Fatigue crack growth and fracture toughness properties of low fluence neutron-irradiated Type 316 and Type 304 stainless steels

Small compact-tension specimens of Type 316 plate and Type 304 forging have been irradiated in the High Flux Reactor (HFR) at Petten, The Netherlands, up to a fluence level of 2 x 10/sup 24/ neutrons (n) . m/sup -2/ (E > 0.1 MeV) at 573 K. Post-irradiation fatigue crack propagation tests and J-integral fracture toughness tests have been performed at the irradiation temperature. Additional tests were made at the higher temperatures of 723 and 823 K. The two materials exhibited almost identical crack growth curves after irradiation, despite the chemical and microstructural differences between the materials in the as-fabricated condition. The irradiation caused a slight increase of the resistance to crack growth under cyclic and monotonic loading at 573 K. This is attributed to irradiation hardening for displacement damage, the latter being observed in the form of the well-known black dots. The additional tests showed modest degradation of the resistance at 723 and 823 K, but the fatigue crack growth resistance decreased significantly under low frequency loading at 823 K because of helium-enhanced intergranular crack formation. The results are compared with fatigue and toughness curves for Type 304 and Type 316 plate and welds after similar irradiations in the HFR and in thermal material test reactors in the United Kingdom (UKMTR). The closely corresponding curves show the same type of results after the various irradiations. It is concluded that low fluence (less than 1 dpa, displacements per atom) irradiations at temperatures below 700 K, have a slight beneficial effect on the crack growth resistance of austenitic stainless steels. Nevertheless, the fatigue crack growth rates of Type 316 plate and Type 304 forging are still higher than for weld metal, although the toughness properties are better than for weld metal