Fracture Toughness of Irradiated Stainless Steel Alloys

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Abstract
The postirradiation fracture toughness responses of Types 316 and 304 stainless steel wrought products, cast CF8 stainless steel, and Type 308 stainless steel weld deposit were characterized at 427°C using JR curve techniques. Fast neutron irradiation of these alloys caused an order-of-magnitude reduction in Jc and two-orders-of-magnitude reduction in the tearing modulus at neutron exposures above 10 dpa, where radiation-induced losses in toughness appeared to saturate. Saturation Jc values for the wrought materials ranged from 28 to 31 kJ/m2; the weld exhibited a saturation level of 11 kJ/m2. Maximum allowable flaw sizes for highly irradiated stainless steel components stressed to 90% of the unirradiated yield strength are on the order of 3 cm for the wrought material and 1 cm for the weld. Electron fractographic examination revealed that irradiation displacement damage brought about a transition from ductile microvoid coalescence to channel fracture, associated with local separation along planar deformation bands. The lower saturation toughness value for the weld relative to that for the wrought products was attributed to local failure of ferrite particles ahead of the advancing crack that prematurely initiated channel fracture.