Shot peening: Almen intensity, coverage and the difference from cleaning

Controlled shot peening is a mechanical process intended to introduce beneficial compressive residual stress near a surface. It is considered for cyclically loaded components where surface crack initiation or growth matters. Cleaning or cosmetic blasting primarily targets scale, contamination or texture. The operations can look similar while their controls and acceptance evidence differ. This guide addresses requirements for a peening specification, rather than choosing glass beads or alumina for a decorative finish.

Media impacts create local plastic deformation. Restraint from underlying material contributes to compressive residual stress near the surface. The result depends on material, heat-treatment condition, geometry and process. No universal fatigue-life improvement applies to every part, and peening cannot repair a poor design or every pre-existing defect. Uniform appearance alone is insufficient evidence. If fatigue performance is the purpose, define process controls and suitable evidence instead of relying only on a photograph.

The Almen system uses a standard strip peened on one side. Arc height is measured under a specified procedure to determine and verify intensity. Specify strip type, units and intensity range, together with the applicable saturation or verification procedure. An arbitrary arc-height reading at an arbitrary exposure time is not automatically valid intensity. Almen is a process-control indicator, not a direct residual-stress measurement on the component; it cannot be converted indiscriminately into MPa or surface roughness Ra.

Identify mandatory peening zones, prohibited zones and permitted transitions. Evaluate precision bores, threads, thin edges, sealing faces and fits separately. Masking must protect the intended area without accidentally shielding a required treatment zone. For difficult geometry, establish media access and the method for verifying results. A changed external appearance or a nozzle passing near a feature is not a coverage guarantee.

Peening is often placed after major shaping operations, but the actual sequence must follow design requirements and the approved process. Subsequent grinding, polishing or cutting may remove treated surface material. Later heating can also alter residual stress. If finishing remains, specify permissible removal and how its influence will be assessed. Check dimensions, distortion, roughness and cleanliness as appropriate, especially on thin parts. Do not change the sequence merely to simplify packing or improve appearance.

Provide material, heat-treatment condition, standard and revision, treatment zones, media specification, intensity range with Almen strip type and units, coverage, masking, final condition and required records. Specify residual-stress measurement or fatigue testing separately if the design requires them; a peening certificate does not automatically include those tests. VietTech performs CNC machining and inspection within confirmed capability and coordinates specialist surface treatments with partners. For shot peening, partner availability, process and scope must be confirmed before commitment. This article does not imply an in-house peening line.

Reviewed on 19 October 2026 against the SAE listings for J443_202512 on intensity, J2277_202301 on coverage and Curtiss-Wright’s process-control guidance. These documents serve different purposes; the order must identify the applicable editions. This guide supports specification review and RFQ preparation, not replacement of a complete standard or qualified procedure. Do not select intensity, coverage or expected life from an illustration or from an unrelated part merely because the material is the same.