Microstructure evolution of fine-grained heat-affected zone in type IV failure of P91 welds

A microstructure analysis has been conducted on the fine-grained heat-affected zone (FGHAZ) of P91 welds that have undergone sequential thermomechanical histories: the as-welded (AW), after a postweld heat treatment, and after a creep rupture test. The Type IV failure is observed to have initiated by wedge-type cavities that coalesce to form an intergranular separation with a high local deformation at the outer edge of the FGHAZ. The ratio of the high-angle grain boundary (HAGB) frequency to low-angle grain boundary (LAGB) frequency in the FGHAZ increases from the aswelded 0.48 to the crept 1.06, resulting in an increased grain size from 1.2 μm (AW) to 3.1 μm (crept). The area fraction of the fully recrystallized ferrite grains increases from 14% (AW) to 45% (crept). Coarse, undissolved, Cr-rich M₂₃C₆-type carbides remain on prior austenite grain boundaries and packet/block boundaries after welding. V (C, N) and Nb (C, N) carbonitrides distribute at HAGBs. The MoC/Mo₂C and M₇C₃ precipitates form a grain boundary network that is believed to help the nucleation of cavities during creep. The kernel average misorientation (KAM) maps show the higher local strain level in the particle-containing grains. Straining variation of the particle-containing grains and particle-free grains in the crept FGHAZ can be traced back to the degree of austenitization in the welding thermal cycle. Creep deformation mismatch between fully recrystallized grains that are free of precipitates and the unrecrystallized grains that contain carbide particles seems to have contributed to nucleation of the initial creep cavities.