Imagine a metal surface no longer vulnerable to cracks and corrosion, but instead fortified like an impenetrable fortress. This transformation is made possible through shot peening, a precision surface treatment that breathes new life into metals, enabling them to withstand even the harshest environments.

Shot peening, also known as shot blasting, is a cold-working process that significantly enhances the mechanical properties of metals and composite materials. At its core, the technique involves bombarding a workpiece with high-velocity spherical particles—typically made of metal, glass, or ceramic—to induce plastic deformation on the surface. This creates a compressive stress layer that acts like invisible armor, effectively suppressing the initiation and propagation of microcracks while dramatically improving fatigue life and corrosion resistance.

• Extended Fatigue Life: The most notable advantage of shot peening is its ability to dramatically increase a metal component's fatigue life. By introducing compressive stresses, it counteracts the detrimental effects of tensile stresses, delaying crack formation. In some cases, peened components show fatigue life improvements of up to 1,000%.
• Stress Corrosion Resistance: The compressive layer not only fights fatigue but also combats stress corrosion cracking—a destructive phenomenon caused by the combined action of tensile stress and corrosive environments. Shot peening eliminates surface tensile stresses, substantially reducing this risk.
• Surface Defect Mitigation: The process can repair minor surface imperfections like scratches and microcracks through plastic deformation, blunting these flaws and minimizing their impact on performance.
• Surface Finish Enhancement: Beyond mechanical improvements, shot peening can refine a metal's surface texture. By adjusting media size and process parameters, manufacturers can achieve various aesthetic finishes. In architecture, the technique often creates subtle, matte metal surfaces.

The magic of shot peening lies in its creation of compressive stress. When high-speed media impacts the surface, it causes localized plastic deformation. This deformation attempts to expand the surface material, but surrounding unaffected metal constrains this expansion, generating compressive stresses at the surface. To maintain equilibrium, balancing tensile stresses develop deeper in the material. However, since cracks typically originate at surfaces, the compressive layer proves far more valuable in preventing failure.

Two key metrics govern shot peening effectiveness:

Measured using standardized test strips, intensity reflects the kinetic energy transferred during peening. Higher intensity creates greater compressive stress, but excessive energy can cause detrimental overworking. The Almen scale—developed by John Almen—quantifies intensity by measuring the curvature of thin metal strips after peening. When a strip deforms by 10%, doubling the exposure time should produce another 10% deformation if process consistency is maintained.

This percentage indicates how thoroughly the surface has been impacted. While 100% coverage means every area has been struck at least once, multiple passes ensure uniform stress distribution. Interestingly, at 150% coverage, about 52% of surface points endure five or more impacts, rising to 84% at 200% coverage. Achieving optimal coverage depends on media geometry, material hardness, and exposure time—softer media on harder materials require longer processing.

Shot peening involves three primary stages:

1. Surface Preparation: Cleaning removes contaminants like oils and rust, ensuring proper media contact.
2. Peening: Automated systems propel media via compressed air (pneumatic) or centrifugal force (wheel blast), precisely controlling intensity, coverage, and duration.
3. Post-Treatment: Additional processes like cleaning, passivation, or coating may follow to enhance corrosion resistance or appearance.

Emerging techniques like ultrasonic peening, water jet peening, and laser peening offer superior precision for specialized applications.

• Cast Steel Shot: The most common option, offering high hardness and durability for general applications.
• Glass Beads: Softer media for delicate surfaces requiring fine finishes.
• Ceramic Beads: Exceptional wear and corrosion resistance for extreme conditions.
• Cut Wire Shot: Economical steel particles initially effective but often conditioned to remove sharp edges.

• Aerospace: Enhances fatigue life of turbine blades, landing gear, and other safety-critical components—sometimes achieving tenfold lifespan improvements.
• Automotive: Strengthens engine crankshafts, connecting rods, and transmission gears for improved reliability.
• Manufacturing: Extends service life of springs, bearings, and cutting tools.
• Construction: Improves corrosion resistance and aesthetics of metal facades and bridges.

• Spring Manufacturing: Critical for high-cycle components like valve springs, where performance demands sometimes push materials beyond standard limits—extreme racing springs may survive just two quarter-mile passes before requiring replacement.
• Steel Strip Processing: Graded peening from center to edges under controlled pressures (up to 90 psi for some stainless steels) creates compressive layers that combat cracking in strip materials.
• Peen Coating: Hybrid processes like NASA's "shot peen plating" embed coatings—from solid lubricants to bioceramics—by combining peening with powder deposition. Emerging techniques like Temperature-Modulated Collision-Mediated Coating (TM-CMC) even enable polymer and antibiotic applications.

X-ray diffraction and hardness profiling map subsurface stress distributions, revealing how factors like part geometry, material properties, and peening parameters influence outcomes. Proper process development prevents surface stress drops that could compromise performance—sometimes requiring multi-stage treatments to optimize the stress gradient.

Ultimately, shot peening's ability to impart beneficial compressive stresses through controlled kinetic energy transfer makes it indispensable for enhancing metal component performance across countless industries.