Heat Treatment Industry
In the metals industry, a blanketing or protective gas is used in many heat treatment processes. The drying of this gas is imperative to produce a more uniform metal; a required grade is made with precision and the furnace in which the heat treating is done works with greater regularity.
Steel is sometimes annealed in a controlled atmosphere prepared by the partial combustion of natural gas. An exothermic base gas is an inert gas generated when natural gas is burned with a controlled amount of air which produces mostly nitrogen with 0.2% - 0.5% combustibles and 0% - 0.2% maximum oxygen. During the combustion, a considerable amount of water vapor is formed. The gas is cooled and then dried by a desiccant dryer. The gas then blankets the steel during heat treating to prevent oxidation.
In aluminum heat treating or annealing, an exothermic gas is used. Too much moisture in the furnace atmosphere can cause oxidation of the alloying constituents. The amount of moisture is extremely critical whenever a metal is exposed to processing. This includes not only heat treating but polishing; carburizing; and welding of titanium, stainless, and other alloys.
A hydrogen atmosphere is used in copper-brazing furnaces for annealing highly oxidizable metals. Where even a slight amount of moisture is extremely detrimental, nickel, nickel steel, and monel wires must be annealed in these furnaces to avoid discoloration.
Another heat treating atmosphere used to a large extent is cracked ammonia. Anhydrous ammonia is dissociated into a gas resulting in three parts of hydrogen to one part of nitrogen. Most cracking units are highly efficient so that the degree of dissociation is usually 99.75% or 99.95%. Since one volume of ammonia yields two volumes of the mixed gas, the ammonia content is 0.125 to 0.025 percent by volume respectively, or 2500 and 500 PPM weight. A molecular sieve dryer is used to remove water and the ammonia.
A gas of increasing importance in the heat treating of metals is HNX gas. This gas has excellent properties in the bright annealing process and is nearly neutral with regard to carburization when treating steels with different carbon contents. Thus, the gas can be used universally. This makes the gas distribution within the workshop easier and enables the purchase of larger and more economical gas production plants.
HNX gas is produced by the controlled combustion of fuel gas. This gas must be dried before being used in a furnace.