shot peening process
Introduction to shot peening
The process of shot peening is used to produce a decorative finish and to modify mechanical properties of materials. Shot peening is a cold working process in which the surface of the material is bombarded with small spherical media called shot. Each piece of shot striking the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. In order for the dimple to be created, the surface fibres of the material must be yielded in tension. Below the surface, the fibres try to restore the surface to its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression.
(click for larger image)Overlapping dimples develop an even layer of material in residual compressive stress parallel to the surface of the material. This compressive stress region may be serveral times greater than any imposed tensile stress, such as that encountered in bending, and markedly improves fatigue life of the part in service.
(click for larger image)Why shot peening prevents fractures
Material failures are mostly caused from fractures, or cracks, which start at the surface of the material. These fractures are caused from over stressing or bending past the materials yield strength, fatigue from continuous or repeated stresses, or from corrosion weakening the surface. When forces are applied to any material, tension stress and compressive stress regions form. Fracures only occure in tension stress regions, never in compression stress regions.
By shot peening material, internal compressive stress areas are formed which oppose these tension stress regions and aid to prevent fractures in the material. The compressive stress areas in a shot peened surface are a residual effect and can profoundly increase the service life of the mareial.
Effects of shot peening
Benefits obtained by shot peening are the result of the effect of the compressive stress and the cold working induced. Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitation. Benefits obtained due to cold working include work hardening, intergranular corrosion resistance, surface texturing, closing of porosity and testing the bond of coatings. Both compressive stresses and cold working effects are also benefits in the application of shot peening in forming metal parts. (see shot peen forming)
Example for High Strength Steel
Samples as a Function of the Ultimate Tensile Strength of Steel
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Increased Resistance to Fatigue
of Samples Manufactured from Ground Parts.
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In addition, the shot peening action improves the distribution of stresses in surfaces that have been disturbed by grinding, machining, or heat treated. Particularly effective on ground surfaces, shot peening changes the tensile stresses that grinding imposes in a metal surface to beneficial compressive stresses. Also especially effective in reducing the deleterious stress-concentration effects of notches, fillets, forging pits, surface defects, and decarburization.
Wide usage of shot peening
The high level of performance of the jet plane and the modern automobile would be impossible without beneficial residual stresses gained from shot peening. Such stresses could be used to a much greater extent in the manufacture of industrial and domestic machinery, leading to savings in weight and greater serviceability along with lowering over-all cost. Springs are made stronger and lighter, hardware becomes stronger and more reliable, gears and bushings wear less, axles and shafts stay straighter, and structural parts resist fatigue and corrosion. However, the above explanation does not include every possible application of shot peening. The shot peening treatment can be applied to every kind of metal product and utilized in a wide range of technical fields. In general, shot peening allows for stronger and more durable parts made with lighter and sometimes less material.
The advancement in shot peening technology has allowed for giant leaps in manufacturing and quality. For more information on shot peening, feel free to contact us at any time. For details on our specific capabilities, please visit the shot peening services page.
