Glossary | A & B (The vocabulary of the coating and heat-treating industries—explained)

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Acm, A1, A2, A3, A4

The shortened version of Aecm, Ae1, Ae2, Ae3, and Ae4, all of which are defined under Transformation Temperature.

Abradable Coating

A coating that is designed to rub down against the action of a mating surface to form a tight gas or air seal.

Abrasion Resistance

The measurement of a material's resistance to abrasion or wear.


A substance—or describing a substance—that is used for grinding, honing, lapping, superfinishing, polishing, pressure blasting or barrel finishing. Abrasives include natural materials such as garnet, emery corundum and diamond, as well as electric-furnace products like aluminum oxide, silicon carbide and boron carbide.

Abrasive Blast

A pressurized stream of hard metal or oxide grit material used to clean and/or roughen surfaces prior to coating.

Abrasive Wear

Wear caused by hard, sharp particles.

Accm, Ac1, Ac2, Ac3, Ac4

Defined under Transformation Temperature.

Acetylene C2H2

Unsaturated aliphatic hydrocarbon gas. Used as a fuel gas in combustion thermal spray processes, welding and cutting. Acetylene has the highest flame temperature and requires the smallest amount of oxygen to form a neutral flame.

Acicular Ferrite

A highly sub-structured, conequiaxed ferrite that forms upon continuous cooling by a mixed diffusion and shear mode of transformation, and which begins to form at a temperature slightly higher than upper bainite's temperature transformation range. It is distinguished from bainite in that it has a limited amount of carbon available, which means that there is only a small amount of carbide present.

Acoustic Emission

A sound or ultrasound pulse that is generated by the initiation or propagation of a crack in materials and/or coatings as a result of those materials and/or coatings being subjected to stress. Transducers, which detect acoustic emissions, are used to discover these cracks.


The measurement of how well an object or substance (such as a coating) adheres to, or remains on, a surface—without spalling, flaking or cracking. Adhesion is likely the single most important property of a coating.

Adhesion Scratch Test

Currently the most commonly used method of assessing the adhesion of a coating or film. It is used to analyze organic and inorganic, soft and hard coatings. Example subjects are thin and multilayer PVD, CVD, PACVD, TD, photoresist, paint, lacquer and other types of films and coatings that are used in many applications, including optical, micro-electronic, protective and decorative. Substrates can be hard or soft, and include metals, alloys, semiconductors, glass, and refractive and organic materials. In a scratch test, stresses are introduced at the interface between coating and substrate. This is achieved by pressing a diamond stylus on the sample surface with a normal load FN. As the sample is displaced at constant speed, the resulting stresses at the interface cause the coating to flake or chip. The minimum load at which a specific failure event is recorded is called the Critical Load (Lc).

Adhesive Wear

Wear caused by sliding surfaces, where surface asperities interact with and adhere to one another.

Aecm, Ae1, Ae2, Ae3, and Ae4

Defined under Transformation Temperature.

Aerated Bath Nitriding

A type of liquid nitriding in which air is pumped through the molten bath to both create agitation and increase chemical activity.

Age Hardening

When an increase in hardness is the result of aging, usually after rapid cooling or cold working.

Age Softening

A spontaneous decrease in strength and hardness when left at room temperature. This happens in certain strain-hardened alloys, especially those of aluminum.


A group of particles that have adhered to one another.

Agglomerated powder

A mechanically mixed combination of fine particles composed of different materials, held together with an organic binder and formed into power particles.


Any change in the properties of certain metals and alloys that occurs at ambient or moderately elevated temperatures after hot working or a heat treatment (quench aging in ferrous alloys, natural or artificial aging in ferrous and nonferrous alloys) or after cold working operation (strain aging). The change in properties is often (but not always) due to the chemical composition of the metal or alloy. See also: age hardening, age softening, interrupted aging, overaging, precipitation hardening, precipitation heat treatment, progressive aging, and step aging.

Air Cap

A component of a thermal spray gun; used for shaping the air flow for atomizing the wire or rod feedstock.

Air Classification

The process of separating powder into particle-sized fractions using a stream of air that travels at a controlled velocity.

Air-Hardening Steel

Steel containing sufficient carbon and other alloying elements to harden fully during cooling in air or other gaseous media from a temperature above its transformation range. The terms should be restricted to steels that are capable of being hardened by cooling in air in fairly large sections, about 2 in. (50 mm) or more in diameter.

Alkyd Resin

A type of polyester resin used in paints and other surface coating. The original alkyd resins were made by copolimerizing phathalic anhydride with glycerol, to give a brittle cross. It is a linked polymer.


A near synonym for polymorphism. Allotropy is generally restricted to describing polymorphic behavior in elements, terminal phases, and alloys whose behavior closely parallels that of the predominant constituent element.


A substance with metallic properties that is composed of two or more chemical elements. At least one of these elements must be a metal.

Alloy Steel

Steel containing specified quantities of alloying elements (other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus) within the limits recognized for construction alloy steels, added to effect changes in mechanical properties or physical properties.

Alloying Element

An element which is added to a metal (which remains within the metal) to effect changes in properties.

Alpha Ferrite

The lower of two ferrite regions on some equilibrium diagrams.

Alpha Iron

The body-centered cubic form of pure iron, stable below 910°C (1670°F).


Aluminum Oxide compound used in both abrasive blasting as an abrasive and in thermal spraying as a consumable feedstock (powder and rod) for the production of coatings. Alumina is a hard wear resistance ceramic and can be alloyed with various amounts of titania (titanium dioxide) to improve certain properties.

Aluminization (using gas)

High temperature (approx 900°C) pack or gaseous diffusion of aluminum into the surface of a component to enhance its resistance to high temperature corrosion and oxidation.

Aluminization (using hot dip)

A liquid aluminum submersion technique at temperatures above 600°C that deposits a layer of aluminum (usually on steel sheet) to enhance its resistance to corrosion.

Aluminization (using thermal spray)

The thermal spray coating of aluminum, usually on substrates of steel or nickel chromium alloys, which are subsequently heat treated to aluminize the surface.

Aluminum (Al)

Used to deoxidize steel and control grain size. Grain size control is affected by forming a fine dispersion with nitrogen and oxygen which restricts austenite grain growth. Aluminum is also an extremely effective nitride former in nitriding steels.

Aluminum Ion Plating

The process by which aluminum is deposited, via a vacuum evaporative process, onto what is being plated. This provides galvanic corrosion resistance, and is normally followed by a passivation treatment.


Non-crystalline, or devoid of regular structure.


A negatively charged ion.


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 purposes of such changes can include improvement of machinability, facilitation of cold work, improvement of mechanical or electrical properties and increase in the 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.
    In ferrous alloys, annealing usually is done above the upper critical temperature, but the time-temperature cycles vary widely in both maximum temperature attained and in cooling rate employed, depending on composition, material condition, and results desired. When applicable, the following commercial process names should be used: black annealing, blue annealing, box annealing, bright annealing, cycle annealing, flame annealing, full annealing, graphitizing, intercritical annealing, isothermal annealing, malleablizing, order hardening, process annealing, quench annealing, spheroidizing, subcritical annealing.
    In nonferrous alloys, annealing cycles are designed to: (a) remove part or all of the effects of cold working (recrystallization may or may not be involved); (b) cause substantially complete coalescence of precipitates from solid solution in relatively coarse form; or (c) both, depending on composition and material condition. Specific process names in commercial use are final annealing, full annealing, intermediate annealing, partial annealing, recrystallization annealing, stress-relief annealing, anneal to temper.

Anneal to Temper

A final partial anneal that softens a cold worked, nonferrous alloy to a specified level of hardness or tensile strength.

Annealing Carbon

Fine, apparently amorphous carbon particles formed in white case iron and certain steels during prolonged annealing. Also called temper carbon.

Annealing Twin

A twin formed in a crystal during recrystallization.


The positive electrode in a plasma spray torch (gun). Generally made from copper but can be lined with tungsten to increase life. See also cathode.

Anodic Coating

A coating that becomes the anode in an electrochemical cell with the substrate (cathode). Zinc and aluminum are the only metals anodic to iron and steel that are in common use for thermal spraying.


The production of an oxide layer on aluminum alloys. The process is electrolytic, a typical electrolyte being sulfuric acid. Treatment at room temperature produces thin, decorative layers with some corrosion protection. Treatment at 0°C produces hard, thicker layers (up to 100μ) with wear resistance. They can be post sealed to give improved corrosion resistance.

Antireflection (AR) Coating

A coating with a reflection of zero; used for lenses. It is usually fabricated with interference oscillations of a HeNe (Helium Neon) laser.

Apparent Density

The weight of a unit volume of powder or coating.

Apparent Hardness

The value obtained by testing a coating or sintered material with standard indentation hardness equipment. See macro hardness. Since the reading reflects a composite of pores and solid material, (which may be particles relatively poorly bonded together) it is usually lower than that of an equivalent solid wrought or cast material. Not to be confused with particle hardness. See micro hardness.

Arcm, Ar1, Ar2, Ar3, Ar4, Ar’, Ar”

Defined under Transformation Temperature.

Arc Blow (plasma)

The deflection of an electric arc, under the influence of the magnetic field associated to the flow of electrons (electric charge), which results in arc instability. Arc blow can be reduced in plasma welding by an appropriately designing electrodes (namely cathode tip and anodic nozzle plenum) in terms of shape and size.

Arc Wire Spraying

A thermal spray process in which two electrically conducting wires are brought together to form an electric arc. The consequent molten metal is then projected by an air stream towards the work piece to form a coating.

Argon (Ar)

Monatomic noble gas, atomic number 18, one of the most inert elements. Commonly used as a plasma gas for plasma spraying and providing inert environments for many processes.

Artificial Aging

Aging above room temperature, as opposed to at room temperature (natural aging).

As Cast

Literally: 'As it was when it was cast.' Refers to metal which has not received finishing (beyond gate removal or sandblasting) or treatment of any kind including heat treatment after casting. Similar terms are as drawn, as forged and as rolled.

Athermal Transformation

A reaction that proceeds without benefit of thermal fluctuations; that is, thermal activation is not required. In contrast, a reaction that occurs at constant temperature is an isothermal transformation; thermal activation is necessary in this case and the reaction proceeds as a function of time.


The gaseous environment in which the metal being treated is heated for processing. The purpose of an atmosphere is to prevent its contents from changing chemically. For example, it prevents the surface chemistry of steel from changing through the addition or removal of carbon, nitrogen, hydrogen, and oxygen. It also is used to add certain metallic elements such as chromium, silicon and sulfur.

Atomic Force Microscopy (AFM)

Alternate name: Scanning Probe Microscopy. An extremely accurate and versatile technique for measuring structures or surface forces. A cantilever (a very fine sensor tip mounted to the end of a small deflecting spring) is brought into contact with the sample surface to be investigated. The sensor tip is moved across the surface in numerous line scans, measuring roughness by feeling hills and valleys (the surface's topography) as it moves. The accuracy and detail of the results allow for imaging of the surface.


The process by which molten material is dispersed into particles either by a rapidly moving gas/liquid stream or by mechanical dispersion.

Atomized Powder

The product of the atomization process.

Auger Electron Spectroscopy (AES)

An electron-beam technique used to identify near-surface elements. Well used in thin film science.


The thermomechanical treatment of steel in the metastable austenitic condition below the recrystallization temperature, which is followed by quenching to obtain martensite and/or bainite.


A heat treatment for ferrous alloys. A part is quenched from the austenitizing temperature, quickly enough to avoid formation of ferrite or pearlite, down to a temperature just slightly higher than Ms. It is kept at this temperature until its transformation to bainite is complete.
Although austempered steel is also designated as bainite in austempered ductile iron (ADI), it actually consists of two phase mixtures containing ferrite and carbide whereas austempered ductile iron consists of two phase mixtures containing ferrite and 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.

Austenite Steel

Any steel containing sufficient alloy to produce a stable austenitic (gamma iron) crystalline structure at ambient temperatures.

Austenitic grain size

The size by the grains of steel when heated to the austenitic region; may be revealed by appropriate etching of cross sections after cooling to room temperature.


The process by which austenite is formed. A ferrous alloy is heated into the transformation range (partial austenitization) or above it (complete austenitization). When used without qualification, the term implies complete austenitization.


To produce a stable, protective oxide on steel parts by treating them in a pressurized, high-temperature atmosphere of steam.

Autogenous Welding

The method of uniting two pieces of metal by melting their edges together without solder or any added welding metal, as by the thermite process that employs a medium of finely divided aluminum powder and oxide or iron by which a temperature of some 2982.2°C (5400°F) is obtained.



A metastable aggregate that consists of dispersed carbide in ferrite. It is formed by the transformation of austenite at a temperature between the transformation temperatures of Ms and Pearlite. Its appearance has the form of relatively coarse ferrite laths that are surrounded by carbides. In upper bainite (bainite formed at a relatively high temperature), these carbides are precipitated as platelets. Lower bainite is more acicular and resembles tempered martensite.

Bainitic Hardening

A quench-hardening treatment that results principally in the formation of bainite.

Banded Structure

A layering effect that is sometimes developed during the hot rolling of steel.


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

Base Coats for Electroplating

A coating of PVD that is applied to a material so that it can be more easily electroplated. Such materials are difficult to electroplate because of rapid oxide formation. Once this base coat has been applied, it can be built up using electrodeposition. For example, a material like nickel or copper is used for this base coat on materials like titanium, uranium, and zirconium before they are electroplated.

Batch Furnace

A furnace used for heat treating that can only accomodate one load at a time. Batch-type furnaces are necessary for large parts such as heavy forgings, and are preferred for complex alloy grades requiring long cycles. See also: car furnace, horizontal batch furnace.

Belt Furnace

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

Beta Annealing

A process that creates a beta phase by heating certain titanium alloys to a temparature that will allow this phase to form. Once this phase has been established, the alloys are cooled at a rate that is designed to prevent the phase's decomposition.


A solid, semi-finished round or square that has been hot-worked. It is usually smaller than a bloom. It is also a general term for wrought starting stock for forgings or extrusions.


A cementing medium that is used in the production of composite or agglomerate powders.

Black Annealing

The process of box annealing or pot annealing ferrous alloy sheet, strip, or wire to impart a black color to the oxidized surface.

Black Oxide

A black finish produced by immersing metal in hot oxidizing salts or salt solutions.

Blank Carburizing

A process that simulates carburizing without introducing carbon. This is usually accomplished by using an inert material in place of the carburizing agent, or by applying a suitable protective coating to the ferrous alloy.

Blank Nitriding

A process that simulates nitriding, but without introducing nitrogen. This is usually accomplished by using an inert material in place of the nitriding agent or by applying a suitable protective coating to the ferrous alloy.


A pressurized stream of particulates (ceramic, plastic, metal, etc.) applied on a surface to clean, peen or abrade.

Blended Powder

A powder consisting of two or more different powder materials, thoroughly mixed.


A semi-finished, hot-rolled, rectangular product. The width of the bloom is no more than twice its thickness, and its cross-sectional area is usually greater than or equal to 36 square inches.

Blue Annealing

The process of heating hot-rolled, ferrous steel in an open furnace to a temperature within its transformation range and then cooling it, for the purpose of softening the metal. The formation of a bluish oxide on the surface is incidental.

Blue Brittleness

Brittleness exhibited by some steels after being heated to a temperature within the range of about 205°C – 370°C (400°F – 700°F), particularly if the steel is worked at the elevated temperature. Killed steels are virtually free of this kind of brittleness.


The process of subjecting the scale-free surface of a ferrous alloy to the action of air, steam, or other agents at a suitable temperature, thus forming a thin, blue film of oxide and improving the appearance and resistance to corrosion. Note: This term is ordinarily applied to sheet, strip, or finished parts. It is used also to denote the heating of springs after fabrication to improve their properties.


This represents the state of adhesion between the coating and the substrate. Its strength depends on the details of the thermal spraying process and the materials used. Bonding mechanisms may be mechanical, physical, metallurgical or a combination of these.

Bond Coat

A coating applied as an intermediary between the main (top) coating and the substrate in order to improve the bond strength.


A thermochemical treatment involving the enrichment of the surface layer of an object with borides. This surface-hardening process is performed below the Ac1 temperature.

Boron (B)

An element. When it is added to a material, usually so that it amounts to .0005% – .003% of the whole, it can significantly increase the material's hardenability—especially for low-carbon alloys. It does not affect the strength of ferrite, so no ductility, formability or machinability is sacrificed in the annealed state.


The diffusion of boron into the surface of a component (usually steel) by a high temperature (approx. 900°C) gas or pack process. It produces hard phases within the surface, typically 100μm deep.

Boundary Condition

The loads, displacements, temperatures, densities etc. at the periphery of the domain or mesh in a numerical simulation.

Box Annealing

A type of annealing that occurs within a sealed container under conditions that minimize oxidation. When box-annealing a ferrous alloy, the charge is slowly heated to a temperature that is usually below but sometimes above or within the transformation range. It is then cooled slowly. This process is also called close annealing or pat annealing. See black annealing.


Creases or ridges, usually in "untempered" or in aged material, where the yield point has been exceeded. Depending on the origin of the break, it may be termed a cross break, a coil break, an edge break, or a sticker break.

Bright Annealing

A type of annealing that occurs within a protective medium to prevent discoloration of the bright surface. Compare with blank nitriding.

Brinell Hardness Test

A test that determines a material's hardness. A hard steel/carbide ball, of a particular diameter, is forced into a material 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.

Brine Quenching

A quench in which brine (salt water-chlorides, carbonates, and cyanides) is the quenching medium. The salt addition improves the efficiency of water at the vapor phase or hot stage of the quenching process.

Brittle Fracture

The separation of a solid accompanied by little or no macroscopic plastic deformation. Typically, brittle fracture occurs by rapid crack propagation with less expenditure of energy than for ductile fracture.

Brittle Tempering Range

Some hardened steels show an increase in brittleness when tempered in the range of about 450°F to 700°F, even though some tempering causes softening.


(1) Permanent damage to a metal or alloy that is caused by over-heating until either incipient melting or intergranular oxidation occurs. See grain-boundary liquidation.
(2) In grinding, getting the work hot enough to cause discoloration or to change the microstructure by tempering or hardening.

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