Ageing of UNS6625 Grade 2 Alloy and Its Welds for Steam Generator Applications

No data were existing concerning the fracture toughness and crack propagation properties of UNS6625 grade 2 alloy or 625 weld metal after long thermal exposures at 520°C. To have a realistic idea of the worst, ageing was performed on base metal and Gas Tungsten Arc (GTA) weld metal up to 10 000 h at 520°C and 700°C. Structures and fracture features examinations, Charpy tests, fracture toughness (20, 520, 700°C) and fatigue crack growth tests (520°C) were performed. The drastic drop of E_CVN after ageing at 520°C corresponds to a K_1J fall in the same order for the base metal and the joint (≤ 100 MPa $m$⁠) in relationship with the development level of the γ″ Ni₃Nb phase with a ductile rupture feature. In comparison, the fracture toughness curves at room temperature of the 700°C ageings show an asymptotic trend beyond 6000 h, with K_1J ≥ 130 MPa $m$ after 10 000 h and a brittle/ductile rupture feature (δ Ni₃Nb phase +carbides). The J/Δ 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 after prolonged use of UNS6625 base metal and its welds at 520°C of pressure shell materials is satisfactory regarding an in service risk of brittle rupture.