Assessment and quantification of damage in the grade 91 steel partially transformed zone

Damage in the grade 91 steel partially transformed zone has historically been associated with many different types of features in the microstructure. Susceptible features responsible for the nucleation of damage are frequently cited to include particles such as laves phase, coarse M23C6, inclusions, nitrides, or interactions between creep strong and creep week grains, grain boundaries and potentially other sources. Few studies have attempted to link these observations on a statistically relevant basis using 2D or 3D microscopy techniques. In this study, 2D assessment using scanning electron microscopy (SEM) and quantification techniques such as energy dispersive x-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) are utilized in combination with 3D serial sectioning of large volumes using plasma focused ion beam milling (P-FIB) and simultaneous EDS to evaluate an interrupted cross-weld creep test. The cross-weld creep test was performed using a feature-type geometry, in parent material susceptible to the evolution of creep damage and interrupted at an estimated life fraction of 70%. The findings from these evaluations provide perspective on the features in the microstructure responsible for the nucleation and subsequent growth of the observed damage.