A multiaxial model for the evolution of the microstructure and for the mechanical behaviour of a 12% chromium steel under creep conditions

In creep-resistant steels several mechanisms contribute to the degradation of creep properties under long-time service conditions. Most important are the coarsening of the carbide and subgrain structure and grain-boundary cavitation. If creep cavities on grain-boundaries grow by the constrained diffusive mechanism, partly cavitated boundary facets act mechanically like microcracks. Cell models are worked out numerically to explore the influence of a distribution of microcracks on the constitutive response of a creeping solid. The results confirm the predictions of analytical estimates based on the differential self-consistent method of Rodin and Parks. The Rodin and Parks model is combined with a Chaboche-type model in order to provide a comprehensive model covering primary, secondary and tertiary creep under arbitrary loading conditions. Measured carbide coarsening and subgrain evolution enter through a microstructurally motivated softening equation. The model parameters are adjusted to a set of creep curves for a 12% chromium steel (X 20 CrMoV 12 1) and to quantitative microstructural measurements. The combined model is implemented in the finite element code ABAQUS. Test on compact specimens are successfully modeled. (orig.)