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.