An improved continuum damage constitutive model is presented to describe the creep behavior of centrifugally cast 20Cr32Ni1Nb stainless steel. In order to determine the internal softening mechanisms of the steel, microstructural observations of damage in 20Cr32Ni1Nb steel are made using the long-term iso-thermal aging treatment. Based on the physics of microstructural processes, a two state variable theory which represents two damage mechanisms related to particle coarsening and microcrack is employed to account for tertiary creep. The kinetic equation which describes the coarsening of the precipitates with time is coupled with Liu-Murakami creep damage constitutive model. Good agreement between predicted and experimental data proves the validity of the proposed model. Compared with the failure times predicted by power law based models, such as Kachanov-Robotnov and Liu-Murakami model, life predictions based on the proposed model are found to be more reasonable over a wide stress range. Additionally, the proposed model is also applied to analyze the accumulation and development of damage in 20Cr32Ni1Nb manifold components.

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