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 analyzing 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 M 23 C 6 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 − 5 5 ∘ 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 − 2 0 ∘ 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. • The study investigated creep cavity nucleation in ferritic P91 material at early and late creep stages, analyzing interrupted and failed creep tests at 4%, 10% and 35.6% strain. • Pre-existing voids were found in the reference microstructure prior to creep testing and were associated with elongated manganese sulphide (MnS) inclusions. • Manganese sulfide (MnS) inclusions were found to be highly susceptible to damage and initiate cavity nucleation during early creep life stages. • M 23 C 6 carbides and Laves phase at the interface between MnS inclusions and ferrite matrix facilitated cavity nucleation. • Grain boundary misorientation and localized deformation measured in terms of KAM, were strongly correlated with cavitation. • The Schmid factor showed no statistically significant correlation with cavitation. • A dense dislocation structure observed in early creep stages was reduced at the failure stage, likely due to dislocation relief during cavity formation and crack propagation.