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Old 09-02-2004, 01:43 PM
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Lightbulb Shot Peening - Interesting Read

Saw this on shot-peening. I was trying to get idea of exactly what it is and why it is good. Thought I would share from mrtrally.com



" MRT Shot Peening
WHAT IS IT?
Shot peening is the process of cold forming the surface of a part by means of a propelled stream of round hardened steel shot. The result of this process is a uniformly dimpled surface, the roughness being determined by the shot size and the peening intensity. The process is non-abrasive.

The shot peening process is used to improve the fatigue properties of the part by the introduction of compression forces in the surface layer. The presence of this surface compressive force serves to retard the initiation and growth of fatigue cracks.

Fatigue failures are easily recognised, and almost always start from a focal point on the surface. These focal points are stress raisers such as fillets, holes, keyways, seams, laps, tool marks, scratches or structure variations. When fatigue failures are encountered these stress raisers should be eliminated where possible.

The size of the shot used is related to the size of the smallest fillets / openings that the shot must penetrate, and the peening intensity required. The peening intensity is monitored regularly by the use and measurement of Almen test strips. The peening intensity is normally selected from military specification S-13165A, on the basis of the material strength and the section thickness. The recommendations are based on the lowest peening intensity capable of producing the desired surface compressive stress. The intensity may be considered excessive in thin parts if the tensile forces in the core material outweigh the beneficial compressive forces induced at the surface.

The process should not be confused with shot blasting which is a cleaning process using broken shot as an abrasive. The surface finish resulting from shot blasting is coarse, and can result in a reduction in the fatigue performance of the part.

WHAT ARE THE BENEFITS?
Reported increases in the life of springs are in the order of 400% to 1200%, depending on the extent of peening already imparted on the spring. Peening of valve springs from a vintage engine resulted in an increase in the installed spring pressure, and an extension of life of an otherwise irreplaceable part.

Shot peening increases the fatigue life of gears, and case histories have revealed over 500% increase in life. Other tests on drive pinions give 40% to 414% life increases with peening. In many instances satisfactory transmission or rear axle gears are shot peened, and then the loads increased under racing conditions.

Other parts that are shot peened include conrods, axles, torsion bars, marine propellers and rudders, and exhaust megaphones. All have benefited from increased fatigue life, or an increase in service loads.

Previously peened parts that exhibit damage that has been dressed back (eg. conrods dressed after a lifter failure) can be re-peened to restore their original fatigue resistance.

The peening process also has applications in forming hardened materials, and straightening bent parts.

After peening the material should not be heat-treated, machined or straightened, as the effectiveness of the peening process will be reduced."

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