Numéro : 2432 - Year : 2004
Ageing at 520° c of the usns6652 grade 2 and its welding for steam generator
François CORTIAL, DCN Propulsion - CESMAN - La Montagne (France)
Alloy UNS6625 grade 2 has been choiced by DCN Propulsion as the one material to make a steam generator working in severe conditions: high number of cycles, 60 bar max., water-steam-gas combustion environment, 520°C (pressure shell), in service period 18000 h, in relationship with both its good characteristics for the application: in temperature tensile properties, creep resistance, corrosion behaviour and the knowledge we had of its working processes and particularly the arc welding. No data were existing concerning the fracture toughness properties of 625 alloy base metal or 625 weld metal after long thermal exposures at temperatures less than 550°C (renowned as harmless). In reality Charpy impact energy is used as a toughness indicator but it is of no significance to conduct any rupture behaviour analysis for pressure vessels. Owing to time and cost constraints and in the aim to have a realistic idea of the worst we have performed ageing treatments on base metal and TIG weld metal (Tungsten Inert Gas) up to 10000 h at 520°C and 700°C that is known to be the most detrimental temperature for toughness. The drastic drop of EKV after ageing at 520° C corresponds to a K1J fall in the same order for the base metal and the joint (100 MPa ) in relationship with the development level of the "Ni3Nb phase with a ductile rupture feature. In comparison, the fracture toughness curves at room temperature of the 700°C ageing show an asymptotic trend since 6000 h, with KlJ 130 MPa after 10000 h and a brittle/ductile rupture feature ( Ni3Nb phase + carbides). The J/a curves at 520°C of the 520°C aged states reveal a saturation effect. Finally, in spite of the metallurgical changes that take place, the behaviour at 520°C of pressure shell materials give satisfactory guarantee regarding an in service risk of brittle rupture.
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