Nitride inclusions in titanium ingots: A study of possible sources in the production of magnesium-reduced sponge

An experimental program is being conducted to investigate possible sources of nitride defects in the production of magnesium-reduced titanium sponge, and to determine if such defects are capable of surviving consumable-electrode arc melting. Pilot plant experiments have shown that both air-burned and nitrided sponge pieces lose their color and texture when seeded into the process during both the reduction cycle and the distillation cycle. Such defect pieces act as nuclei for sponge growth during the reduction and are often found partially or completely clad by newly-formed sponge. Simulated air leaks during both reduction and distillation cycles and the incorporation of air-contaminated magnesium have been studied. The resulting sponge has a very uneven distribution of nitrogen. Some regions show nitrogen concentration as high as 12 pct and contain є Ti₂N, or Δ TiN, or both. Alpha-stabilized inclusions have been found in ingots produced from sponge made from dross-contaminated magnesium and from sponge contaminated by air during vacuum distillation. Although the nitride in the sponge is in the form of a fine powder, powder pockets, or soft friable lumps, it is capable of surviving single and in some cases, double arc melting. A theory of the mechanism of survival of these defects is proposed.