Shot peening is a cold working process performed by bombarding the
surface of a part with small spherical media which results in a thin
layer of high magnitude residual compressive stress at the surface. This
stress may improve the bending
fatigue strength of a gear tooth as much as 25 percent. It is becoming
an accepted practice to specify shot peening on carburized and other
gears. Because the process increases bending fatigue
strength, it may be used either to salvage or upgrade a gear design.
Contact fatigue strength may also be improved in some instances by shot peening, but
quantitative data to substantiate this condition is limited. Shot
peening should not be confused with grit and shot blasting, which are
Machinery used for shot peening should be automatic and provide means
for propelling shot by air pressure or centrifugal force against the
work. Mechanical means for moving the work through the shot streamby
either translation or rotation, or both, should be provided. Machinery
must be capable of consistently reproducing the shot peening intensity
and coverage required.
Regardless of the type of equipment used, the gear must be rotated on
its axis while exposed to the shot stream.
For optimization of shot peening of
gears, nozzle type equipment is
generally preferred because of the ability to vary the angle of shot
impingement and, therefore, achieve more uniform intensity along the tooth form. This type of equipment is generally used for high performance
gearing, although centrifugal wheel equipment is often used for very
- Process Control.
Because it is difficult to directly measure the effects of shot peening
on a part, a high degree of process control is essential to assure
- Intensity Control.
Intensity refers to the kinetic energy with which the peening media
strikes the part. This energy controls the depth of the peening effect.
It is measured by shot peening a flat, hardened steel strip called an
Almen Strip, in the same manner as the part will be peened. The strip is
held flat on an Almen block placed in the representative location during
the peening operation. When released from the block, the strip will bow
convexly on the peened surface. The amount of bow is measured in inches
with a gauge and is called the arc height. There are three
classifications of Almen Strips, N, A, and C, which have thicknesses of
0.031 inch (0.8 mm), 0.051 inch (1.3 mm) and 0.0938 inch(2.4 mm)
respectively. Strips are SAE 1070 cold rolled spring steel, hardened and
tempered to 40-50 HRC. Flatness tolerance is +-0.0015 inch (+-0.04mm).
An intensity determination must be made at the beginning, at intervals
of no more than four hours and at the end of each production run.
Whenever a processing procedure is developed for a new part, an
intensity curve must be developed which establishes the time required to
reach peening saturation of the Almen strip. This is accomplished by
shot peening several strips at various times of exposure to the shot
stream and plotting the resulting arc heights. Saturation is defined as
that point at which doubling the time of exposure will result in no more
than a 10 percent increase in arc height.
- Shot Control.
Shot size and shape must be carefully controlled during the shot peening
process, tominimize the number of fragmented particles caused by
fracturing of the shot. These fragmented particles can cause surface damage. Also, as a result of lower mass, fragmented shot particles will
lengthen the time to reach a specified peening intensity. Periodic
inspection of the shot is required to control shot size and shape within
specification limits. When these limits are reached, the shot should be
classified and separated to restore size and shape integrity as shown in
- Coverage Control.
Coverage refers to the percentage of indentation that occurs on the
surface of the part. One hundred percent coverage is de fined as uniform
dimpling of the original part surface as determined by either visual examination using a 10X magnifying glass or by using a fluorescent
tracer dye in a scanning process. In the latter process, full coverage
has been achieved when no traces of the dye remain when viewed under
ultraviolet light. A minimum of 100 percent coverage is required on any
shot peened part.
Coverage must be related to the part, not the Almen strip. The actual
part must be examined for complete coverage in all areas specified to be
shot peened. The peening time required to obtain 100 percent coverage
should be recorded. The time required to obtain multiples of 100 percent
coverage is thatmultiple times the time to reach 100 percent coverage
(200 percent, 300 percent, etc.).
- Design Consideration.
The following sections describe items that the designer should include
in a shot peening specification.
- Governing Process Specification.
Acommonly referenced shot peening specification is MIL-S-13165B which
identifies materials, equipment requirements, procedures, and quality
control requirements for effective shot peening. The SAE Manual on Shot
Peening, SAE-J808a-SAE HS84, may also be used.
- Shot Size and Type.
Shot type and size selection depends upon the material, hardness, and
geometry of the part to be peened. Shot types available are cast steel
(S), conditioned cut wire (CW), glass bead, and ceramic. Most shot
peening of ferrous materials is accomplished with cast steel shot.
Cast steel shot is available in two hardness ranges: 45-55 HRC, and
55-62 HRC. When peening gears higher in hardness than 50 HRC, the harder
shot should be specified to achieve higher magnitudes of compressive
The intensity governs the depth of the compressive layer and must be
specified as the arc height on the A, C, or Nstrip.
The range of arc height is generally 0.004 inch (0.10 mm)wide, but it
can be specified to a closer tolerance for more repeatable results.
Figure 5-8 illustrates the depth of the compressive layer on steel at 31
and 52 HRC hardness according to intensity.
In most cases, 100 percent coverage is adequate. In some instances, it
may be desirable to specify multiples of 100 percent in an attempt to
achieve more blending of a poorly machined surface. A typical statement
in a blueprint specification is “100 percent minimum coverage.”
- Masking. At times, it is desirable to mask finished machined
areas of the part from shot impingement.
Typical masked areas would be finished bores or bearing surfaces. If
masking is required, this should be stated in the shot peening
requirements and defined on the drawing, with masked area tolerances
- Drawing Example.
A typical example of drawing or blueprint specification for shot peening
would be as follows:
Shot peen area(s) indicated with S170 cast steel shot to an intensity of
0.010-0.014A per MIL-S-13165B; Mask area(s) indicated (if necessary).
Other areas optional. Use 55-62 HRC shot, 100 percent minimum coverage.
- General Comments.
Additional comments for shot peening include the following:
- All magnetic particle or dye penetrant inspections should be
performed before shot peening.
The plastic flow of the surface as a result of peening will tend to
obscure minute cracks.
- All heat treating operations must be performed prior to shot peening
as high temperatures [over 450_F(232_C)] will thermally stress relieve
the peening effects.
- Generally all machining of areas to be peened are complete prior to
shot peening. It is possible to restore surface finish in peened areas
(and retain beneficial effects) by lapping, honing, or polishing, if
material removal is limited to 10 percent of the depth of compressive
- Compressive residual stress levels produced by shot peening can be
quantitatively measured by X-ray diffraction. Currently this must be
measured on a cut sample in a laboratory X-ray diffraction unit.
Portable units are under development.
- When there are significant machining marks in the tooth roots, it is
desirable to achieve an intensity sufficient to produce a depth of
compressive stress to negate the stress riser effect of the machining
mark. However, shot diameter should not exceed 50 percent of the fillet
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