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

Abstract

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 30kg, 7mm-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: 290massppm, C: 70massppm, N: 110massppm. 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 (500nm) 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.