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
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.
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