Oxidation behavior of a V–4Cr–4Ti alloy during the commercial processing of thin-wall tubing

Because of the high solubility and mobility of oxygen in vanadium, composition control during the fabrication of thin (0.25 mm) wall tubing from vanadium alloys by cold drawing and annealing, presents a technological challenge. During intermediate annealing at 1000 °C in the 10−4 Torr vacuum regime, oxygen penetration into the tube wall is controlled by the development of a semi-protective surface oxide (linear-parabolic oxidation conditions); oxygen-hardened surface layers lead to a high incidence of surface cracking during the final stages of cold drawing. In the 10−5 Torr regime, under linear kinetic oxidation conditions, rapid oxygen penetration results in unacceptably high levels of oxygen pick-up (∼1500 wppm). In the 10−7 Torr vacuum regime, molecular impingement rates are reduced to the point where overall oxygen pick-up is reduced to <100 wppm. Improved cleaning/gettering procedures also restrict carbon and nitrogen pick-up to very low levels.