Shot peening

thumb|Difference between non shotpeened metal and shotpeened metal

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.

In machining, shot peening is used to strengthen and relieve stress in components like steel automobile crankshafts and connecting rods. In architecture it provides a muted finish to metal.

Shot peening is similar mechanically to sandblasting, though its purpose is not to remove material, but rather it employs the mechanism of plasticity to achieve its goal, with each particle functioning as a ball-peen hammer.

Details

Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. Its main application is to avoid the propagation of microcracks in a surface. By putting a material under compressive stress, shot peening prevents such cracks from propagating.

Shot peening is often called for in aircraft repairs to relieve tensile stresses built up in the grinding process and replace them with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, and shot coverage, shot peening can increase fatigue life up to 1000%.

SAE International's includes several standards for shot peening in aerospace and other industries.

Process and equipment

Popular methods for propelling shot media include air blast systems and centrifugal blast wheels. In the air blast systems, media are introduced by various methods into the path of high pressure air and accelerated through a nozzle directed at the part to be peened. The centrifugal blast wheel consists of a high speed paddle wheel. Shot media are introduced in the center of the spinning wheel and propelled by the centrifugal force by the spinning paddles towards the part by adjusting the media entrance location, effectively timing the release of the media. Other methods include ultrasonic peening, cavitation water jet peening, and laser peening (which does not use media).

Industrial shot peening processes are commonly applied in controlled environments to enhance fatigue resistance and improve surface performance of metal components.

Media choices include spherical cast steel shot, ceramic bead, glass bead, or conditioned (rounded) cut wire. Cut wire shot is preferred because it maintains its roundness as it is degraded, unlike cast shot, which tends to break up into sharp pieces that can damage the workpiece. Cut wire shot can last five times longer than cast shot. Because peening demands well-graded shot of consistent hardness, diameter, and shape, a mechanism for removing shot fragments throughout the process is desirable. Equipment is available that includes separators to clean and recondition shot and feeders to add new shot automatically to replace the damaged material. Shot peening may also be used for a cosmetic effect. One cosmetic use is that surface roughness resulting from the overlapping dimples causes light to scatter upon reflection. As peening typically produces larger surface features than sand-blasting, the resulting effect is more pronounced.

Springs

Shot peening is a crucial process in spring making. Types of springs include leaf springs, extension springs, and compression springs. The most widely used application is for engine valve springs (compression springs) due to high cyclic fatigue. In an OEM valve spring application, shot peening ensures longevity. Automotive makers are shifting to higher performance valve spring designs, which have higher stress, 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 exceed 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.

Steel belts

Shot peening is used on steel belts to extend lifespan, reduce cracks, and create layers of compressive stress. Compressed shot creates an indentation on the cold-worked metal, introducing high compressive stress. Overlapping indentations create/cause a continuous layer of residual stress. Because most fatigue and stress corrosion failures originate at the surface, the compressive stress layer reduces surface cracks and may also extend the belt's lifespan. Peening starts from the center of a section and progresses towards the edge of the belt. The process typically starts with a low pressure and increases in steps until a noticeable effect is seen in the curve of the belt. For a precipitation-hardened stainless-steel belt, the required pressure can be as high as 90 PSI.

Peen Plating

Shot peening and abrasive blasting can apply materials on metal surfaces. When the shot or grit particles are blasted through a powder or liquid containing the desired surface coating, the impact plates or coats the workpiece surface. The process has been used to embed ceramic coatings, though the coverage is random rather than coherent. 3M developed a process where a metal surface was blasted with particles with a core of alumina and an outer layer of silica. The result was fusion of the silica to the surface. The process known as peen plating was developed by NASA. Fine powders of metals or non-metals are plated onto metal surfaces using glass bead shot as the blast medium. The process has evolved to applying solid lubricants such as molybdenum disulfide to surfaces. Biocompatible ceramics have been applied this way to biomedical implants. Peen plating subjects the coating material to high heat in the collisions with the shot and the coating must also be available in powder form, limiting the range of materials that can be used. To overcome the problem of heat, a process called Temperature Moderated-Collision Mediated Coating (TM-CMC) has allowed the use of polymers and antibiotic materials for peened coatings. The coating is presented as an aerosol directed to the surface at the same time as a stream of shot particles. The TM-CMC process is still in the R&D phase of development.

Compressive residual stress

A sub-surface compressive residual stress profile is measured using techniques such as x-ray diffraction and hardness profile tests. The X-axis is depth in mm or inches, and the Y-axis is residual stress in ksi or MPa. The maximum residual stress profile can be affected by the factors of shot peening, including: part geometry, part material, shot material, shot quality, shot intensity, and shot coverage. For example, shot peening a hardened steel part with a process and then using the same process for another unhardened part could result in over-peening, which causes a sharp decrease in surface residual stresses, but not affecting sub-surface stresses. This is critical because maximum stresses are typically at the surface of the material. Mitigation of these lower surface stresses can be accomplished by a multi-stage post process with varied shot diameters and other surface treatments that remove the low residual stress layer.

The compressive residual stress in a metal alloy is produced by the transfer of kinetic energy (K.E.) from a moving mass (shot particle or ball peen) into the surface of a material with the capacity to plastically deform. The residual stress profile is also dependent on coverage density. The mechanics of the collisions involve properties of the shot hardness, shape, and structure; as well as the properties of the workpiece. Factors for process development and the control for K.E. transfer for shot peening are: shot velocity (wheel speed or air pressure/nozzle design), shot mass, shot chemistry, impact angle and work piece properties. As an example: if very high residual stresses were needed, the following would be required: large diameter cut-wire shot, a high-intensity process, direct blast onto the workpiece, and a very hard workpiece material.