Jargon – Our Small Glossary


Different and unclear notions of heat treatment terms often lead to misunderstanding, uncertainty and sometimes even unwanted component properties. For example, what should be understood from the term ‘case hardening depth (CHD)’ following a case hardening process?

  • In this depth, should there also be a hardness of 60 HRc corresponding to the required surface hardness?
  • Maybe it is the depth, at which the hardness has sunken to that of the components core?

A clear definition of the CHD can, for example, be found in DIN EN 10052, which defines the terms for the heat treatment of ferrous materials. Our small glossary of “the mutual language” describes terms in reference to this standard. It should help to prevent misunderstanding and faults. The selection of terms is based on our practical experience. We welcome your suggestion for further additions.



Describes a procedure in which the work piece is heated to a specific temperature at which it is soaked and then cooled.


Isothermal transformation to bainite. This process consists of austenitising and finally quenching to a temperature above the beginning of the martensite temperature (Ms) at such a rate that any formation of ferrite and pearlite is prevented, and soaked at this temperature to partially or completely transform the austenite to bainite. The final cooling to room temperature is not at any specific rate.


γ-Mixed crystal – a solid solution of one or more elements in γ -iron, which exists in a stable state between 911 and 1392°C. The crystalline structure is face-centred cubic.


A heat treatment step in which the material is brought to a temperature, so that it becomes austenitic.



A constituent, which occurs between the temperature range of pearlite and martensite formation. In this constituent, ferrite is saturated with carbon, where the carbon is partly secreted as carbides.



Enrichment of a work piece with carbon and nitrogen through a thermochemical treatment above 723°C. The medium in which carbonitriding occurs should be specified, e.g. carbonitriding in gas (gas carbonitriding), carbonitriding in salt bath (salt bath carbonitriding).


Enrichment of the surface of a work piece with carbon through thermochemical treatment in the austenitic state. The medium in which carburising occurs should be specified, e.g. carburising in gas (gas carburising), carburising in plasma (plasma carburising).

Case hardening

A process consisting of carburising or carbonitriding with subsequent hardening.

Case hardening depth (CHD)

The case hardening depth is the vertical distance between the surface and the layer, whose hardness (according to Vickers) is HV1 = 550. Other test forces and limits may be used according to agreement.

Compound layer

It occurs during thermochemical treatment, directly on the surface and consists of one or more chemical compound(s). These are formed by the element(s) diffused and certain elements of the base material, e.g. a layer of nitride formed during nitriding.


The temperature of a work piece is reduced; this may occur in one or more steps. The medium in which cooling occurs should be specified, e.g. furnace, air, gas, oil, water.


Diffusion annealing

Is a special annealing process to prevent inhomogeneity in the work piece. It is also described as solution or homogenising annealing.

Direct hardening

From the carburising or carbonitriding temperature, hardening occurs through direct quenching. If necessary, first cooling to a temperature most appropriate for hardening occurs.

Double hardening

Following direct hardening, a second hardening at a lower temperature occurs.



A solid solution (mixed crystal) of one or more elements in α-iron, which pure iron below 911°C is in a stable state. The crystalline structure is face-centred cubic.



A process consisting of austenitising and quenching, in which an increase in hardness occurs through the transformation of austenite to martensite, and if necessary, to bainite.

Hardening and tempering

A process consisting of quench hardening and tempering at high temperatures in order to reach the required combination of the mechanical properties, particularly good toughness and ductility.


A process in which a work piece reaches the specified temperature in the entire cross-section through heating (temperature at the surface is reached) and equalising (temperature at the core is also reached).

Heat treatment

A process in which the process of a work piece is partially or completely exposed to thermal cycles, in order to change the properties and/or structure. If necessary, during the process the chemical composition of the work piece is altered (thermochemical treatment).


Induction hardening

Partial heating of a work piece via electromagnetic induction, using a medium or high frequency electric field, then quenching. Induction hardening is preferably applied for heat-treated steels.



It is formed through a non-diffusion transformation of austenite. Like ferrite, the crystalline structure is body-centred. In comparison to ferrite, it is saturated with carbon. Amongst other things, this results in a hardness increase.

Martensite starting temperature (Ms)

The temperature at which Martensite formation commences.



Enriching the surface of a component with nitrogen through a thermochemical treatment. If the process is carried out in a medium to which a certain quantity of oxygen has been added, ‘oxynitrding’ is referred to. The medium in which nitriding occurs should be specified, e.g. nitriding in gas (gas nitriding), nitriding in plasma (plasma nitriding).

Nitriding hardness depth (NHD)

The nitriding hardness depth is the vertical distance of the surface of a nitrided or nitrocarburised work piece to the point at which it corresponds to the hardness of a specified hardness limit (GL). The hardness limit (GL) is in general specified according to Vickers as HV 0.5. GH = (actual core hardness + 50) x HV 0.5 (each rounded to 10 HV).


Enrichment of the surface of a work piece with nitrogen and carbon in the formation of a compound layer through thermochemical treatment. The medium in which nitrocarburising occurs should be specified, e.g. nitrocarburising in gas (gas nitrocarburising), nitrocarburising in plasma (plasma nitrocarburising).


This process consists of autenitising and then cooling in still air. Other mediums with corresponding cooling speed can be used.



A constituent of ferrite and carbide lamella, which is formed during the transformation of austenite with eutectoid composition. The formation temperature is above the range for bainite formation.



A process in which cooling occurs more rapidly than in still air. The conditions in which quenching should be carried out should be specified precisely, e.g. quenching in gas (gas quenching), quenching in oil (oil quenching), quenching in water (water quenching).


Single hardening

Hardening occurs following cooling of the carburising or carbonitriding temperature to room temperature.


The stage of a temperature-time phase, in which a constant temperature is kept. Which temperature is meant should be specified. E.g. of the furnace, the work piece surface or the entire cross-section.

Soft annealing

A process to reduce the hardness of a work piece to the specified value.

Stress relieving

Internal stress is relieved substantially, without significant changes to the structure. The process consists of heating and soaking at a suitable temperature and then cooling at an appropriate rate.



A process, which in general occurs following hardening or another heat treatment, in order to bring the properties to the required level. It involves heating one or more times to a predefined temperature, in general <700°C, soaking at this temperature and finally cooling at an appropriate rate. In general, tempering leads to a reduction or hardness, in some cases to an increase of hardness.

Thermochemical treatment

Heat treatment to chemically alter the composition of the base material through exchange of matter with a suitable medium, for example, in carburising.