Shot Peening

Shot Peening

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 heat treated 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 cleaning operations.

Equipment.
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 high volume production.
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 repeatability.
1. 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.
2. 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 MIL-S-13165B.
3. 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.
1. 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.
2. 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 stress.
3. Intensity.
  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.
4. Coverage.
  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.”
5. 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 given.
6. 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.
7. 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 layer.
  - 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 radius.