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Research Papers: Materials and Fabrication

Cavity Growth Simulation in 2.25Cr–1Mo Steel Under Creep-Fatigue Loading

[+] Author and Article Information
Takashi Ogata

 Central Research Institute of Electric Power Industry, 2-11-1 Iwadokita, Komae, Tokyo 201-8511, Japantogata@criepi.denken.or.jp

J. Pressure Vessel Technol 130(3), 031404 (Jun 11, 2008) (6 pages) doi:10.1115/1.2937735 History: Received May 02, 2006; Revised April 19, 2007; Published June 11, 2008

High temperature components in thermal power plants are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of a quantitative evaluation method of cavity growth is important for reliable maintenance of these components. In this study, a creep-fatigue test was carried out at 600°C on 2.25Cr–1Mo steel in a scanning electron microscope, and continuous observation of cavity growth behavior during the test was made. Based on the cavity growth observation, existing cavity growth models were modified and the simulated results using the modified model were compared to the observed cavity growth behavior. From the observation, spherical shape cavities initiate and grow up to their length of 2μm on the grain boundaries at the initial stage of damage, and then these cavities change their shape to cracklike and grow until their length reaches around 10μm. Finally, cracklike cavities coalesce with each other to form one microcrack along a grain boundary. It can be concluded that cavity growth rates are controlled by diffusion and power law creep under constrained conditions, based on the theoretical consideration of cavity growth mechanism. Through these discussions, a new cavity growth model was proposed by modifying conventional models. Both spherical and cracklike cavity growth rate equations were derived from the modified cavity growth model. It was indicated that the measured cavity growth rate was well predicted by the growth rate equations, derived from the modified model, and a cavity growth simulation result corresponds to the change in the maximum cavity size with number of cycles under the creep-fatigue loading.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 2

Appearance of high-temperature fatigue testing machine in a SEM. (a) View of the equipment; (b) specimen attachment portion.

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Figure 3

Stress wave form in the creep-fatigue test

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Figure 4

Observed cavities and microcracks by a SEM during the creep-fatigue test. (a) 705 cycles; (b) 952 cycles; and (c) 1308 cycles.

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Figure 1

Specimen geometry used in the test

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Figure 9

Comparison of cavity growth rates between spherical and cracklike cavities, from the proposed (CDP) model

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Figure 5

Schematic representation of cavity growth model

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Figure 6

Comparison of cavity growth rate between different cavity growth models

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Figure 7

Comparison of cavity growth rates between different growth models and observed cavities in the test

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Figure 8

Comparison between cavity growth simulation and the maximum cavity observed in the test

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