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Austempering
Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ''ausferrite''. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.
== History ==
The austempering of steel was first pioneered in the 1930s by Edgar C. Bain and Edmund S. Davenport, who were working for the United States Steel Corporation at that time. Bainite must have been present in steels long before its acknowledged discovery date, but was not identified because of the limited metallographic techniques available and the mixed microstructures formed by the heat treatment practices of the time. Coincidental circumstances inspired Bain to study isothermal phase transformations. Austenite and the higher temperature phases of steel were becoming more and more understood and it was already known that austenite could be retained at room temperature. Through his contacts at the American Steel and Wire Company, Bain was aware of isothermal transformations being used in industry and he began to conceive of new experiments 〔Bhadeshia, H. K. D. H., "Bainite in Steels: Transformations, Microstructure, and properties" second edition, IOM Communications, London, England, 2001〕
Further research into the isothermal transformation of steels was a result of Bain and Davenport's discovery of a new microstructure consisting of an "acicular, dark etching aggregate." This microstructure was found to be "tougher for the same hardness than tempered Martensite".〔Bain, Edgar C., "Functions of the Alloying Elements in Steel" American Society for Metals, Cleveland, Ohio, 1939〕 Commercial exploitation of bainitic steel did not become common overnight. Common heat treating practices at the time featured continuous cooling methods and were not capable, in practice, of producing fully Bainitic microstructures. The range of alloys available produced either mixed microstructures or excessive amounts of Martensite. The advent of low-carbon steels containing boron and molybdenum in 1958 allowed fully Bainitic steel to be produced by continuous cooling.〔〔Irvine, K.J. and Pickering, F.B JISI 188, 1958.〕 Commercial use of bainitic steel thus came about as a result of the development of new heat treating methods, those that involve a step holding the work piece at a fixed temperature for a period of time sufficient to allow transformation became collectively known as austempering.
One of the first uses of austempered steel was in rifle bolts during World War II.〔http://www.appliedprocess.com〕 The high impact strength possible at high hardnesses, and the relatively small section size of the components made austempered steel ideal for this application. Over subsequent decades austempering revolutionized the spring industry followed by clips and clamps. These components, which are usually thin, formed parts do not require expensive alloys and generally possess better elastic properties than their tempered Martensite counterparts. eventually austempered steel made its way into the automotive industry where one of its first uses was in safety critical components. The majority of car seat brackets and seat belt components are made of austempered steel because of its high strength and ductility.〔 These properties allow it to absorb significantly more energy during a crash without the risk of brittle failure. Currently, austempered steel is also used in bearings, mower blades, transmission gear, wave plate, and turf aeration tines.〔 In the second half of the twentieth century the austempering process began to be applied commercially to cast irons. Austempered Ductile Iron (ADI) was first commercialized in the early 1970s and has since become a major industry.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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