Gases used in annealing primarily include nitrogen, hydrogen, and argon, often in combination with hydrocarbons or other gases to create specific atmospheres suitable for different types of metal treatments.
Nitrogen and Nitrogen-Hydrocarbon Blends: Nitrogen is often used in annealing processes, though pure nitrogen (100%) is rarely used due to its inability to exclude air (oxygen) effectively. To create a more suitable atmosphere, nitrogen is commonly blended with small percentages of hydrocarbon gases like methane, propane, or propylene, or combined with methanol. These blends are carefully controlled and often follow specific recipes to ensure the desired metallurgical effects are achieved without causing oxidation or other unwanted reactions.
Hydrogen: Hydrogen is another key gas used in annealing, especially for bright annealing and the treatment of stainless steel, alloy steel, and non-iron materials. It is typically used in a dry form, which is 98-99.9% pure. When combined with nitrogen or argon, hydrogen helps create an atmosphere that prevents oxidation and promotes a clean, bright surface finish on the metal. This is particularly important for materials that require high surface quality, such as stainless steel.
Argon: Argon is used in conjunction with hydrogen or as a standalone gas in annealing processes. It is an inert gas that helps maintain a stable, non-reactive atmosphere within the furnace. When used with hydrogen, argon aids in bright annealing and the treatment of various steels and non-iron materials. Its primary role is to prevent oxidation and other chemical reactions that could degrade the metal's surface or properties.
Purified Exothermic Gases: Purified rich exothermic gas, with low levels of CO2, is used for short-cycle annealing of medium- and high-carbon steels. This gas has a high carbon monoxide (CO) content, which can be beneficial in some annealing processes due to its high carbon potential. However, for long-cycle annealing, the high CO content can lead to soot deposits and other surface effects, necessitating the use of purified lean exothermic gas.
Purging: Before the annealing process begins, purging is performed to remove any residual oxygen or other reactive gases from the furnace atmosphere. Nitrogen or lean exothermic gas is commonly used for this purpose, and the process is considered complete when the oxygen level is less than 1%. This step is crucial to prevent oxidation and ensure the stability of the annealing atmosphere.
In summary, the choice of gas or gas blend used in annealing depends on the specific requirements of the metal being treated, the desired metallurgical outcomes, and the type of annealing process being employed. Nitrogen, hydrogen, and argon are fundamental to creating the controlled atmospheres necessary for effective annealing.
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