Annealing Atmospheres from the Combustion Products of Gaseous Fuels A. .G. HOTCHICISS General Electric Company, Schenectady, N. Y. velopment of protective atmospheres has been so great that bright annealing in products of combustion is now universal. To compete with the controlled flame of the gas- or oilfired furnace, the air atmosphere in electric furnaces was first displaced with hydrogen, hydrogen-nitrogen mixtures, or hydrocarbon gases cracked with steam. These gases were expensive, some were difficult to use, and, in general, all were unavailable to the average user. The electric furnace manufacturer was forced to find an atmosphere easily produced in
.The history of the art of producing annealing furnace atmospheres which will not affect the surface qualities of metals is briefly given. The development and design of equipment for producing controlled atmospheres from combustion gases are traced, and the equipment illustrated by photographs and diagrams. The behavior of various fuels in such equipment and the cost of controlled atmosphere gas from the fuels are discussed. Purity requirements of atmospheres for various uses are stated, and methods and equipment for removing such impurities as water, sulfur dioxide, nitric oxide, and carbon dioxide are described. The applications of atmospheres of various compositions to the treatment of specific metals are discussed.
T
EN years ago protective atmospheres made from products of combustion in fuel-fired heat-treating furnaces were obtained by adjusting the burners for a “soft flame” which operated with a deficiency of air. The main object was to heat the work, the atmosphere control being secondary. The resuIt was reduced oxidation and a step in the right direction, but a long way from the ultimate of complete elimination of scaling. Annealing of stacks of low-carbon sheets or coils of strip was accomplished by covering the charge with a heavy cast cover, sand- or mud-sealed, into which an atmosphere of raw coke-oven or natural gas (sometimes treated) was introduced immediately after the charge was removed from the furnace, and WRS maintained during the cooling cycle. The result was oxidized edges and outer surfaces and soot deposits. This atmosphere was expensive and unsatisfactory and required care in its use. High-carbon coiled strip was buried in cast-iron borings, in cast pots with heavy covers whose combined weights were about equal to the weight of the steel being annealed. The reaction of entrapped air with the borings produced the atmosphere to prevent oxidization and decarburization during the annealing cycle. Today all types of materials are completely bright-annealed under thin alloy-metal covers which act only to retain the relatively cheap protective atmosphere around the work, add practically nothing to the amount of material heated, and thus reduce to a minimum the time of heating the charge. About 1931 and 1932 when industry was flat on its back and looking desperately for new and better products and cost reducing methods, the use of protective atmospheres began to receive considerable attention. Since that time the de-
FIGURE 1 (Aboue). ONE OF THE FIRSTATMOSPHEREGAS PRODUCERS FIGURE2 (Below). COMBUSTION-TYPE ATMOSPHERE-GAS CONVERTER(2000 CUBICFEETPER HOUR)FOR REFORMING CITY GAS,NATURAL GAS, PROPANE, BUTANE, OR DIESOCIATED AMMONI.4
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1941 c.-.--
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