Investigation of creep cavitation mechanisms in ferritic Grade 91 steel

The majority of premature failures in Grade 91 steel components used in high-temperature applications, such as power plants, are attributed to creep cavity nucleation. This study examined creep cavity nucleation in ferritic P91 ex-service material during its early formation stages through interrupted creep tests at 4% and 10% strain, as well as in later stages by analysing a failed creep-tested specimen with 35.6% strain at failure. While cavity growth under high-temperature exposure did not require applied stress, cavity interlinkage was more pronounced in high-stress regions. It was found that manganese sulfide (MnS) inclusions were highly prone to damage and were responsible for the nucleation of the first cavities during early creep life stages. This process was facilitated by the presence of M₂₃C₆ carbides and Laves phase located at the interface between the MnS inclusions and the ferrite matrix. Grain boundary misorientation was highly associated with cavitation with grain boundaries of misorientations $45-55^{\circ }$ being the predominant type in the ferritic microstructure and, consequently, the most frequently cavitating. Although less frequent in the microstructure, lower-angle GBs with misorientations $<15-20^{\circ }$ exhibited the highest cavitation ratios. Localized deformation was found to be strongly correlated with cavitation, whereas the Schmid factor did not exhibit a statistically significant link to damage. A dense dislocation structure, observed using TEM and SEM imaging at early creep life stages, was significantly reduced at the failure stage, likely due to dislocation relief during cavity formation and crack propagation.