There are many types of atmosphere generators for a wide variety of purposes. The most common Atmosphere Generators used in the Heat Treating Industry are generally classified as ‘Exothermic’ or ‘Endothermic’. Some of the other types are also included here as they are used in some aspects of the heat treating industry.
An important aspect of the heat treatment of materials is the effect of the furnace atmosphere on the stock being heated. In most cases the need is to minimize or eliminate the undesirable effects of the furnaces gases, such as oxidation or decarburization. But, there are processes in which the interaction between the gases and the stock is the reason for the process, such as the carburizing of steel.
In direct fired furnaces only a limited control of furnace atmosphere is possible. This is adequate for some low-temperature heat treatment processes, like stress-relieving of low-carbon steels, an increasing number of heat-treatment processes are carried out in indirect-fired furnaces. In these the stock is separated from the heating gases by a muffle or radiant tubes and the working chamber is fed with a separately generated prepared atmosphere.
The terms ‘oxidizing’, ‘neutral’ and ‘reducing’ are often applied to furnace atmospheres. For more information on the actual reactions produced by heat treating, see Heat Treat Chemistry.
With the exception of a few metallurgical processes which require the provision of inert gasses from cylinders or the use of a vacuum, the vast majority of industrial protective atmospheres are derived from the combustion or reforming of fuel gas and contain some or all of the following constituents: nitrogen, carbon dioxide, carbon monoxide, hydrogen, water vapor and hydrocarbons. (There will also be sulphur dioxide if the fuel source is oil.)
Typical cylinder gases are pure hydrogen, argon and ammonia. Other gases may also be provided from cylinders if the process is very small. However, at some point it becomes inconvenient and expensive to buy gas in cylinders so atmosphere gases will be produced on site.
Exothermic Generators combust natural gas using either a very small amount of air (Rich) or a large amount of air (Lean) depending on what atmosphere is being generated. Exothermic Generators consist essentially of a pre-mix burner, combustion chamber, and atmosphere cooler.
Lean Exothermic gases contain N2, CO2 and H2O.
Rich Exothermic gases contain H2, CO, CO2 and CH4.
Rich Exothermic gases may be used ‘raw’ (direct) or ‘stripped’ (further processed) to concentrate the specific gas that is needed.
See the Gas Summary Chart for a listing of specific gases and processes.
The description of a typical Lindberg Generator:
Exothermic gas essentially composed of 12.0% carbon dioxide and balance nitrogen (typical analysis when using 1000 BTU/CF natural gas). May be operated slightly on the oxidizing side of perfect combustion (0.5, to 1.0%, oxygen max. ) without trace of combustibles. Operation on the rich side of perfect combustion yields no oxygen — Atmosphere on rich side of perfect combustion may be adjusted to yield properties desired, from no combustibles to 18%, combustibles (typical analysis – carbon dioxide 5.0%, carbon monoxide 10.5%. hydrogen 12.5%, methane 0.5%, and nitrogen 71.5%).
Endothermic Atmospheres are produced by the catalysed reforming of fuel gases with a small amount of air. Air and gas from a machine-premix system are fed into an externally heated nickel-chrome steel or refractory retort containing nickel-impregnated refractory as a catalyst. The generator temperature is normally between 1,000C and 1,150C. Normal air-gas ratios on natural gas are 2.4 : 1.
Carbon is deposited on the catalyst and needs to be burned off periodically by reducing the retort temperature below 700C and passing air through the retort. The burn-off period depends largely on the dew point of the process, but is typically about 3 weeks. After burn-off, the nickel catalyst is partially oxidized and needs to be reduced to metallic nickel by ‘conditioning’ or passing through a mixture of the correct air-gas ratio for a few hours.
Endothermic gases contain CO, H2, and N2, with trace amounts of H2O, CO2 and hydrocarbons. This process is sensitive to the composition of natural gas (feed stock). For gas carburizing processes, a few percent of natural gas is re-injected to increase carbon content.
See the Gas Summary Chart for specific gases and applications.
Endothermic Generator produces protective atmosphere for:
- Hardening of carbon and tool steels
- Annealing and normalizing of carbon steels
- Carbon restoration
Membrane generators may be used alone or in combination with combustion type generators. In the simplest term, they are filters.
Membranes separate gases by the principle of selective permeation across the membrane wall. For polymeric membranes, the rate of permeation of each gas is determined by its solubility in the membrane material, and the rate of diffusion through the molecular free volume in the membrane wall. Gases that exhibit high solubility in the membrane, and gases that are small in molecular size, permeate faster than larger, less soluble gases.
The ability of a membrane to separate two gases is determined by its “selectivity”, the ratio of permeabilities of the two gases. The higher the selectivity, the more efficient the separation and less energy is needed to run the system.
The Lindberg HYAM® system produces atmosphere by dissociating anhydrous ammonia into components of 75% hydrogen and 25% nitrogen by passing the ammonia over a heated catalyst. Raw ammonia is supplied as a vapor from cylinders or tanks. The dissociation chamber is heated between 1,850° F to 1,900° F (1010° C to 1038° C) using a fully automatic (ELECTRIC) heating system.
- Bright annealing
- Bright copper and silver brazing
- Bright tempering of ferrous and ferrous metals
- Protection of molybdenum heating elements
- Fluxless soldering
- Glass to metal sealing
See the Atmosphere Gas Table below as a PDF File
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