Heat Treating Definitions

If you are unfamiliar with our processes, or would like to learn more, the glossary below covers some of our primary heat treatment terms and definitions. Still have questions? Please contact us.

Age hardening

A process of aging, usually following rapid cooling or cold working, that increases hardness and strength and ordinarily decreases ductility.


A substance composed of two or more chemical elements, at least one of which is metal, which has metallic properties.

Alloy steel

A steel which achieves specific properties by the addition of alloying elements.


Heating to and holding at a suitable temperature, often at or above transformation, and then cooling at a rate suitable to produce specific microstructure results. This can result in reduced hardness, improved machinability, easier cold working, a desired microstructure, or other mechanical or physical properties.

Atmosphere (protective)

The gases surrounding the work in high temperature apparatus in metallurgical practice. The character of the protective atmosphere in usually to prevent high temperature oxidation of the work surface. The atmosphere may also be enriched with component gasses to produce specific chemical reactions with the work surface, such as carburizing and nitriding, also known as Controlled Atmospheres.


A solid solution of one or more elements in facecentered cubic iron. Unless indicated otherwise, the solute is generally assumed to be carbon.


Heating a ferrous alloy above its transformation range resulting in the formation of austenite.


Joining metals by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal. This may be accomplished by means of a torch (torch brazing), in a furnace (furnace brazing), or by dipping in a molten flux bath (dip or flux brazing).

Bright annealing

Annealing performed in a controlled atmosphere or vacuum so that surface oxidation is reduced, and the surface remains relatively bright. A common bright annealing atmosphere is hydrogen or dissociated ammonia (nitrogenhydrogen).


A carbon compound with one or more metallic elements.

Carbon steel

A metal alloy of iron and carbon, without substantial amounts of other alloying elements. Carbon content usually limited to 1.00%, silicon limited to 0.60%, and manganese to 1.65%. Also called plaincarbon steel.


The controlled atmosphere process of adding carbon to the surface of steel with the intent of making the surface layer harder to a specific case depth, often in the range of 0.005 to 0.080 inch depth. The controlled atmosphere component achieving this surface reaction is carbon monoxide, usually provided in the atmosphere gas and increased by the addition of methane.

Case hardening

A heat treatment in which the surface layer of a piece is made substantially harder than the interior either by (a) altering its composition and properties with a controlled atmosphere heat treatment, or (b) by selectively heating only the part surface by means of flame or induction, in both cases leaving the interior soft and ductile and thus creating a hard metal “case”.

Cast iron

An iron and carbon alloy, containing above 1.7% carbon.


A part produced by solidifying molten metal in a mold.


An element added to ferrous alloys which results in increased hardenability and corrosion resistance.

Cold rolled

Rolling at or near room temperature, used as a finishing process.

Controlled cooling

The predetermined manner of cooling from an elevated temperature, usually through a transition temperature, to achieve specific properties.


The internal portion of the metal part, as distinguished from the surface. The distinctions important in case hardened parts or in parts of large crosssection size.


The conversion of refined metal to its more stable oxide resulting in the gradual destruction of materials.


A sample piece of metal used to determine the properties of the more elaborate metal part after heat treating through hardness testing and microanalysis.


The result of heating in an atmosphere that reacts with the carbon in the part surface, causing the loss of carbon from the surface of a ferrous alloy.


Applied stress resulting in a change of dimensions.


The process by which materials disperse, usually in the direction from a high concentration to a low concentration. In atmosphere heat treating, diffusion is the limiting rate by which carbon or nitrogen achieve case depth from the surface.

Ductile iron (nodular iron)

A cast iron that is treated with a special alloy while in its liquid state so that the graphite is spherical rather than flake.


The ability of a material to deform without fracturing, measured by elongation or reduction of area in a tensile test, or by other means.


The increase in gage length upon tensile testing fracture, usually expressed as a percentage of the original gage length.


The loss of a material's ductility.


A weakness that develops in metal structures leading to fracture under repeated or fluctuating stresses, in some cases measured in millions of cycles.

Fatigue strength

Range of maximum stress that can be applied to the material for a specific number of cycles without causing fatigue failure, the stress being completely reversed within each cycle unless otherwise stated.

File hardness

Surface hardness determined by the use of a file of standardized hardness under the assumption that a metal which cannot be scored is as hard as, or harder than the file. This is a quick and relatively effective way of measuring surface hardness, although one of the the least accurate forms of hardness testing.


Two surfaces with a concave joint.

Flame hardening

The process of hardening the surface of a metal part by heating it with a high temperature flame followed by rapid cooling.


A chemical cleaning, flowing, and/or purifying agent. Often used as a protective covering to prevent the formation of, or facilitate the removal of, undesirable substances.


Hammering or pressing hot metal into a desired shape.


A component of a solid metal microstructure in which the space lattice pattern is continuous. The boundary of a grain is at the intersection with another grain of different lattice orientation.

Grain size

An index number corresponding to the average dimension of a grain when the individual sizes are fairly uniform. Grain sizes are reported in terms of number of grains per unit area or volume, average diameter, or as a grainsize number determined by area measurements.


One of the crystal forms of carbon.


The property that determines the ability of a ferrous alloy to harden.


The property of a substance determined by its ability to resist abrasion or indentation by another substance. Often measured in Rockwell or Brinell Numbers.

Heat treatment

Heating and cooling a solid metal or alloy to obtain desired conditions or properties.

Impact test

Fracturing a test bar at high velocity to determine the energy absorbed during failure.

Induction hardening

Using electrical induction to heat the surface of steel above the transformation range, followed by rapid cooling.

Isothermal annealing

An annealing process in which the steel is heated to the austenitizing temperature. The steel is then cooled to a temperature below the lower criticaltemperature and held for a specified amount of time in order to produce a homogenous structure in the material. This is mainly applicable for low carbon and alloy steels to improve their machinability.

Knoop hardness

Microhardness determined from the resistance of metal to indentation by a pyramidal diamond indenter. This test is used in very light loads under a microscope for metal parts of small size and thickness.


A photographic reproduction of any object that may be slightly reduced, of actual size, or magnified, not exceeding ten diameters.


The quality that determines the ease of shaping metal.


This microstructure occurs on cooling when steel is quenched from its austenite range fast enough to suppress upper transformation products.

Mechanical properties

Those properties, used to classify and identify a material, strength, ductility, hardness, impact resistance, and fracture toughness, are the most common properties used as a standard for material certification.


Visual examination by microscope and study of microstructures of metals and alloys.


The science of working or heating metals so as to give them certain desired characteristics or properties.

Microhardness test

Hardness test used when test samples are very small or thin, or when small regions in a composite sample or plating are to be measured.


Using a microscope at magnifications over ten diameters to determine the structure of polished and etched alloys and metals. See Knoop hardness.


Surface hardening a ferrous alloy by absorption of nitrogen in the surface at subcritical temperatures.


This process is used to eliminate all effects of prior processes and to render the part uniform in structure. The process consists of heating a steel to a temperature about 100F above the critical range and cooling in still air.

Isothermal annealing

An annealing process in which the steel is heated to the austenitizing temperature. The steel is then cooled to a temperature below the lower criticaltemperature and held for a specified amount of time in order to produce a homogenous structure in the material. This is mainly applicable for low carbon and alloy steels to improve their machinability.

Physical properties

Any property that is measurable, whose value describes a physical system, such as density, conductivity, thermal expansion, etc.


Using chemical or electrochemical processes to clean metal surfaces.


The measure of how much of a material is open space, as a result of fine holes or pores.

Post heating

Heating welded metal immediately after welding resulting in tempering, stress relief, or providing a controlled rate of cooling to minimize formation of a hard or brittle structure.

Precipitation hardening

Also known as Age Hardening, the material is heated to a specific temperature and held for a certain time period. This increases the hardness and yield strength of the material by means of a change in microstructure accomplished by the formation of metal alloy precipitates. It can be very beneficial for hardening finished machined parts with critical tolerances.


Used to minimize thermal shock on complex parts. This process consists of heating parts to a temperature below the austenitizing temperature prior to austenitizing.

Process annealing

Softening the alloy or metal for further cold working by heating a it to a temperature close to, but below, the lower limit of the transformation range and then cooling.


Rapid cooling from an elevated temperature in various mediums.

Reduction in area

The difference between the original crosssectional area of a tensile test piece and that of the smallest necked area at the point of fracture. This result is usually stated as percentage of the original area and is a measure of ductility.


The surface oxidization formed on a metal by heating or casting in air or other oxidizing atmospheres.

Shear strength

Maximum shear load a material can withstand before failure occurs divided by its cross sectional area. A shear load is a force that produces a sliding failure along a plane that is parallel to the direction of the force.


Assuring the desired metallurgical reaction occurs to all parts in a furnace load by holding them at temperature for a sufficient time.


Joining two or more metals by the melting and flowing of alloys that have relatively low melting points into a joint, most commonly leadbased or tin base solders.


An alloy of iron and carbon in which the carbon content is between about 0.05% and 1.0% and is malleable at some temperature in the solid state.

Stress relieving

Reducing residual stresses in a metal object by heating to a temperature below the lower critical austenitizing temperature and holding for a sufficient time. Applied to relieve stresses induced by casting, quenching, normalizing, machining, cold working or welding.

Stress corrosion cracking

Spontaneous failure of metals, usually by slow crack propagation under corrosion and stress conditions.

Stress risers

A location in an object where stress is concentrated, as a result of sharp changes in contour or surface defects such as cracks.

Residual stresses

Stresses that remain in a material after the cause of the stress has been removed.


A soft annealing process that forms an even distribution of spheroidal carbides in the steel, which will make the material softer and tougher. This is used to increase the material’s machinability.


A process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening to reduce the excess hardness in the material. This is done by heating the metal to a temperature below the materials critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For example, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.

Tensile strength

Measurement of the force required to pull a material to the point where it breaks.

Thermal stresses

Stresses in metal resulting from nonuniform heating and cooling. This result can occur when a part has large cross section differences or when the material is heated or cooled too rapidly.


The ability of a metal to deform plastically and to absorb energy in the process before fracture. Toughness is a combination of strength and ductility.

Thermal expansion

The tendency of material to change in volume in response to a change in temperature.

Vacuum brazing

A high­end joining technology that results in parts with extremely strong joints and with no residual corrosive flux. It is a process in which two base metals, such as aluminum plates, are joined using a filler metal that possesses a melting point below that of the base metal.

Vacuum furnace

A furnace using low atmospheric pressures rather than a protective gas atmosphere like typical heat-treating furnaces. Vacuum furnaces are categorized as hot wall or cold wall, depending on the location of the heating and insulating components.