Effects of Thermal Aging and Low Dose Neutron Irradiation on the Ferrite Phase in a 308L Weld

Abstract

The integrity of reactor internal components made of austenitic stainless steel welds with a duplex structure can potentially be affected by thermal aging and/or neutron irradiation induced embrittlement. There have not been sufficient studies on the long-term service performance of SS welds in light water reactors. In this study, thermal aging was performed at 400 °C for up to 2220 h on a 308L weld, and the irradiation was conducted in the Halden reactor at ~315 °C to 0.08 dpa (5.6 × 1019 n/cm2, E > 1 meV). The microstructural evolution of the ferrite phase was characterized using atom probe tomography (APT) and auxiliary transmission electron microscope studies. Spinodal decomposition and Ni-Mn-Si solute clusters were observed in both the thermally aged and neutron irradiated 308L welds. As compared with thermal aging, low dose neutron irradiation induced similar spinodal decomposition with slightly larger concentration wavelength and amplitude. The solute clusters in irradiated ferrite phase also show a larger mean size, a wider size distribution, but a lower number density as compared with those in thermally aged ferrite phase. In addition, the neutron irradiation significantly promotes segregation of trace elements, particularly phosphorus, at the Ni-Mn-Si solute clusters.