Microstructure Evolution of Fine-Grained Heat-Affected Zone of Gr.92 Steel Welded Joint During Creep

The microstructure evolution of fine-grained heat-affected zone (FGHAZ) of Gr.92 steel welded joint has been investigated during creep using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The microstructure evolution strongly depended on the applied stress level. At the stress level of 90 MPa, the grains of the FGHAZ were coarsened after creep. When the stress level decreased to 70 MPa, the grains of the FGHAZ were found to be further refined during creep owing to continuous dynamic recrystallization (subgrains rotation). This was verified by the increase in boundary length featuring random misorientation angles after creep. Recrystallized microstructure was only observed for the FGHAZ of welded joints after creep, but not for the A_c3 heat-affected zone (HAZ) simulated samples which exhibited homogeneous microstructure even at the low applied stress of 50 MPa. In high stress regime the creep strength of the welded joints was almost the same as that of Gr.92 steel owing to the mechanical constraint originating from the heterogeneous creep property of welds. In low stress regime, the creep strength of the welded joint was inferior to that of Gr.92 steel and approached that of the A_c3 HAZ simulated sample as the applied stress decreased. This was caused by the relaxation of the constraint owing to grain boundary sliding. Recrystallization was regarded as a sign of relaxation of constraint and deterioration of creep property for high-Cr heat-resistant steel welded joints.