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

Thermal Aging Phenomena in Cast Duplex Stainless Steels

“…[2][3][4][5][6][7]9 However, a complete embrittlement (or zero ductility) above room temperature has not been observed in the thermally aged CASS alloys for LWR conditions. Many past studies have been carried out at temperatures higher than the common operating temperature range, i.e., in accelerated aging conditions, to obtain detectable property change within short time periods, [2][3][4][5][6][7][10][11][12][13][14][15][16][17][18][19][20][21][22] and it has become customary to simulate the microstructural processes in aging during service by aging at 400°C. [10][11][12][13][14][15][16][17][18][19][20][21][22] However, as a variety of aging mechanisms are found to strongly depend on aging temperature, we need to be conscious on the relevance of those accelerated aging experiments to the actual service conditions in LWR power plants.…”

“…For cast stainless steels, the main microstructural mechanisms of thermal aging at <500°C are associated with the precipitation of additional phases in the ferrite: (a) formation of a Cr-rich a¢-phase through Spinodal decomposition, (b) precipitation of a G-phase (Ni, Si-rich) and M 23 C 6 carbide, and (c) additional precipitation and/ or growth of existing carbides and nitrides at the ferrite/austenite phase boundaries. [2][3][4][5][6][7][8][9]13,17,19,[29][30][31][32][33][34][35][36] In the austenite matrix phase, thermal aging induces various precipitations but usually causes a negligible to moderate effect on the mechanical properties of the phase. [3][4][5][6][7] The effect on toughness, in particular, is less pronounced in the austenitic phase.…”

“…[2][3][4][5][6][7]29 The primary embrittlement mechanism, the Spinodal decomposition of ferrite, should have higher activation energy because of the short range and fast characteristics of the process. 3,7,17,19,31,32,35 The strengthening in ferrite is caused primarily by Spinodal decomposition of ferrite to form the Cr-rich a¢-phase, and consequently the kinetics of thermal embrittlement should be primarily controlled by the size and spacing of the a¢-phase.…”

“…Further, numerous aging experiments in the past have been performed at or above 400°C to investigate microstructural evolution and mechanical behaviors and to predict long-term CASS behaviors. [2][3][4][5][6][7][10][11][12][13][14][15][16][17][18][19][20][21][22] However, such highly-accelerated aging treatment above the operation temperature range is yet to be thoroughly validated for simulation of the lower temperature (300°C) phenomena in the current fleet of nuclear power plants. 9 The current consensus might be that, although the vast majority of the current nuclear power plants are expected to be operational for extended lifetimes of 60 years or potentially longer, 1 the behaviors of CASS alloys during such prolonged periods have remained largely unknown.…”

Evaluation of hardening behaviors in ion-irradiated Fe–9Cr and Fe–20Cr alloys by nanoindentation technique

“…The irradiation-induced microstructural evolution and mechanical property changes of F/M steels become the research hot spot recently [3,5e7,13,14]. In addition, ferrite phases in duplex stainless steels (DSS), cast austenitic stainless steels (CASS) or weld of austenitic stainless steels, generally contain >20 wt% Cr, are well known to be sensitive to thermal aging embrittlement, which is induced by spinodal decomposition of ferrite into Cr-rich a 0 and Crdepleted a domains [16,17]. Neutron/ion irradiation was found to induce spinodal decomposition in the ferrite of as-cast microstructure, and also affect the spinodal decomposition in the thermally aged cast alloys [18e20].…”