Microstructure evolution and impact fracture behaviors of Z3CN20-09M stainless steels after long-term thermal aging

Z3CN20-09M steels of primary circuit piping in Daya Bay Nuclear Power Plant were studied on the microstructure evolution and fracture behaviors after thermal aging at 400 °C for up to 20,000 h. The impact toughness of aged materials decreases a lot with aging time, and the impact fracture features change from ductile dimples into brittle cleavages in ferrite and tearing in austenite. Nano-indentation tests indicate that hardness in ferrite continuously increases with aging time. After long-term aging, ferrite decomposes into coherent Cr-rich and Fe-rich domains, and extensive G-phases precipitate homogeneously in ferrite, but no precipitate in austenite. Spinodal decomposition in ferrite leads to the thermal aging embrittlement in CASS. G-phase, with the Fm-3m space group and the lattice parameter of 1.14 nm, adopts a cube-on-cube orientation relationship with the ferrite matrix.