Shot peening is a process used to create a compressive residual stress layer and modify the mechanical properties of metals and composites. A surface is blasted with shot; metallic, glass, or round ceramic particles with enough force to create plastic deformation.
What is Shot Peening?
Shot peening is effective through creating a plasticity, rather than in sandblasting, relying on abrasion. The shot particle acts as a ball-peen hammer. This method is less destructive and removes less material than sandblasting.
How does shot peening work?
Peening causing changes in the mechanical properties of the substrate. Crucially, it aims to avoid the propagation of micro-cracks from a surface, an advantage of exposing the material to a layer of compressive stress.
In aerospace, repairs via this process are used to relieve tensile stresses built up in the grinding process. By altering the variables; geometry, material, shot material, shot quality and intensity, shot peening can increase fatigue life significantly.
Plastic deformation induces a residual compressive stress in a peened surface, along with tensile stress in the interior. Surface compressive stresses confer resistance to metal fatigue and to some forms of stress corrosion.
Shot peening intensity
This is an important variable. J Almen noticed, during the development of the process that shot peening made the side of the sheet metal bend and stretch. He named the Almen strip a measure of the compressive stresses. The intensity of the blast is measured by the deformation on the Almen strip. As the strip reaches 10% deformation, the Almen strip is then hit with the same intensity for twice the amount of time. If the strip deforms another 10%, then you can calculate the intensity of the blast stream. The Almen round, developed by R. Bosshard, is a second method to analyse the intensity.
Shot peening coverage
This is the percentage of the surface indented once or more. The stream is cone-shaped and therefore the shot blasts the surface at various angles. A series of repeat, overlapping passes improves coverage, although variation in stripes still occurs. Alignment of the stream with the Almen strip is crucial. 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.
Shot peening process and equipment
Common methods for passing shot media are air blast systems and centrifugal blast wheels. Media can be introduced into the path of high pressure air and accelerated through a nozzle. 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. Several alternative methods are ultrasonic peening, wet peening, and laser peening.
Materials used for shot peening
Shot options are spherical cast steel shot, ceramic bead, glass bead or conditioned cut wire. Cut wire shot is preferred because it maintains its roundness as it is degraded, unlike cast shot which tends to disintegrate. Cut wire shot can lasts longer too. Separators clean and recondition shot during the process and feeders add new shot replacing any damaged material.
Wheel blast systems are typically satellite rotation models. There are overhead monorail systems as well as reverse-belted models. Workpiece holding equipment includes rotating index tables, loading and unloading robots.
Cut wire shot
This is a metal shot where small particles are fired by a compressed air jet. It is a low-cost manufacturing process, as the basic shot is relatively cheap. Cut shot is manufactured from high quality wire. The particles are conditioned to remove the sharp corners produced during the cutting process. Depending on application, various hardness ranges are available, with the higher the hardness of the media the lower its durability.
What is compressive residual stress (shot peening)?
A compressive residual stress profile is measured using techniques such as x-ray diffraction. The maximum residual stress profile can be affected by the several factors. 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 will result in a decrease in surface residual stresses, but won’t affect sub-surface stresses. This is critical because maximum stresses are typically at the surface of the material.
The compressive residual stress in a metal alloy is produced by the transfer of kinetic energy from a shot mass into the surface of a material. The residual stress profile is also dependent on coverage density. The mechanics of the collisions involve properties of the shot hardness, shape, and structure. Factors for process development and the control for kinetic energy transfer for shot peening are: shot velocity, shot mass, shot chemistry, impact angle and workpiece properties. Therefore, for very high residual stresses you would want to use large diameter cut-wire shot, a high-intensity process, direct blast onto the workpiece, and a very hard workpiece material.
Advantages and benefits
The advantages of shot peening is to extend the service life of a component, increasing resistance to fatigue e.g. corrosion fatigue and stress corrosion. Additionally, the material benefits from the reduction in probability of further propagation of cracks. The compressive stresses will resist metal fatigue. Another advantage is that the process actually assists tensile stressed parts from within the material. In summary, the extension of life of a component can be magnified, if an optimised process is used.
Applications of shot peening
Applications include being used on gear parts, connecting rods, crankshafts, rock drills, and turbine blades. It is used in foundries for sand removal and descaling. Its descaling action can be used in the manufacturing of steel products such as strip, plates, sheets, wire, and bar stock.
This is a key process in spring making. Types of springs include extension springs and compression springs. A common application for manufacture of engine valve springs due to high cyclic fatigue. Shot peening ensures longevity. 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.
It is also used for aesthetics. The surface roughness results in light scatter upon reflection.
Shot peening used for coatings and plating
This process can apply additive materials on metal surfaces. If applied through a powder or liquid, the impact plates the workpiece. The process has been used to embed ceramic coatings. Fine powders of metals or non-metals are plated onto metal surfaces using glass bead. The process has evolved to applying solid lubricants such as molybdenum disulphide to surfaces.
Shot peen plating
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. Temperature moderated-collision mediated coating (TM-CMC) has allowed the use of polymers and antibiotic materials as peened coatings, considering the excessive temperatures involved. TM-CMC process is still at a low technology readiness level.
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