Creep-Induced Microstructural Changes in Large Welded Joints of High Cr Heat Resistant Steel

The creep damage process of high-Cr steel welded joints is characterized by the formation and growth of creep voids prior to the initiation of cracking, and this formation and growth accounts for a large proportion of creep life. Therefore, there has been much investigation into the detection and quantitative evaluation of creep voids and microcracks in welded joints for use in remaining life assessment. However, the microstructure around creep voids and microcracks is still not well known. In this study, a creep test on a large welded joint in Mod.9Cr-1Mo steel was conducted under 60 MPa at 650 °C. The observation of the microstructure in the heat-affected zone (HAZ) was made for the specimen interrupted the creep tests at 25% of a rupture life. The microstructure around the creep void was characterized using an electron backscatter diffraction pattern (EBSD) method. It was founded that creep voids formed and developed along random high angle grain boundaries that were not subject to K-S orientation relationship in the martensitic transformation. In addition, the initially formed void promoted preferential dynamic recovery and dynamic recrystallization in its surrounding microstructure, followed by sub-boundary formation.