2.3 Indirect Sintering.
The green compacts (no presintering necessary) are placed inside a cylindrical or basketlike heating element of the furnace (constructed of Mo or preferably W). In a radial direction to the outside, the furnace is adapted with radiation shields (inner shields made of W, out sheet made of Mo), which protect the furnace wall and concentrate the heat to the center. A vacuum system is necessary to empty the furnace prior to hydrogen flooding. Maximal dimension depends on furnace size. Compacts of and desired shape can be sintered. In order to achieve even shrinkage, the compacts have to be placed on green tungsten shims. A slow heating rate is essential; otherwise, surface densification will occur too early, not allowing the outgassing of the interior. Internal stress will be built up, resulting in cracks. This is particularly important for large parts, such as tungsten billets for forging or rolling. Holding times are essential. In practice, the sintering schedule is adjusted to the specific requirements. Proper furnace loading is also important, since the shielding of parts by others can lead to different densification rates and less uniformity.
Common sintering temperatures are between 2000 and 2700℃ (max). Sintering times of 8 to 24 hours, or even more, are common. Furnaces having uniform hot zones of up to 1.2m and 0.14m2 are available. Weights in the range of a thousand kg can be sintered in one batch. For very high loads, deformation of the part can take place due to gravity.
Advantages are: no loss of material, no dimensional restrictions, and high capacity. Disadvantages are: longer heating times, less purification, Iower efficiency of heating, and higher maintenance costs.
Please visit our website www.tungsten-powder.com to get more details.
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