Gas carburizing consists of heating and holding low carbon alloy steel
(0.07-0.28 percent Carbon) at normally 1650-1800_F
(899-982_C) in a controlled atmosphere which causes additional carbon to
diffuse into the steel (typically 0.70-1.10 percent carbon at the
Gear blanks to be carburized and hardened are generally preheated after
the initial anneal by a subcritical anneal at 1100_F-1250_F (590-675_C),
normalize, normalize and temper or quench and temper to specified
hardness before carburize hardening.
This is done for machinability, dimensional stability and possible grain
refinement considerations. An intermediate stress relief before final
machining before carburizing may be used to remove residual stress from
After carburizing for the appropriate time, gearing will usually be
cooled to 1475-1550_F (802-843_C), held at temperature to stabilize
while maintaining the carbon potential, and direct quenched. Gearing may
be atmosphere cooled after
carburizing to below approximately 600_F (315_C) and then reheated in
controlled atmosphere to 1475-1550_F (802-843_C) and quenched. After
quenching, gearing is usually tempered at 300-375_F (149-191_C). Gearing
may be subsequently given a refrigeration treatment to transform
retained austenite and retempered.
Carburized and hardened gearing is used when optimum properties are
required. High surface hardness, high case strength, favorable
compressive residual stress in the hardened case, and suitable core
properties based on selection
of the appropriate carburizing grade of steel, result in the highest
AGMA gear tooth ratings for contact stress, pitting resistance and root
Carburized gear ratings are higher than the ratings for through hardened
and other types of surface hardened gearing because of higher fatigue
strength. Improved load distribution can be obtained by subsequent hard
gear finishing. Conventional hard gear finishing (skiving and grinding)
results in some sacrifice of beneficial compressive stress at the
surface and substantially increases costs.
gearing is used in enclosed gear units for general industrial
use, high speed and aerospace precision gear units and also large open
gearing for mill applications. Carburized gearing is also used for
improved wear resistance. Specified finish operations after hardening
depend upon accuracy and contact requirements for all applications.
Carburizing technology is well established and the available equipment
and controls make it a reliable process. Surface hardness, case depth,
and core hardness can be specified to reasonably close tolerances, and
the quality can be audited.
Some gearing does not lend itself to carburize hardening because of
distortion. Gearing which distorts and cannot be straightened without
cracking, rack gears, thin sections, complex shapes, parts not designed
for finishing or where finishing is cost prohibitive, present
manufacturing problems. Press
quenching after carburizing can be used to
minimize distortion. Selected areas of gearing can be protected from
carburizing (masked) to permit machining after hardening, or can be
machined after carburizing and slow cooling before hardening.
Gearing beyond 80 inch (2032 mm) diameter is difficult to carburize due
to the limited number of available furnaces for processing. Maximum size
of carburize gearing is currently in the 120 inch (3048 mm) diameter
range. Most of this large
gearing requires tooth finishing (skiving
and/or grinding) after carburizing and hardening.
Material selection is an integral part of the design process. Selection
should be made on the basis of material hardness and hardenability,
chemistry, cleanliness, performance, and economical considerations.
Performance criteria include, but are
not limited to, the following: toughness, notch sensitivity, fatigue
strength, bending strength, pitting wear resistance, and operational
- Control With Test Bars.
Test bars are used to show that the case properties and, when required,
core properties meet specifications. Test bars should be of the same
steel type as the gear(s), but not necessarily the same heat. Bars
through all heat treatments, including all post hardening treatments.
Consideration should be given to evaluation of that portion of the case
that is not removed during tooth finishing.
A section, with a ground and polished surface (normal, at mid length of
a test bar), is considered satisfactory for determining effective case
depth of carburized helical and spur gearing to 4 1/2DP. The test bar
should have minimum dimensions of 5/8 inch (16 mm) diameter by 2 inch
(50 mm) long. One inch (25 mm) diameter ¢2.0 inch (50 mm) long bar may be
coarser pitch carburized gearing to 1.5 DP. The size of the bar
for coarser than 1.5 DP gearing should be mutually agreed upon, and
should approximate the inscribed diameter at mid height of the tooth
cross section. The bar length should be 2-3 times the diameter.
Test discs or plates may also be used whose minimum thickness is 70
percent of the appropriate test bar diameter. The minimum in scribed
diameter on a test disc (or plate dimensions) should be a minimum of
three times its thickness.
The recommended test bar diameter for bevel gearing is to be
approximately equal to the inscribed diameter of the normal tooth
thickness at mid face width.
When disagreement exists as to the properties obtained on the test bar
and the parts, an actual part may be sectioned for analysis.
- Case Hardness.
Case hardness should be measured with microhardness testers which
produce small shallow impressions, in order that the hardness values
obtained are representative of the surfaces or area being tested. Those
testers which produce Diamond
Pyramid or Knoop hardness numbers (500 gram load) are recommended. When
measuring directly on the surface of a case hardened part or test bar,
superficial or standard Rockwell A or C scale may be used. Other
instruments such as Scleroscope or Equotip are also used when
penetration hardness testers can not be used. Consideration must be
to the case depth relative to the depth of the impression made by the
Low readings can be obtained when the indentor penetrates entirely or
partially through the case.
Microhardness tests for surface hardness should be made on amounted and
polished cross-section at a depth of 0.002 to 0.004 inch (.05 to .10 mm)
below the surface. Care must be taken during grinding and polishing not
to round the edge being inspected and not to temper or burn the ground
NOTE: Direct surface hardness readings (ASTM E18-79) or file checks at
the tooth tip or flank will generally confirm the case hardness.
However, if secondary transformation products are present below the
first several thousandths of the case, direct surface checks will not
necessarily indicate their presence.
Microhardness inspection 0.002 to 0.004 inch (.05 to .10 mm) from the
edge on a polished cross section of the tooth is more accurate.
This type of inspection may be necessary for accurate micro-hardness
readings near the surface.
- Core Hardness.
When required, core hardness may be determined by any hardness tester,
giving consideration to the size of the specimen.
Occasionally banding, which results from the steel melting practice, can
cause variations in core hardness during testing with amicrohardness
tester. These variations should not fall below the minimum, when core
hardness is specified.
- Case Depth - Effective.
The procedures used to prepare the cross sectioned specimen for case
hardness (refer to 5.3.3) should be used to prepare
the specimen for case depth evaluation. The microhardness traverse
should be started 0.002 to 0.004 inch (.05 to .10 mm) below the surface
and extend to at least 0.01 inch (.25mm) beyond the depth at which 50HRC
is obtained. Usually an interval of 0.005 inch (0.13 mm) is used. Care
should also be exercised in establishing the perpendicular to the mid
tooth point when starting the traverse. Effective case depth at roots
are typically 50-70 percent of mid tooth height case depths, and tips may
be 150 percent of mid tooth height case depths.
When steels of high hardenability such as 4320, 4327, 8627, 4820, 9310,
and 3310 are used for
fine pitches, the high through hardening
characteristics of the steel may prevent obtaining a hardness less than
50 HRC across the tooth section. The case depth should then be
determined in the following manner: Measure the base material hardness
at mid tooth height at the mid face. For each one HRC point above 45 HRC,
one HRC point should be added to the 50 HRC effective case depth
criterion (example, core hardness equals 47 HRC, effective case depth
should bemeasured at 52HRC).
NOTE: Through carburized
fine pitch teeth have several disadvantages.
Favorable compressive surface stresses are lowered. Excessive tooth
distortion and a loss of core ductility can also occur. Parts of this
type should be carefully reviewed for case depth specifications and for
use of lower hardenability steels such as 4620 and 8620.
- Case Carbon Content.
Surface carbon content may be determined from a round test bar by taking
turnings to a depth of 0.005 inch (0.13 mm).
Spectrographic techniques have also been developed for this purpose.
Carbon gradient can also be determined on the bar by machining chips at
0.002 to 0.010 inch (0.05 to 0.25 mm) increments through the case,
depending on accuracy desired and depth of case. Grinding in steps
through the case would be used with spectrographic techniques.
Test specimens should be carburized with the parts. Care should be
exercised to maintain surface integrity during cooling or in tempering
for subsequent machining. Bar should be straightened to within 0.0015
inch (0.038 mm) (TIR) before machining.
Test specimens must be clean and machined dry. Care must be taken to
ensure that the turnings are free of any extraneous carbonaceous
materials prior to analysis.
The microstructure may be determined on a central normal section of the
test bar or tooth, preferably mounted, after being properly polished and
Microstructure will vary with the core hardness as related to steel
hardenability, section size and quench severity.
To aid in obtaining the above characteristics, the heat treater should
be given the following as a minimum:
When additional characteristics are required, the following additional
items may be specified in whole or part:
- Case depth range.
- Surface hardness range.
- Core hardness. Approximate minimum tooth core hardness, which can be
obtained from some typical carburizing grades of steel and good agitated
- Core microstructure.
- Case microstructure.
- Surface carbon content.
- Subzero treatment.
- Areas to be free of carburizing by appropriate masking by copper
plating or use of commercial stop-off compounds.
- Carburizing Process Control.
Precision carburizing requires close control of many factors including:
- Temperature Control.
Furnace equipment with temperature uniformity, close temperature
control, and accuracy of temperature recording and control instruments.
Controls should be checked and calibrated at regular intervals.
Furnaces should be capable of maintaining a carburizing atmosphere with
controllable carbon potential. Instrumentation for
continuous atmosphere control is preferred, but other approved methods
may be used.
- Subzero Treatment (Retained Austenite Conversion Treatment).
When the surface hardness is low due to excessive retained austenite in
the case microstructure, it may be necessary to refrigerate the parts to
transform the retained austenite to martensite. The refrigeration
treatment may vary from 20_F
(-7_C) to -120_F (-84_C). To minimize microcracking, parts should be
tempered before and after refrigeration.
NOTE: Caution should be exercised in the use of refrigeration treatment
on critical gearing. Microcracks can result which can reduce fatigue
strength to a moderate degree. Use of refrigeration may require
agreement between the customer and supplier.
- Carbide Control.
When high surface carbon results in a heavy continuous carbide network
in the outer portion of the case, parts should be reheated to typically
1650_F(900_C)in a lower carbon potential atmosphere, typically 0.60
percent carbon, to diffuse
and break up the excess carbide. Carbide networks should be avoided
whenever possible as they tend to reduce fatigue strength of the
Surface decarburization as defined for carburized
gearing is a reduction
in the surface carbon in the outer 0.005 inch (.13 mm) below the
specified minimum. This is characterized by an increase in carbon
content with increasing depth; for example, when the peak carbon content
Gross decarburization can be readily detected microscopically as a
lighter shade of martensite and clearly defined ferrite grains. Hardness
in this area will be substantially lower.
Partial decarburization will result in a lighter shade of martensite,
but may not show discernible ferrite. It will result in reduced hardness
if the carbon content falls below approximately 0.60 percent.
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