Damage mechanics-based creep model for 9–10%Cr ferritic steels

The key microstructural features that control creep in advanced 9–10%Cr steels are elucidated. Conclusive evidence for the presence and coarsening of nano-scale MX carbonitrides which contribute to the long-term stability of these materials is presented. It is also shown that subgrain structures resulting from the martensitic transformation contribute to the basic strength by generating internal stresses through an anelastic bowing mechanism. Kinetic equations that describe the evolution of these microstructural features with time and strain, derived from exhaustive experimental measurements for two steels, are incorporated into a creep equation set based on the damage mechanics philosophy. It is shown that the anomalously high activation energy reported in the literature for creep of ferritic steels can be traced to the fact that temperature dependence of subgrain growth had been ignored hitherto. This model is shown to be able to predict creep data for two steels accurately up to tens of thousands of hours over which data are available.