Effects of Neutron Irradiation on Fatigue and Creep-Fatigue Crack Propagation in Type 316 Stainless Steel at 649°C

Abstract

The fatigue and creep-fatigue crack propagation performance of Type 316 stainless steel has been investigated following fast neutron (n) irradiation to fluence levels of 1.5 X 10 2 2 and ∼5 X 10 2 2 n/cm 2 (>0.1 MeV), ∼6 and 26 displacements per atom (dpa), respectively, at 649°C. The purpose of this work was to evaluate the effects of neutron fluence and temperature on the crack propagation resistance and failure mode of the steel. The results from fatigue tests of the annealed steel show that irradiation to both fluence levels at 649°C produced no significant effect on the crack propagation rate when compared with unirradiated steel tested at 649°C. For the 20 percent cold-worked steel, however, irradiation to the lower fluence level increased the crack propagation rate while at the higher fluence level the crack propagation rate was only slightly increased relative to the unirradiated material. For tests conducted using creep-fatigue cycling, the effect of a 1-min hold time at the maximum cyclic load was to produce a marked increase in the crack propagation rate of the annealed steel irradiated to the lower fluence level. At the higher fluence level, the inclusion of a 2-min hold time in the creep-fatigue cycle resulted in a further increase in the crack propagation rate. The effect of a 1-min hold time in the irradiated, 20 percent cold-worked steel was to significantly increase the crack propagation rates, particularly at the low fluence level at 649°C. Scanning electron microscope examination of the fracture surfaces of the tested specimens revealed that the failure mode of the specimens which exhibited increased crack propagation rates was primarily intergranular while a transgranular mode was observed for specimens with lower crack propagation rates. The results point toward a synergistic relationship between thermomechanical history, precipitate formation, and hold time effects as the responsible mechanism for the crack propagation performance.