Heat Treating Terms


Aging: The process of soaking solution heat treated parts at a moderately elevated temperature – or for some alloys and tempers, at room temperature – to enhance strength and corrosion resistance properties.


AISI: Abbreviation for American Iron and Steel Institute.


Allotropy: The existence of an element in two or more physical forms. The most common elements having this property are carbon, sulfur and phosphorus.


Alloy: A substance that has metallic properties and is composed of two or more metals.


Annealing: A thermal treatment used to soften metal. Typically a process used on products that require bending or forming.


ASTM International: An international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services.


Austenite: A solid solution of iron and carbon (and sometimes other elements) in which gamma iron characterized by a face-centered cubic crystal structure is the solvent.


Austenitizing: The process of forming austenite by heating a ferrous alloy above the transformation range.


AWS: Abbreviation for American Welding Society.


Brittleness: The tendency of a material to break suddenly instead of deforming under load. Cast iron and glass, for example, are brittle.


Bright Annealing / Bright Hardening: The process of annealing or hardening in a protective medium to prevent oxidation and keep the part surface bright.


Carburizing: A process that introduces carbon into a heated solid ferrous alloy by having it in contact with a carbonaceous material. The metal is held at a temperature above the transformation range for a period of time. Carburizing is generally followed by quenching to produce a hardened case.


Case Hardening: A process in which a ferrous alloy is hardened so that the surface layer, or case, becomes considerably harder than the interior, or core. Examples of case hardening processes are carburizing followed by quenching, cyaniding, carbonitriding, nitriding, flame hardening and induction hardening.


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


Cementite: A compound of iron and carbon also known as iron carbide.


Cryogenic Treatment: The process of exposing steel to suitable subzero temperatures (-85 C or -120 F) for the purpose of obtaining desired conditions or properties, such as dimensional or microstructural stability. This process is also known as cold treatment and deep freezing.


Decarburization: The loss of carbon from the surface of a ferrous alloy as a result of heating it in the presence of a medium (such as oxygen) that reacts with the carbon.


Degreasing: The process of removing oil and grease from surfaces to which they have adhered.


Diffusion: The process of intermingling atoms or other particles within a solution. In solids, diffusion 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), a movement of vacancies or a direct exchange of atoms to neighboring sites.


Drawing: A term sometimes used for the process of tempering hardened steel.


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


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; the condition appears to be one of rest rather than one of change.


Ferrite: A solid solution of one or more elements in body-centered cubic iron.


Flame Hardening: A process for hardening the surface of hardenable ferrous alloys by using an intense flame to heat a part’s surface layers above the upper transformation temperature, then immediately quenching the part.


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 metal surface that shows a typical crystalline pattern. Fatigue fractures, however, often display a smooth, clamshell appearance.


Grain: Individual crystals in metals.


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


Hardening: The process of increasing a metal’s hardness by suitable treatment, usually involving heating and cooling. More specific terms include aging, case hardening, flame hardening, induction hardening, precipitation hardening and quench hardening.


Hardness: The ability of a metal to resist being permanently deformed. There are three categories of hardness: resistance to penetration, resistance to abrasion, and elastic hardness.


Homogeneous Carburizing: The use of carburizing to convert a low-carbon ferrous alloy to one of uniform and higher carbon content throughout the section.


Inclusions: Particles of impurity 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 workpiece is heated by electromagnetic induction to above the upper critical temperature and immediately quenched.


Macroscopic Details: 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.


Martensite: An unstable constituent that forms when steel is heated and quenched. Martensite forms without diffusion and only below a certain temperature, known as the Ms temperature. Martensite is the hardest of the transformation products of austenite, having an acicular (needlelike) microstructure.


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


Microscopy: The use of, or investigation with, a microscope.


Microstructure: The structure of polished and etched metal specimens, as seen through a microscope.


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


Nonferrous: Metals other than iron or iron alloys.

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


Oxidation: The slow or rapid reaction of oxygen with other elements; burning. Overoxidation of metals can occur when they are heated under oxidizing conditions, often resulting in permanent damage.


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 a slightly elevated temperature.


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


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.


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


Solution Heat Treatment: Heating an alloy to a suitable temperature, holding the alloy at that temperature long enough to cause one or more constituents to enter into solid solution and then cooling the alloy 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 other elements. Carbon steel is an alloy of iron and less than 2 percent carbon; it can also contain some impurities and small amounts of other alloying elements. 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. Depending on the load, this ability can be called tensile strength, compressive strength or shear strength. See stress.


Stress: The load per unit of area on a stress-strain diagram.

Tensile stress refers to an object loaded in tension; tensile stress denotes the longitudinal force that causes the fibers of a material to elongate.

Compressive stress refers to an object 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 an object as the result of an applied torque or torsional load.


Stress Relief: The reduction of residual stress in a metal part by heating the part 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: In ferrous metals, the stress relief of steels hardened by quenching for the purpose of toughening them and reducing their brittleness.


Thermal Expansion: An increase in the dimensions of a material due to an increase in movement of atoms caused by increased temperature.


Thermal Stress: A shear stress that is induced in a material due to unequal heating or cooling rates. Thermal stress can occur, for example, when there are different rates of expansion and / or contraction between the interior and exterior surfaces of a metal that is being heated or cooled.


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: The ability of a metal to resist rupture. Toughness is generally measured in terms of notch toughness, which a metal’s ability to resist rupture from impact loading when a notch is present. To test toughness, a standard test specimen containing a prepared notch is inserted into the vise of a machine, called the Izod-Charpy testing machine. This machine consists of a weight on a swinging arm or pendulum. When the arm is released, it 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 temperature at which a material in one phase transforms into another phase. For example, when ferrite or alpha iron reach their transformation temperature, they transform into austenite or gamma iron.


Vacuum Annealing: Annealing carried out at subatmospheric pressure.


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