EFFECTS OF CYCLIC LOADING, DYNAMIC STRAIN-AGING AND NEUTRON IRRADIATION ON FRACTURE BEHAVIOR OF A516 GRADE 70 AND OTHER STEELS

摘要

Ferritic steels commonly used for pressure vessels and reactor supports in light water reactors (LWRs) are known to exhibit radiation embrittlement in terms of decreased toughness and increased DBTT following exposure to neutron radiation. Recent work indicated decreased toughness during reverse-cyclic loading that has implications on reliability of these structures under seismic loading conditions. Moreover, dynamic strain aging (DSA) results in decreased ductility and toughness. We summarize some of our work on these aspects along with synergistic effects, of interstitial impurity atoms (IIAs) and radiation induced point defects that result in interesting beneficial effects of radiation exposure at appropriate temperature and strain-rate conditions. Radiation-defect interactions were investigated on pure iron, Si-killed mild steel, A516 and reactor vessel steels (A533B). While dips in fracture toughness are observed in A533B steel in the DSA region, A516 steel exhibited at best a plateau. Load-displacement curves during J1c tests on CT specimens did show load drops in the DSA regime. The effect of load ratio (R) on J versus load-line displacement curves for A516 steel indicated decreased J1c as load ratio is decreased. The hardness and the ball-indentation tests were performed to characterize the strain hardening at the crack tip. In addition, studies on fast vs total (thermal + fast) neutron spectra revealed unexpected results due to the influence of radiation exposure on source hardening component of the yield stress; grain-size of pure iron plays a significant role in these effects.