Hardening is a heat treatment used on ferrous metals to improve mechanical properties. Austemperating involves heating metal to an austenitic state, cooling it in a salt bath, and maintaining it at a specific temperature. The process produces bainite or ausferrite, which are stronger and more wear-resistant than martensite. The process was patented in 1933 but only became cost-effective in the 1960s.
Hardening is a form of heat treatment used on ferrous metals, such as iron and steel, to improve the mechanical properties of the metal. The metal is heated to an austenitic state and then cooled rapidly, or cooled, but kept at a high enough temperature to prevent martensite from forming for an extended period. Tempered metals have higher strength, toughness, and resistance to distortion, wear, and hydrogen embrittlement, and are often used in machine parts.
In the first part of the austemperating process, the metal is heated to a temperature between 1.350° F (about 732° C) and 2.462° F (about 1.394° C). This causes it to undergo a phase transition which modifies the crystalline structure where the iron atoms are arranged, transforming it into austenite. The austenite is then cooled, usually in a bath of molten nitrate salt, and cooled to a temperature between 459°F and 750°F (about 232°C and 399°C). It is then maintained at this temperature for a period of time ranging from a few minutes to several hours. The residence time of the metal in the salt bath and the precise temperatures used in both phases vary depending on the composition of the ferrous metal and the desired mechanical properties in the final product.
The austenite process differs from conventional heat treatment, which rapidly cools austenite in water or oil, usually to room temperature. This produces a form of steel called martensite. Martensite is quite hard but very brittle and requires further heat treatment, a process called tempering, to become ductile enough to be used.
Generally, the result of the autemperature depends on the material used. Hardened steel becomes a form of steel called bainite, which is more ductile than martensite and requires no additional tempering. It is also stronger, tougher and more wear resistant 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 products of standard heat treatment.
The austemperature process was patented by EC Bain and ES Davenport in 1933. It produced high quality steel, but the process was originally quite expensive and not cost effective for most uses. This limited their use to producing 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 steelmaking greatly reduced production costs, that austemperature became an economically viable means of producing steel for large-scale commercial use.
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