Fracture Toughness of Aged Stainless Steel Primary Piping and Reactor Vessel Materials

The ductile fracture toughness behavior of FFTF primary piping and reactor vessel construction materials was characterized using the multiple-specimen J_R-curve technique before and after 10,000-hr thermal aging treatments. The test materials included Types 304 and 316 stainless steel (SS) and Types 308 and 16–8–2 SS welds. In the unaged condition, these alloys exhibited very high J_c initiation toughness and tearing modulus values at elevated temperatures (427–538°C). The fracture resistance for the 316 SS piping was found to be dependent on orientation; J_c values for the axial (C–L) direction were 60 to 70 percent lower than those for the circumferential (L–C) orientation. The lower fracture properties in the C–L orientation resulted from premature failure of stringers aligned in the axial direction. Thermal aging at 427° C caused no degradation in fracture resistance, while 482 and 566° C agings resulted in a modest 10 to 20 percent reduction in J_c for both base and weld metals. Residual toughness levels after aging are adequate for precluding any possibility of nonductile fracture. Hence, conventional stress and strain limits, such as those provided by the ASME Code, are sufficient to guard against ductile fracture for SS components that have accumulated 10,000-hr exposures at or below 566° C. Metallographic and fractographic examinations revealed that the degradation in fracture properties was associated with aging-induced second-phase precipitation.