Production Sintering Practices Sintering Equipment

Self-resistance heating has been used extensively for tungsten because of the extremely high temperatures required for practical sintering. Typically, rods with cross sections less than 645 mm2 (1 in.2) are heated to 3000 °C (5430 °F) or higher. Densification to greater than 90% of theoretical can occur in less than 2h.

Sintering is done under a hydrogen atmosphere in a water-cooled, copper-jacketed enclosure. Tungsten contacts are required, and one contact must be able to move as the bar shrinks. Bar cross sections must be uniform or sintering temperature will vary along the bar length. Temperatures and densities are always low within several inches of the clamped ends because of the heatsink effect of the contacts. Sintering of molybdenum by this method has been replaced by resistance and induction heated furnaces.

Resistance element heating furnaces are frequently used for hydrogen sintering of molybdenum and tungsten at 1800 °C (3270 °F). Furnaces having uniformly hot zones up to 1.2 m (4 ft) long and 0.14 m2 (1.5 ft2) are available. Loads of up to several thousand pounds can be sintered.

A typical furnace consists of water-cooled entrance and exit zones and a central zone of alumina refractory brick that supports molybdenum heating element rods. Furnace loads are supported on molybdenum slabs that are conveyed through the entrance and exit zones by powered rolls. The load is moved through the hot zone by a walking beam that oscillates above and below the hearth and that carries the load forward during the portion of the cycle that is above the hearth. Varying the frequency of oscillation varies the rate of forward movement of the parts in the furnace. Smaller furnaces with lighter loads can be operated by mechanically stoking a series of boats through the furnace.

Induction radiant heating is used when temperatures above 1800 °C (3270 °F) are required and also when very large (greater than 405 mm, or 16 in. diam) parts are involved. Molybdenum alloy billets weighing up to 4500 kg (10,000 lb) have been sintered with this type of heating mode.

A typical furnace consists of a large-diameter water-cooled chamber that contains a cylindrical susceptor ring of tungsten or molybdenum. The susceptor is inductively heated, and the compact is heated by radiation from the susceptor. Depending on the construction materials and loads, sintering temperatures ii 2400 °C (4350 °F) can be obtained.

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