Behavior of duplex stainless steel casting defects under mechanical loadings

Several components in the primary circuit of pressurized water reactors are made of cast duplex stainless steels. This material contains small casting defects, mainly shrinkage cavities, due to the manufacturing process. In s afety analyses, the structural integrity of the components is studied under the most severe assumptions: presence of a large defect, accidental loadin gs and end-of-life material properties accounting for its thermal aging emb rittlement at the service temperature. The casting defects are idealized as semi-circular surface cracks or notches that have envelope dimensions. In order to assess the real severity of the casting defects under mechanical l oadings, an experimental program was carried out. It consisted of testing, under both cyclic and monotonic solicitations, three-point bend specimens c ontaining either a natural defect (in the form of a localized cluster of ca vities) or a machined notch having the dimensions of the cluster's envelope . The results show that shrinkage cavities are far less harmful than envelo pe notches thanks to the metal bridges between cavities. Under fatigue load ings, the generalized initiation of a cluster of cavities (defined when the cluster becomes a crack of the same global size) is reached for a number o f cycles that is much higher than the one leading to the initiation of a no tch. In the case of monotonic loadings, specimens with casting defects offe r a very high resistance to ductile tearing. The tests are analyzed in orde r to develop a method that takes into account the behavior of casting defec ts in a more realistic fashion than by an envelope crack. Various approache s are investigated, including the search of equivalent defects or of criter ia based on continuum mechanics concepts, and compared with literature data . This study shows the conservatism of current safety analyses in modeling casting defects by envelope semi-elliptical cracks and contributes to the d evelopment of alternative approaches. (C) 2000 Elsevier Science S.A. All ri ghts reserved.