Microstructural parameters associated with cavity nucleation in martensitic Grade 91 steel under creep conditions

Creep cavity formation in Grade 91 steel components is a serious challenge that energy applications face, as cavities grow, coalesce, form propagating cracks and eventually lead to failure. This study investigates the microstructural parameters associated with creep cavity nucleation in ex-service martensitic P91 material. Three uniaxial and one double-notched creep tested samples were investigated, interrupted at 0.5 % and 1 % and failed at 3.7 % strain fractions, in order to inspect the evolution of damage in early and late stages of the creep life. A correlative SEM-EBSD technique was employed, revealing that most cavities nucleated along the parent austenite grain boundary (PAGB) structure, with grain boundary junctions exhibiting increased susceptibility to damage. Although PAGBs with misorientation angles of 15–35° and 60° were not the predominant boundary type in the parent microstructure, they exhibited high cavitation ratios. Manganese sulfide (MnS) inclusions were found to be highly prone to cavity nucleation, while smaller precipitates, such as M₂₃C₆ carbides, Laves phase and MX-type nitrides, facilitated cavity formation at the MnS/martensite matrix interface. Since the vast majority of grains were soft, no clear correlation between the Schmid Factor and cavitation could be established.