Through Hardening Processes
Through Hardening Processes|
Through hardened gears are heated to a required temperature and cooled
in the furnace or quenched in air, gas or liquid. Through hardening may
be used before or after the gear teeth are formed.
There are generally three methods of heat treating through hardened
gearing. In ascending order of hardness for a particular type of steel
they are; annealing, normalizing (or normalizing and tempering), and
quenching and tempering. Modifications of quench hardening, such as
austempering and martempering, occur infrequently for steel gearing and
are, therefore, not discussed. Austempering is used, however, for
through hardened (approximately 300 to 480 HB) ductile cast iron
Annealing consists of heating steel or other ferrous alloys to 1475-1650_F
(802-899_C), and furnace cooling to a prescribed temperature [generally
below 600_F (316_C)]. Annealing may be the final treatment (when low
hardness requirements permit) or is typically a pretreatment applied to
the cast or wrought gear blank in the “rough.” It results in low
hardness and provides improved machinability and dimensional stability
(minimum residual stress).
Normalizing consists of heating steel or other ferrous alloys to 1600-1800_F
(871-982 _C) and cooling in still or circulated air. Normalizing results
in higher hardness than annealing, with hardness being a function of
grade of steel and the part section thickness. However, with plain
carbon steels containing up to about 0.4 percent carbon, normalizing
does not increase hardness significantly more than annealing, regardless
of section size.
Alloy steels are normally tempered at 1000-1250_F (538-677_C) after
normalizing for uniform hardness, dimensional stability and improved machinability.
- Normalizing and Annealing for Metallurgical
The normalizing and annealing processes are frequently used,
either singularly or in combination, as a homogenizing
for alloy steels. These processes are used in wrought steel to reduce
metallurgical non-uniformity such as segregated alloy microstructures
(banding) and distorted crystaline microstructures from mechanical
Cycle annealing is a term applied to a special normalize/temper process
in which the parts are rapidly cooled to 800-1000_F (427-538_C) after
normalizing at 1600-1750_F (871-954_C), followed by a 1200_F (649_C)
temper with controlled
cooling to 600_F (316_C).
- Quench and Temper.
The quench and temper process on ferrous alloys involves heating to form
austenite at 1475-1600_F (802-871_C), followed
by rapid quenching. The rapid cooling causes the
gear to become harder
and stronger by formation of martensite. The gear is then tempered to a
specific temperature, generally below 1275_F(691_C), to achieve the
desired mechanical properties. Tempering reduces the material hardness
and mechanical strength but improves the material ductility and
toughness (impact resistance). Selection of the tempering temperature
must be based upon the specified hardness range, material composition,
and the as
quenched hardness. The tempered hardness varies inversely
with tempering temperature. Parts are normally air cooled from tempering
The hardness and mechanical properties achieved from the quench and
temper process are higher than those achieved from the normalize or
The quench and temper process should be specified for the following
- When the
gear application stress analysis indicates that the
hardness and mechanical properties for the specified material grade can
best be achieved by the quench and temper process.
- When the hardness and mechanical properties required for a given
gear application can be achieved more economically by quench and temper
of a lower alloy steel, than by normalizing or annealing.
- When it is necessary to develop mechanical properties (core
properties) in sections of the part which will not be altered by
subsequent heat treatments (for example
nitriding, flame hardening,
induction hardening, electron beam hardening, and laser hardening).
- Processing Considerations.
The major factors of the quench and temper process that influence
hardness and material strength are:
The steel carbon content determines the maximum surface hardness which
can be achieved, while the alloy composition determines the hardness
gradient which can be achieved through the part.
- Material chemistry and hardenability
- Quench severity
- Section size
- Time at temperature
Tempering lowers hardness and strength, which improves ductility and
toughness or impact resistance. The tempering temperature must be
carefully selected based upon the specified hardness range, the quenched
hardness of the part,
and the material. The optimum tempering temperature is the highest
temperature possible while maintaining the specified hardness range.
Hardness after tempering varies inversely with the tempering temperature
used. Parts are normally air cooled from the tempering temperature.
CAUTION: Some steels can become brittle and unsuitable for service if
tempered in the temperature range of 800-1200_F
(425-650_C). This phenomenon is called “temper brittleness” and is
generally considered to be caused by segregation of alloying elements or
precipitation of compounds at ferrite and prior austenite grain
If the part under consideration must be tempered in this range,
investigate the specific material’s susceptibility to temper brittleness
and proceed accordingly. Molybdenum content of 0.25-0.50 percent has
been shown to eliminate temper brittleness in most steels. Temper
brittleness should not be confused with the tempering embrittlement
from tempering in a lower range (500-600_F) often referred to as “500_F
- Designer Specification.
The designer should specify the following on the drawing.
- Grade of steel
- Quench and temper to a hardness range.
The hardness range should be a 4 HRC or 40 HB point range. The designer
should not specify a tempering temperature range on the drawing. It is
best to specify a hardness range and allow the heat treater to select
the tempering temperature to obtain the specified hardness. Specifying
both tempering temperatures and hardness ranges on a drawing causesan impractical situation for the heat treater. Tempering below 900_F(482_C)
should be approved by the purchaser.
- Any testing required.
For example, hardness tests, or any non-destructive tests such as
magnetic particle inspection or dye penetrant inspection, including the
frequency of testing.
- Specified Hardness.
The specified hardness of through hardened gearing is generally measured
on the gear tooth end face and rim section. Historically, this has been
interpreted to mean that the specified hardness must be met at this
location. Designers often interpret this to mean that minimum hardness
is to be obtained at the roots of teeth for gear rating purposes. Since
depth of hardening depends upon grade of steel (hardenability),
controlling section size and heat treat practice, achieving specified
hardness on these surfaces may not necessarily insure hardness at the
roots of teeth. If
gear root hardness is critical to a specific design
criteria, the gear tooth root hardness should be specified. However,
care should be taken to avoid needlessly increasing material costs by
changing to a higher hardenability steel where service life has been
- Maximum Controlling Section Size.
The maximum controlling section size is based upon the hardenability of
alloy steel for through hardened
- Additional Information.
For more information, consult the following:
The ASM Handbook, Volume 4, Heat Treating, 8th or 9th edition.
Military specification MIL-H-6875 and Mil-STD-1684.
- Stress Relief.
Stress relief is a thermal cycle used to relieve residual stresses
created by prior heat treatments, machining, cold working, welding, or
other fabricating techniques. The ideal temperature range for full
stress relieving is 1100-1275_F (593-691_C). Lower temperatures are
sometimes used when 1100_F (593_C) temperatures would reduce hardness
below the specified minimum. Lower temperatures with longer holding
times are sometimes used.
NOTE: Stress relief below1100_F(593_C) reduces the effectiveness. Stress
relief below 900_F(482_C) is not recommended.
- Heavy Draft, Cold Drawn, Stress Relieved Steel Bars.
Heavy draft, cold drawn, stress relieved bars may be used as an
quench and tempered steel. However, fatigue properties of
this steel may not be equivalent to quench and tempered steel with the
same tensile properties.
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