Shot peening is a manufacturing process that strengthens metal surfaces by bombarding them with tiny round particles at high speed. Each particle strikes the surface hard enough to create a small dimple, and the accumulated effect of millions of these impacts leaves behind a layer of compressive stress that makes the metal far more resistant to cracking and fatigue. It’s one of the most widely used surface treatments in aerospace, automotive, and energy industries.
How Shot Peening Works
The core idea is surprisingly simple. Small spherical particles, typically ranging from 0.1 to 3 millimeters in diameter, are propelled against a metal surface using compressed air, centrifugal wheels, or pressurized water. When each particle hits, it plastically deforms a tiny spot on the surface, meaning the metal is permanently pushed inward at that point. But the metal just below the surface hasn’t been deformed. It tries to spring back, creating a tug-of-war between the stretched surface layer and the underlying material.
This tug-of-war is exactly the point. When the shot rebounds, the elastic material beneath pushes back against the plastically deformed surface, locking in compressive residual stress. Think of it like bending a paperclip partway: when you let go, internal forces remain in the metal even though nothing is actively pushing on it. That locked-in compression is what gives shot-peened parts their extra strength, because cracks need tensile (pulling-apart) stress to open and grow. A surface already under compression essentially has a built-in buffer against cracking.
Types of Peening Media
The particles used in shot peening come in three main materials, each suited to different jobs.
- Steel shot is the most common choice for heavy-duty applications. Cast or cut steel particles deliver high impact energy and are effective at inducing deep compressive stress layers. They’re the go-to for gears, springs, and structural components where maximum fatigue resistance matters.
- Ceramic beads are made from zirconia or similar materials. They’re harder and more uniform than steel, making them a good fit for high-precision aerospace parts and applications where iron contamination of the surface isn’t acceptable, such as titanium or nickel alloy components.
- Glass beads operate at lower pressures and produce a smoother, brighter surface finish with a slightly dimpled profile. They’re often chosen when the part needs both light peening and a clean aesthetic, or when the material is too soft or thin for steel shot.
What It Does to Fatigue Life
The practical payoff of shot peening is measurable and significant. A NASA study on carburized steel spur gears found that shot-peened gears lasted 1.6 times as long as identical gears that weren’t peened, based on surface pitting fatigue. The shot-peened gears reached 30.1 million stress cycles before 10% of them showed pitting, compared to 18.8 million cycles for the untreated gears. At the 50% failure mark, the gap held: 67.5 million cycles versus 46.1 million.
The residual stress measurements explain why. At the depth where shear stress peaks inside the gear tooth, the shot-peened gears showed 40% higher compressive stress than the standard gears. Near the very surface, at a depth of just 13 micrometers, compressive stress was 350% higher. That shallow, intense compression is exactly where fatigue cracks typically start, which is why peening is so effective at delaying them.
Shot Peening vs. Sandblasting
Shot peening is often confused with shot blasting and sandblasting, but the goals are fundamentally different. Shot blasting fires particles at metal to clean off rust, scale, and corrosion. Sandblasting (grit blasting) uses angular abrasive particles to smooth or strip surfaces. Both are essentially cleaning or surface preparation steps.
Shot peening, by contrast, isn’t about removing material or cleaning anything. It’s a cold-working process designed to strengthen the part. The round shape of peening media is critical: angular particles would cut into the surface and create stress concentrators, the opposite of what you want. Peening creates controlled dimples that induce compressive stress, increasing fatigue strength, preventing crack initiation, and reducing brittleness. A blasted part looks cleaner. A peened part is structurally tougher.
Protection Against Stress Corrosion Cracking
Beyond pure fatigue, shot peening provides a strong defense against stress corrosion cracking, a failure mode where the combination of tensile stress and a corrosive environment causes cracks to form and grow in metal that would survive either condition alone. Because peening replaces surface tensile stress with compressive stress, it removes one of the two ingredients needed for this type of cracking.
Research on austenitic stainless steel weld joints showed that shot peening to 100% coverage remarkably enhanced resistance to stress corrosion cracking. The peening also caused grain refinement at the surface and triggered a phase transformation that further hardened the material. These layered effects, compression plus microstructural changes, make peened surfaces significantly more durable in harsh chemical or high-temperature environments like those found in oil and gas pipelines or nuclear power systems.
How Coverage Is Measured
Coverage refers to the percentage of the surface that has been struck at least once. In practice, 98% measured coverage is considered full coverage and labeled as 100%. Reaching true mathematical 100% would take a very long time because of the random distribution of impacts, so the industry standard accepts that small 2% margin.
When specifications call for 200% coverage, the part is peened for twice the time it took to reach 100%. This doesn’t mean every spot is hit exactly twice; it means the treatment is more thorough, building a more uniform stress layer. Higher coverage levels are common for critical parts like turbine blades and landing gear, where the consequences of a fatigue crack are severe.
Controlling the Process With Almen Strips
Shot peening intensity, meaning how aggressively the process is deforming the surface, is measured using thin standardized metal strips called Almen strips. A flat strip is clamped to the part (or to a test fixture using the same peening parameters), then peened. The compressive stress on the peened side causes the strip to curve, and the height of that arc is measured with a specialized gauge.
To find the true peening intensity, multiple strips are peened at different exposure times and the arc heights are plotted on a saturation curve. Saturation time is defined as the point where doubling the exposure produces no more than a 10% increase in arc height. The arc height at that saturation point is the Almen intensity for that setup. This gives engineers a repeatable, objective number they can specify and verify, ensuring every part receives the same treatment regardless of which machine or operator runs the job. The main industry specifications governing this process, SAE AMS 2430 and AMS 2432, require Almen-based intensity verification.
Effect on Surface Finish
Shot peening inevitably roughens the surface. The overlapping dimples raise the average roughness (Ra) compared to a machined or ground starting surface. Ra increases steeply during early peening and then plateaus as coverage builds, but it can actually decrease slightly with very extended peening times as peaks get flattened by repeated impacts.
For many applications, the added roughness is acceptable because the fatigue benefits far outweigh the cosmetic change. But when tight surface finish tolerances matter, such as on bearing surfaces or aerodynamic components, secondary finishing operations like polishing or superfinishing may follow peening. Researchers have noted that standard roughness measurements like Ra become less useful for distinguishing between peened surfaces at higher coverage levels, and that volume-based surface parameters give a more meaningful picture of what peening has actually done to the surface topography.
Where Shot Peening Is Used
The process is standard practice anywhere metal parts face cyclic loading, vibration, or corrosive conditions. In aerospace, turbine blades, landing gear, wing skins, and engine shafts are routinely peened. Automotive applications include transmission gears, connecting rods, crankshafts, and suspension springs. In the energy sector, shot peening treats oil and gas drilling components, gas turbine parts, and nuclear plant hardware.
Medical implants like hip and knee replacements also benefit from peening, since the compressive surface layer helps the implant resist fatigue in the body’s corrosive saline environment over decades of use. Even everyday items like coil springs in your car’s suspension owe their longevity partly to shot peening applied during manufacturing.