Peening shot and beads are small metal balls or glass beads that are used to impact compressive stresses into part surfaces and to deburr sharp edges without removing material. Induced compressive stress improves fatigue strength, the maximum stress that a material can sustain for a specific number of cycles without failure. The compressive residual stress improves fatigue strengths significantly - as long as other factors such as surface roughness and flaws are eliminated. Consequently, penning shot and beads are blasted at the material’s surface. Upon impingement, the part surface is compressed by the shot peen media. Shot peening media may be heat-treated to harden or soften the peening shot and beads as necessary. Hardening increases media life. Softening may reduce surface damage or marring on softer alloys.
Peening shot and beads are usually made of cast iron, ceramic materials, glass, metal, porcelain, steel, stainless steel, or zinc. Cast iron peening shot and cast iron beads consist ferrous alloys with varying amounts of carbon. Ceramic peening shot and ceramic beads are hard, dense, inorganic materials and media. Glass peening shot and glass beads have very sharp edges; however, these edges may not be as durable as those of crystalline minerals. Metal peening shot and metal beads are made of aluminum, cast iron, steel, stainless steel, or zinc. Steel peening shot and steel beads, stainless steel peening shot and stainless steel beads, and zinc peening shot and zinc beads are commonly available.
Specifications for peening shot and beads include type or shape, media size, and grading system. Choices for type or shape include balls or beads, cut wire, cast shot, and rounded shot. Cut wire shot and rounded or preconditioned products are usually made of metals such as steel. Cast shot may not be perfectly round unless conditioned. Pre-conditioned peening shot and beads are rounded to a radius that eliminates any sharp edges or corners that would otherwise scratch or mar the parts. In terms of media size, peening shot and beads are specified by either media dimension A or media dimension B. There are several grit grades and shot sizes. Choices include ANSI bonded, ANSI or CAMI coated, FEPA F, FEPA P, JIS, mesh-graded, micron-graded, and SAE. ANSI grading systems are used in the United States, while FEPA grading systems are used in Europe. JIS is an acronym for Japanese Industrial Standard.