Machinery's Handbook, 31st Edition
Heat Treatment of Steel 459 Austempering: Quenching from a temperature above the transformation range, in a medium having a rate of heat abstraction high enough to prevent the formation of high- temperature transformation products, and then holding the alloy, until transformation is complete, at a temperature below that of pearlite formation and above that of martensite formation. Austenitizing: Forming austenite by heating into the transformation range (partial austenitizing) or above the transformation range (complete austenitizing). When used without qualification, the term implies complete austenitizing. Baking: Heating to a low temperature in order to remove entrained gases. Bluing: A treatment of the surface of iron-base alloys, usually in the form of sheet or strip, on which, by the action of air or steam at a suitable temperature, a thin blue oxide film is formed on the initially scale-free surface, as a means of improving appearance and resistance to corrosion. This term is also used to denote a heat treatment of springs after fabrication, to reduce the internal stress created by coiling and forming. Carbon Potential: A measure of the ability of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon content of the steel exposed to it. In any particular environment, the carbon level attained will depend on such factors as temperature, time, and steel composition. Carbon Restoration: Replacing the carbon lost in the surface layer from previous pro cessing by carburizing this layer to substantially restore the original carbon level. Carbonitriding: A case-hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous atmosphere of such composition as to cause simultaneous absorption of carbon and nitrogen by the surface and, by diffu sion, create a concentration gradient. The process is completed by cooling at a rate that produces the desired properties in the workpiece. Carburizing: A process in which carbon is introduced into a solid iron-base alloy by heating above the transformation temperature range while in contact with a carbonaceous material that may be a solid, liquid, or gas. Carburizing is frequently followed by quench ing to produce a hardened case. Case: 1) The surface layer of an iron-base alloy that has been suitably altered in compo sition and can be made substantially harder than the interior or core by a process of case hardening; and 2) the hardened surface layer of a piece of steel that is large enough to have a distinctly softer core or center. Cementation: The process of introducing elements into the outer layer of metal objects by means of high-temperature diffusion. Cold Treatment: Exposing to suitable subzero temperatures for the purpose of obtaining desired conditions or properties, such as dimensional or microstructural stability. When the treatment involves the transformation of retained austenite, it is usually followed by a tempering treatment. Conditioning Heat Treatment: A preliminary heat treatment used to prepare a material for a desired reaction to a subsequent heat treatment. For the term to be meaningful, the treatment used must be specified. Controlled Cooling: A term used to describe a process by which a steel object is cooled from an elevated temperature, usually from the final hot-forming operation in a predeter mined manner of cooling to avoid hardening, cracking, or internal damage. Core: 1) The interior portion of an iron-base alloy that after case hardening is substan tially softer than the surface layer or case; and 2) the relatively soft central portion of certain hardened tool steels. Critical Range or Critical Temperature Range: Synonymous with Transformation Range , which is preferred. Cyaniding: A process of case hardening an iron-base alloy by the simultaneous absorp tion of carbon and nitrogen by heating in a cyanide salt. Cyaniding is usually followed by quenching to produce a hard case.
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