The effects of helium on the embrittlement and hardening of boron doped EUROFER97 steels

The role of helium in a process of embrittlement and hardening of RAFM steels was investigated in EUROFER97 based experimental heats, ADS2, ADS3 and ADS4, that were doped with different contents of natural B and the separated 10B-isotope (0.008–0.112 wt.%). The neutron irradiation of the boron doped and the reference RAFM steels was performed in the Petten High Flux Reactor up to 16.3 dpa at different temperatures between 250 and 450 °C. The embrittlement behaviour and hardening was investigated by instrumented Charpy-V tests with subsize specimens. Irradiation lead to generation of 84, 432 and 5580 appm He in ADS2, ADS3 and ADS4 steels, respectively. At irradiation temperatures Tirr ≤ 350 °C the boron doped steals show progressive embrittlement and reduction of toughness with increasing helium amount. The analysis of the hardening vs. embrittlement behaviour at Tirr ≤ 350 °C reveals hardening nature of embrittlement for helium contents up to 84 appm and indicates the possible existence of additional non-hardening embrittlement mechanisms beyond the helium-induced hardening at helium contents of 432 appm. At Tirr = 450 °C the investigated steels exhibit non-hardening embrittlement for helium contents up to 432 appm. The heat with 5580 appm helium showed brittle behaviour at all irradiation temperatures, most probably due to non-homogeneous helium distribution.