Removal of Oxygen from Commercially Pure Titanium Melt via a Two-Step Plasma-Arc Melting Process Using Hydrogen

Abstract

Oxygen is one of the principal impurities in off-grade Ti sponges and Ti scraps. To promote the effective use of these low-cost materials, developing reliable technologies for removing oxygen from Ti is essential. This study investigated the removal of oxygen from commercially pure Ti (CP Ti, Gr. 2) with an initial oxygen content of 0.127 mass% using a two-step plasma-arc melting process consisting of hydrogen plasma-arc melting and subsequent Ar plasma-arc melting. In the first step, plasma-arc melting under an Ar-H₂ atmosphere with an H₂ partial pressure of 0.5 atm resulted in dissolved hydrogen content of 2 mass% in the subsurface region of the Ti melt. This high hydrogen content could be attributed to the high hydrogen potential of the atomic hydrogen (H) gas generated in the plasma. Second, Ar plasma-arc melting at a plasma current of 300 A reduced the oxygen content in the subsurface region of the Ti melt to 0.034 mass%. When the plasma current was increased to 600 A, the oxygen content decreased to approximately 0.05 mass%. Although the reduction was less pronounced than that observed at 300 A, the oxygen content decreased more uniformly over a deeper region of the melt. Thermodynamic considerations suggest that the dissolved hydrogen introduced during the first step functioned as a deoxidizer, with the deoxidation driven by the different water vapor partial pressures between the first and second steps. This study clarifies the underlying deoxidation mechanism and the influence of melting conditions on oxygen removal.