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Shot Peening is an operation that improves fatigue life and stress corrosion resistance by creating beneficial residual surface stresses that inhibits and/or delays crack initiation and propagation. It is a method of cold working in which compressive stresses are induced into the exposed surface layers of metallic parts and structures by impingement to improve the fatigue life of metal components.

The most common mechanical method used to treat the surfaces is shot peening, where spherical media is propelled by air or a centrifugal wheel and directed at a metal surface at high velocity and controlled parameters.

A void or heavy tooling mark in the surface of a part is a stress concentration point or stress riser, such that its crack propagates and migrates (sometimes to failure) in a part with fatigue loading.

The shot peening process has been a standard mitigating remedy in most cases for post-machining, deburring/polishing and heat treating of high fatigue components.

Common peening media are cast steel shot, conditioned cut wire, ceramic and glass beads.

Note: Shot peening is not applied in order to compensate for deficiencies in other steps of the manufacturing process.

Shot peening may increase the component’s life by 500-1000%.

Shot Peen Control Parameters

Standard operating guidelines have to be met when shot peening. Otherwise, you are just simply shot blasting.  The correct choice of the peening equipment, media, and process parameters are crucial to meeting the specified callout.  At Halo Metal Prep, our team is highly skilled and able to recognize the factors that may affect the shot peening process.

Factors that affect the process are:

  • Shot size and hardness

  • Media volume & flow

  • Rpm/Hrz

  • PSI

  • Nozzle/wheel to work piece distance

  • Angle of impingement

  • Blast pattern

  • Cycle time for coverage

  • Constant media classification (not just periodic sieve tests)

  • Workpiece geometry, material type, hardness and thickness

  • Performing almen strip intensity checks

  • Maintaining a Comprehensive Preventive Maintenance Program

Shot Peening Increases Resistance to:

  • Fatigue failure due to repeated alternating or cyclic stresses

  • Corrosion fatigue

  • Hydrogen Assisted Cracking (HAC)

  • Cavitation erosion compared to that of raw/machined specimens

  • Stress Corrosion Cracking (SCC) common in the alloys/corrosive environment

  • Galling by providing lubricant retention

  • Inter Granular Corrosion (IGC) resistance on some metals

  • Fretting caused by asperity or unevenness of two surfaces

  • Stress risers or flaws in the metal that causes an accumulation of stresses in that area creating micro-cracks that migrate.

Shot Peening Helps With:

  • Enabling lighter designs

  • Surface texturing for bonding

  • Masking & closing of porosity

  • Surface finish and aesthetics

  • Work hardening on softer metals

  • Increasing strength & durability

  • Deburring & radiusing

  • Plasma and welding slag removal

Components which are normally shot peened include springs, wheels, turbine parts (blades, shafts), propellers, aircraft wings and landing gear, valves, gears, crankshafts, connecting rods, pistons, cylinder heads, and bolts.

Specs and Coverage

Shot Peen per SAE J3141 refers to the specification administered by the Society of Automotive Engineers (SAE) titled Surface Vehicle Standard. Shot Peening, which describes required peening media, equipment, preparation, procedure, and quality assurance provisions, among other requirements. Inspect for 100% coverage in accordance with SAE J2277 refers to the specification administered by the SAE titled Surface Vehicle Recommended Practice: Shot Peening Coverage. It describes the procedure for coverage determination and inspection methods.

Peen Scan

A method to determine shot peen coverage with a fluorescent tracer fluid, whereby the part to be peened is coated with the fluorescent dye, which under ultraviolet light glows yellow. After peening, if there is no trace of the coating, the coverage has exceeded 100%. This procedure is used to determine length of cycle time.

Lot Control

Per customer-initiated lot numbers. Certifications will reflect individual lots.

Almen Strip Testing

The shot peening process is widely used to induce compressive residual stresses in fatigue critical surfaces, and its intensity is calculated by placing Almen strips near the intended target and measuring their post-peening curvature or arch height.

The Almen strip is a small hardened and tempered steel test strip, which curves on the exposed side when submitted to the intensity of the blast stream. This amount of upward deflection is then measured by an Almen Gage and its value is used to determine if proper intensity has been achieved. This process may have to be repeated several times. Once the proper arch height is reached, the parameters are recorded and can be used again for future production.

Profilometer

In such cases when customers call out specific surface finish requirements and/or maximum ra values, a profilometer is used to verify the surface roughness after peening or blasting treatments.

Contact Halo Metal Prep today to learn more about our premium quality shot peening services.