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What is Shot Peening ?

Shot peening, also known as shot blasting, is a cold work process used to finish metal parts to prevent fatigue and stress corrosion failures and prolong product life for the part. In shot peening, small spherical shot bombards the surface of the part to be finished. The shot acts like a peen hammer, dimpling the surface and causing compression stresses under the dimple. As the media continues to strike the part, it forms multiple overlapping dimples throughout the metal surface being treated. The surface compression stress strengthens the metal, ensuring that the finished part will resist fatigue failures, corrosion fatigue and cracking, and galling and erosion from cavitation.


Shot Peening Methods and Media

Shot peening is commonly undertaken using air blast systems or centrifugal blast wheels. Air blast systems use high pressure air to fire the shots through a nozzle at the workpiece. A centrifugal blast wheel uses a high speed paddle wheel which propels the peening media using centrifugal force by adjusting the media entrance location to time the release of the media. Other shot peening method include ultrasonic peening, wet peening and laser peening, which doesn’t use media.

The peening media that can be used include cast steel shot, ceramic or glass beads and cut wire. Cut wire shot is generally preferred since it maintains its spherical shape as it degrades, unlike cast shot, which can break into sharp pieces and damage the workpiece. Cut wire can last five times as long as cast shot and is also relatively inexpensive. However, the use of equipment to remove shot fragments during the process can help by cleaning and reconditioning shot and feeders and replacing damaged media.


Applications

Shot peening is used across a wide range of industries to improve the surface properties of components, including medical, aerospace and automotive. Example components include [1]:

· Gear Parts      · Camshafts           · Springs         · Connecting rods

· Crankshafts    · Gearwheels         · Pistons          · Drill bits

· Propeller shafts  · Compressor blades                  · Turbine blades

· Landing gear      · Epidural probes

In addition to these applications, shot peening can be used for sand removal in foundries, decoring, descaling, and surface finishing for castings used in engine blocks and cylinder heads.

The process is also widely used to relieve tensile stresses created through work hardening in aircraft repairs. Where processes such as grinding can create tensile stresses, shot peening can replace these with beneficial compressive stresses. Depending on factors such as shot quality, material, intensity and coverage, shot peening can increase fatigue life by as much as 1000%.

Shot peening can also be used for cosmetic purposes, creating a roughness that scatters light from the surface more effectively than with processes like sand blasting.

The process can also be used to apply material to metal surfaces through the use of an additional powder or liquid with the desired surface coating. In this application, the shot is fired through the powder or liquid which then coats the workpiece due to the impact of the peening progress.This has been used to embed ceramic coatings and apply solid lubricants to surfaces, although the coverage is random due to the chaotic nature of the peening process.



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