GLOSSARY


Age Hardening -
A process where an increase in hardness and strength can occur in certain metals and alloys, usually after a heat treatment with rapid cooling or a cold working operation.  Learn more..

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.  Learn more..

Austempering - A heat treatment for ferrous alloys that consists of quenching a part from the austenitizing temperature at a rate fast enough (usually in molten salt) to avoid upper transformation products (ferrite and pearlite) to a temperature just above the Ms and held at that temperature until transformation to bainite is complete.  Learn more..

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.

Austenitizing - The process of forming austenite in ferrous materials by heating above the transformation temperature range.

Bainite - A microstructural constituent in ferrous alloys resulting from transformation of austenite at temperatures below the pearlite range but above the Ms.

Brazing - A thermal treatment of joining metals by fusion of nonferrous alloys that have melting points above 800*F but lower than those of the metals being joined.  Learn more..

Carbon Restoration - A thermal process of replacing the carbon lost in the surface layer of a ferrous part from previous processing by adding carbon to this layer to bring it back to essentially the same carbon level.  Learn more..

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 absorption of carbon and nitrogen into the surface of the part and, by diffusion, create a concentration gradient. The process is completed by cooling at a rate that produces the desired properties in the workpiece.  Learn more..

Carburizing - A case hardening process in which a suitable ferrous material is heated to a temperature usually above Ac3 and exposed to a suitable carbonaceous material that may be a solid, liquid or gas to allow absorption and diffusion of carbon into the surface of the part. This produces a carbon gradient extending inward from the surface. Once the carbon has penetrated to the desired depth, the case is typically hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then re-austenitizing and quenching.  Learn more..

Case Hardening - A generic term covering several processes applicable to ferrous materials in which the surface layer of the material is made substantially harder than its interior. This heat treatment can either 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 or can be selective hardened on the surface, i.e. induction or flame hardening. The processes case hardening applies to are carburizing, nitriding; ferritic nitrocarburizing and carbonitriding. The use of the applicable specific process name is strongly recommended to eliminate confusion.  Learn more..

Cold treatment - A thermal process of cooling and holding at a temperature below ambient typically to allow the transformation of retained austenite to martensite.

Cryogenic treatment - A thermal treatment whereby materials are brought to temperatures in the order of -300*F and held at that temperature long enough to improve the properties of the materials.

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.

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.

Ferritic nitrocarburizing (FNC): - A surface treatment process for ferrous metals involving diffusion of nitrogen and carbon into workpiece at temperatures below the lower critical temperature and, by diffusion, create a concentration gradient. Nitrocarburizing is typically done to provide an antiscuffing surface and improved fatigue resistance. Learn more..

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.

Freezing - A thermal process of cooling and holding at a low temperature (typically between -90*F and -150*F) to facilitate the transformation of retained austenite to martensite or stabilize the material for subsequent precipitation hardening.  Learn more..

Gas Nitriding - A case hardening process where nitrogen is absorbed into the surface of ferrous alloy at temperatures below the lower critical temperature. Parts are atmosphere cooled to ambient and do not need to be quenched for hardening.

Heat treatment - The heating and cooling of a solid metal or alloy in such a way as to obtain desired conditions or properties. This is a generic term that covers the full range of thermal processes. 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. Learn more..

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 which nitrogen ions are diffused into a workpiece in a vacuum through the use of high-voltage electrical energy. Synonymous with plasma nitriding or glow-discharge nitriding.

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 phase transformation from austenite of ferrous materials. Martensite is characterized by an acicular pattern in the microstructure. 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 (Mf)..

Ms - Martensite start. The temperature at which martensite starts to form during the cooling process.

Mf - Martensite finish. The temperature at which martensite formation on cooling is essentially finished.

Nitriding - The process of absorbing nitrogen into the surface layer of a solid ferrous alloy by holding at a suitable temperature (below the lower critical temperature) in a nitrogen rich atmosphere, usually ammonia. Quenching is not required to produce a hard case.

Nitrocarburizing - A surface treatment process for ferrous metals involving diffusion of nitrogen and carbon into workpiece at temperatures below the lower critical temperature and, by diffusion, create a concentration gradient. Nitrocarburizing is typically done to provide an anti-scuffing surface and improved fatigue resistance. (same as Ferritic Nitrocarburizing).

Normalizing - A process of heating a ferrous alloy to a suitable temperature above the transformation range and then cooling at a rate simulating "still air cooling" to a temperature substantially below the transformation range. Learn more..

Oil Quenching - The rapid cooling of carbon steel in an oil bath to facilitate hardening. Oils are categorized as conventional, fast, martempering, or hot quenching. Learn more..

Pearlite - A microstructural constituent in ferrous alloys that is a 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.  Learn more..

Precipitation hardening - A thermal treatment that causes hardening by the precipitation of a constituent from a supersaturated solid solution. See also age hardening. Learn more..

Press quenching - A quenching operation 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.

Retained austenite (RA) -A microstructural constituent in ferrous alloys that can result from the hardening process when the quenching process does not go below the Mf temperature for the material. This is a metastable constituent that can transform with subsequent thermal processing or under stress.

Rockwell hardness test - An indentation hardness test based on the depth of penetration of a specified penetrator into the specimen under fixed conditions controlled by ASTM standards.

Salt bath heat treatment - Heat treatment performed in a bath of molten salt.  Learn more..

Straightening - A process by which a metal part is moved to achieve a specified tolerance, typically following the heat treatment of the material. This can be a cold mechanical process performed on a straightening press or it can involve the use of heat and a fixture or die. Learn more..

Stress Relieving - A thermal treatment of heating a part to a suitable temperature, holding long enough to reduce internal residual stresses, and then cooling slowly enough to minimize the development of new residual stresses.  Learn more..

Sub-critical anneal - A thermal treatment of heating a ferrous alloy to a temperature just below the lower critical temperature and holding, followed by slow cooling to reduce hardness and mechanical properties. Learn more..

Tempering - A thermal treatment of heating a hardened or normalized ferrous alloy to a temperature below the transformation range (lower critical temperature) for the purpose of decreasing hardness and increasing toughness or ductility. Learn more..

Thermal hydraulic flattening - A process where parts are subjected to hydraulic pressure during a stress relieving or temper operation for the purpose of making them more uniformly flat. Learn more..

Toughness - The ability of a material to absorb energy and deform plastically before fracturing.

Vacuum furnace - A furnace that uses low atmospheric pressures instead of positive pressure protective gas atmospheres like most heat-treating furnaces. Vacuum furnaces remove the air in the furnace and replace it with a low pressure atmosphere of an inert gas. Learn more..