Quantification of the effect of early microstructural degradation during creep of 9Cr-1Mo-NbV steels at 600 °C

A microstructure-based model for the prediction of the creep resistance at 600 °C of 9%Cr steels was applied to literature data, in order to clarify the reasons for the drop in creep strength observed for certain heats of P91 steels. The analysis demonstrated that the observed sigmoidal behaviour of the time to rupture vs. applied stress graph can be explained by the progressive and early dissolution of the fine MX particles which, in the lath interior, interact with dislocations obstructing their mobility. The same phenomenon, whose effects in part eclipse those of the recovery of the substructure, i.e. the lath/subgrain growth and the reduction in free dislocation density, explains the abrupt decrease in the room temperature hardness as measured after rupture with different test durations.