TUNGSTEN METAL POWDER PRODUCTION-PUSH-TYPE FURNACE-1

2. Push-Type Furnace

Metal-boats are charged with oxide to a height ranging from a few mm up to several cm and are pushed in stages through the furnace in corrosion-resistant steel tubes at specific time intervals. By introducing a new boat into the tube, the row in front is pushed forward by the length of a boat. Hydrogen in excess flows either co- or countercurrent to the tungsten flow direction. The hydrogen is not only responsible for the reduction process itself but serves also to remove the water vapor formed and also acts as protecting atmosphere in the cooling zone. The “wetted” hydrogen leaving the furnace is dried to a desired dew point and recycled to the furnace . As indicated, hydrogen having higher dew points can also be fed into the furnace.

Hydrogen has to be applied in large excess, which guarantees a fast flow over the powder layer. The excess depends on the desired grain size (smaller for coarse and higher for fine powder). The range is somewhere between 2.5 and 40 times stoichiometric.

Multitube furnaces (14 to 18 tubes arranged in two rows) are frequently in use today. The boat material, in most cases, is an iron alloy high in Ni and Cr (lnconel). More seldom, because of the high price, boats are made of TZM (molybdenum alloy with Ti, Zr, and C) or pure tungsten.

The big disadvantage of the iron alloys is that diffusion of the elements occurs into the contacting tungsten powder layer. In this respect, Ni is the most dangerous element although widely used. Ni rapidly diffuses over the tungsten grains, thereby weakening the surface of the bottom and wall of the boats. With time, a Ni, Fe, Cr, and W containing scale is formed. This scale sticks more or less firmly to the boat. After several travels through the furnace, it gets thicker and partly breaks off, contaminating heterogeneously the tungsten powder. Bigger-scale particles can be separated by the always applied screening process following the reduction, but the smaller particles remain in the tungsten powder. The higher the temperature and humidity, the more pronounced the scale formation. Cast alloy material (coarse microstructure) shows enhanced scale formation compared to boats made of rolled sheet. Alloys containing Co instead of Ni are more resistant, but the high price of Co makes them unacceptable for boats. Co containing alloys are only in use as tubes in rotary furnaces.

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