Shot peening
Controlled surface cold working: residual stress, Almen intensity, coverage and component qualification.
Tags: processes, finishing, surface-enhancement
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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
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, SAE J2277 coverage scope
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
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
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, Progressive Surface FAQ
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, Progressive Surface FAQ
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
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
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
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
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
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
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.