Peening is a metalworking process that strengthens metal surfaces by hitting them repeatedly with small, controlled impacts. Each impact creates a tiny dent, plastically deforming the surface layer and locking in compressive stress that resists cracking and fatigue. It’s one of the most widely used techniques for extending the life of metal parts in aerospace, automotive, energy, and medical industries.
How Peening Works
Metal parts under repeated stress eventually develop tiny surface cracks that grow over time until the part fails. Peening counteracts this by changing the stress state of the surface layer. When a small projectile or tool strikes the metal, it pushes the surface material outward, creating plastic deformation. The material beneath tries to spring back but can’t fully recover, so the surface ends up locked in a state of compression. This compressive layer acts like a shield: it physically squeezes surface cracks shut and makes it much harder for new ones to form or spread.
The process is a form of cold working, meaning it reshapes the metal without heat. Beyond the stress benefits, peening also hardens the surface and can refine the grain structure of the metal at a microscopic level, further improving strength and wear resistance.
Types of Peening
Shot Peening
Shot peening is the most common form. The metal surface is bombarded with high-velocity spherical media, typically cast steel shot with a hardness rating between 40 and 55 on the Rockwell C scale. Other media options include stainless steel, glass beads, ceramic beads, aluminum oxide, and silicon carbide. The choice depends on the part being treated: softer glass beads work for lighter applications, while hard steel or ceramic shot is used when deeper compression is needed. Machines deliver the shot using either compressed air nozzles or spinning centrifugal wheels that fling the media at the workpiece.
Hammer Peening
Hammer peening uses a pneumatic or manual hammer with a rounded tip to strike the metal surface directly. It’s particularly valuable for treating welds, where the welding process leaves behind tensile residual stresses that can reach the yield strength of the material and cause premature cracking. Hammer peening at the weld toe (the junction where the weld meets the base metal) can convert those dangerous tensile stresses into beneficial compression. Research has shown that the fatigue resistance of welded joints can double after hammer peening, and the process also reduces distortion in welded parts. Robotic hammer peening systems have proven effective for large structural components where manual treatment would be inconsistent.
Laser Shock Peening
Laser shock peening fires a high-power laser pulse at the metal surface, generating a plasma that produces an intense shock wave. This wave drives compressive stress much deeper into the material than conventional shot peening can reach, and multiple laser passes increase both the depth and magnitude of the compressed layer. The technique also refines the microstructure of the surface. It’s used on high-value parts where maximum fatigue resistance justifies the higher cost, such as jet engine components and critical aerospace structures.
Ultrasonic Peening
Ultrasonic peening uses vibrating pins or needles driven at ultrasonic frequencies to strike the surface. It’s especially effective on welded joints in structural steel. In testing on Q345 steel (a common structural grade), ultrasonic peening increased surface hardness by 64%, from 150 HV to 246 HV. At a stress range of 240 MPa, the fatigue life of ultrasonically peened welds reached 1.4 million cycles compared to just 223,000 cycles for untreated welds, a 6.3-fold improvement. The process also smooths sharp weld toes into gentler profiles, reducing the stress concentration that triggers cracks.
Where Peening Is Used
In aerospace, peening is standard for turbine blades, stators, drums, shafts, and spinners. These components endure extreme cyclic loading, and the compressive surface layer is often the difference between a part lasting its intended service life and failing prematurely. In automotive manufacturing, crankshafts, connecting rods, gear wheels, coil springs, suspension springs, and piston heads are routinely peened. Medical devices including bone screws, dental implants, hip implants, and knee replacement components also undergo peening to improve durability inside the body. Oil and gas drill bits, threaded drill components, and power generation turbine parts round out the list of common applications.
How Intensity Is Measured
Peening intensity isn’t measured on the actual part. Instead, engineers use standardized test strips called Almen strips, made from SAE 1070 spring steel. These flat strips are clamped into a holder and exposed to the same peening conditions as the workpiece. The bombardment causes the strip to curve, and the height of that curve (the “arc height”) is measured with a specialized gauge.
Almen strips come in three thicknesses: N strips (0.79 mm) for low-intensity work, A strips (1.29 mm) for medium intensity, and C strips (2.38 mm) for high-intensity peening. To determine the official intensity value, multiple strips are peened at different exposure times to build a saturation curve. The saturation point is reached when doubling the exposure time produces only a 10% increase in arc height. The arc height at that saturation point is the peening intensity. Industry standards like SAE J442, most recently revised in February 2026, define the requirements for the equipment and supplies used in these measurements.
What Happens When Peening Goes Wrong
More peening is not always better. Over-peening, whether from oversized shot, excessive velocity, or too many passes, can cause real damage. The surface roughness increases to the point where it creates new stress concentration sites, essentially trading one crack-initiation problem for another. In severe cases, surface folds or micro-cracks develop. The material can also soften rather than harden if the plastic deformation is too extreme, and ductility drops as the surface becomes brittle.
Non-uniform residual stress fields are another risk. If the peening coverage is uneven, some areas may retain tensile stress while adjacent zones are in compression, creating unpredictable weak points. Proper parameter selection, including media size, velocity, coverage percentage, and exposure time, is what separates peening that triples fatigue life from peening that shortens it. In one study on 3D-printed stainless steel, optimized severe shot peening tripled the fatigue limit from 200 MPa to over 600 MPa, showing just how dramatic the benefit can be when the process is dialed in correctly.