Age Hardening: Hardening by aging, usually after rapid cooling or cold working.

Age Softening: Spontaneous decrease of strength and hardness that takes place at room temperature in certain strain-hardened alloys, especially those of aluminum.

Aging: The process of holding metals at room temperature or at a predetermined temperature for the purpose of increasing their hardness and strength by precipitation; aging is also used to increase dimensional stability in metals such as castings.

Air Hardening Steel: An alloy steel which does not require quenching from a high temperature to harden, but which is hardened by simply cooling in air from above its critical temperature range.

AISI: Abbreviation for American Iron & Steel Institute.

Allotropy: The ability of a material to exist in several crystalline forms.

Alloy: A substance that has metallic properties and is composed of two or more chemical elements of which at least one is a metal. Aluminum Dip Brazing: The joining of aluminum parts using an aluminum alloy, above 450C and in a salt environment. The salt acts as a flux or cleaning agent for the surfaces being joined.

Anneal: To heat and then cool a material, or for treatments intended to alter the mechanical or physical properties to produce a definite microstructure. Property changes include removing stress, inducing softness, altering ductility or toughness, or changing electric or magnetic properties.

ASMi: Abbreviation for American Society of Metals International.

ASTM: Abbreviation for American Society for Testing Materials.

Atmosphere: The gaseous environment in which the metal being treated is heated for processing. Atmospheres are used to protect from chemical change, or to alter the surface chemistry of steel through the addition or removal of carbon, nitrogen, hydrogen, and oxygen and to add certain metallic elements as chromium, silicon, sulphur, etc.

Austempering: Hardening steel by quenching from the austenitizing temperature into salt, and transforming the austenite into bainite. Austempering often increases ductility and toughness.

Austenite: A solid solution of iron and carbon (and sometimes other elements), obtained by heating the material to a temperature above the upper critical temperature (or transformation temperature).

Austenitizing: The process of forming austenite by heating an iron/carbon alloy above its transformation temperature. The austenitizing temperature varies for different grades of carbon, alloy and tool steels.

AWS: Abbreviation for American Welding Society.


Bainite: A decomposition or transformation product of austenite.

Bark: An older term used to describe the decarburized skin that develops on steel bars heated in a non-protective atmosphere.

Belt Furnace: A continuous-type furnace that uses a mesh-type or cast-link belt to carry parts through the furnace.

Beta Annealing: Producing a beta phase by heating certain titanium alloys in the appropriate temperature range, and then cooling at a rate to prevent its decomposition.

Brazing: The joining of two or more metals with the use of a dis-similar material or alloy above 450C. Brazing may be done in air, or in a protective atmosphere such as vacuum or hydrogen.

Brittleness: The property of materials that will not deform under load but tend to break suddenly; for example, cast iron and glass are brittle. Brittleness is the property opposite to plasticity. Bright Annealing / Bright Hardening: Annealing in a protective medium or atmosphere to prevent discoloration of the bright surface.

Brinnell Hardness Test: A test for determining the hardness of a material by forcing a hard steel or carbide ball of specified diameter into it under a specified load. The result is expressed as the Brinell Hardness number, which is the value obtained by dividing the applied load in kilograms by the surface area of the resulting impression in square millimeters.


Car Furnace: A batch-type furnace using a car on rails to enter and leave the furnace area. Car furnaces are used for lower stress relieving ranges.

Carburizing: Adding carbon to the surface layer of steel by heating the steel below its melting point, while in contact with carbonaceous solids, liquids or gases.

Case Hardening: A process in which a ferrous alloy is hardened so that the surface layer or case is made considerably harder than the interior or core. Some case-hardening processes are carburizing, cyaniding, carbonitriding, carbo-nitriding, nitriding, flame hardening, and induction hardening.

Cast Iron: Iron containing 2 to 4.5 percent carbon, silicon, and other trace elements. It is used for casting objects into molds. Cast iron is somewhat brittle.

Cementite: Also known as iron carbide, a compound of iron and carbon (Fe3C).

Controlled Cooling: Cooling from an elevated temperature in a pre-determined manner, to avoid hardening, cracking or internal damage, or to produce desired mecrostructure or mechanical properties.

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

Critical Temperature: The preferred term used by metallurgists is transformation temperature. The lower A1 and the upper A3 temperatures are the boundaries of the transformation range in which ferrite transformations into austenite.

Cryogenic Treatment: Often called cold treatment or deep freezing. Steel is exposed to suitable subzero temperatures (-85 C or lower) for the purpose of obtaining desired conditions or properties such as dimensional or micro- structural stability. May also reduce retained austenite.</p>


Decarburization: When steel is heated in the presence of a medium such as air, oxygen or hydrogen, there is a loss of carbon at the surface of the steel. This loss of carbon, or decarburization, results in a reduction of surface hardness of the steel.

Diffusion: The process of inter-mingling atoms or other particles within a solution. In solids, it is a slow movement of atoms from areas of high concentration toward areas of low concentration. The process may be (a) migration of interstitial atoms such as carbon, (b) movement of vacancies, or (c) direct exchange of atoms to neighboring sites.

Dip Brazing: The joining of aluminum parts using an aluminum alloy, above 450C and in a salt environment. The salt acts as a flux or cleaning agent for the surfaces being joined.

Drawing: A term used interchangeably with tempering, and refers to reheating hardened (and usually quenched) steel to a temperature below the lower critical temperature, followed by cooling. Drawing slightly reduces the hardness of the steel, but increases ductility and makes the part less brittle.

Ductility: The property of a material to deform permanently, or to exhibit plasticity without rupture, while under tension.


Elasticity: The ability of a material to return to its original form after a load has been removed.

Equilibrium: A condition of balance in which all the forces or processes that are present are counterbalanced by equal or opposite forces or processes where the condition appears to be one of rest rather than of change.

Equilibrium Diagram: A graphical representation of the temperature, pressure and composition limits of phase fields in an alloy system as they exist under conditions of complete equilibrium. In metal systems, pressure is usually considered constant.

Eutectic: (1) 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 eutectic reaction.


Fatigue: Failure by progressive fracture caused by repeated applications or reversals of stress.

Ferrite: A magnetic form of iron. A solid solution in which alpha iron is the solvent, characterized by a body-centered cubic crystal structure.

Fixturing: The placing of parts to be heat treated in a constraining or semi-constraining apparatus to avoid heat-related distortions. See racking.

Flame Hardening: A heat treatment method used to harden the surface of some parts via the use of a flame, followed by an oil quench.

Forging: The shaping of metal by hammering or pressing. Although forging may be used to shape malleable metals in the cold state, the application of heat increases plasticity and permits greater deformation without inducing undue strain in the metal.

Fracture: A ruptured surface of metal that shows a typical crystalline pattern. Fatigue fractures, however, often display a smooth, clam-shell appearance.


Grain: Individual crystals in metals.


Hardenability: The property that determines the depth and distribution of hardness in a ferrous alloy induced by heating or quenching.

Hardening: Increasing hardness of metals by suitable treatment, usually involving heating and cooling. More specific terms include age hardening, case hardening, flame hardening, induction hardening, precipitation hardening, and quench hardening.

Hardness: The property of a metal to resist being permanently deformed. This is divided into three categories: the resistance to penetration, abrasion, and elastic hardness.

Homogeneous Carburizing: Use of carburizing process to convert low-carbon ferrous to allow one of uniform and higher carbon content throughout the section.

Hydrogen Brazing: The joining of materials using an alloy, above 450C and in a hydrogen environment. The hydrogen acts as a flux or cleaning agent for the surfaces being joined.

Hydrogen Embrittlement: The brittleness induced in steel by the absorption of atomic hydrogen, most commonly from a pickling or plating operation.


Inclusions: Particles of impurities that are usually formed during solidification and are usually in the form of silicates, sulfides, and oxides.

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


Knoop Hardness: Microhardness determined from the resistance of metal to indentation by a pyramidal diamond indenter, having edge angels of 172, 30 and 130 degrees, making a rhombohedral impression with one long and one short diagonal.


Latent Heat: Thermal energy absorbed or released when a substance undergoes a phase change.

Liquid Penetrant Inspection: A type of nondestructive inspection that locates discontinuities that are open to the surface of a metal by first allowing a penetrating dye or fluorescent liquid to infiltrate the discontinuity, removing the excess penetrant, and then applying a developing agent that causes the penetrant to seep back out of the discontinuity and register as an indication. Liquid penetrant inspection is suitable for both ferrous and nonferrous materials, but is limited to the detection of open surface discontinuities in nonporous solids.


Macroscopic: Structural details on an object that are large enough to be observed by the naked eye or with low magnification (about 10x).

Macrostructure: The structure of metals as revealed by macroscopic examination.

Magnetic Annealing: Annealing a part, usually made of a high-nickel alloy, in order to change its magnetic properties, including magnetic permeability.

Maraging: A precipitation hardening treatment applied to a special group of iron-0base alloys to precipitate one or more intermetallic compounds in a matrix of essentially carbon free martensite.

Martempering (Marquenching): A method of hardening steel by quenching from the austenitizing temperature into salt, and then cooling in air and then tempering. The interrupted cooling allows for reduced distortion in some situations.

Martensite: An unstable constituent that is formed by heating and quenching steel. It is formed without diffusion and only below a certain temperature. Martensite is the hardest of the transformation products of austenite, having an acicular, or needlelike, microstructure.

Metallurgy: The science and study of the behaviors and properties of metals and their extraction from their ores.


Neutral Hardening: To harden carbon steel parts by heating them to the proper temperature in atmospheres such as pure nitrogen, nitrogen and natural gas, or nitrogen-methanol, followed by quenching.

Nitriding: The process of adding nitrogen to the surface of a steel. Nitriding develops a very hard case after a long time at comparatively low temperature.

Nitrocarburizing: Any of several processes in which both nitrogen and carbon are absorbed into the surface layers of a ferrous material. Nitrocarburizing is performed primarily to provide an anti-scuffing surface layer and to improve fatigue resistance.

Nonferrous: Metals other than iron or iron alloys, such as aluminum, copper or titanium.

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

NTMA: Abbreviation for The National Tooling and Machining Association.


Oil Hardening: Quench-hardening treatment involving cooling in oil.

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

Oxidation: (1) A reaction in which there is an increase in valence resulting from a loss of electrons. (2) A corrosion reaction in which the corroded metal forms an oxide; usually applied to reaction with a gas containing elemental oxygen, such as air.


Precipitation Hardening: A process of hardening an alloy by heat treatment, in which a constituent precipitates from a supersaturated solid solution while at room temperature or at some slightly elevated temperature.


Quenching: The process of rapid cooling metal alloys for the purpose of hardening. Quenching media include air, oil, water, molten metals, and fused salts.


Racking: A term used to describe the placing of parts to be treated or joined on a rack or tray. This is done to keep parts in a proper position to avoid heat-related distortions and to keep the parts separated. See fixturing.

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.


SAE: Abbreviation for The Society of Automotive Engineers.

Scale: The surface oxidation on metals that is caused by heating in air or in other oxidizing atmospheres.

Soaking: A prolonged heating of metal at a predetermined temperature to create a uniform temperature throughout its mass.

Soldering: The joining of two or more metals with the use of a dis-similar material or alloy below 450C.

Solubility: The degree to which one substance will dissolve in another.

Solution Heat Treatment: Heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooling rapidly enough to hold these constituents in solution.

Spheroidizing: A process in which carbon steel is held for a period of time at just under the transformation temperature. An aggregate of globular carbide is formed from other microstructures such as pearlite.

Stainless Steel: An alloy of iron containing at least 11 percent chromium and sometimes nickel that resists almost all forms of rusting and corrosion.

Steel: An alloy of iron and less than 2 percent carbon plus some impurities and small amounts of alloying elements is known as plain carbon steel. Alloy steels contain substantial amounts of alloying elements such as chromium or nickel besides carbon.

Strain: The unit deformation of a metal when stress is applied.

Strength: The ability of a metal to resist external forces. This is called tensile, compressive, or shear strength; depending on the load. See stress.

Stress: The load per unit of area on a stress-strain diagram. Tensile stress refers to an object loaded in tension, denoting the longitudinal force that causes the fibers of a material to elongate. Compressive stress refers to a member loaded in compression, which either gives rise to a given reduction in volume or a transverse displacement of material. Shear stress refers to a force that lies in a parallel plane. The force tends to cause the plane of the area involved to slide on the adjacent planes. Torsional stress is a shearing stress that occurs at any point in a body as the result of an applied torque or torsional load.

Stress Relief: (1) Uniform heating of a structure or portion thereof to a sufficient temperature to relieve the major portion of the residual stresses, followed by uniform cooling. (2) The reduction of residual stress in a metal part by heating it to a given temperature and holding it there for a suitable length of time. This treatment is used to relieve stresses caused by welding, cold working, machining, casting, and quenching.


Tempering: Reheating quenched steel to a temperature below the critical range, followed by any desired rate of cooling. Tempering is done to relieve quenching stresses, or to develop desired strength characteristics.

Thermal Expansion: The increase of the dimension of a material that results from the increased movement of atoms caused by increased temperature.

Thermal Stress: Shear stress that is induced in a material due to unequal heating or cooling rates. The difference of expansion and contraction between the interior and exterior surfaces of a metal that is being heated or cooled is an example.

Tool Steel: A special group of steels that are designed for specific uses such as heat-resistant steels that can be heat treated to produce certain properties, mainly hardness and wear resistance.

Toughness: Generally measured in terms of notch toughness, which is the ability of a metal to resist rupture from impact loading when a notch is present. A standard test specimen containing a prepared notch is inserted into the vise of a testing machine. This devise, called the Izod-Charpy testing machine, consists of a weight on a swinging arm. The arm or pendulum is released, strikes the specimen, and continues to swing forward. The amount of energy absorbed by the breaking of the specimen is measured by how far the pendulum continues to swing.

Transformation Temperature: The temperatures at which one phase transforms into another phase; for example, ferrite or alpha iron transforms into austenite or gamma iron.


Vacuum Annealing: Annealing carried out at less-than-atmospheric pressure.

Vacuum Brazing: The joining of materials using an alloy, above 450C and in a vacuum environment. The vacuum acts as a flux or cleaning agent for the surfaces being joined.

Vacuum Furnace: A furnace using low atmospheric pressures instead of a protective gas atmosphere. Vacuum furnaces are categorized as hot wall or cold wall, depending on the location of the heating and unsulating components.


White Layer: The compound layer that forms as a result of the nitriding process.

Work Hardness: Hardness developed in metal resulting from cold working.