Creep behavior and microstructure evolution of creep cavities in the heat-affected zone of P92 steel joints

The Type IV fracture of P92 steel joints is characterized by the aggregation of micro-cavities leading to creep cracking, which presents a significant risk to the operational safety of P92 steel pipelines in ultra-supercritical plants. Consequently, it is essential to examine the evolution of creep cavities in the heat-affected zone of joints to provide early warning of potential cracking risks. This study characterized the hardness, microstructures, and electron backscatter diffraction (EBSD) images for two different P92 steel joints exhibiting Type IV fracture, as well as for a P92 steel joint without fracture. By building upon the understanding of creep damage mechanisms in homogeneous materials and integrating practical observations from P92 steel pipeline joint failures, we elucidated the microstructural evolution of creep cavities within the heat-affected zone of these heterogeneous materials using a micromechanics-based model. Our findings indicate that the nucleation of creep cavities varies across different regions of the joint, influenced by their distinct creep characteristics. Additionally, the growth of these cavities is regulated by the hydrostatic stress and stress triaxiality factor, while cavity coalescence is primarily governed by axial stress. To mitigate Type IV fracture, it is crucial to enhance creep resistance in the heat-affected zone and minimize axial stress levels.