Tungsten Carburization

Conventional production (Carburization)

Tungsten powder is reacted with carbon at temperatures between 1300 and 1700℃ in a hydrogen atmosphere. The average particle size and size distribution of the starting W powder determine the particle size and size distribution of WC. Only a slight increase in size occurs due to the change in density from 19.3 g•cm-3 (W) to 15.7 g•cm-3(WC). In addition a certain agglomeration (local sintering), always occurs, especially at higher temperatures.

In any case, carbon black is always more impure than tungsten powder, particularly in regard to the alkali metals, Ca, Si, Mg, and S. Part of these trace elements are volatilized during carburization (the percentage depending on temperature). This is why finer WC powders (lower carburization temperature) are usually more impure than coarser powders.

The two components (W and C) must be blended thoroughly prior to carburization. This is done in different types of equipment, like V or double cone blenders, mixing ball mills, or high-energy mixers. An even blend is of importance because, during carburization, carbon atoms can only move via diffusion or as methane molecules over very short distances. Pelletizing or compacting enhances diffusion and increases the furnace capacity.

In producing submicron WC powder, small amounts of grain-growth-inhibiting substances (inhibiting WC grain growth during carburization and especially during hardmetal sintering) are sometimes added to the W+C charge prior to blending. The usual chromium or vanadium are added either as oxide or as carbide. The addition of oxide must be considered in calculating the carbon balance, because additional carbon will be consumed for reduction and carburization of the metal oxide.

Carbon vessels are filled with the powder blend. Depending on the furnace type, either boats or boxes made of dense graphite are used. The vessels are covered with a graphite cover to avoid any contamination, and pass trough the furnace.

Push-type furnaces equipped with heated tubes or channels are mainly used. Construction material for the tubes and channels is either alumina or graphite, and the heating elements are made of molybdenum wire or graphite. Both materials have advantages, but also significant disadvantages which shorten their lifetime. The advantage of graphite is its high chemical stability against trace elements which evaporize during carburization; its disadvantage is the slow but constant reaction with hydrogen and water vapor. In contrast, alumina is very stable against hydrogen and water vapor, but reacts with alkali metals (evaporating from the powder blend), which finally weaken the ceramic by decreasing the melting point.

The furnace tubes and the heating elements are swept with dry hydrogen, which acts as protective atmosphere for the product as well as for the sensitive furnace parts. Moreover, it carries away a certain amount of the impurities which evaporate from the product and leads to a purification. Finally, it favors the carburization reaction by intermediately forming methane molecules. The latter is of special importance in carburizing coarse tungsten powder.

Carburization temperatures vary between 1300 and 1700℃, mainly depending on the average particle size of the powder. The smaller the particle size, the lower can the temperature be maintained. Lower carburization temperature leads to a higher degree of lattice defects, and consequently to a higher reactivity during sintering, which is undesirable especially for submicron grades. On the other hand, very fine powders tend to grow already during carburization at higher temperature. Therefore, a compromise has to be made in carburizing submicron powder.

Usual retention times in the hot furnace zone are between 1 and 2 hours. The exothermic heat of reaction can be used in the rear part of the hot zone to maintain the temperature without any heating.

After heating is completed, the vessels pass a cooling zone, still under hydrogen, and are discharged at room temperature. More modern furnaces are equipped with locks, and charging and discharging is done automatically. By using locks, no air is allowed to enter during loading and unloading, thus avoiding reactions with oxygen or moisture. For WC powders with average particle sizes below 0.5um (ultrafine grades), special care must be taken due to their pyrophorictiy, and handling is commonly done under inert gas.

For submicron WC powders, an extended milling process is applied, especially in cases where the subsequent wet milling procedure for graded powder preparation is not very intense (attritor milling). The WC milling can be performed either in optimized ball mills (hardmetal balls; optimal ball milling conditions avoid any contamination from the steel walls and keep the abrasion of the hardmetal balls at a minimum) or in jet mills in comination with a sifter. The main reason for this milling is to destroy any coarser WC particle (<2um) which might be responsible for coarse WC crystals in the sintered structure. Furthermore, heterogeneous impurity particles (graphite from the carburizing vessel and Fe-Ni-Cr containing particles from the reduction boat scale) are finely divided and distributed. This type of milling does not effectively influence the WC average particle size.

The physical parameters are not only responsible for the microstructure after sintering, but also for the shrinkage behavior during the sintering period. Therefore, they have to be kept constant within very close limits.

By far, the biggest percentage of WC is produced by this method.

If you have any interest in tungsten carbide powder, please feel free to contact us by email: sales@chinatungsten.com or by telephone: +86 592 5129696.

 

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