Shot Peening,Shot Peening,Shot Peening,Shot Peening,Shot Peening
SHOT PEENING is a method of cold working in which compressive stresses are induced in the exposed surface layers of
metallic parts by the impingement of a stream of shot, directed at the metal surface at high velocity under controlled
conditions. It differs from blast cleaning in primary purpose and in the extent to which it is controlled to yield accurate
and reproducible results. Although shot peening cleans the surface being peened, this function is incidental. The major
purpose of shot peening is to increase fatigue strength. The process has other useful applications, such as relieving tensile
stresses that contribute to stress-corrosion cracking, forming and straightening of metal parts, and testing the adhesion of
silver plate on steel.
Peening Action
When individual particles of shot in a high-velocity stream contact a metal surface, they produce slight, rounded
depressions in the surface, stretching it radially and causing plastic flow of surface metal at the instant of contact. The
effect usually extends to about 0.13 to 0.25 mm (0.005 to 0.010 in.) but may extend as much as 0.50 mm (0.02 in.) below
the surface. The metal beneath this layer is not plastically deformed. In the stress distribution that results, the surface
metal has induced or residual compressive stress parallel to the surface, while metal beneath has reaction-induced tensile
stress. The surface compressive stress may be several times greater than the subsurface tensile stress. This compressive
stress offsets any service-imposed tensile stress, such as that encountered in bending, and improves fatigue life of parts in
service markedly.
Peening action improves the distribution of stresses in surfaces that have been disturbed by grinding, machining, or heat
treating. It is particularly effective on ground or machined surfaces, because it changes the undesirable residual tensile
stress condition that these processes usually impose in a metal surface to a beneficial compressive stress condition. Shot
peening is especially effective in reducing the harmful stress concentration effects of notches, fillets, forging pits, surface
defects, and the low-strength effects of decarburization, and the heat-affected zones of weldments.
Strain Peening. The magnitude of residual stress that can be induced by shot peening is limited. In hard metals, it is
slightly more than half the yield strength. A higher residual stress, approaching the full yield strength, can be obtained by
strain peening, which consists of peening the surface as it is being strained in tension. The effectiveness of strain peening
is limited to parts such as springs, gears, and shafts, that are subjected to unidirectional service loads.
Surface Coverage and Peening Intensity
The workpiece surface being peened is affected by the amount of the target surface peened and the effectiveness of the
peening action on that target surface.
Surface coverage is a measure of how completely an area has been hit by the myriad of impinging shot particles.
Without 100% coverage or saturation, the improvement in fatigue characteristics conventionally produced by shot
peening is not obtained.
As stated in SAE Recommended Practice J443, "Procedure for Using Shot Peening Test Strip," a definite and quantitative
relationship between coverage and exposure time exists, which may be expressed as follows:
Cn = 1 - (1 - C1)n
where C1 is the percentage of coverage (decimal) after 1 cycle, Cn is the percentage of coverage (decimal) after n cycles,
and n is the number of cycles.
This relationship indicates that coverage approaches 100% as a limit. Accurate measurements above 98% coverage are
difficult to obtain, but a measurement at a lower degree of coverage serves as a means of determining the exposure time
or equivalent time required to obtain any desired coverage. Because accurate measurement can be made up to 98% coverage, this value is arbitrarily chosen to represent full coverage or saturation.

Measurement of Coverage. Direct methods for measuring coverage include visual methods and the Straub method.
One of the indirect methods is the Valentine method, which involves layer removal.
Visual methods, although not quantitative, are almost universally used. The simplest of these consists of visual inspection,
with or without the aid of optical (10×) magnification of the surface of the peened part. This method may be
supplemented by a series of reference photographs illustrating various percentages of coverage.
Another visual method consists of preparing a transparent plastic replica of the peened surface and comparing it, by
means of photographic projection, with reference replicas having various percentages of coverage.
The Straub method consists of exposing a polished surface to the shot stream, projecting the surface at a magnification of
50 diameters on the ground glass of a metallographic camera, tracing the images of the indented areas on translucent
paper, and measuring the total area and the indented area with a planimeter. Percentage of coverage is expressed as the
ratio of indented area to total area multiplied by 100. About 15 min is required to make one measurement.
The Peenscan method is offered in lieu of visual inspection in MIL-S-13165 and consists of painting a part before peening
with a dye sensitive to ultraviolet light, shot peening the part, inspecting the part under the ultraviolet light for any missed
areas, shot peening the part, and reinspecting the part under ultraviolet light. Complete removal of the dye indicates 100%
coverage of the part.
The Valentine method consists of making a duplicate of the part from low-carbon steel, peening the part, annealing it for
several hours to promote recrystallization and grain growth, and relating peening coverage to the amount and continuity
of grain growth by metallographic examination of cross-sectional areas.

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