Cincinnati Steel Treating
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Heat Treating Definitions (Alphabetical)

age hardening: Hardening by aging, usually after rapidcooling or cold working.

annealing: A generic term denoting a treatment, consisting of heating to and holding at a suitable temperature followed by cooling at a suitable rate, used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be, but is not confined to: improvement of machinability, facilitation of cold work, improvement of mechanical or electrical properties, and/or increase in stability of dimensions. When the term is used without qualification, full annealing is implied. When applied only for the relief of stress, the process is properly called stress relieving or stress-relief annealing.

austenite: A solid solution of one or more elements in face-centered cubic iron. Unless otherwise designated (such as nickel austenite), the solute is generally assumed to be carbon.

boronizing (bodding): A surface treatment procedure in which boron atoms are diffused into the workpiece by thermochemical treatment for purpose of increased hardness, wear and corrosion resistance.

carbonitriding: A case hardening process in which a suitable ferrous material is heated above the tower transformation temperature in a gaseous atmosphere of such composition as to cause simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, create a concentration gradient. The process is completed by cooling at a rate that produces the desired properties in the workpiece.

carburizing: Absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature usually above Ac3, in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching.

case hardening: A generic term covering several processes applicable to steel that change the chemical composition of the surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient. The processes commonly used are carburizing and quench hardening; cyaniding; nitriding; and carbonitriding. The use of the applicable specific process name is preferred.

cementation: The introduction of one or more elements into the outer portion of a metal object by means of diffusion at high temperature.

cooling curve: A curve showing the relation between time and temperature during the cooling of a material.

cryogenic treatment (cold treatment): Treatment carried out after quenching to transform retained austenite into martensite, involving cooling and holding at a temperature below ambient.

decarburization: Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface.

dilatometer: An instrument for measuring the linear expansion or contraction in a metal resulting from changes in such factors as temperature and allotropy.

embrittlement: The severe loss of ductility or toughness or both of a material, usually a metal or alloy. Many forms of embrittlement can lead to brittle fracture. Many forms can occur during thermal treatment or elevated-temperature service (thermally induced embrittlement).

eutectic: An isothermal reversible reaction in which a liquid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system. (2) An alloy having the composition indicated by the eutectic point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectic reaction.

ferritic nitrocarburizing: A surface treatment process of ferrous metals involving diffusion of nitrogen and carbon into workpiece during ferritic phase, as opposed to austentic phase, for purpose of increased fatigue limit.

flame hardening: A process for hardening the surfaces of hardenable ferrous alloys in which an intense flame is used to heat the surface layers above the upper transformation temperature, whereupon the workpiece is immediately quenched.

gas nitriding: A case hardening process where nitrogen is added to surface of ferrous alloy at relatively low temperature.  Need for quenching is eliminated.

heat treatment: Heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties. Heating for the sole purpose of hot working is excluded from the meaning of this definition.

induction hardening:  A surface-hardening process in which only the surface layer of a suitable ferrous workpiece is heated by electromagnetic induction to above the upper critical temperature and immediately quenched.

induction heating: Heating by combined electrical resistance and hysteresis losses induced by subjecting a metal to the varying magnetic field surrounding a coil carrying alternating current.

ion nitriding: A method of surface hardening in whichnitrogen ions are diffused into a workpiece in a vacuumthrough the use of high-voltage electrical energy.Synonymous with plasma nitriding or glow-dischargenitriding.

ledeburite: The eutectic of the iron-carbon system, the constituents being austenite and cementite. The austenite decomposes into ferrite and cementite on cooling below the Ar1.

liquid nitrocarburizing: A nitrocarburizing process (where both carbon and nitrogen are absorbed into the surface) utilizing molten liquid salt baths below the lower critical temperature.

marquenching (martempering): (1) A hardening procedure in which an austenitized ferrous workpiece is quenched into an appropriate medium whose temperature is maintained substantially at the Ms of the workpiece, held in the medium until its temperature is uniform throughout—but not long enough to permit bainite to form— and then cooled in air. The treatment is frequently followed by tempering. (2) When the process is applied to carburized material, the controlling Ms temperature is that of the case. This variation of the process is frequently called marquenching.

martensite: A generic term for microstructures formed by diffusionless phase transformation in which the parent and product phases have a specific crystallographic relationship. Martensite is characterized by an acicular pattern in the microstructure in both ferrous and nonferrous alloys. In alloys where the solute atoms occupy interstitial positions in the martensite lattice (such as carbon in iron), the structure is hard and highly strained; but where the solute atoms occupy substitutional positions (such as nickel in iron), the martensite is soft and ductile. The amount of high-temperature phase that transforms to martensite on cooling depends to a large extent on the lowest temperature attained, there being a rather distinct beginning temperature (Ms) and a temperature at which the transformation is essentially complete (M1).

metallurgy: The science and technology of metals and alloys. Processmetallurgy is concerned with the extraction of metals from their ores and with refining of metals; physical metallurgy, with the physical and mechanical properties of metals as affected by composition, processing, and environmental conditions; and mechanical metallurgy, with the response of metals to applied forces.

nitriding: Introducing nitrogen into the surface layer of a solid ferrous alloy by holding at a suitable temperature (below Ac1 for ferritic steels) in contact with a nitrogenous material, usually ammonia or molten cyanide of appropriate composition. Quenching is not required to produce a hard case.

nitrocarburizing: Any of several processes in which both nitrogen and carbon are absorbed into the surface layers of a ferrous material at temperatures below the lower critical temperature and, by diffusion, create aconcentration gradient. Nitrocarburizing is done mainly toprovide an antiscuffing surface layer and to improvefatigue resistance. Compare with carbonitriding.

normalizing: Heating a ferrous alloy to a suitable temperature above the transformation range and then cooling in air to a temperature substantially below the transformation range.

oil quenching: Hardening of carbon steel in an oil bath. Oils are categorized as conventional, fast, martempering, or hot quenching.

pearlite: A metastable lamellar aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures above the bainite range.

phase diagram: A graphical representation of the temperature and composition limits of phase fields in an alloy system as they actually exist under the specific conditions of heating or cooling (synonymous with constitution diagram). A phase diagram may be an equilibrium diagram, an approximation to an equilibrium diagram, or a representation of metastable conditions or phases.

precipitation hardening: Hardening caused by the precipitation of a constituent from a supersaturated solid solution. See also age hardening.

press quenching: A quench In which hot dies are pressed and aligned with a part before the quenching process begins. Then the part is placed in contact with a quenching medium in a controlled manner. This process avoids part distortion.

pyrometer: A device for measuring temperatures above range of liquid thermometers.

Rockwell hardness test: An indentation hardness test based on the depth of penetration of a specified penetrator into the specimen under certain arbitrarily fixed conditions.

salt bath heat treatment: Heat treatment carried out in a bath of molten salt.

spalling: A chipping or flaking of a surface due to any kind of improper heat treatment or material dissociation.

straightening: A process by which a long, slender metal part is straightened to a specified tolerance, typically following hardening of the material.  This can be a cold mechanical process performed on a straightening press or it can involve the use of heat in a fixture or die.

stress relieving: Heating to a suitable temperature, holding long enough to reduce residual stresses, and thencooling slowly enough to minimize the development of new residual stresses.

vacuum furnace: A furnace using low atmospheric pressures instead of a protective gas atmosphere like most heat-treating furnaces. Vacuum furnaces are categorized as hot wall or cold wall, depending on the location of the heating and insulating components.