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What is Shot Peening

Shot peening is a cold working process in which the surface of a part 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. Overlapping dimples develop an even layer of metal in residual compressive stress.

It is well known that cracks will not initiate or propagate in a compressively stressed zone. Since nearly all fatigue and stress corrosion failures originate at the surface of a part, compressive stresses induced by shot peening provide considerable increases in part life. The maximum compressive residual stress produced at or under the surface of a part by shot peening is at least as great as half the yield strength of the material being peened. Many materials will also increase in surface hardness due to the cold working effect of shot peening.

Benefits 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 used in the application of shot peening in forming metal parts.


What is Glass Bead Peening

While steel shot is used for heavy duty peening applications, glass bead peening, being lighter than steel shot, are used for low to medium peening operations. Glass beads are spheres of uniform size and hardness formulated of chemically inert soda-lime glass. The impact of the beads removes foreign substances from the base surface without contamination and dimensional change. Glass bead peening produces a clean, bright, satin finish, without dimensional change of the parts. Available in a wide range of sizes, glass beads are primarily used in blasting cabinets for honing, polishing, peening, blending, finishing, removing light burrs and cleaning most light foreign matter.

Benefits of Glass Bead Peening

Glass beads are used for peening and the removal of almost any surface blemish. They can be used safely for texturing or producing attractive cosmetic finishes, without damaging the base material. Glass beads being totally inert, there is no risk of corrosion or any contamination whatsoever. Unless excessive blasting occurs, there is no alteration in the dimensions of the treated surfaces. For delicate thin-walled parts and thin welds, peening with glass bead abrasive material provides the right balance of stress relief without over-stressing and causing damage. Research has shown that with the proper glass bead peening procedure, you can effectively restore the original fatigue life of many materials in their mid-life state. Strict control of both manufacturing and grading processes ensures consistency of finish and material structural properties. It is one of the most versatile blast peening materials available.

What is Abrasive Blasting

Abrasive blasting is used for a variety of surface cleaning and texturing operations, mostly involving metallic target materials. Sand is the most widely used blasting abrasive. Other abrasive materials include coal slag, smelter slags, mineral abrasives, metallic abrasives, and synthetic abrasives.

Examples of some of the many applications:

Type of medium       Applications
Glass beads             Decorative blending; light deburring; peening; general cleaning; texturing; noncontaminating
Aluminum oxide        Fast cutting; matte finishes; descaling and cleaning of coarse and sharp textures
Garnet                      Noncritical cleaning and cutting; texturing; noncontaminating for brazing steel and stainless steel
Crushed glass          Fast cutting; low cost; short life; abrasive; noncontaminating
Steel shot                General-purpose rough cleaning (foundry operation, etc.); peening
Steel grit                  Rough cleaning; coarse textures; foundry welding applications; some texturing
Cut plastic                Deflashing of thermoset plastics; cleaning; light deburring
Crushed nutshells    Deflashing of plastics; cleaning; very light deburring; fragile parts

Benefits of Abrasive Blasting

Media particle shapes fall broadly into the categories of shot and grit. Spherical particles distribute their impact over a larger area, moderating the impact and potentially creating a round-bottomed dimple in the surface. Sometimes called a peened finish, the effect of shot blast treatment is likely to be a semi-reflective sheen appearance. With angular grit media, the impact may be concentrated on a point of the particle or a sharp edge, generating an etched, matte finish that is characteristically bright, but non-reflective. In terms of surface modification capabilities, comparing peened versus etched surfaces created by different-shaped particles, the contrast is not so much in the final texture or depth of impression, but in the nature of the indentations in the surface and its reflectivity. Particle size has an important effect on the number of impacts per second of blasting, so it is advisable to use media of the smallest screen size that will do the work, in order to reduce process time to a minimum. Larger particles may be capable of creating bigger indentations and more texture in the surface, whereas smaller ones produce dimples or angular dents of lesser diameter.