Insight into the creep-fatigue interaction and remaining creep damage mechanisms in different micro-regions of 9%Cr steel welded joints

The deformation and damage mechanisms of welded joints have gained high interest for their widespread use in power plants. The inter-critical heat-affected zone (ICHAZ) is generally identified as the weakest region in the whole welded joint. However, the micro-regions' nanoindentation and creep results in the present work reveal that the weakest position moves to base material after fatigue loading. It is found from electron backscatter diffraction (EBSD) analysis that the early saturated grain growth in the ICHAZ contribute to the movement of the weakest region. It is important to note that compared with the creep strength in the as-received condition, prior fatigue loading enhances the creep deformation. It is also observed from three-dimensional X-ray tests that large fatigue cycles and long hold time of fatigue loading accelerate the nucleation of creep voids. Finally, a fatigue damage parameter based on the nanoindentation microhardness is proposed, which is validated to be capable of capturing the effect of various loadings on the subsequent creep life and creep fracture location. These results benefit the understanding of the creep behaviour of welded joints after fatigue damage and provide a guide for the design and damage evaluation of welded components.