Mechanical properties of a forged ferritic steel vessel shell containinghydrogen flakes
Résumé
During the 2012 outage of the Doel 3 and Tihange 2 nuclear power plants (NPP), in-service inspectionsrevealed the existence of numerous quasi-laminar indications in the reactor pressure vessel(RPV). The indications were quickly identified as being hydrogen flakes formed in ghost lines at themanufacturing stage.Independently of the Doel 3 and Tihange 2 NPP utility (1), IRSN conducted a test program aiming atverifying that the presence of hydrogen flakes is not symptomatic of a weaker material. AREVA NPprovided the investigated material, namely two blocks of a steam generator vessel shell in 18MND5steel, so called VB395. This shell was rejected because of an incident which occurred during the degassingheat treatment. One block was a sound block without flake - the reference block - and thesecond block included a high density of hydrogen flakes. Those blocks were provided after quenching,tempering and simulated Post Weld heat treatment.Tensile tests and fracture toughness tests were performed in the ductile-to-brittle transition range aswell as in the ductile range of the materials. In the flaked block, different regions were identified andcharacterized material without flake nearby the flaked region, material between the hydrogen flakesand material in a large zone within the flaked region. Particular attention was paid to the sampling sothat the quenching effect related to the vessel shell thickness is the same for all the specimens. Torefine the interpretation of the test results, chemical composition behind the crack front of the specimenswas measured after fractographic examinations of the broken surfaces.