Furnace Atmospheres for Metal Processing ApplicationsMr. Tom Phillips, Air Products Inc.
The focus of this presentation will be in the following three areas:
1) Traditional metal processing atmospheres
2) Now metal processing atmospheres
3) Troubleshooting atmosphere related metal processing problems
Most furnace atmospheres are based on bulk Nitrogen and Oxygen. The metal processing types using atmospheres are: Annealing, Brazing, Neutral Hardening, Carburizing, Sintering, and CVD and PVD coating application. Furnace atmospheres belong to one of three classes: Protective, Reactive, and Vacuum (or lack of atmosphere). The function of these atmospheres is to:
1) Protect against oxidation
2) Aid in Reduction
3) Remove organic vapors
4) Provide a controlled amount of carbon
5) Transfer heat.
Specific constituents of atmospheres play the following roles:
Nitrogen - inert, can nitride stainless, titanium, or tantalum.
Hydrogen - reducing, decarburizing, can hydride aluminum and titanium, high thermal transfer
Carbon Monoxide - carburizing, reducing
Carbon Dioxide - decarburizing, oxidizing
Oxygen - oxidizing
Water vapor - oxidizing, decarburizing
Argon - inert
Cx Hy - carburizing and provides hydrogen source
Helium - inert, high thermal transfer
Amonia - nitriding
In the oxidation-reduction reactions of most metal-metal oxides, oxygen is removed from the oxide by hydrogen to form water vapor. This reaction is completely reversible, and its direction is determined by the dew point of the atmosphere.
Traditional atmospheres are based on Industrial gas (mostly methane). The common ones are called 'Exothermic", "Endothermic", and "Cracked ammonia" (which is ammonia based). Exothermic atmosphere is traditionally the cheapest to produce. It is made by partially combusting methane in air. Once initiated, it is self-sustaining (eg. exothermic) and produces heat. The typical resulting atmosphere is about 70-98% Nitrogen, 2-20% Hydrogen, 2-20% Carbon Monoxide and 1-6% Carbon dioxide.. It is used primarily as a protective atmosphere. Endothermic is produced by heating industrial gas (methane) over a nickel catalyst in air. The typical resulting atmosphere is 40% Nitrogen, 40% Hydrogen, and 20% Carbon monoxide. Heat must be constantly supplied and the output monitored to maintain control of this atmosphere. Higher in CO, this is often used for carburizing. Ammonia is cracked into 75% Hydrogen 25% Nitrogen by heating over a catalyst. This atmosphere is very dry (low dew point), and used to promote sintering of many metals.
These "traditional" atmospheres can be replaced by "blends" of "pure" gasses which are produced by various means to ensure high purity. Distillation of air, pressure screen absorption, membrane separation, and other methods are used. "Cracked ammonia" is the most direct to duplicate as a mixture of pure Hydrogen and pure Nitrogen. Usually the Hydrogen level is kept lower than the natural cracked product because of the relatively high cost of the "pure hydrogen" used for the mixture. "Lean Exothermic" atmosphere is usually replaced by 1-3% Hydrogen in Nitrogen. This renders a very useful protective atmosphere which is non-flammable. "Rich Exothermic" atmosphere is usually replaced with 4-19% hydrogen in Nitrogen with small additions of methanol or hydrocarbon to create the desired carbon potential.
The primary advantages of "mixed" gasses are: Consistent Quality, Consistent Purity, Flexibility of equipment, Better efficiency (gas does not need to be generated at a fixed generation level whether needed or not), and improved safety. The primary disadvantage is cost per cubic foot. This disadvantage can actually be a savings however if a generator is forcing many cubic feet to be wasted.
Common problems with furnace atmospheres include:
Sooting (improper de-lubrication)
Oxidation / Discoloration
Decarburization (when not desired)
Carburization (when not desire)
Undersintering
Dimension control
Carbon control
Composition of metal and microstructure
By judiciously injecting differing gas mixtures at differing points in continuous furnaces, many of these problems can be overcome which is what is meant by the "flexibility" of the mixed gas systems.
Finally here are some troubleshooting tips:
Parts are discolored which could be due to an air leak or water leak in the cooling zone.
To determine an air or water leak.
Reduce the temperature of the furnace main chamber to 1900F.
Run a strip of Steel and a strip of Copper through.
If an air leak exists, both the steel and copper will oxidize.
If a water leak exists, the steel will oxidize, but the copper will not.
If an air leak has been determined, look at the color of the oxidation on the steel. If the color is "straw" to "brown" the leak is at the exit end of the furnace. If the color is grey, it is in the hot zone. If parts come out with a "frosted" finish, oxidation is occurring in the front (or preheat) end of the furnace.