Microstructural change of a 9Cr steel longitudinal welded tube under internal pressure creep loading

In this study, the microstructure of a base metal, and the heat-affected zone (HAZ) and weld metal of a 9Cr steel longitudinal welded tube with various internal pressure creep damage levels was studied by transmission electron microscopy and energy dispersive X-ray spectroscopy. Each portion of the steel weldment clearly changed with damage level. The present results indicate that the recovery of the microstructure is faster in HAZ than in other portions, and thus creep deformation preferentially occurs in HAZ. Moreover, it was revealed that stress accelerates the growth of M23C6 precipitates as well as the reduction of dislocation density, consequently promoting recovery. It was also confirmed that the so-called type IV failure is reasonably explained by precipitate strengthening.