Work

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

Year: 2025

Type: article

Abstract: • Nano-scale cavities were observed in martensitic Grade 91 steel in early creep life stage after 0.5 % strain. • The majority of creep cavities were observed on prior austenite grain boundaries (PAGBs). • The misorientation angle of PAGBs exhibited strong association with creep cavitation. PAGBs with misorientations of 15–35° and 60° were the least frequent in the parent microstructure but exhibited the highest cavitated ratios with triple junctions showing increased suceptibility to cavitation. • Localized deformation measured in temrs of Kernel Average Misorientation (KAM) exhibited strong association with creep cavitation. • Manganese sulphide (MnS) inclusions and complex MnS-Al 2 O 3 inclusions were found to be highly susceptible to damage. 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 M 23 C 6 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.

Source: Materialia

DOI: 10.1016/j.mtla.2025.102599