Deformation behavior, microstructure evolution, and creep damage mechanism of the novel 9Cr-3W-3Co-1CuVNbB steel CMT+P welded joint during creep

To promote the widespread application of 9Cr-3W-3Co-1CuVNbB steel in ultra-supercritical (USC) power plants, this study systematically investigated the creep deformation behavior, microstructural evolution, and damage mechanisms of its welded joints. The results showed that welded joints prepared using Cold Metal Transfer with Pulse (CMT+P) exhibited superior creep resistance compared to those fabricated using Electron Beam Welding (EBW). Analysis based on Norton’s power law indicated that the deformation mechanisms of the welded joints were highly stress-dependent: under high-stress conditions, creep deformation was primarily governed by back stress, whereas under low-stress conditions, it was dominated by dislocation motion. Specifically, under high-stress creep conditions, the welded joints exhibited excellent microstructural stability, with fractures occurring in the weld metal (WM). Transmission electron microscopy (TEM) analysis revealed that dislocations accumulated extensively at martensitic laths (MLs) interfaces, leading to cracking of the lath structure. In contrast, under low-stress creep conditions, the intercritical heat-affected zone (ICHAZ) became the weak point. Dislocation tangles formed around coarsened precipitates, promoting cavity nucleation. As creep progressed, these cavities expanded along the precipitate/matrix interfaces and interconnected, eventually forming macroscopic cracks and cavity bands, ultimately resulting in brittle Type IV cracking. Classical nucleation theory analysis further indicated that dislocation density gradients and M₂₃C₆ carbide coarsening were the primary factors accelerating precipitate coarsening. This study innovatively established a composite creep damage mechanism incorporating both back stress and dislocation motion deformation mechanism, elucidating the competitive damage mechanisms of CMT+P welded joints under multi-stress fields.