# Shot peening

Controlled surface cold working: residual stress, Almen intensity, coverage and component qualification.

Reviewed: 2026-11-01

Tags: processes, finishing, surface-enhancement

A hypothetical Almen response rises from **0.200 mm at T** to **0.218 mm at 2T**, or **9.0%**. The **9.0%** rise is within the illustrative **10%** comparison. Use the full fitted curve to establish saturation. Separately, **80 seconds** to complete coverage gives **100 seconds** under an illustrative **125% time convention**. Verify coverage on the part and retain the qualified intensity and exposure requirements.

## Function and process boundary

Shot impacts plastically deform the surface and leave residual compression. Curtiss-Wright ties process plans to geometry, masking, base material and distortion risk. [Curtiss-Wright shot peening](https://surfacetechnologies.curtisswright.com/our-services/shot-peening)

Do not specify abrasive cleaning and assume the resulting texture qualifies as shot peening. A cleaned or uniformly matte surface is not evidence of a controlled intensity, coverage or residual-stress profile. SAE's peening references address intensity and coverage as distinct controls; they do not define a visual cleanliness grade for coating preparation. [SAE J443 intensity scope](https://saemobilus.sae.org/standards/j443_202512-procedures-determining-verifying-peening-intensity), [SAE J2277 coverage scope](https://saemobilus.sae.org/standards/j2277_202301-shot-peening-coverage-determination)

**AMS2430U**, revised April 9, 2018, covers shot peening of part surfaces by impingement of media including metallic, glass or ceramic shot. Its public scope is a starting point for identifying the required process specification, not a substitute for the full contract requirements or a component qualification. State the ordered revision rather than using an unqualified “to AMS2430” note. [SAE AMS2430U](https://saemobilus.sae.org/standards/ams2430u-shot-peening)

## Three different controls

| Quantity | What it establishes | What it does not establish alone |
| --- | --- | --- |
| Almen intensity | A process-control value derived using a saturation curve and the prescribed strip/gauge procedure | Actual component residual stress or a fatigue allowable |
| Saturation | A feature of the strip-response curve used in intensity determination | Complete coverage of the part |
| Coverage | Extent of the actual surface impacted, evaluated by the specified procedure | Correct intensity or a qualified service life |

An Almen strip is peened on one side and bows; its arc height is measured with a gauge. Wheelabrator explains the conventional saturation criterion as no more than a 10% arc-height increase when exposure time doubles. Use that explanation to understand the curve, then apply the applicable full intensity procedure. It is not permission to infer a saturation point from one isolated pair of readings. [Wheelabrator shot-peening explanation](https://www.wheelabratorgroup.com/my-application/application-by-process/what-is-shot-peening/)

SAE **J443_202512**, revised December 11, 2025, defines derivation and verification of peening intensity. Progressive Surface explicitly warns that an arc height obtained at the part's processing time need not equal intensity: time needed for coverage can differ from strip saturation time. Keep strip type, units, fixture position and the approved curve/verification method with the result. A bare arc-height number is insufficient to reproduce the control. [SAE J443](https://saemobilus.sae.org/standards/j443_202512-procedures-determining-verifying-peening-intensity), [Progressive Surface FAQ](https://www.progressivesurface.com/shot-peening-faqs/)

SAE **J2277_202301**, revised January 13, 2023, addresses coverage determination and its relationship to exposure. Its scope warns that inadequate **or excessive** coverage can harm fatigue strength and component life. More peening is therefore not automatically better. Progressive Surface distinguishes actual-part coverage inspection from saturation-curve determination and explains 125% coverage as 1.25 times the time established for complete coverage. A time multiplier above 100% does not mean more than the entire geometric area exists. [SAE J2277](https://saemobilus.sae.org/standards/j2277_202301-shot-peening-coverage-determination), [Progressive Surface FAQ](https://www.progressivesurface.com/shot-peening-faqs/)

## Original bounded control example

Suppose a hypothetical fitted saturation curve gives an arc height of **0.200 mm** at exposure time T and **0.218 mm** at 2T, using the same strip type and fixed process conditions:

`relative increase = (0.218 − 0.200) / 0.200 × 100 = 9.0%`.

This pair meets the illustrative ≤10% comparison. It does not prove that T is the correctly determined saturation time, establish the complete curve, or qualify the process under J443. The 0.200 mm value is strip response, **not** a claim of 0.200 mm compressive-layer depth. [Wheelabrator saturation explanation](https://www.wheelabratorgroup.com/my-application/application-by-process/what-is-shot-peening/)

Independently, assume actual-part inspection establishes complete coverage after **80 seconds** under an approved, unchanged setup. Applying the supplier's illustrative 125% time convention gives `1.25 × 80 = 100 seconds`. This arithmetic does not establish that the extra 20 seconds is suitable for the component. The drawing and qualified process must authorize the coverage requirement. [Progressive Surface coverage convention](https://www.progressivesurface.com/shot-peening-faqs/)

## Media, access and process sequence

Media selection includes material, size distribution, shape and hardness. Curtiss-Wright's applications guide identifies cast steel, conditioned cut wire, ceramic and glass media, with substrate condition influencing the resulting residual-stress profile. These are not interchangeable merely because two setups produce similar strip readings. Establish the approved media and its condition-control requirements for the part. [Curtiss-Wright applications guide, 10th edition, printed page 11](https://www.cwst.com/sites/default/files/Resources/tac/shot-peening/MIC-CWST-ShotPeeningApplications10thEdition.pdf)

Access must be qualified at the required surfaces. A fillet, recess or bore may not receive the same stream as an exposed face. The same guide describes internal lances and deflectors for internal surfaces; an external witness strip does not by itself demonstrate treatment of a hidden feature. Define nozzle/part motion, orientation, fixture contacts and masking boundaries in the processing plan rather than inventing a universal hole-depth capability. [Curtiss-Wright applications guide, printed pages 40–41](https://www.cwst.com/sites/default/files/Resources/tac/shot-peening/MIC-CWST-ShotPeeningApplications10thEdition.pdf)

Peening can also change shape: Curtiss-Wright uses that effect deliberately in peen forming. For fatigue treatment, preserve final dimensional requirements and evaluate thin sections and distortion separately; a successfully treated surface does not automatically mean an acceptable final contour. [Curtiss-Wright peen forming](https://cwst.com/our-services/shot-peening/shot-peen-forming)

Later processing matters. The applications guide cautions that heat-induced relaxation depends on material, temperature and time. Review subsequent heating and any removal of the treated surface with the process authority. Do not derive a universal allowable bake temperature or permissible grinding depth from an unrelated alloy example. [Curtiss-Wright applications guide, printed page 34](https://www.cwst.com/sites/default/files/Resources/tac/shot-peening/MIC-CWST-ShotPeeningApplications10thEdition.pdf)

## Qualification and release

Separate repeatable processing from demonstrated component performance. Preserve substrate alloy/condition, critical surfaces, media identity, intensity range and strip designation, coverage requirement/method, approved equipment and motion, masking, verification records and final inspection. Relate representative test pieces to the production geometry and processing setup.

Where the design depends on a residual-stress profile, fatigue result or surface condition, define that requirement and its qualification method separately. An Almen record is useful process evidence, but it is not a direct measurement of the component's stress-versus-depth distribution. The handoff should also identify final dimensions, roughness requirements, permitted subsequent operations and any re-peening/rework authority.

Source versions: SAE records identify J443's December 2025 revision, J2277's January 2023 revision and AMS2430U's April 2018 revision. The Curtiss-Wright guide identifies **10th edition** on its cover and **©2020** on printed page 5; no more specific issue date is inferred. Supplier web explanations are not controlled replacements for the ordered specifications. The worked calculations above are original illustrations.

## Related reference

[Surface-finishing processes](index.md)