Role and significance of source hardening in radiation embrittlement of iron and ferritic steels

Radiation effects on ferritic steels used for pressure vessels and pure iron are investigated to examine the role of the source hardening term responsible for the yield point phenomena and dynamic strain-aging (DSA). The majority of the radiation hardening stems from friction hardening, and the source hardening term decreased with exposure to neutron radiation apparently due to the interaction of the interstitial impurities with radiation produced defects. This decrease in the source hardening suppressed DSA which in turn led to increased ductility with a simultaneous increase in the strength in the temperature range of DSA in the unirradiated condition. While the source hardening term was evaluated from an extrapolation of the work-hardening region to the elastic line for the ferritic steels, the grain-size variation of the yield strength in pure iron allowed a direct evaluation and demonstrated their equivalence. The influence of low-energy (Cd-cutoff) neutrons was studied by comparing radiation effects in specimens with and without Cd-wrapping. Inclusion of thermal neutrons along with fast resulted in a small decrease in the source hardening with a slight increase in the friction hardening which revealed a critical grain size below which exposure to total (fast and thermal) neutron spectrum resulted in a slight reduction in the yield stress compared to the exposure to only fast neutrons. This grain-size effect is shown to be in line with known radiation effects on friction and source hardening terms along with the observation that low-energy neutrons have a non-negligible effect on the mechanical properties of steels. In ferritic steels, however, despite their small grain size, exposure to total neutron spectrum yielded higher strengths than exposure to only fast neutrons. (orig.)