Correlation between microstructure and hardening of 9Cr ferritic/martensitic steels irradiated in spallation neutron source

Significant hardening can be caused in ferritic/martensitic (FM) steels because of intensive displacement damage and transmutation helium/hydrogen when irradiated with high-energy protons and spallation neutrons. It is important but difficult to correlate microstructure and corresponding hardening of the FM steels irradiated in the Swiss spallation neutron source (SINQ). Based on the results of the China low activation martensitic (CLAM) steel and comparison with other 9Cr FM steels irradiated in SINQ up to 20.8 dpa/328°C, total hardening of 9Cr FM steels was considered to be made up of helium bubbles, defect clusters/loops and α′ phase. Barrier strength of helium bubbles and clusters/loops in the CLAM specimens were obtained and investigated. Results show that the hardening of 9Cr FM specimens irradiated in SINQ increased with dose up to 20.8 dpa. After annealing at 600°C, the hardening of FM specimens approximately showed a linear relationship with dose. Annealing made the clusters/loops in FM steels almost completely disappear under all irradiation conditions. In contrast, annealing changed the bubble size slightly and changed the bubble density to some degree. Barrier strength of helium bubbles in the CLAM specimens approximately increased linearly with size below 1.79 nm. Barrier strength of clusters/loops in the CLAM specimens also increased from 0.23 to 0.48 when their size increased from 3.5 nm to 19.1 nm. The obtained barrier strength of helium bubbles and loops in the CLAM steel is useful to explain the composition of irradiation hardening of other FM specimens irradiated in SINQ.