Glossary | H – K (The vocabulary of the coating and heat-treating industries—explained)

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Hafnium Nitride (HN)

Gold-colored ceramic, typically applied as a thin coating. Very hard (3500Hv). Used on cutting tools and forming tools and other surfaces needing wear resistance.

Half Hard

A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength about midway between that of dead soft and full-hard tempers.

Hard Chrome Plating

The electrolytic deposition of chromium to form a very hard (1000Hv), tough coating with good wear resistance. The structure is micro-cracked.

Hard Temper

See full-hard temper.


The relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured as the distance below a quenched surface where the metal exhibits a specific hardness (50 HRC, for example) or a specific percentage of martensite in the microstructure.


Increasing the hardness by suitable treatment, usually involving heating and cooling. When applicable, these more-specific terms should be used: age hardening, flame hardening, induction hardening, laser hardening, precipitation hardening, and quench hardening.


The application of a cladding or coating of material that is designed to resist wear.


Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting. Indentation hardness may be measured by various hardness tests, such as Brinell, Rockwell, and Vickers.

Hardness Profile

Hardness as a function of distance from a fixed reference point (usually from the surface).

Heat Resistant Coating

Any coating which, during normal use, must withstand temperatures of at least 204°C (400°F).

Heat Tinting

Coloration of a metal surface through oxidation by heating to reveal details of the microstructure.

Heat-Treatable Alloy

An alloy that can be hardened by heat treatment.

Heat-Treating Film

A thin coating or film, usually an oxide, formed on the surface of metals during heat treatment.

Heat Treatment

A combination of heating and cooling operations which is applied to a metal or alloy to produce desired properties and microstructures. Heating for the sole purpose of hot working is excluded from the meaning of this definition.

Helium (He)

A monatomic, noble and gaseous element with an atomic number of 2; the most inert element. It is used as a plasma gas in plasma spraying.

High-Alloy Steel

Ferrous alloy with more than 12 weight percent of noncarbon additions.

High Pressure Mold

A strong, high-density mold that is used in pneumatic, hydraulic or other squeeze processes.

High Strength Low Alloy Steel (HSLA)

Steel with relatively high strength and impact properties. The carbon level is low, and alloying additions make up significantly less than 5% of its total mass.

High Velocity Oxy-fuel Spraying (HVOF)

A thermal spray process in which the spray powder particles are injected into a jet, formed by the combustion of oxy-fuel, that heats and accelerates them toward the work piece.


The high temperature/high pressure consolidation of a powder metallurgy component or thermally sprayed coating. Density is greatly increased, and metallurgical changes provide enhanced corrosion and wear resistance properties.


The portion of a thermal cycle during which an object is kept at a constant temperature.

Holding Temperature

The temperature at which an object is kept throughout the holding portion of a thermal cycle.

Holding Time

The duration of the holding portion of a thermal cycle.

Hollow Cathode Discharge (HCD)

The deposition of a thin film made from high quality TiN, CrN, TiCN or TiCrN onto an object.

Homogeneous Carburizing

The action of using a carburizing process to convert a low-carbon ferrous alloy to one of higher, more uniform carbon content.


Holding at high temperature and using diffusion to eliminate chemical segregation.

Horizontal Batch Furnace

A versatile batch-type furnace that can give light or deep case depths. Because the parts are not exposed to air, resulting surfaces can be almost entirely free of oxides.

Hot Dip Coating

A metallic coating obtained by dipping the substrate metal into a molten metal.

Hot Quenching

An imprecise term used to cover a variety of quenching procedures in which a quenching medium is maintained at a prescribed temperature above 70°C (160°F).

Hot-Wire Analyzer

An electrical atmosphere analysis device that is based on the fact that the electrical resistivity of steel is a linear function of carbon content over a range from 0.05% C to saturation. The device typically measures the carbon potential of furnace atmospheres. This term is not to be confused with hot-wire test, which measures heat extraction rates.

Hydrogen (H2)

A diatomic and gaseous element with an atomic number of 1; the lightest element. It is a very reactive and powerful reducing agent that is used as a secondary plasma gas in the plasma spraying process and also as a fuel gas in combustion thermal spray processes (CWS, CPS and HVOF).

Hydrogen Embrittlement

The cracking or severe loss of ductility owing to the presence of hydrogen in a metal. Metals can absorb hydrogen while being electroplated, pickled etc. Its use as a secondary gas in plasma spraying has no effect on substrates and on the majority of coatings, one exception being coatings made from titanium.


(adjective) Tending to absorb water; the opposite of hydrophobic.


(adjective) Tending to repel or lacking an affinity for water; the opposite of hydrophilic.


A biocompatible ceramic. It may be deposited by plasma spray onto orthopedic implants in order to increase the bone-implant contact. It may also be sprayed over a porous titanium coating to promote bone ingrowth.

Hypereutectic Alloy

In an alloy system exhibiting a eutectic, an alloy whose composition contains an excess of alloying element for its eutectic composition and whose equilibrium microstructure contains some eutectic structure. Compare with hypoeutectic alloy.

Hypereutectoid Alloy

In an alloy system exhibiting a eutectoid, an alloy whose composition contains an excess of alloying element for its eutectoid composition and whose equilibrium microstructure contains some eutectoid structure. Compare with hypoeutectoid alloy.

Hypereutectoid Steel

Steel wherein carbon makes up more than the eutectoid percentage (0.83%) of its mass.

Hypoeutectic Alloy

In an alloy system exhibiting a eutectic, an alloy whose composition contains an excess of base metal for its eutectic composition and whose equilibrium microstructure contains some eutectic structure. Compare with hypereutectic alloy.

Hypoeutectoid Alloy

In an alloy system exhibiting a eutectoid, an alloy whose composition contains an excess of base metal for its eutectoid composition and whose equilibrium microstructure contains some eutectoid structure. Compare with hypereutectoid alloy.


Ideal Critical Diameter (DI)

Under an ideal quench condition, the bar diameter that has 50% martensite at the center of the bar when the surface is cooled at an infinitely rapid rate (that is, when H = ∞, where H is the Grossmann number, or the quench severity factor).

Immersed-Electrodes Furnace

A furnace used for the liquid carburization of parts. It heats molten salt baths via electrodes immersed in the bath. Compare with submerged-electrode furnace.

Impact Test

A test to determine the behavior of materials when subjected to high rates of loading; usually in bending, tension or torsion. The quantity measured is the energy absorbed in breaking the specimen by a single blow, as in the Charpy or Izod tests.


A process resulting in the continuous impact of liquid or solid particles on a solid surface.

Impingement Corrosion

A form of erosion-corrosion that is generally associated with the impingement of a high-velocity flow of liquid that contains air bubbles against a solid surface.


An element unintentionally allowed in a metal or alloy. Some impurities have little effect on properties; others will grossly damage the alloy.


Particles of Impurities (usually oxides, sulphides, silicates and such) which separate from the liquid steel and are mechanically held during solidification. In some grades of steel, inclusions are made intentionally high to aid machinability.

Indentation Method (for hardness and modulus measurements)

A method of measuring quantitative hardness and modulus values. An indenter tip with a known geometry is driven into the place on the material that is to be tested by applying an increasing normal load. When reaching a preset maximum value, the normal load is reduced until partial or complete relaxation is achieved. This procedure is performed repetitively, and at each stage of the experiment the position of the indenter in relation to the sample surface is precisely monitored. For each loading/unloading cycle, the applied load value is plotted with respect to the corresponding position of the indenter. The resulting load/displacement curves provide data specific to the mechanical nature of the material under examination. This data is used to discern the results of the test.

Indium-Tin Oxide (ITO)

A transparent, semiconducting material used as an electrode on flat-panel displays and other electro-optical devices.

Induction Hardening

A surface-hardening process in which only the surface layer of a suitable ferrous work piece is affected. It is heated to above 900°C via electromagnetic induction, above the upper critical temperature, and then immediately quenched.

Induction Heating

The heating of a electrically conductive material using an induction coil that produces alternating magnetic fields. The magnetic fields cause alternating electric currents to flow through the material and cause heating by resistance. It is used in many heating processes including (but not limited to) induction fusing, induction plasma and induction hardening.

Induction Tempering

The tempering of steel using low-frequency electrical induction heating.

Infrared Analyzer

An atmosphere-monitoring device that measures the presence of gas (usually carbon monoxide, carbon dioxide, and methane) based on the specific wavelength absorption of infrared energy.


A casting of a simple shape which can be used for hot working or remelting.

Injection Molding

The injection of molten metal or other material, under pressure, into molds.


A part, usually formed from metal, which is placed in a mold and may become an integral part of the casting.

Intense Quenching

Quenching in which the quenching medium cools the part at a rate at least 2.5 times faster than still water. See Grossmann chart.

Interconnected Porosity

A network of pores that exists throughout a coating, including the coating's surface.

Intercritical Annealing

Any annealing treatment that involves heating to and holding at a temperature between the upper and lower critical temperatures in order to achieve partial austenitization, followed by either slow cooling or holding at a temperature below the lower critical temperature.


(adjective) Existing between crystals or grains. Also called intercrystalline. Contrast with transgranular.

Intergranular Corrosion

Preferential corrosion at / adjacent to the grain boundaries of a metal or alloy.

Intergranular Cracking

Cracking that occurs between crystals or grains in a polycrystalline aggregate. Also called intercrystalline cracking. Contrast with transgranular cracking.

Intergranular Fracture

Fracture that occurs between crystals or grains in a polycrystalline aggregate. Also called intercrystalline fracture. Contrast with transgranular fracture.

Intermediate Annealing

Annealing wrought metals at one or more stages during manufacture and before final treatment.

Internal Friction

Ability of a metal to transform vibratory energy into heat; generally refers to low stress levels of vibration. Damping has a broader connotation since it may refer to stresses approaching or exceeding yield strength.

Internal Oxidation

The formation of isolated particles of corrosion products beneath the surface of a metal or coating. This occurs as a result of preferential oxidation of certain alloy constituents by inward diffusion of oxygen, nitrogen, sulfur, etc.

Internal Shrinkage

A void, or network of voids, within a casting; caused by inadequate feeding of that section during solidification.

Internal Stresses

Stresses within a part that generally occur when it is cooling.

Interrupted Aging

Aging at two or more temperatures, by steps, and cooling to room temperature after each step. Compare with progressive aging and step aging.

Interrupted Quenching

A quenching procedure in which the work piece is removed from the first quench while its temperature is still substantially higher than that of the quenchant, and then subjected to a second quenching system with a different cooling rate from the first.

Interval Test

A test that measures the heat extraction rates of various quenchants. It measures the increase in a quenchant's temperature when a standard bar of metal is quenched for five seconds. Faster quenchants will exhibit greater increases in temperature.

Ion Carburizing

A surface-hardening process in which carbon ions are diffused into a work piece in a vacuum through the use of high-voltage electrical energy. Synonymous with plasma carburizing and glow-discharge carburizing.

Ion Implantation

A process in which a beam of positive ions is projected towards and into a surface. It is carried out in partial vacuum, and the ions diffuse into the surface layer of the substrate. Typically this is carried out with nitrogen, giving a nitrided effect.

Ion Nitriding

A surface-hardening process in which nitrogen ions are diffused into a work piece in a vacuum through the use of high-voltage electrical energy. Synonymous with plasma nitriding and glow-discharge nitriding.

Ion Plating

A process in which positive ions produced in a glow discharge are attracted to the substrate, which is connected as the cathode. The ions are typically made by evaporation.

Ion Scattering Spectroscopy (ISS)

A surface-sensitive technique wherein the depth of analysis can be limited to the outermost atomic layer. In ISS, a low-energy, monoenergetic beam of ions is focused onto a solid surface, and the energy of the scattered ions is measured at a fixed angle. A primary beam of an inert gas (He+, Ne+, Ar+) is usually employed, and since the collision with the atoms on the surface can be considered as elastic, the energy loss identifies the element on the surface.

Irregular Powder

Powder made up of asymmetrical particles.

Isothermal Annealing

Austenitizing a ferrous alloy and then cooling to and holding at a temperature at which austenite transforms to a ferrite carbide aggregate that is relatively soft.

Isothermal Transformation

A change in phase that takes place at a constant temperature. The time required for transformation to be completed, and in some instances the time delay before transformation begins, depends on the amount of supercooling below / superheating above the equilibrium temperature for the same transformation.

Isothermal Transformation (IT) Diagram

A set of curves drawn using logarithmic time and linear temperature as coordinates, which define, for each level of temperature, the beginning and end of the transformation of the initial phase under isothermal conditions.


Jobbing Foundry

A foundry engaged in the manufacture of numerous types of castings.

Jominy Test

Synonymous with end-quench hardenability test.



A liquid fuel that is used in some HVOF thermal spray processes.

Killed Steel

Steel treated with a strong deoxidizer to reduce oxygen to a level where no reaction occurs between carbon and oxygen during solidification.


Free graphite that forms in molten hypereutectic cast iron as it cools. In castings, the Kish may segregate toward the cope surface, where it lodges at or immediately beneath the casting surface.

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