Tensile properties and microstructure of 9Cr-1Mo martensitic steels containing a high helium concentration

Abstract
Tensile tests and microstructural examinations were performed on 100 m thick specimens of 9Cr-1Mo (EM10) and modified 9Cr-1Mo (T91) martensitic steels homogeneously implanted with 23 MeV particles to a concentration of 5000 appm. Two implantation temperatures were selected, 250 and 550°C, which correspond respectively to the lower (higher) bound of the operation temperature range foreseen for the window of Accelerator Driven Systems devoted to waste transmutation. 250°C is also the maximum operating temperature of the ESS (European Spallation Source) window. The specimens were tested at room and implantation temperatures and the fracture surfaces were characterized using scanning electron microscopy. It was found that implantation at 250°C induces a very strong hardening of both materials together with a total loss of ductility. Embrittlement was also observed for the specimens implanted at 550°C, however the ductility loss was much less severe. Preliminary Transmission Electron Microscopy (TEM) observations are presented. Helium bubbles were observed in the specimens implanted at 550°C but none could be detected in the samples implanted at the lower temperature. However, based on results of Small Angle Neutron Scattering experiments performed on samples implanted together with the tensile specimens, it is proposed that the high degree of hardening following implantation at 250°C is due to the formation of a high density of tiny helium bubbles. It is furthermore suggested that the brittle, intergranular fracture mode displayed by these specimens results from the combined effects of pronounced intragranular hardening and weakening of Prior Austenite Grain (PAG) boundaries due to helium.