Heat Treatment Terminology: A to Z


The process of holding metals at room temperature or at a predetermined temperature for the purpose of increasing their hardness and strength by precipitation.

Air Hardening

The hardening process consists of heating the components above the critical (normalizing) temperature, holding at this temperature for one hour per inch of thickness cooling at a rate fast enough to allow the material to transform to a much harder, stronger structure, and then tempering.


A heat treatment process used to soften the metal and produce desired changes in its microstructure. Annealing is used to improve machine-ability, dimensional stability, relieve stresses and define the crystalline grain structure of the material.


Austempering is the process of heat treatment where the steel is heated to austenitizing temperature, held there for sufficient period of time to produce homogeneous 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).


The process of forming austenite by heating an iron/carbon alloy above its transformation temperature.


The constituent produced when austenite transforms at a temperature below that at which pearlite is produced and above that at which martensite is formed. This constituent may be produced by austempering.

Blank Carburizing (Nitriding)

Nitriding is a surface hardening treatment, where nitrogen is added to the surface of steel parts either using a gaseous process where dissociated ammonia as the source or an ion or plasma process where nitrogen ions diffuse into the surface of components.

Bright Annealing

A process of annealing usually carried out in a controlled furnace atmosphere so that surface oxidation is reduced to a minimum and the surface remains relatively bright.


Heating to such a high temperature that the properties of the material are permanently impaired by incipient fusion of oxide penetratiom


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.

Carburizing Depth

It is the margin distance from the carburized workpiece surface at which the carbon content reaches the specified limit, typically C = 0.35 %. Commonly, the carburization depth  is around 1 mm, with deep carburization it is up to 2 mm.

Case Hardening

Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal at the surface.

Cooling Rate

Cooling rates are usually expressed in terms of the time taken for product to become ‘3⁄4 cooled’ or ‘7⁄8 cooled’. This is calculated as the time for 3⁄4 or 7⁄8 of the initial difference in temperature between the product and the cooling medium to be removed.

Cyaniding (Cyanide Hardening)

Cyaniding is a case-hardening process that is fast and efficient; it is mainly used on low-carbon steels. The part is heated to 871–954 °C (1600–1750 °F) in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide.


Decarburization is a loss of carbon in the surface-adjacent zone of the material (Gunnarson, 1963). Contributing factors are high temperatures and large amounts of oxygen in the atmosphere. At higher temperatures, the diffusion rate of carbon is so large that longer annealing times lead to decarburization.


Shape Distortion (warpage) as a result of heat treatment is a result of processing or design issues rather than the expected phase changes of the material. Rapid heating can cause stresses to develop in parts because of excessive temperature gradients.


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.

Flame Hardening

Flame hardening is a heat treatment process where oxyfuel gas flames are directly impinged onto the gear-tooth surface area to be hardened which is then subjected to quenching. It results in a hard surface layer of martensite over a softer interior core. Its cost is considerably less than induction hardening.

Full Annealing

Full annealing is the process by which the distorted cold-worked lattice structure is changed back to one that is strain-free through the application of heat. This is a solid-state process and is usually followed by slow-cooling in the furnace. Recovery is the first stage of annealing.


Hardenability describes how deep a metal can be hardened upon quenching from high temperature, and can also be referred to as the depth of 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.

Hardening Temperature

The temperature that metal is heated to the point of hardening.

Hardness Penetration

In a hardened work piece, the depth to which a certain hardness is present.

Heating time

The time until the surface of a work piece being heated attains the desired temperature.

Heat Temperature

Heating and cooling a solid metal in such a way as to obtain certain condition or properties. Heating for the sole purpose of hor working is excluded from the meaning of this definition.

Holding Time

Time during which a work piece is held at a certain temperature afrer the core has reched this temperature (does not comprise heating time).


Heating a metal to high temperature in order to ensure uniform distribution of components. This process is typically used to reduce chemical segregation and coring in cast structures and to produce a more homogeneous microstructure in hot worked materials.

Immersion Hardening

Hardening the surface of a work piece subsequent to heating the case by short-time immersion in high-temperature metal of salt baths.

Immersion Time

Time between immersion of a work piece and withdrawal from the bath.

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.

Interrupted Quenching (Time Quenching)

Martempering is also known as stepped quenching or interrupted quenching. In this process, steel is heated above the upper critical point (above the transformation range) and then quenched in a salt, oil, or lead bath kept at a temperature of 150-300 °C.

Isothermal Annealing

Isothermal annealing is a heat treatment process similar to complete annealing with similar output, including creating pieces with reduced residual stress, improved machinability, and homogenized grain structures.


Annealing white cast iron in such a way that some or all of the combined carbon is transformed to graphite or, in some instances, part of the carbon is removed completely.


Martempering is also known as stepped quenching or interrupted quenching. In this process, steel is heated above the upper critical point (above the transformation range) and then quenched in a saltoil, or lead bath kept at a temperature of 150-300 °C.


A constituent formed in steels by rapid quenching, consisting of a supersaturated solid solution of carbon in iron. It is formed by the breakdown of austenite when the rate of cooling is large enough to prevent pearlite forming.


Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on titanium, aluminium and molybdenum.


Normalizing involves heating a material to an elevated temperature and then allowing it to cool back to room temperature by exposing it to room temperature air after it is heated. This heating and slow cooling alters the microstructure of the metal which in turn reduces its hardness and increases its ductility.


Heating a metal to such a high temperature that its properties are impaired. When the original properties cannot be restored by further heat treating, the overheating is known as burning.

Overheating Sensitivity

Heat intolerance is an unusual sensitivity to heat. People with heat intolerance may feel hot when others feel comfortable or even cold. They may also have an unusual response to heat, such as intense sweating or anxiety. Heat intolerance is not a disease, but it can be a symptom of an underlying medical condition.


A patent is an exclusive right granted for an invention, which is a product or a process that provides, in general, a new way of doing something, or offers a new technical solution to a problem. To get a patent, technical information about the invention must be disclosed to the public in a patent application.


Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons.

Precipitation Hardening (Age Hardening)

Age hardening, also known as precipitation hardening, is a type of heat treatment that is used to impart strength to metals and their alloys. It is called precipitation hardening as it makes use of solid impurities or precipitates for the strengthening process.


To heat something to a particular temperature before putting it to be heated inside.

Process Annealing

Process annealing is carried out intermittently during the working of a piece of metal to restore ductility lost through repeated hammering or other working. Full annealing is done to give workability to such parts as forged blanks destined for use in the machine-tool industry.


In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties.

Quenching Temperature

The process of quenching is a progression, beginning with heating the sample. Most materials are heated to between 815 and 900 °C (1,500 to 1,650 °F), with careful attention paid to keeping temperatures throughout the workpiece uniform.

Quenching Time

Quench time is a measure of how long a steel casting spends at high temperature during quenching. While castings cool, the quench water warms; therefore, there is a one-to-one relationship between the two.

Shock Tempering

Shock hardening is a process used to strengthen metals and alloys, wherein a shock wave produces atomic-scale defects in the material’s crystalline structure.


The purpose of the soaking stage is to keep the metal at the appropriate temperature until the desired internal structure takes shape.

Solution Heat Treatment

Solution treatment is a heat treating process that heats alloys to a specific temperature, sustaining that temperature long enough to cause one or more constituents to enter into a solid solution and then rapidly cooled to maintain the solution’s properties.

Spheroidizing (Spheroidize Annealing)

Spheroidize Annealing is a type of annealing process involving controlled heating and cooling to produce a microstructure consisting of spherical (spheroidal) carbides in a ferrite matrix.

Stabilizing Treatment

Thermal Stabilization involves heating the alloy to higher temperatures and maintaining the desired temperature for a set amount of time.

Stress Relieving

Stress Relieving is the treatment of a metal or alloy by heating to a predetermined temperature below its lower transformation temperature followed by cooling in air.

Subcritical Annealing

Sub-critical annealing consists of heating the steel to below the lower critical temperature. This type of annealing is mainly carried out in the temperature range 630° – 700°C to reduce hardness by allowing recrystallisation of the microstructure to occur.

Sub-zero Treatment

Sub-zero treatments are treatments in which components are cooled below room temperature. There can be many reasons to do this, but the main ones are to remove retained austenite from quenched components or tools, to increase the wear resistance of tools or to stabilise the component.

Temper Brittleness

Temper embrittlement refers to the decrease in notch toughness of alloy steels when heated in, or cooled slowly through, a temperature range of 400°C to 600°C.


Process of improving the characteristics of a metal, especially steel, by heating it to a high temperature, though below the melting point, then cooling it, usually in air.


Through hardening, also known as neutral hardening, is the process of strengthening a steel alloy using a rapid quench for increased hardness throughout the material.

Transformation Range (Critical Range)

A temperature range in which an internal change takes place within a metal. Also termed transformation range.