Tungsten wire production

Tungsten filament production mostly uses ammonium paratungstate (APT) as raw material. The general process involves calcining APT in air at around 500°C to produce tungsten trioxide, or slightly reducing it in hydrogen at around 450°C to produce blue tungsten oxide. Tungsten filaments for incandescent lamps require the addition of small amounts of potassium oxide, silicon dioxide, and aluminum oxide to the tungsten trioxide or blue tungsten oxide; the total amount of these three should not exceed 1%.

The doped tungsten oxide is then reduced to metallic tungsten powder using hydrogen. The reduction process generally involves two steps: first, reduction to tungsten dioxide (brown tungsten oxide) at around 630°C, and second, reduction to metallic tungsten powder at around 820°C. The purpose of these two reduction steps is to ensure the added potassium is fully utilized and to control the powder particle size. The resulting doped tungsten powder is then pressed into long, thin rectangular strips in a specially designed mold. A tungsten bar is sintered by passing an electric current through hydrogen gas and heating it using self-resistance (reaching temperatures of approximately 3000℃). After sintering, the density of the tungsten bar can reach over 85% of the theoretical value. This tungsten bar can then be forged into tungsten rods with a diameter of approximately 3mm using a rotary forging method, and further processed into tungsten wires of various thicknesses using a die-drawing method.

When the diameter of the tungsten wire reaches the micrometer level, it is difficult to accurately measure its diameter using conventional calipers. Therefore, internationally, the thickness of tungsten wires with a diameter of less than 0.2mm is usually expressed by the weight of a 200mm long section of the wire.