Concept of The Gear Heat Treatment
|Concept in Gear Heat Treatment
- Annealing - Full.
Full 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). This
treatment forms coarse lamellar pearlite, the best
microstructure for machinability of low and medium
carbon steels. Unless otherwise stated, annealing is
assumed to mean full annealing.
- Annealing - Spheroidizing.
annealing is a process of heating and cooling steel
that produces a globular carbide in a ferritic matrix.
This heat treatment results in the best machinability
for high carbon (0.60 percent carbon or higher) and
Austempering is a heat treat process
consisting of quenching a ferrous alloy (steel or
ductile iron) from a temperature above the transformation
range in amediumhaving a rate of cooling
sufficiently high to prevent high temperature transformation
products, and maintaining the alloy temperature
within the bainitic range until desired transformation
is obtained. The bainitic transformation
range is below the pearlitic range, but above themartensitic
range. Austempering is applied to steels and,
more recently in the development stage for ductile
iron gearing .
Austenite in ferrous alloys is amicrostructural
phase consisting of a solid solution of carbon
and alloying elements in face-centered cubic
crystal structured iron.
- Austenitizing Temperature.
The temperature at
which ferrous alloys undergo a complete microstructural
phase transformation to austenite.
Bainite is a microstructural phase resulting
from the transformation of austenite, and
consists of an aggregate of ferrite and iron carbide.
Its appearance is feathery if formed in the upper portion
of the bainite transformation range, and acicular
if formed in the lower portion.
Carbon is the principal hardening element
in steel, and it’s amount determines the maximumhardness
obtainable. Generally as carbon is increased,
tensile strength and wear resistance increase;
however, ductility and weldability decrease.
A modified form of gas carburizing,
in which steel (typically plain carbon and very low alloy) is heated
(788-899_C) in an ammonia enriched carburizing
atmosphere. This results in simultaneous absorption
of carbon and nitrogen, which results in the formation
of complex nitrides in a high carbon case.
- Carburizing Gas.
Gas carburizing consists of
heating and holding low carbon or alloy steel (less than 0.30 percent
carbon) at 1650-1800_F
(899-982_C) in a controlled carbonaceous atmosphere,
which results in the diffusion of carbon into
the part (0.70-1.00 percent carbon is typically obtained
at the surface). Temperatures above 1800_F
(982_C) may be ultilized in specialized equipment
such as vacuum carburizers. After carburizing, parts are either cooled
to 1475-1550_F (802-843_C) and
held at this temperature to stabilize and then direct
quenched; or slow cooled and reheated to 1475-1550_F (802-843_C) and quenched.
- Case Depth of Carburized Components.
case depth for carburized gearing may be defined in
several ways including effective case depth, etched
case depth, total case depth, and depth to 0.40 percent
carbon. The carburized case depth referred to in
this Manual will be effective case depth. Carburized
case depth terms are defined as follows:
- Effective case depth.
The effective case
depth is the hardened depth to HRC 50 at 0.5 tooth
height and mid face width, normal to the tooth surface.
- Etched case depth.
Etched case depth is determined
by etching a sample cross-section with nitric
acid, and measuring the depth of the darkened
area. The etched case approximates the effective
case. Hardness survey is preferred for contral purposes.
- Total case depth.
he total case depth is the
depth to which the carbon level of the case has decreased
to the carbon level of the base material. This
is approximately 1.5 times the effective case depth.
- Case depth to 0.40 percent carbon.
case depth is less frequently referred to as the depth
to 0.40 percent carbon. This depthmay bemeasured
by analyzing the carbon content or estimating based
on microstructure. Estimating based on microstructure
ignores the hardenability of the base material
and is not as accurate ameasurement as directly analyzing
the carbon level. There is poor correlation between microstructure
readings and material strength
gradients using this method.
- Case Depth of Flame or Induction Harden Components.
This is defined as the depth at which the
hardness is 10HRC points below the minimum specified
- Case Depth of Nitrided Components.
case depth is defined as the depth at which the hardness
is equivalent to 105 percent of the measured
core hardness. The case depth is determined by amicrohardness
tester and measured normal to the tooth
surface at 0.5 tooth height and mid face width.
- Case Hardness.
Case Hardness is the microhardness measured perpendicular to the tooth surface
at a depth of 0.002 to 0.004 inches (0.05 to 0.10mm) at 0.5 tooth height and mid face width.
Cementite is a hard microstructure
phase otherwise known as iron carbide (Fe3C) and
characterized by an orthorhombic crystal structure.
- Combined Carbon.
The amount of carbon in
steel or cast iron that is present in other than elemental
- Core Hardness.
Core Hardness for
tooth design purposes is the hardness at the intersection
of the root diameter and the centerline of the
tooth at mid face width on a finished gear.
- D.I. (Ideal Critical Diameter).
Ideal critical diameter
is the diameter which, when quenched in an
infinite quench severity (such as ice brine), will result
in a microstructure consisting of 50 percent martensite
of the center of the bar.
Decarburization is the reduction
in surface carbon content of a gear or test piece
during thermal processing.
- Ferrite (alpha).
Ferrite is a microstructural
phase consisting of essentially pure iron, and is characterized
with a body centered cubic structure.
- Flame Hardening.
Flame Hardening of steel
gearing involves oxyfuel burner heating to 1450-1650_F (788-899_C) followed by quenching
- Grain Size.
Grain size is specified as either
coarse (grain size 1 through 4) or fine (grain size 5
through 8), determined according to ASTM E112.
Graphite is carbon in the free state
with a shape described as either flake, nodule, or
spheroid. The graphite shape classifies the type of
cast iron as either gray, ductile, or malleable.
An indication of the depth to which a steel will harden during
The process of increasing hardness,
typically through heating and cooling.
- H-Band Steels.
H-Band steels are steels which
are produced and purchased to a specified Jominy
- Induction Hardening.
Induction hardening of
gearing is the selective heating of gear
teeth profiles to 1450-1650_F
(788-899_C) by electrical inductance
through the use of a coil or single tooth inductor
to obtain the proper heat pattern and temperature,
followed by quenching and tempering.
- Jominy End Quenching Hardenability Test.
The standard method for determining the hardenability
of steel.The test consists of heating a standard
one inch (25 mm) diameter test bar to a specified
temperature, placing the specimen in a fixture so
that a stream of water impinges on one end, cooling
the specimen to room temperature, grinding flats,
and measuring the hardness at 1/16 inch (1.6mm) intervals
starting at the quenched end.
Martensite is the diffussionless
transformation of austenite to a body centered tetragonal
structure, characterized by an acicular
Microstructure is the material
structure observed on a sample polished to a mirror
finish, etched, and viewed at 100X or higher magnification.
- Nitriding (Aerated Salt Bath).
This term includes
a number of heat treat processes in which nitrogen
and carbon in varying concentrations are absorbed
into the surface of a ferrousmaterial at a temperature
below the austenitizing temperature [1000-1150_F (538-621_C)], while submerged in a
gas stirred and activated molten chemical salt bath.
These processes are used mainly for improved wear
resistance and fatigue strength.
- Nitriding (Gas).
Surface hardening process in
which alloy steel, after machining following quench
and tempering, is subjected to a cracked ammonia furnace atmosphere at
causing nitrogen to be absorbed into the surface,
forming hard iron nitrides.
Nitrocarburizing is a gaseous
heat treatment in which both nitrogen and carbon
are absorbed into the surface of a ferrousmaterial at
a temperature below the austenitizing temperature [1000-1150_F (538-621_C)]. Nitrocarburizing is
done mainly for antiscuffing and to improve surface
Normalizing consists of heating steel or other ferrous alloys to 1600-1800_F
(871-982_C) and cooling in still or circulated air.
Normalizing is used primarily to obtain a uniform microstructure.
Pearlite is a microstructure consisting
of lamellar layers of ferrite and cementite, with a
body centered cubic crystal structure.
- Quench and Temper.
The quench and temper
process on ferrous alloys involves heating a part to the austenite
transformation state at 1475-1650_F (802-899_C), followed by rapid cooling (quenching).
The part is then reheated (tempered) to a specific
temperature generally below 1275_F (690_C) to
achieve the desired mechanical properties for the
- 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. Maximum stress relief
is achieved at 1100_F (593_C) minimum.
- Surface Hardness.
Surface Hardness is the
hardnessmeasured directly on the surface. To obtain
accurate results on shallow case hardened parts, a superficial
test must be used.
Tempering is reheating a hardened
part to a specified temperature, generally below
1275_F (690_C) to reduce hardness and increase
- Test Coupon.
A test coupon is an appropriately
sized sample(often a bar) used generally for surface
hardening treatments. It should be of the same specified
material grade, with regard to composition and
hardenability limits, as the gear it represents. The
test coupon should be heat treated along with the
gears it represents.
- Through Hardening.
Through hardening is a
term used to collectively describe methods of heat
treatment of steel other than surface hardening techniques.
These include: annealing, normalizing (or
normalizing and tempering) and quenching and tempering. Depth of hardening is dependent
upon hardenability, section size and heat treat
NOTE: Through hardening does not imply that
the part has equivalent hardness throughout the entire
Transformation Temperature. The temperature
at which a change in microstructure phase occurs.
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