost overlapped regardless of creep testing conditions and chemical variations to the two alloy systems.

• Journal of Mechanical Science and Technology
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• v.16 no.11
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• pp.1420-1427
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• 2002

The Monkman-Grant (M-G) and its modified parameters were evaluated for type 316LN and modified 9Cr-Mo stainless steels prepared with minor element variations. Several sets of creep data for the two alloy systems were obtained by constant-load creep tests in 550～650$\^{C}$ temperature range. The M-G parameters, m, m', C, and C' were proposed and discussed for the two alloy systems. The m value of the M-C relation was 0.90 in type 316LN steel and 0.84 in modified 9Cr-Mo steel. The m' value of the modified relation was 0.94 in type 316LN steel and 0.89 in 9Cr-Mo steel. Although creep fracture modes and creep properties between type 316LN and modified 9Cr-Mo steels showed a basic difference, the M-G and its modified relations demonstrated linearity quite well. The m' of modified relation almost overlapped regardless of the creep testing conditions and chemical variations in the two alloy systems, and the parameter m' was closer to unity than that of the M-G relation.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.16 no.1
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• pp.49-55
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• 2020

Creep rupture data for Alloy 690 steam generator tubes in a pressurized water reactor are essentially needed to demonstrate a severe accident scenario on thermally-induced tube failures caused by hot gases in a damaged reactor core. The rupture data were obtained using the tube specimens under different applied-stress levels at 650℃, 700℃, 750℃, 800℃, and 850℃. Important creep constants were proposed using various creep laws in terms of Norton power law, Monkman-Grant (M-G) relation, damage tolerance factor (λ), and Zener-Hollomon parameter (Z). In addition, a creep activation energy (Q) value for Alloy 690 tube was reasonably determined using experimental data. Creep behaviors such as creep strength, creep rates, rupture elongation showed the results of temperature dependence well. Modified M-G plot improved a correlation of the creep rate and rupture life. Damage tolerance factor for Alloy 690 tubes was found to be λ =2.20 in an average value. Creep activation energy for Alloy 690 tube was optimized for Q=350 (kJ/mol). A plot of Z parameter obeyed a good linearity, and the same creep mechanism was inferred to be operative in the present test conditions.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.2
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• pp.296-303
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• 2001

Kachanov-Rabotnov(K-R) creep damage theory was reviewed, and applied to design a creep curve for type 316LN stainless steel. Seven coefficients used in the theory, i.e., A, B, k, m, λ, r, and q were determined, and their physical meanings were analyzed clearly. In order to quantify a damage parameter ($\omega$), cavity amount was measured in the crept specimen taken from interrupted creep test with time variation, and then the amount was reflected into K-R damage equations. Coefficient λ, which is regarded as a creep tolerance feature of a material, increased with creep strain. Mater curve with λ=2.8 was well coincided with an experimental one to the full lifetime. The relationship between damage parameter and life fraction was matched with the theory at exponent ${\gamma}$=24 value. It is concluded that K-R damage equation was reliable as the modelling equation for type 316LN stainless steel. Coefficient data obtained from type 316LN stainless steel can be utilized for life prediction of operating material.

• Journal of Mechanical Science and Technology
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• v.15 no.11
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• pp.1463-1471
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• 2001

The Kachanov and Rabotnov (K-R) creep damage model was interpreted and applied to type 316LN and HT-9 stainless steels. Seven creep constants of the model, A, B, $textsc{k}$, m, λ, ${\gamma}$, and q were determine d for type 316LN stainless steel. In order to quantify a damage parameter, the cavity was interruptedly traced during creep for measuring cavity area to be reflected into the damage equation. For type 316LN stainless steel, λ= $\varepsilon$R/$\varepsilon$* and λf=$\varepsilon$/$\varepsilon$R were 3.1 and increased with creep strain. The creep curve with λ=3.1 depleted well the experimental data to the full lifetime and its damage curve showed a good agreement when r=24. However for the HT-9 stainless steel, the values of λ and λf were different as λ=6.2 and λf=8.5, and their K-R creep curves did not agree with the experimental data. This mismatch in the HT-9 steel was due to the ductile fracture by softening of materials rather than the brittle fracture by cavity growth. The differences of the values in the above steels were attributed to creep ductilities at the secondary and the tertiary creep stages.