Integrated effect of thermal ageing and low flux irradiation on microstructural evolution of the ferrite of welded austenitic stainless steels

Artikel i vetenskaplig tidskrift, 2021

With the purpose to quantify microstructural changes with respect to ageing degradation, the microstructure of aged type 308 stainless steel welds with a ferrite content of 5-7% has been analysed using atom probe tomography. The weld metal of the core barrel of a decommissioned light water reactor, irradiated during operation of the reactor to 0.1 dpa, 1 dpa and 2 dpa at 280-285°C (231,000 h), are compared to two similar thermally aged welds. In the ferrite of the irradiated welds, there is spinodal decomposition into Cr-rich α’ and Fe-rich α, with a similar degree of decomposition for all investigated doses, amplitudes of 21-26% and wavelengths between 6 and 9 nm. The ferrite of the thermally aged material showed evidence of decomposition when aged at 325°C (an amplitude of 13-14% and wavelength of 5 nm), but not when aged at 291°C, thus the irradiation significantly increases the rate of spinodal decomposition. There is G-phase (Ni Si Mn ) precipitation in the ferrite of all the weld metals except the one that was thermally aged at the lowest temperature. After irradiation to 1 and 2 dpa, the G-phase is considerably more well developed than after 0.1 dpa or thermal ageing.