Shot Peening
Also: peening, controlled shot peening, AMS 2430, AMS 2432, computer-monitored shot peening, SAE J2441, MIL-S-13165, Almen peening, glass bead peening, ceramic shot peening
Shot peening blasts a part with round steel, ceramic or glass shot at a controlled Almen intensity, putting the surface in compression for fatigue life.
18 shops tagged Shot peening among 3,387 finishing and heat treating companies in the Noramark directory
Shot peening is a cold-working process. Round shot, usually cast steel, conditioned cut wire, glass bead or ceramic, strikes the surface at a controlled velocity; each impact stretches the skin plastically into a small dimple, and the elastic material underneath pushes back. The result is a layer of residual compressive stress, typically 0.1-0.5 mm (0.004-0.020") deep, with a peak often around half the tensile strength of the material. Fatigue cracks start at a surface in tension; a compressive skin delays their start and slows their growth, and it also resists stress-corrosion cracking and fretting.
The process is controlled by intensity and coverage. Intensity is measured with Almen strips (SAE J442): thin spring-steel coupons in N, A or C thickness are peened on one side and bow in proportion to the energy. The arc height at saturation, the point where doubling the exposure raises it by no more than 10%, is the intensity, called out as "0.008-0.012A". Coverage is the percentage of the surface dimpled, 100% minimum and often 200% on critical parts, checked under magnification or with a fluorescent tracer. AMS 2430 is the aerospace spec for automatic peening, AMS 2432 adds computer monitoring of the process variables, SAE J2441 is the general industry practice, and aerospace peeners hold Nadcap accreditation.
The payoff is large on the right parts: springs, gears, shafts, crankshafts, connecting rods, landing gear, turbine disks and blade roots, and welded joints. Peening also recovers the fatigue debit of grinding, EDM, hard chrome plating and anodizing. It does nothing for a part that is not cyclically loaded, and it is easily undone: machining or grinding after peening removes the layer, and heating near the stress-relief temperature relaxes it. The surface comes out dimpled and matte, not cosmetic.
At a glance
| Typical tolerances | No material is removed. Surfaces grow slightly, by a few microns, and thin sections distort, since one-sided peening bends a part the way it bends an Almen strip. Intensity is called out as a band of Almen arc height, typically 0.10 mm (0.004") wide, such as 0.20-0.30 mm A (0.008-0.012A); steel parts commonly fall between about 0.15 and 0.50 mm A (0.006-0.020A), and thin or soft parts use the thinner N strip. Coverage is 100% minimum. Threads, bearing bores, seal faces and tight fits are masked, and flatness on parts thinner than about 3 mm (0.12") is hard to hold unless both sides are peened. |
|---|---|
| Size limits | Cabinet and CNC peening machines commonly take parts up to about 1-1.5 m (40-60"); large aerospace shops peen landing gear and wing skins in dedicated machines, and portable equipment peens welds and structures in place. Springs, gears and small parts are peened in batch tumble machines. The shot must fit the geometry: shot diameter should be no more than half the smallest fillet radius to be peened, and small bores need angled or internal nozzles. |
| Surface finish | A uniform, dimpled matte surface. Roughness rises with shot size and intensity, typically to about 1.6-6.3 µm Ra (63-250 µin); fine glass or ceramic shot at low intensity stays nearer 1 µm Ra. The skin is cold worked, which raises the hardness of work-hardening alloys such as austenitic stainless and adds little on already hard parts. Steel shot leaves iron on aluminum, titanium and stainless unless it is cleaned off chemically. Where the drawing allows, parts can be lightly lapped, honed or superfinished afterwards, taking only a small fraction of the compressive layer. |
| Lead time | 3-7 business days at a peening shop for repeat parts; 1-3 weeks for a new aerospace part that needs masking tooling, a program, saturation curves and first-article approval. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
High-strength steels are the main users: 4340, 4140 and 300M, carburized 8620 gears, spring steels and bearing steels. Stainless (17-4 PH, 15-5 PH, and 304 and 316 against stress-corrosion cracking), titanium (Ti-6Al-4V), aluminum (7075, 7050, 2024) and nickel superalloys (Inconel 718) are peened in aerospace, with glass, ceramic or stainless shot, or with steel shot followed by chemical cleaning, to avoid iron contamination. The benefit grows with the strength of the material; soft, annealed low-carbon steel gains little.
What drives the cost
- Masking: tape, boots, plugs and fixtures for every area that must not be peened, usually the largest labour item
- Intensity and coverage: higher coverage and multiple intensities mean more machine time
- Area and geometry: fillets, bores and slots that need small shot, special nozzles or robot programs
- Process control and certification: Almen strip records, saturation curves, coverage checks, AMS 2432 monitoring, Nadcap
- Media: ceramic, glass and stainless shot, and cleaning to remove iron contamination on non-ferrous parts
- Quantity: first-article setup is spread across the run; batch-peened springs and gears cost little each
When to use it
- Cyclically loaded parts: springs, gears, shafts, crankshafts, connecting rods, landing gear, turbine disks and blade roots
- Welds and weld toes in fatigue-loaded structures
- Recovering the fatigue debit of grinding, EDM, hard chrome plating or anodizing on high-strength parts
- Resisting stress-corrosion cracking in stainless, aluminum and high-strength steel
- Parts whose drawing or aerospace spec calls for peening to AMS 2430 or AMS 2432
When not to
- A cosmetic matte finish: that is bead blasting, which is cheaper
- Thin or flexible parts that will distort, unless both sides are peened and some distortion is acceptable
- Parts that will be machined, ground or heated near their stress-relief temperature after peening
- Precision seal faces, bores and threads that cannot be masked
- Soft, low-strength parts and parts without cyclic load, where the benefit is small
Design tips
- Call out the full process: "Shot peen per AMS 2430, cast steel shot S230, intensity 0.008-0.012A, 100% coverage minimum, areas as shown".
- Show the areas to be peened and the areas to be masked on the drawing, and mark zones where overspray is acceptable.
- Keep the shot diameter no larger than half the smallest fillet radius in the peened area, and size fillets with that in mind.
- Sequence it: heat treat, then finish machining and grinding, then peen, then plate or anodize if required.
- On aluminum, titanium and stainless, specify glass, ceramic or stainless shot, or require removal of iron after steel shot.
- Specify AMS 2432 computer-monitored peening for flight-critical parts, and a Nadcap-accredited source when the customer requires one.
- Inspect for cracks (penetrant or magnetic particle) before peening, not after: peening can close a surface crack and hide it.
Frequently asked questions
- What is shot peening used for?
- To raise fatigue life and resist stress-corrosion cracking by putting the surface of a part in compression. It is standard on springs, gears, shafts, crankshafts, landing gear, turbine parts and welds, and on high-strength parts that have been ground, plated or anodized.
- What is Almen intensity?
- The measure of peening energy. A thin spring-steel Almen strip is peened on one side and bows; its arc height at saturation, read on an Almen gauge, is the intensity. "0.008-0.012A" means an arc height of 0.008-0.012 inch on the A strip.
- What is the difference between shot peening and bead blasting?
- Shot peening is a controlled process, with specified media, intensity verified on Almen strips, and measured coverage, that produces a known compressive layer. Bead blasting is a cosmetic or cleaning process with no such control, and it earns no fatigue credit on a drawing.
- How much does shot peening improve fatigue life?
- It depends on the part, the material and the load. Gains of several times in life at a given stress, or roughly 20-50% in fatigue strength, are commonly reported for high-strength steel parts. The gain is largest on notched, ground or plated parts and small on soft material.
- What is the difference between AMS 2430 and AMS 2432?
- AMS 2430 covers automatic shot peening, with intensity and coverage verified by Almen strips and inspection. AMS 2432 adds computer monitoring of the process variables, such as shot flow, air pressure or wheel speed, and part and nozzle motion, on every cycle. Flight-critical parts often call out AMS 2432.
