The purpose of this section is to provide information, means of
specifying, and inspection of nitrided gearing. This section covers the
selection and processing of materials, hardnesses obtainable, and
definitions and inspection of depth of hardening.
Conventional gas nitride hardening of gearing, which has had a quench
and temper pretreatment and is usually finish machined, involves heating
and holding at a temperature between 950-1060_F (510-571_C) in a
controlled cracked ammonia atmosphere (10 to 30 percent dissociation).
Nitride hardening can also be achieved with the ion nitriding
process. During nitriding, nitrogen atoms are absorbed into the surface
to form hard iron and alloy nitrides. The practical limit on case depth
is about 0.040 inch (1.0mm)maximum, which requires a thorough stress
analysis (for other than wear applications) of the effectiveness of the
coarse pitch gearing.
Nitrided gears are used when gear geometry and tolerances do not lend
themselves to other case hardening methods because of distortion, and
when through hardened gears do not provide sufficient wear and pitting
gears are used on applications where thin, high
hardness cases can withstand applied loads. Nitrided gears should not be
specified if shock loading is present, due to inherent brittleness of
Steels containing chromium, vanadium, aluminum, and molybdenum, either
singularly or in combination, are required in order to form stable
nitrides at the nitriding temperature.
Typical steels suitable for nitriding are 4140, 4150, 4340, the
Nitralloy grades, and steels with chromium contents of 1.00 to 3.00
percent. Aluminum containing grades such as Nitralloy 135 and Nitralloy
N will develop higher case hardness.
Parts to be nitrided must be quenched and tempered to produce the
essentially tempered martensitic microstructure required for case
diffusion. Microstructure must be free of primary ferrite, such as is
produced by annealing and
normalizing, which produces a brittle case prone to spalling. The
nitriding process will cause a slight uniform increase in size. However,
residual stresses from quench and tempering may be relieved at the
nitriding temperature, causing distortion. This should be avoided by
tempering at approximately 50_F (28_C) minimum above the intended
temperature after quenching. In order to minimize distortion of certain
gearing designs, intermediate stress relieving after rough machining at
25-50_F (14-28_C) below the tempering temperature may also be required
prior to finish machining to relieve machining stresses before nitriding.
In alloys such as series 4140 and 4340 steels, nitrided hardness is
lessened appreciably by decreased core hardness prior to nitriding. This
must be considered when selecting tempering or stress relieving
If distortion control is very critical, the newer ion nitriding process
should be considered. Nitriding can be accomplished at lower
temperatures with ion nitriding than those used for conventional gas
Nitriding over decarburized steel causes a brittle case which may spall
under load. Therefore, nitrided surfaces subject to stress should be
free of decarburization. Sharp corners or edges become brittle when
nitrided and should be removed to prevent possible chipping during
handling and service.
Where it is desired to selectively nitride a part, the surfaces to be
protected from nitriding can be plated with dense copper 0.0007 inch
(0.018 mm) minimum thickness, tin plate 0.0003 to 0.005 inch (0.008 to
0.13 mm) thick, or by coating with proprietary paints specifically
designed for this purpose.
Nitrided parts will distort in a consistent manner when all
manufacturing phases and the nitriding process are held constant. The
amount and direction of growth or movement should be determined for each
part by dimensional analyses both prior to and after nitriding.
- Nitriding Process Procedures.
Variables in the nitriding process are the combined effects of surface
condition, degree of ammonia dissociation,
temperature, and time of nitriding. Nitrogen adsorption in the steel
surface is affected by oxide and surface contamination. In order to
guarantee nitrogen adsorption it may be necessary to remove surface
oxidation by chemical or mechanical means.
The nitriding process affects the rate of nitrogen adsorption and the
thickness of the resultant brittle white layer on the surface.
A two stage nitriding process (two temperatures with increased percent
of ammonia dissociation at the second higher temperature) generally
reduces the thickness of the white layer to 0.0005-0.001 inch
(0.013-0.026 mm) maximum. The white layer thickness is also dependent
upon the analysis of steel.
The ion nitride process uses ionized nitrogen gas to effect nitrogen
penetration of the surface by ion bombardment. The process can provide
flexibility in determining the type of compound produced. The process
can also be tailored to better control nitriding of geometric problems,
such as blind holes and small orifices.
- Specific Characteristics of Nitrided Gearing.
Nitriding does not lend itself to every gear application. The nitride
process is restricted by and specified by case depth, surface hardness,
core hardness and material selection constraints.
- Material Selection.
Selection of the grade of steel is limited to those alloys that contain
metal elements that form hard nitrides.
- Core Hardness.
Core hardness obtained in the
quench and temper pretreatment must
provide sufficient strength to support the case under
load and tooth bending and rim stresses. Core hardness requirements
limit material selection to those steels that can be tempered to the
core hardness range with a tempering temperature that is at least 50_F (28_C)
above the nitriding temperature.
- Surface Hardness.
Surface hardness is limited by the concentration of hard nitride forming
elements in the alloy and the core hardness of the
gear. Lower core hardness does not support the hard,thin case as well as
higher core hardness. Lower core hardness will result from less alloy,
larger section size, reduced quench severity and a greater degree of
martensite tempering. Lower core hardness results in a microstructure
which causes a lower surface hardness nitrided case, since it limits the
form high concentration of hard metallic nitrides. Surface hardness will
also increase with increasing nitride case depth.
- Case Depth.
The specified case depth for nitrided gearing is determined by the
surface and sub-surface stress gradient of the design application.
Surface hardness and core hardness will influence the design’s minimum
required case depth. Since the diffusion of nitrogen is extremely slow,
most specifications only specify a minimum case depth requirement.
Case depth should be determined using a microhardness tester. At least
three hardness tests should be made beyond the depth at which core
hardness is obtained to assure that the case depth has been reached.
A test bar, for example 1/2 to 1 inch (13 to 25 mm) diameter with a
length 3¢ the diameter, disc or plate section, can be used for
determining case depth of nitrided parts. The test section must be of
the same specified chemical analysis range and must be processed in the
same manner as the parts it represents.
Sectioning of an actual part to determine case depth need only be
performed when the results of the test bar are cause for rejection, or
the surface hardness of the part(s) is not within 3 HRC points of the
surface hardness of the test bar.
Parts which are to be nitrided should have the following specified:
- Material grade
- Preheat treatment
- Minimum surface hardness
- Minimum total case depth
- Maximum thickness of white layer, if required
- Areas to be protected from nitriding by masking, if required
- Nitriding temperature
- Metallurgical test coupons
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