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HEAT TREATMENT TERMS:


AGING: A change in a metal by which its structure recovers from an unstable condition produced by quenching or by cold working. The change in structure consists in precipitation, often submicroscopic, and is marked by a change in physical properties. Aging which takes place slowly at room temperature may be accelerated by a slight increase in temperature. See also Stress Relieving.
AIR HARDENING: See Hardening, Air.
ANNEALING: A process involving heating and cooling applied usually to induce softening. The term is also used to cover treatments intended to remove stresses, alter mechanical or physical properties, produce a definite microstructure, and remove gases. Certain specific heat treatment of iron-base alloy covered by the term annealing are: Black Annealing, Box Annealing, Bright Annealing, Full Annealing, Graphitizing, Malleablizing, Process Annealing.
ANNEALING, BLACK: A process of box annealing iron-base alloy sheets after hot rolling, shearing and pickling. The process does not impart a black color to the product if properly done. The name originated in the appearance of the hot-rolled material before pickling and annealing.
ANNEALING, BLUE: A process of softening iron-base alloys in the form of hot-rolled sheet, in which the sheet is heated in the open furnace to a temperature within the transformation range and cooled in air, the formation of a bluish oxide on the surface is incidental.
ANNEALING, BOX: A process of annealing which, to prevent oxidation, is carried out in a suitable closed metal container with or without packing material. The charge is usually heated slowly to a temperature below, but sometimes above or within the transformation temperature range and cooled slowly. It is also called Close Annealing or Pot Annealing.
ANNEALING, CLOSE: See Annealing, Box.
ANNEALING, FLAME: A process in which the surface of an iron-base alloy is softened by localized heat applied by a high temperature flame.
ANNEALING, FULL: A softening process in which an iron-base alloy is heated to a temperature above the transformation range and, after being held for a proper time at this temperature, is cooled slowly to a temperature, below the transformation range. The objects are ordinarily allowed to cool slowly in the furnace, although they may be removed from the furnace and cooled in some medium, which assures a slow rate of cooling.
ANNEALING, POT: See Annealing, Box.
ANNEALING, PROCESS: A process commonly applied in the sheet and wire industries, in which an iron-base alloy is heated to a temperature close to, but below, the lower limit of transformation range and subsequently cooled. This process is applied for the purpose of softening for further cold working.
AUSTEMPERING: A trade name for a patented heat treating process consisting in quenching an iron-base alloy from a temperature above the transformation range in a medium having a suitably high rate of heat abstraction, and maintaining the alloy, until transformation is complete, at a temperature which is below that of pearlite formation and above that of martensite formation. The temperature for austenite transformation is chosen on the basis of the properties desired.
BLUEING: 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. The term is also used to denote a heat treatment of springs after fabrication, to reduce the internal stress created by coiling and forming.
BRUNORIZING: A trade name for a special treatment applied to steel rails, which, after cooling to a temperature below the transformation range, are reheated to a temperature slightly above the range, and then are allowed to cool in the air, the ends of rails being partially quenched by jets of compressed air.
BURNT: A term applied to a metal permanently damaged by being heated to a temperature close to the melting point. The damage may involve melting of some constituent or penetration by, and reaction of the metal with, a gas such as oxygen, or by segregation of component elements of the metal.
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 which may be solid, liquid or gas. Quenching to produce a hardened case frequently follows carburizing. The carbonizing is sometimes used erroneously in place carburizing.
CASE: (1) The surface layer of an iron-base alloy which has been suitably altered in composition and can be made substantially harder than the interior or core by a process of case hardening. (2) The term case is also used to designate the hardened surface layer of a piece of steel that is large enough to have a distinctly softer core or center.
CASE HARDENING: See Hardening, Case.
CEMENTATION: The process of introducing elements into the outer layer of metal objects by means of high temperature diffusion.
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 predetermined manner of cooling to avoid hardening, cracking, or internal damage.
CORE: (1) The interior portion of an iron-base alloy, which after case hardening is substantially softer than the surface layer or case. (2) The term core is also used to designate the relative soft central portion of certain hardened tool steels. See also Case.
CRITICAL RANGE OR CRITICAL TEMPERATURE RANGE: Synonymous with Transformation Range, which is the preferred term.
CYANIDE HARDENING: See Hardening, Cyanide.
CYANIDING: A process of case hardening an iron-base alloy by the simultaneous absorption of carbon and nitrogen by heating in a cyanide salt. Quenching to produce a hard case usually follows cyaniding.
DECARBURIZATION: The loss of carbon from the surface of an iron-base alloy as the result of heating in a medium, which reacts, with the carbon.
DRAWING: Steel is "drawn" or tempered by reheating after hardening to some temperature below the critical temperature range and then cooling the steel. This heat treatment is often referred to as drawing, but the term tempering is preferable.
DRAWING BACK: Reheating after hardening to a temperature below the critical range for the purpose of improving the ductility and or lowering the hardness of the steel.
FLAME HARDENING: See Hardening, Flame.
GRAPHITIZING: An annealing process applied to certain iron-base alloys, such as cast iron some steels with high carbon and silicon contents, by which the combined carbon is wholly or in part transformed to graphite or free carbon. See Temper Carbon.
HARDENING: Any process of increasing hardness of metal by suitable treatment, usually involving heating and cooling.
HARDENING, AIR (AIR QUENCHING): A hardening process wherein the steel is heated to the hardening temperature and cooled in air. Unless the steel is high in carbon, or an alloy, or both, it will not show much increase in hardness when air hardened.
HARDENING, AGE: See Aging.
HARDENING, CASE: A process of surface hardening involving a change in the composition of the outer layer of an iron-base alloy followed by appropriate thermal treatment. Typical casehardening processes are Carburizing, Cyaniding, Carbo Nitriding and Nitriding.
    
In order to harden low-carbon steel it is necessary to increase the carbon content of the surface of the steel so that a thin outer "case" can be hardened by heating the steel to the hardening temperature and then quenching it.
   
The process therefore, involves two separate operations. The first is the carburizing operation for impregnating the outer surface with sufficient carbon, and then the second operation is that of heat-treating the carburized parts so as to obtain a hard outer case and, at the same time, give the "core" the required physical properties. The term "casehardening" is ordinarily used to indicate the complete process of carburizing and hardening.
HARDENING, CYANIDE: This process is employed to produce what is known as superficial hardness. This superficial hardening is the result of carburizing a very thin outer skin (which may only be a few thousandths of an inch thick) by immersing the steel in a bath containing sodium cyanide.
HARDENING, FLAME: A process of heating the surface layer of an iron-base alloy above the transformation temperature range by means of a high temperature flame, followed by quenching.
   
This method of hardening is especially applicable to the selective hardening of large steel forging or castings which must be finished-machined prior to heat-treatment, or which because of size or shape cannot be heat-treated by using a furnace or bath.
HARDENING, INDUCTION: A surface hardening process consisting of heating ferrous metals by electro-magnetic induction, followed by immediate quenching.
HARDENING, PACK: The purpose of this process is to protect the work, prevent scale formation, insure uniform heating, and minimize the danger of cracking or warpeage. The work is packed, as in carburizing, and the same type of receptacle is used.
HARDENING, PRECIPATION: A process of hardening an alloy in which a constituent precipitates from a supersaturated solid solution. See Also Aging.
HARDENING, SECONDARY: An increase in hardness following the normal softening that occurs during tempering of certain alloy steels.
HARDENING, SUPERFICIAL: When low carbon steel is subjected to the cyanide hardening process, a very thin but extremely hard surface is obtained, and this is known as superficial hardening. This hard outer skin may be only a few thousandths of an inch thick, and that is the important difference between superficial hardening and ordinary casehardening. See also Hardening, Cyanide.
HEATING, DIFFERENTIAL: A heating process by which the temperature is made to vary throughout the object being heated so that on cooling different portions may have such different physical properties as may be desired.
HEATING, INDUCTION: A process of local heating by electrical induction.
HEAT TREATMENT, SOLUTION: A treatment in which an alloy is heated to a suitable temperature and held at this temperature for a sufficient length of time to allow a constituent in solution. The material is then in a supersaturated, unstable state, and may subsequently exhibit Age Hardening.
HOMOGENIZING: A high temperature heat-treatment process intended to eliminate or to decrease chemical segregation by diffusion.
INDUCTION HARDENING: See Hardening, Induction.
MALLEABLIZING: A process of annealing white cast iron in which the combined carbon is wholly or in part transformed to graphite or free carbon, and, in some case, part of the carbon is removed completely. See Temper Carbon.
NITRIDING: A process of case hardening in which an iron-base alloy by the simultaneous absorption carbon and nitrogen by heating in an atmosphere of ammonia or in contact with nitrogenous material. Surface hardening is produced by the absorption of nitrogen without quenching.
NITRIDING, CARBO: A process of case hardening an iron-base alloy by the simultaneous absorption of carbon and nitrogen by heating in a gaseous atmosphere of suitable composition, followed by either quenching or cooling slowly as required.
NORMALIZING: A process in which an iron-base alloy is heated to a temperature above the transformation range and subsequently cooled in still air at room temperature.
   
Normalizing is intended to put the steel into a uniform unstressed condition of proper grain size and refinement so that it will properly respond to further heat-treatment.
OVERHEATED: A metal is said to have been overheated if, after exposure to an unduly high temperature, it develops an undesirable coarse grain structure but is not permanently damaged. The structure damaged by overheating can be corrected by suitable heat treatment or by mechanical work or by a combination of the two. In this respect it differs from a Burnt structure.
PACK HARDENING: See Hardening, Pack.
PATENTING: A process of heat treatment applied to medium or high carbon steel in wire making prior to the drawing or between draws. It consists of heating to a temperature above the transformation range, followed by cooling to a temperature below that range in air or in a bath of molten lead or salt maintained at a temperature appropriate to the carbon content of the steel and the properties required of the finished product.
PREHEATING:
    (1) A general term used to describe a heating applied preliminary to some further thermal or mechanical treatment.
   
(2) A term specifically applied to tool steel to describe a process in which the steel is heated slowly and uniformly to a temperature below the hardening temperature and is then transferred to a furnace in which the temperature is substantially above the preheating temperature.
QUENCHING: A process of rapid cooling from an elevated temperature, by contact with liquids, gases or solids.
QUENCHING, DIFFERENTIAL: A quenching process by which only certain desired portions of the object are quenched and hardened.
QUENCHING, HOT: A process of quenching iron-base alloys in a medium, the temperature of which is substantially higher than atmospheric temperature.
QUENCHING, POT: A process of quenching carburized parts directly from the carbonizing box or pot.
SANDBERG SORBITIC TREATMENT: A treatment in which carbon steel objects are moderately hardened, either wholly or in part. It consists of cooling the parts to be hardened through the transformation range at a moderately rapid rate by the application of jets of air, steam, or atomized water and then allowing the residual heat in the object to effect a tempering operation.
SOAKING: Prolonged heating of a metal at a selected temperature.
SPHEROIDIZING: Any process of heating and cooling steel that produces a rounded or globular form of carbide in the structure.
STRESS RELIEVING: A process to reduce internal residual stresses in a metal object by heating the object to a suitable temperature and holding for a proper time at that temperature. The treatment may be applied to relieve stresses induced by casting, quenching, normalizing, machining, cold working or welding. Stress relieving is sometimes termed Aging.
SUPERFICIAL HARDENING: See Hardening, Superficial.
TEMPER CARBON: The free or graphitic carbon, which comes out of solution usually in the form of rounded nodules in the structure during Graphitizing or Malleablizing.
TEMPERING:         A process of reheating hardened or normalized steel to a temperature below the transformation temperature range, followed by any desired rate of cooling.
TRANSFORMATION RANGE: In ferrous alloys the transformation range on heating is the temperature interval within which austenite forms. The transformation range on cooling is the temperature interval in which austenite disappears. Distinction must be made between the two ranges. They may overlap but never coincide. The limiting temperatures of the ranges depend on the composition of the alloy and, particularly for the cooling, on the rate of change of temperature.

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