Bending Test on an Aged Cast Duplex Stainless Steel Pipe Containing a Crack in the Heat Affected Zone: Results and Analysis Available to Purchase

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. In order to build the primary loops, these components are welded and the behaviour of the weld heat affected zone (HAZ) is not well known. So a specific program was launched on this topic, involving metallurgical studies, fracture mechanics tests, medium-scale experiments, and finite element analyses. This paper presents the main characteristics and results of an experiment conducted on a 6^″ aged cast pipe. This pipe contained a machined notch in the heat affected zone of a butt-weld and was tested under four-point bending at 300°C. The chemical composition of the steel 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 4 mm) despite the low toughness properties of the aged material. 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 applied bending moment. 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.