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 iro n are investigated to examine the role of the source hardening term respons ible for the yield point phenomena and dynamic strain-aging (DSA). The majo rity of the radiation hardening stems from friction hardening, and the sour ce hardening term decreased with exposure to neutron radiation apparently d ue to the interaction of the interstitial impurities with radiation produce d defects. This decrease in the source hardening suppressed DSA which in tu rn led to increased ductility with a simultaneous increase in the strength in the temperature range of DSA in the unirradiated condition. While the so urce hardening term was evaluated from an extrapolation of the work-hardeni ng region to the elastic line for the ferritic steels, the grain-size varia tion of the yield strength in pure iron allowed a direct evaluation and dem onstrated their equivalence. The influence of low-energy (Cd-cutoff) neutro ns was studied by comparing radiation effects in specimens with and without Cd-wrapping. Inclusion of thermal neutrons along with fast resulted in a s mall decrease in the source hardening with a slight increase in the frictio n hardening which revealed a critical grain size below which exposure to to tal (fast and thermal) neutron spectrum resulted in a slight reduction in t he 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 neut rons have a non-negligible effect on the mechanical properties of steels. I n ferritic steels, however, despite their small grain size, exposure to tot al neutron spectrum yielded higher strengths than exposure to only fast neu trons. This behavior is consistent with the fact that the source hardening is small in these alloys and the radiation effect is due only to friction s tress. (C) 1999 Elsevier Science B.V. All rights reserved.