French investigation of a new V-4Cr-4Ti grade: CEA-J57 - Fabrication and microstructure

In the scope of the development of structural materials for application in future fast neutron fission reactors, a V-4Cr-4Ti alloy, which is the reference alloy for the first wall/blanket in lithium fusion reactor, was fabricated. After vacuum arc melting of the starting ingot of 30 kg, 7 mm-thick plates were processed by forging and cold rolling down to 54% thickness reduction with intermediate heat treatment at 1050 °C. The pollution by interstitial impurities such as O, N and C was well controlled and was quite comparable to previous grades by US and Japan - O: 290 mass ppm, C: 70 mass ppm, N: 110 mass ppm. An additional cold-rolling was achieved on a small specimen to get a reduction in thickness of 87%. The recrystallization was investigated through heat treatments between 600 °C and 1200 °C. Hardness measurement on annealed samples suggested the recrystallization to occur around 800 °C for 54% thickness reduction and at a lower temperature for 87% thickness reduction. The minimum hardness was obtained after annealing at 950-1000 °C, the microstructure being fully recrystallized and the hardness isotropic. Above this temperature, hardness increase was significant. Large Ti-rich precipitates (500 nm) formed during forging and oriented along the rolling direction were easily observed but, on the contrary to previous studies, no small Ti-(O,C) precipitates could be detected.