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Austempering is a form of heat treatment used on ferrous metals, such as iron and steel, to improve the metal's mechanical properties. The metal is heated until it reaches an austenitic state and then rapidly cooled, or quenched, but kept at a temperature high enough to prevent the formation of martensite for an extended period. Austempered metals have improved strength, toughness, and resistance to distortion, wear, and hydrogen embrittlement, and are often used in machine parts.
In the first part of the austempering process, the metal is heated to a temperature of between 1,350° F (about 732° C) and 2,462° F (about 1,394° C). This causes it to undergo a phase transition that changes the crystalline structure where the iron atoms are arranged, turning it into austenite. The austenite is then quenched, usually in a bath of molten nitrate salt, and cooled to a temperature of between 459° F and 750° F (about 232° C and 399° C). It is then kept at that temperature for a time period ranging from several minutes to several hours. The amount of time the metal is kept in the salt bath and the precise temperatures used in both phases vary according to the composition of the ferrous metal and the mechanical properties desired in the final product.
The austempering process differs from conventional heat treating, which rapidly quenches the austenite in water or oil, usually to room temperature. This produces a form of steel called martensite. Martensite is quite hard but highly brittle and requires further heat treating, a process called tempering, to become ductile enough to use.
Generally, the result of austempering depends on the material used. Austempered steel becomes a form of steel called bainite, which is more ductile than martensite and does not require additional tempering. It is also stronger, tougher, and more resistant to wear for a given hardness than martensitic steels. Austempered ductile iron results in a structure called ausferrite, which has greater strength relative to its ductility than the products of standard heat treatment.
The austempering process was patented by E.C. Bain and E.S. Davenport in 1933. It produced high-quality steel, but the process was originally quite expensive and not cost-effective for most uses. This limited its use to the production of high-performance parts that required extreme toughness and resistance to distortion, such as gun components. It was not until the 1960s, when technological advances in steel working greatly reduced production costs, that austempering became an economically viable means of producing steel for large-scale commercial use.
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