Long-term thermal aging performance of welding structures in primary coolant pipes for nuclear power plants

Abstract

Welding structures of primary coolant pipes were subjected on a series of accelerated thermal aging test at 400 °C until 30,000 h to investigate the resulting microstructure, impact and nanoindentation properties change. The results indicated that after long-term thermal aging, a mottled structure and precipitated G-phases were observed in ferrite phases both of straight pipes and welded joints, leading to a decrease in impact energy and an increase in nanohardness. Moreover, until thermal aging saturation, impact energy of welding structures was still higher than initial design value (80 J), thus demonstrating a high toughness reserve, but saturated impact energy in welded joints was higher than that in straight pipes owing to lower ferrite content. Furthermore, there were higher-density spinodal decompositions and more precipitated G-phases in welded joints than those in straight pipes owing to micro-defects generated from welding process, resulting in a higher nanohardness in ferrite phases of welded joints.