Effect of long-term aging on the fracture toughness of primary coolant piping material Z3CN20.09M

In this study, a series of accelerated thermal aging tests were performed on primary coolant piping materials (Z3CN20.09M) at 400 degrees C for 0, 2000, 5000 and 18,000 h to investigate the resulting microstructure change and fracture toughness. To further assess their fracture toughness, 0.2 mm offset line method and stretch zone width method were both employed to determine the fracture toughness. The results indicated that after long-term thermal aging, round shaped particles and characteristic morphology of spinodal decomposition were found in ferrite, thus resulting in the thermal aging embrittlement of Z3CN20.09M. Subsequently, the J-R curves and J-T curves all decreased significantly with the increasing thermal aging time. Furthermore, the fracture toughness parameters J(50), J(SZW(2D)), J(SZW(3D)) and dJ/d Delta a were first decreased rapidly and then slow saturated. In addition, the J(SZW(2D)) and J(SZW(3D)) values were significantly lower than J(Q) while the reduction of the critical crack size based on J(SZW(2D)) and J(SZW(3D)) parameters was about 30% compared with J(Q), and thus J(SZW) was relatively close to the onset of crack initiation.