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Furnace Atmospheres
Furnace atmospheres are important because of the potential impact on the process. Many different "atmospheres" are available to use, ranging from air, to pure gases, to combinations of gasses, to vacuum, or the removal of most of the "atmosphere". The process requirements will generally drive the type of atmosphere, and thus type of furnace construction used. The following will give examples of the types of atmospheres and where they may be used. Please note that the list is not all inclusive, but should give an idea of how many of the more common types of atmospheres are used.. 

Air is very common to use in furnaces where either the amount of oxidation is acceptable, or in cases where the material being heated will not oxidize any further. It is probably the most common atmosphere. When a furnace is designed to operate only in air, the construction does not have to be gas tight, and hence the construction is generally less expensive than for an equivalent "Protective Atmosphere" furnace where air is not the primary atmosphere.

Endothermic gas in created by mixing a small amount of air with natural gas (or other hydrocarbon gas) and heating it to a temperature (typically 1850°F / 1000°C) in the presence of a catalyst to break down the complex hydrocarbon into Hydrogen, Carbon Monoxide, Carbon Dioxide, Nitrogen, and some moisture (H2O). Typically the gas has a composition of 27-37% H2, 16-20% CO, a small amount of CO2 (0-2%) and the balance N2. The mixture that too lean (deficient in Oxygen) to burn, and thus must be heated in order for the reaction to take place. For natural gas the ratio of Air to Natural gas is in the range of 2.5:1 to 3.5:1. The most common use of this atmosphere is in Heat Treating of ferrous parts to prevent oxidation. Endothermic is also a "carrier" gas that is used in combination with natural gas or other hydrocarbon source to be able to proved a carbon rich, carburizing atmosphere. Ammonia can also be added to provide a nascent nitrogen source for carbonitriding. Nascent nitrogen is present when ammonia breaks down. It is the atomic form, very reactive, and quickly either bonds to another nitrogen atom, or to a ferrous part, forming a Iron Nitride molecule on the surface.

Exothermic gas is produced over a wider range than Endothermic. As the name implies, this gas is created by the Exothermic reaction of burning natural gas (or similar hydrocarbon gas) with air at a ratio can sustain combustion. As the mixture burns, it gives off heat. Using Natural Gas as the hydrocarbon results in a typical gas composition of 2-15% H2, 2-10% CO, 5-11%CO2 and the balance N2. The mixture is ignited and burns giving off heat in an exothermic. For natural gas the ratio of Air to Natural gas is in the range of 6:1 to 10.5:1. As the resulting gas has a fairly high content of water (H2O) from the combustion process, dryers are sometime added after the Exothermic Generator to reduce to moisture content.

Dissociated Ammonia
Ammonia gas (NH3) is heated to approximately 1650°F / 900°C as it is passed over a catalyst (iron oxide coated refractory) in order to produce 25% N2 and 75% H2.

For pure hydrogen gas atmospheres, Hydrogen is usually purchased from a gas supplier in bulk and delivered to the furnace. being very flammable, caution must be used in handling.

Nitrogen is an inert atmosphere that is generally delivered in bulk by a gas supplier. While inert, standard commercial grades do contain some oxygen, and this oxygen combined with oxygen on the part surface, will cause some oxidation the work piece. It is often "Mixed" on site with Hydrogen or other reducing gas to eliminate this oxidation. 

Argon is another "inert" gas, similar to Nitrogen, but generally argon will have a lower "tramp" oxygen content. Argon is heavier than air and thus is also used as a blanket cover gas where "gravity" can be used to help hold it in place.

Vacuum conditions are not really an atmosphere, but rather a lack of one. When Vacuum is used the level of vacuum (less and less atmosphere as the vacuum level increases) is the key property. At high vacuum levels and temperatures, even metallic elements can become gasses. In high vacuum levels, there can be a strong pull to remove the surface atoms from the work piece. This is measured in partial pressures and as in chemical reactions, the system tries to become balanced. If the vacuum level is maintained, then any products that are evaporated will be removed from the vacuum vessel by the pumps (and potentially contaminate the pump system).

Water vapor can be used in some industrial processes. Below 1300°F / 700°C Copper can be annealed in a steam atmosphere. Tempering ferrous material in steam can create a "blue" oxide that can protect against rusting, gun barrels use this process. One must take into account the condensation of the steam if temperatures are lowered.



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