Influence of post irradiation annealing on the mechanical properties and defect structure of AISI 304 steel

The effect of post irradiation annealing on the mechanical properties and the radiation induced defect structure was investigated on stainless steel, of type AISI 304, that was irradiated up to 24 dpa in the decommissioned Chooz A reactor. The material has been investigated both in the as-irradiated state as well as after post irradiation annealing. In the as-irradiated specimen the typical radiation induced defects were found as well as NI3SIprecipitates. Annealing at 400°C had almost no effect on the radiation induced defects, but annealing at 500°C resulted in the immediate unfaulting of the Frank loops. As to the mechanical properties, annealing at 400°C did not strongly affect the yield strength and ductility, although the fraction of intergranular fracture during slow strain rate tensile tests (SSRT), under pressurised water reactor (PWR) condition, was significantly reduced. Annealing at 500°C reduced the yield strength and restored substantially the ductility and the strain hardening capability. The microstructure investigated by transmission electron microscopy correlates to the mechanical test results. It was found that the observed defect changes after post irradiation annealing provide a reasonable explanation for the observed changes of the mechanical properties obtained from SSRT under PWR chemical conditions.