Fracture mechanics evaluations of neutron irradiated Type 321 austenitic steel

Abstract

The effects of fast reactor irradiation at temperatures of ~ 230° C and ~ 400° C on the fracture toughness and associated strength changes, induced in solution treated Type 321 stainless steel have been characterised using instrumented impact test procedures. The studies cover irradiation exposures in the range 16 to 43 displacements per atom (dpa) and test temperatures of 23-500° C. Irradiation results in significant but not catastrophic reductions in fracture toughness, together with radiation hardening effects. Both the dose and test temperature dependence of the toughness changes are sensitive to irradiation temperature. Thus, whilst maximum toughness loss occurs at or below 16 dpa for both irradiation temperatures, the 400° C-irradiation condition is associated with subsequent saturation of the toughness change, whereas for 230° C-irradiation measurable but low on-going toughness degradation occurs up to 43 dpa. The fracture toughness characteristics correlate with fractographic observations which demonstrate retention of a predominantly ductile fracture mode after irradiation, but with dramatic refinement in the scale of microvoid coalescence associated with TiC precipitates. It is suggested that the fracture mechanism after irradiation is controlled primarily by the irradiation-induced precipitate distribution, and furthermore, that the maintenance of ductile fracture, and hence good toughness, up to high irradiation damage levels is a consequence of inhibition of incipient channel fracture processes by the TiC particles. The application of general yield fracture mechanics to calculate critical defect sizes for unstable fracture in fast reactor wrappers is illustrated. These assessments demonstrate the importance of considering net section yield as an alternative failure criterion in thin section components. Finally, the use of empirical equations as a design philosophy to predict irradiation-induced toughness changes is briefly considered. © 1986.