Effect of Cyclic Loads on the Fracture Behavior ofStainless Steel Pipes with High and Low Sulfur Contents

摘要

During the Second Intemational Piping Integrity Research Group (IPIRG-2) program, it was found that sulfurcontent has a significant influence on fracture toughness properties of stainless steel pipe. Two heats of Type 304 stainlesssteel pipe (designated A8i and A8ii) had radically different J-R curves. The main difference in chemical compositionbetween the two heats was the sulfur content. For both heats, J-R curve testing was performed for a variety of load histories:quasi-static monotonic, dynamic monotonic, quasi-static cyclic (R =-0.3 and-1.0), and dynamic cyclic (R =-0.3) loading. Itwas found that regardless of load history, the J-R curves for the low sulfur heat (0.002 percent by weight) were about twice ashigh as the corresponding J-R curves for the higher sulfur heat (0.019 percent).Subsequent to the IPIRG-2 program, a sensitivity study was undertaken in which the effect of sulfur content on loadcarryingcapacity of stainless steel pipe was studied. During that study heats of stainless steel with varying amounts of sulfurwere found in the PIFRAC database. The sulfur contems varied from 0.002 percent to 0.030 percent (maximum allowablefor Type 304 stainless per the ASTM standard). A very consistent trend was observed when examining the fracturetoughness data as a function of sulfur content. The heats with sulfur contents less than 0.015 percent had J-R curves thatwere about twice as high as those with sulfur contents greater than 0.015 percent. Fracture analyses were then conducted fora series of sample pipe cases to see what effect these differences had on the load-carrying capacity of both through-wall andsurface-cracked pipe. It was found that the predicted load-carrying capacity of the higher sulfur (lower toughness) stainlesssteels were about 10 to 15 percent less than those for the lower sulfur (higher toughness) stainless steels.In addition to the above analyses, three cyclically-loaded pipe-system experiments were conducted during the IPIRGand Battelle Integrity of Nuclear Piping (BINP) programs. Each experiment involved one of the two heats of A8 stainless.The cyclic load histories for the three experiments were quite different, i.e., a single-frequency excitation and two verydistinct seismic histories. The key finding from these experiments was that the load history had minimal (10 to 15 percent)impact on the load-carrying capacity. This finding must be qualified, however, because this may not have been the case if alower toughness material, for which cyclic effects on toughness may be more pronounced, had been used.