The Effect of Manufacturing Process on Residual Stresses of Pipe Bends

Feeder pipes in CANDU^® reactors are an integral part of the circulating heat transport system, connecting the in-reactor fuel channels with the primary heat transport pipes. The feeder pipes are 1.5” to 3.5” in nominal size and made out of SA-106 Grade B material. A typical CANDU 6 station has 760 feeder pipes, half of which are located at the fuel channel inlet and the other half are at the fuel channel outlet. In one of the CANDU 6 stations, 2 feeder bends were replaced due to through-wall cracks and 6 others were replaced due to partial cracks detected by non-destructive examination techniques. In all cases, cracks were axial in orientation and were either at the inside surface of the bend flanks (approximately 60° from the intrados symmetry plane) or at the outside surface of the bend extrados. Examination of cracked elbows and measurement of residual stresses indicated that residual stress had a significant role in these failures, since the cracks were associated with locations of high residual stress. Hence, a significant effort was spent to measure the distribution and magnitude of residual stresses at feeder pipe bends. Residual stress in feeder bends is inevitably introduced during the bending process and significantly affected by the manufacturing and heat treatment techniques. Among the techniques used, hot-forming, intrados heating (warm-bending) and compression boosting are investigated. In this paper, the effect of the manufacturing process on the residual stresses of pipe bends is discussed. It was found that, among the bend forming techniques used, the intrados heating (warm-bending) technique results in the highest residual stresses.