Full-Scale Test on an Aged Cast Duplex Stainless Steel Lateral Connection: Results and Analysis

Abstract

Some components (elbows, pump casings and lateral connections) of the primary loop of French PWRs are made of static cast duplex stainless steels. This kind of steel may age even at relatively low temperatures (in the temperature range of PWR service conditions), depending on the material composition. An important consequence of this ageing process is the decrease in the ductility and fracture toughness of the material. It is feared that an embrittlement, associated with the occurrence of casting defects, may increase the risk of failure. Therefore, an extensive programme has been launched by EDF in co-operation with Framatome ANP, in order to determine acceptability criteria for operating cast stainless steel components. This programme relies on a large R&D effort, involving metallurgical studies, large-scale experiments, development of specific finite element tools and J-estimation schemes, and research of methods to assess the ageing state of in-service components. This paper presents the main characteristics and results of an experiment conducted on an aged cast 45 degree lateral connection. This connection contained a machined notch at the acute corner and was tested under internal pressure. The chemical composition was chosen to obtain a fast thermal ageing and low fracture toughness properties. During the test, the defect initiated and grew subsequently by ductile tearing. The test showed that it was possible to obtain a significant amount of stable crack growth (about 2.5 mm) despite the low toughness properties of the aged material. The pressure reached at the end of the test was about twice the in-service pressure. A detailed fracture mechanics analysis, based on finite element calculations, was performed. These calculations fairly simulated the overall behaviour of the tested structure, gave a conservative prediction of the crack initiation pressure and well predicted the crack size associated with the maximum pressure. These tests and their detailed analyses contribute to validate and justify the methodology used in the integrity assessment of in-service cast duplex stainless steel components.