Vacuum Ref 11

Vacuum is essentially a lack of atmosphere. It is used mainly for sintering stainless steels, tool steels, carbides, magnetic alloys, and metals such as titanium, zirconium, uranium, tantalum, and other refractory metals and compounds that react with hydrogen-, nitrogen-, and carbon-monoxide-bearing atmospheres.

Vacuums are being used increasingly for high-temperature sintering of conventional ferrous P/M parts. Under vacuum, care must be taken not to lower the pressure in the furnace below the vapor pressure of the constituents of the alloy to be sintered so that depletion does not occur.

Vacuum cannot be used in the conveyor furnaces conventionally used by the sintering industry. Currently, almost all vacuum sintering furnaces are batch types. Some continuous (compartmentalized) conveyor vacuum furnaces are being introduced to the P/M industry.

Green compacts of carbides, refractory metals, and other materials are sometimes heated under a protective or reducing atmosphere in a separate furnace to drive off volatile lubricants before entering the vacuum furnace for sintering. This minimizes the contamination of heater elements and heater element supports that are frequently associated with vacuum furnaces.

Nitrogen-Base Atmospheres (Ref 13, 14, 15, 16, 17, 18, 19, 20, 21, 22)

The main constituent of the recently developed nitrogen-base systems is molecular nitrogen. Molecular nitrogen is obtained from air, which consists of --78% nitrogen, 21% oxygen, 0.93% argon, 0.03% carbon dioxide, and small amounts of rare gases such as neon and helium.

Nitrogen is produced commercially by cryogenic and noncryogenic methods. Air is first filtered to remove particulates and is then compressed and passed through a heat exchanger for cooling to remove water and carbon dioxide. The air is then rapidly depressurized to an ultracold liquid that is distilled to separate out the high-purity molecular nitrogen gas. The gas is either piped directly to the sintering plant or liquefied for long-distance shipping. One cubic meter of liquid nitrogen equals 697 m3 of nitrogen gas.

Nitrogen is also produced in noncryogenic generators located at the sintering plant. In noncryogenic production, a stream of compressed air is passed through a bed of special adsorbent, such as a carbon molecular sieve maintained at ambient temperature. Oxygen, water vapor, and carbon dioxide are preferentially retained on the adsorbent, but nitrogen (with a small amount of oxygen) flows through it. As the adsorbent bed becomes nearly saturated with oxygen, water, and carbon dioxide, the system automatically switches to a second bed without interrupting nitrogen delivery.

The system regenerates the first bed by reducing the bed pressure and releasing the adsorbed bases. The regenerated bed is then ready to repeat the cycle. All adsorption and desorption activities occur at ambient temperatures rather than at the low temperatures used in cryogenic plants. To reduce the oxygen content of the nitrogen, the product stream with added hydrogen is passed through a catalytic deoxidizer and a dryer to produce high-purity nitrogen. Powder metallurgy fabricators with a nitrogen consumption greater than 100 nrVh (3500 ft Vh) could use an on-site noncryogenic plant for economic reasons. These plants have been introduced into the market only recently.

Whether produced cryogenically or noncryogenically, nitrogen is readily available and economical. Additionally, it provides a consistent source of high technical quality, high-purity product—maximum oxygen content is 10 ppm (typically 2 ppm) and dew point is -65 °C (-85 °F). Because nitrogen is molecular rather than atomic, it is essentially inert to steel and other compositions that are usually sintered in P/M plants.

Inert nitrogen has a density close to that of air (Table 3); consequently, it effectively prevents air from entering the furnace, thus protecting porous P/M parts from harmful contact with air during sintering. Assuming an adequately airtight furnace, an atmosphere of 100% nitrogen is suitable for sintering aluminum and some less critical ferrous and nonferrous metal parts. Nitrogen alone, however, will not reduce particle surface oxides or effectively control surface carbon of steel parts.

With small, controlled additions of active gases, nitrogen atmospheres can perform all functions required of sintering atmospheres. Furthermore, the amount and type of active ingredients can be varied to change the level of reactivity of the atmosphere. The most important active ingredient required for sintering of commonly produced P/M parts is hydrogen.

Systems that use low dew point hydrogen sources and that maintain other oxidants at low levels require only small amounts of reducing gas (hydrogen) to create an effective and efficient nitrogen-base atmosphere. Such atmospheres are potentially highly reducing to metal oxides. Typically, nitrogen-base systems consist of essentially inert nitrogen and small amounts of one or more active gaseous ingredients.

Conventional atmosphere generators are not required in such systems; instead, feedstock gases (nitrogen and active gases) are piped through a flow panel into the furnace. The gases entering the panel regulation system exit as the atmosphere-the metering panel becomes, in effect, an atmosphere generator.

These systems exhibit great flexibility by enabling the ratio and types of active ingredients to be varied easily to suite the various sintering requirements within the plant. Nitrogen systems can also provide different atmospheres in different sections of a furnace, as shown in Fig. 14. The volume output reduction for nitrogen is 100%, considerably more than for endo and exo atmospheres. Nitrogen-base systems contain three basic types of active ingredients, as discussed below.

Fig. 14 Schematic of nitrogen-base zoned atmosphere system. Atmosphere and temperature profiles corresponding to the nitrogen-base atmosphere system are shown.
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