Shot peening
Shot peening is a cold working process used to produce a compressive residual stress layer and modify the mechanical properties of metals and composites.
It entails striking a surface with shot (round metallic, glass, or ceramic particles) with force sufficient to create plastic deformation.
[1][2] In machining, shot peening is used to strengthen and relieve stress in components like steel automobile crankshafts and connecting rods.
John Almen noticed that shot peening made the side of the sheet metal that was exposed begin to bend and stretch.
One can obtain what is referred to as the "intensity of the blast stream" by measuring the deformation on the Almen strip that is in the shot peening operation.
Another operation to gauge the intensity of a shot peening process is the use of an Almen round, developed by R. Bosshard.
Processing the surface with a series of overlapping passes improves coverage, although variation in "stripes" will still be present.
A continuous compressively stressed surface of the workpiece has been shown to be produced at less than 50% coverage but falls as 100% is approached.
Optimizing coverage level for the process being performed is important for producing the desired surface effect.
[8] Wheel blast systems include satellite rotation models, rotary throughfeed components, and various manipulator designs.
If required, the particles are conditioned (rounded) to remove the sharp corners produced during the cutting process.
The "sweet-spot" will directly correlate with the kinetic energy transferred and the material properties of the shot media and workpiece.
[2] It is also used in foundries for sand removal, decoring, descaling, and surface finishing of castings such as engine blocks and cylinder heads.
Its descaling action can be used in the manufacturing of steel products such as strip, plates, sheets, wire, and bar stock.
In an OEM valve spring application, the mechanical design combined with some shot peening ensures longevity.
Automotive makers are shifting to more high performance higher stressed valve spring designs as engines evolve.
In aftermarket high performance valve spring applications, the need for controlled and multi-step shot peening is a requirement to withstand extreme surface stresses that sometimes exceeds material specifications.
The fatigue life of an extreme performance spring (NHRA, IHRA) can be as short as two passes on a 1/4 mile drag racing track before relaxation or failure occurs.
Because peening typically produces larger surface features than sand-blasting, the resulting effect is more pronounced.
3M developed a process where a metal surface was blasted with particles with a core of alumina and an outer layer of silica.
from a moving mass (shot particle or ball peen) into the surface of a material with the capacity to plastically deform.
