Heat
Treatment Of Steel Terminology
Below are
some common heat treating terminology as used by individuals in the steel
industry. These terms are not being
used in a specification and no specific temperatures are identified.
Aging:
Describes a time–temperature-dependent change in the properties of
certain alloys. Except for strain aging and age softening, it is the result of
precipitation from a solid solution of one or more compounds whose solubility
decreases with decreasing temperature. For each alloy susceptible to aging,
there is a unique range of time–temperature combinations to which it will
respond.
Annealing:
A 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 but also to simultaneously produce desired changes in other properties or
in microstructure. The purpose of such changes may be, but is not confined to,
improvement of machinability; facilitation of cold working; improvement of
mechanical or electrical properties; or increase in stability of dimensions. The
time–temperature cycles used vary widely both in maximum temperature attained
and in cooling rate employed, depending on the composition of the
material, its condition, and the results desired.
Bright
Annealing:
Annealing in a protective medium to prevent discoloration of
the bright surface.
Cycle
Annealing:
An annealing process employing a predetermined and closely
controlled time–temperature cycle to produce specific properties or
microstructure.
Flame
Annealing:
Annealing in which the heat is applied directly by a flame.
Full
Annealing:
Austenitizing and then cooling at a rate such that the
hardness of the product approaches a minimum.
Graphitizing:
Annealing in such a way that some or all of the carbon is precipitated as
graphite.
Intermediate
Annealing:
Annealing at one or more stages during manufacture and before
final thermal treatment.
Isothermal
Annealing:
Austenitizing and then cooling to and holding at a temperature
at which austenite transforms to a relatively soft ferrite-carbide aggregate.
Process
Annealing: An imprecise term used to denote various treatments that
improve workability.
Quench
Annealing:
Annealing an austenitic alloy by Solution
Heat Treatment.
Spheroidizing:
Heating and cooling in a cycle designed to produce a spheroidal or
globular form of carbide.
Austempering:
Quenching from a temperature above the transformation range, in a medium
having a rate of heat abstraction high enough to prevent the formation of high
temperature transformation products, and then holding the alloy, until
transformation is complete, at a temperature below that of pearlite formation
and above that of martensite
formation.
Austenitizing:
Forming austenite by heating into the transformation range (partial
austenitizing) or above the transformation range (complete austenitizing). When
used without qualification, the term implies complete austenitizing.
Bluing:
A treatment of the surface of iron-base alloys, usually in the form of
sheet or strip, on which, by the action of air or steam at a suitable
temperature, a thin blue oxide film
is formed on the initially scale-free surface, as a
means of improving appearance and resistance to corrosion. This term is also
used to denote a heat treatment of springs after fabrication, to reduce the
internal stress created by coiling and forming.
Carbon
Potential:
A measure of the ability of an environment containing active
carbon to alter or maintain, under prescribed conditions, the carbon content of
the steel exposed to it. In any particular environment, the carbon level
attained will depend on such factors as temperature, time, and steel
composition.
Carbon
Restoration:
Replacing the carbon lost in the surface layer from previous
processing by carburizing this layer to substantially the original carbon level.
Carbonitriding:
A case-hardening process in which a suitable ferrous material is heated
above the lower transformation temperature in a gaseous atmosphere of such
composition
as to cause simultaneous absorption of carbon and
nitrogen by the surface and, by diffusion, create a concentration gradient. The
process is completed by cooling at a rate that produces the desired properties
in the work piece.
Carburizing:
A process in which carbon is introduced into a solid iron-base alloy by
heating above the transformation temperature range while in contact with a
carbonaceous
material that may be a solid, liquid, or gas.
Carburizing is frequently followed by quenching to produce a hardened case.
Case:
1) The surface layer of an iron-base alloy that has been suitably altered
in composition and can be made substantially harder than the interior or core by
a process of case hardening; and 2) the term case is also used to designate the
hardened surface layer of a piece of steel that is large enough to have a
distinctly softer core or center.
Cold
Treatment:
Exposing to suitable subzero temperatures for the purpose of
obtaining desired conditions or properties, such as dimensional or
microstructural stability. When the treatment involves the transformation of
retained austenite, it is usually followed by a tempering treatment.
Conditioning
Heat Treatment:
A preliminary heat treatment used to prepare a material
for a desired reaction to a subsequent heat treatment.
Controlled
Cooling:
A term used to describe a process by which a steel object is
cooled from an elevated temperature, usually from the final hot-forming
operation in a predetermined manner of cooling to avoid hardening, cracking, or
internal damage.
Core:
1) The interior portion of an iron-base alloy that after case hardening
is substantially softer than the surface layer or case; and 2) the term core is
also used to designate
the relatively soft central portion of certain
hardened tool steels.
Critical
Range
or
Critical
Temperature
Range
: Synonymous with
Transformation
Range
, which is preferred.
Decarburization:
The loss of carbon from the surface of an iron-base alloy as the result
of heating in a medium that reacts with the carbon.
Drawing:
Drawing, or drawing the temper, is synonymous with Tempering,
which is preferable.
Eutectic
Alloy:
The alloy composition that freezes at constant temperature similar
to a pure metal. The lowest melting (or freezing) combination of two or more
metals. The alloy
structure (homogeneous) of two or more solid phases
formed from the liquid eutectically.
Hardenability:
In a ferrous alloy, the property that determines the depth and
distribution of hardness induced by quenching.
Hardening:
Any process of increasing hardness of metal by suitable treatment,
usually involving heating and cooling.
Hardening,
Case:
A process of surface hardening involving a change in the
composition of the outer layer of an iron-base alloy followed by appropriate
thermal treatment. Typical
case-hardening processes are Carburizing,
Cyaniding, Carbonitriding,
and Nitriding.
Hardening,
Flame:
A process of heating the surface layer of an iron-base alloy above
the transformation temperature range by means of a high-temperature flame,
followed by
quenching.
Hardening,
Precipitation:
A process of hardening an alloy in which a constituent
precipitates from a supersaturated solid solution. See also Aging.
Hardening,
Secondary:
An increase in hardness following the normal softening that
occurs during the tempering of certain alloy steels.
Heating,
Differential:
A heating process by which the temperature is made to vary
throughout the object being heated so that on cooling, different portions may
have such different physical properties as may be desired.
Heating,
Induction:
A process of local heating by electrical induction.
Heat
Treatment:
A combination of heating and cooling operations applied to a
metal or alloy in the solid state to obtain desired conditions or properties.
Heating for the sole purpose of hot working is excluded from the meaning of this
definition.
Heat
Treatment, Solution:
A treatment in which an alloy is heated to a
suitable temperature and held at this temperature for a sufficient length of
time to allow a desired constituent to enter into solid solution, followed by
rapid cooling to hold the constituent in solution. The material is then in a
supersaturated, unstable state, and may subsequently exhibit Age
Hardening.
Homogenizing:
A high-temperature heat-treatment process intended to eliminate or to
decrease chemical segregation by diffusion.
Isothermal
Transformation:
A change in phase at constant temperature.
Malleablizing:
A process of annealing white cast iron in which the combined carbon is
wholly or in part transformed to graphitic or free carbon and, in some cases,
part of the carbon is removed completely.
Maraging:
A precipitation hardening treatment applied to a special group of
iron-base alloys to precipitate one or more intermetallic compounds.
Martempering:
A hardening procedure in which an austenitized ferrous workpiece is
quenched into an appropriate medium whose temperature is maintained
substantially at the Ms of the
workpiece, held in the medium until its temperature is uniform throughout but
not long enough to permit bainite to form, and then cooled in air. The treatment
is followed by tempering.
Nitriding:
A process of case hardening in which an iron-base alloy of special
composition is heated in an atmosphere of ammonia or in contact with nitrogenous
material. Surface
hardening is produced by the absorption of nitrogen
without quenching.
Normalizing: A process in which
an iron-base alloy is heated to a temperature above the transformation range and
subsequently cooled in still air at room temperature.
Overheated:
A metal is said to have been overheated if, after exposure to an unduly
high temperature, it develops an undesirably coarse grain structure but is not
permanently damaged. The structure
damaged by overheating can be corrected by suitable heat treatment or by
mechanical work or by a combination of the two. In this respect it differs from
a Burnt structure.
Preheating:
Heating to an appropriate temperature immediately prior to austenitizing
when hardening high-hardenability constructional steels, many of the tool
steels, and heavy sections.
Quenching:
Rapid cooling. When applicable, the following more specific terms should
be used: Direct Quenching, Fog Quenching, Hot Quenching, Interrupted Quenching,
Selective
Quenching, Slack Quenching, Spray Quenching, and Time Quenching.
Direct
Quenching:
Quenching carburized parts directly from the carburizing
operation.
Fog
Quenching:
Quenching in a mist.
Hot
Quenching:
A term used to cover a variety of quenching procedures in
which a quenching medium is maintained at a prescribed temperature above 160
degrees F (71 degrees C).
Interrupted
Quenching:
A quenching procedure in which the workpiece is removed from
the first quench at a temperature substantially higher than that of the
quenchant and is then subjected to a second quenching system having a different
cooling rate than the first.
Selective
Quenching:
Quenching only certain portions of a workpiece.
Slack
Quenching:
The incomplete hardening of steel due to quenching from the
austenitizing temperature at a rate slower than the critical cooling rate for
the particular steel, resulting in the formation of one or more transformation
products in addition to martensite.
Spray
Quenching:
Quenching in a spray of liquid.
Time
Quenching:
Interrupted quenching in which the duration of holding in the
quenching medium is controlled.
Soaking:
Prolonged heating of a metal at a selected temperature.
Stabilizing
Treatment:
A treatment applied to stabilize the dimensions of a workpiece
or the structure of a material such as 1) before finishing to final dimensions,
heating a workpiece to or somewhat beyond its operating temperature and then
cooling to room temperature a sufficient number of times to ensure stability of
dimensions in service; 2 ) transforming retained austenite in those materials
that retain substantial amounts when quench hardened (see cold treatment); and
3) heating a solution-treated austenitic stainless steel that contains
controlled amounts of titanium or niobium plus tantalum to a temperature below
the solution heat-treating temperature to cause precipitation of finely divided,
uniformly distributed carbides of those elements, thereby substantially reducing
the amount of carbon available for the formation of chromium carbides in the
grain boundaries on subsequent exposure to temperatures in the sensitizing
range.
Stress Relieving: A process to
reduce internal residual stresses in a metal object by heating the object to a
suitable temperature and holding for a proper time at that temperature.
This treatment may be applied to relieve stresses induced by casting,
quenching, normalizing, machining, cold working, or welding.
Temper
Carbon:
The free or graphitic carbon that comes out of solution usually
in the form of rounded nodules in the structure during Graphitizing
or Malleablizing.
Tempering:
Heating a quench-hardened or normalized ferrous alloy to a temperature
below the transformation range to produce desired changes in properties.
Double
Tempering:
A treatment in which quench hardened steel is given two
complete tempering cycles at substantially the same temperature for the purpose
of ensuring completion of the tempering reaction and promoting stability of the
resulting microstructure.
Snap
Temper:
A precautionary interim stress-relieving treatment applied to
high hardenability steels immediately after quenching to prevent cracking
because of delay in tempering them at the prescribed higher temperature.
Temper
Brittleness: Brittleness that results when certain steels are held
within, or are cooled slowly through, a certain range of temperatures below the
transformation range.
The brittleness is revealed by notched-bar
impact tests at or below room temperature.
Transformation
Ranges
or
Transformation
Temperature
Ranges
: Those ranges of temperature within which austenite forms during heating
and transforms during cooling. The two ranges are distinct, sometimes
overlapping but never coinciding. The limiting temperatures of the ranges depend
on the composition of the alloy and on the rate of change of temperature,
particularly during cooling.
Transformation
Temperature:
The temperature at which a change in phase occurs. The term
is sometimes used to denote the limiting temperature of a transformation range.
The following symbols are used for iron and steels:
- Accm
-
In hypereutectoid steel, the temperature at which the solution of
cementite in austenite is completed during heating
- Ac1
-
The temperature at which austenite begins to form during heating
- Ac3
-
The temperature at which transformation of ferrite to austenite is
completed during heating
- Ac4
-
The temperature at which austenite transforms to delta ferrite
during heating
- Ae1,
Ae3, Aecm, Ae4 -
The temperatures of
phase changes at equilibrium
- Arcm
-
In hypereutectoid steel, the temperature at which precipitation of
cementite starts during cooling
- Ar1
-
The temperature at which transformation of austenite to ferrite or
to ferrite plus cementite is completed during cooling
- Ar3
-
The temperature at which austenite begins to transform to ferrite
during cooling
- Ar4
-
The temperature at which delta ferrite transforms to austenite
during cooling
- Ms
-
The temperature at which transformation of austenite to martensite
starts during cooling
- Mf
-
The temperature, during cooling, at which transformation of
austenite to martensite is substantially completed
All these changes except the formation of martensite occur at lower temperatures
during cooling than during heating, and depend on the rate of change of
temperature.
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