Shot Peening is a cold working process used to produce a compressive residual stress layer and modify the mechanical properties of metals and composites. This involves impacting the surface with shots (round metal, glass, or ceramic particles) with sufficient strength to create plastic deformation.
In machining, peening shot is used to strengthen and relieve stress on components such as steel car crankshafts and connecting rods. In the architecture it gives a muted touch to the metal.
Peening taking is similar to sandblasting, except that it operates with a plasticity mechanism rather than abrasion: each particle acts as a ball-peen hammer. In practice, this means fewer materials are removed by the process, and less dust is created.
Video Shot peening
Detail
Peening surfaces spread plastically, causing changes in the mechanical properties of the surface. The main application is to avoid spreading microcracks from the surface. Such cracks do not propagate in materials that are under compressive pressure; shooting can cause stress on the surface.
Shooting is often mentioned in the repair of aircraft to relieve the tensile stress built into the grinding process and replace it with a favorable compressive pressure. Depending on part geometry, part material, shooting material, shooting quality, shot intensity, and shooting range, peening photography can increase fatigue life by up to 1000%.
Plastic deformation induces residual compressive stress on peened surfaces, together with tensile stresses in the interior. The surface tension stress provides resistance to metal fatigue and some form of stress corrosion. The tensile stress is deep inside the no-problem part as the tensile stress at the surface because the crack tends to start in the interior.
A study conducted through the SAE SAE Committee for Evaluation and Evaluation shows what shield firing can do for welds compared to welds that do not perform this operation. The study claims that ordinary welding will fail after 250,000 cycles when the fired welds will fail after 2.5 million cycles, and the failure will occur outside the weld area. This is part of the reason that peening shooting is a popular operation with aerospace parts. However, beneficial clumps can breed out at higher temperatures.
Intensity is the key parameter of the firing process of firing. After some process development, analog is required to measure the firing effect of firing. John Almen noticed that the shot peening made the affected metal sheet side begin to bend and stretch. He created the Almen strip to measure the pressure press on the strip created by shooting firing operations. One can obtain the so-called "flurry intensity" by measuring the deformation on the Almen strip present in the firing firing operation. When the strip reaches 10% deformation, the Almen strip is then hit with the same intensity for twice the amount of time. If the lane changes again 10%, then the one gets the intensity of the flow of bursts.
Another operation to measure the intensity of the shot peening process is the use of Almen rotation, developed by R. Bosshard.
Coverage , the percentage of the indented surface once or more, subject to variation due to the angle of the shot burst flow relative to the workpiece surface. The stream is conical, so, the shooting arrives at various angles. Processing a surface with a series of overlaps passes increased coverage, although variations in "line" will still exist. Alignment of the axis of the shot flow with the axis of the Almen strip is important. Continuous continuous pressures of the workpiece have been shown to be produced with a range of less than 50% but dropped as 100% approached. Optimizing the coverage level for the process performed is important to produce the desired surface effect.
SAE International includes several standards for peening shots in the field of aerospace and other industries.
Maps Shot peening
Process and tools
Popular methods for encouraging media shot include an air blast system and a centrifugal blast wheel. In an air blast system, the medium is introduced by various methods into the high-pressure air path and is accelerated through a nozzle directed to the part to be peeled. The centrifugal blast wheel consists of high-speed paddle wheels. The shooting medium is introduced at the center of the spinning wheel and is driven by centrifugal force by the paddle rotating toward that part by adjusting the location of the media entry, effectively setting the media release time. Other methods include ultrasonic peening, wet peening, and peening lasers (which do not use media).
Media options include round cast steel shots, ceramic beads, glass beads or enclosed cut wire (round). Cut wire shot is preferred because it maintains its roundness because it is degraded, unlike cast shots that tend to break into sharp pieces that can damage the workpiece. Cut wire can last five times longer than a cast shot. Because peening demands consistent hardness, diameter, and shape images, a mechanism for removing fragments of shots throughout the process is desirable. Available equipment includes separators for cleaning and remanufacturing shots and feeders to add new shots automatically to replace damaged materials.
A popular method for sorting out damaged/out-of-spec media shots is the use of shot separators. Separator size production consists of various levels of precision wire mesh, of 1 or more sizes to sort, and mechanically shuffled. Some apps require maximum and minimum shooting diameter. To maintain specifications, shooting is slowly introduced where large shots/contamination will be sorted at the first stage, then portraits in the specification are sorted at the second level, then the images are degraded below the last sorted specification. Aperture on the wire mesh is getting smaller in this case. It is possible to attach production separators to peener shots for continuous shooting quality control. The test method uses a similar concept in a much smaller package, where a technician takes a shot sample and then sorts various sizes. Further testing of the sample verifies the quality of the media shot.
The blast wheel system includes satellite rotation models, rotating throughfeed components, and various manipulator designs. There is a monorail system overhead and also a reverse seat belt model. Workpieces holding equipment include rotating index tables, loading and unpacking robots, and jigs that hold many workpieces. For larger workpieces, the manipulator to reposition them to expose the feature to the burst flow of available shots.
Cut the wire shot
Cut wire shot is a metal shot used for firing shot, in which small particles are fired at workpiece by compressed air jet. This is a low cost manufacturing process, because basic raw materials are not expensive. As-cut particles are an effective abrasive because of the sharp edges created in the cutting process; however, the cut shot is not the desired stripping medium, because the sharp edges do not match the process.
Cut shot is made of high-quality wire in which each particle is cut to the same length as its diameter. If necessary, the particles are conditioned (rounded) to eliminate the sharp angles generated during the cutting process. Depending on the application, various ranges of violence are available, with the higher the media hardness the lower its durability.
Other cut-wire shot applications include smoothing and vibration completion.
Coverage
Factors affecting the coverage density include: impact number (shot flow), exposure time, firing properties (size, chemical), and workpiece nature. Coverage is monitored by visual examination to determine percent coverage (0-100%). Coverage beyond 100% can not be determined. The number of individual impacts is directly proportional to the flow of shot, exposure area, and time of exposure. Coverage is not linearly proportional because of the random nature of the process (chaos theory). When 100% coverage is reached, with 1T lighting time, the surface location has been affected multiple times. At 150% (1.5T) coverage, 5 or more impacts occurred in 52% of the sites. At 200% (2T) coverage, 5 or more impacts occurred in 84% of the sites.
Coverage is affected by the geometry of shots and the shots and chemistry of the workpiece. The size of the shot controls how many impacts there are per pound, where smaller shots produce more impact per pound because it takes less exposure time. Soft shots that impact on hard materials will require more lighting time to reach acceptable range compared to hard shots that impact soft material (because harder shots can penetrate deeper, thus creating a bigger impression).
Coverage and intensity (measured with Almen strips) can have a profound effect on fatigue life. This can affect a variety of materials that are usually shot peened. Incomplete or excessive coverage and intensity can lead to reduced fatigue life. Overpeening will cause excessive cold work of the workpiece surface, which can also cause fatigue cracks. Be diligent when developing parameters for coverage and intensity, especially when using materials with different properties (eg Soft metal to harder metals). Testing periods of fatigue on various parameters will produce "sweet spots" where there is an exponential growth close to the peak fatigue period (x = intensity peening or energy flow media, y = time-to-crack or fatigue strength) and rapidly reduce age fatigue due to more intensity or added coverage. The "sweet spot" will be directly correlated with the kinetic energy transferred and the material properties of the media shot and workpiece.
Apps
Peening takes on gears, cam and camshafts, clutch springs, coil springs, connecting rods, crankshafts, gears, leaf springs and suspensions, rock drill, and turbine blades. It is also used in casting for sand removal, decoration, crust, and surface finishing of castings such as engine blocks and cylinder heads. Its crust action can be used in the manufacture of steel products such as strips, plates, sheets, wires, and bar stocks.
Peening is an important process in spring manufacturing. Types of springs include leaf springs, springs extensions, and compression springs. The most widely used application is to spring the engine valve (compression spring) due to high cyclic fatigue. In an OEM valve spring application, mechanical design combined with multiple firing ensure longevity shots. Automotive makers are switching to high-pressure spring designs with higher pressure when engines evolve. In high-performance aftermarket valve spring applications, the need for controlled and multi-step shooting peening is a requirement to withstand extreme surface pressures that sometimes exceed the material specifications. Extreme-life spring fatigue performance (NHRA, IHRA) can be as short as two passes on a 1/4 mile drag racing track before relaxation or failure occurs.
Peening shot can be used for cosmetic effect. The surface roughness resulting from overlapping dimples causes the light to spread during reflection. Because peening usually produces a larger surface feature than sand blasting, the resulting effect is more pronounced.
Awning and abrasive peening shooting can apply the material to the metal surface. When the shot or sand particles are detonated through a powder or liquid containing the desired surface layer, the impact plate or coating the surface of the workpiece. This process has been used to embed a ceramic coating, although the scope is random rather than coherent. 3M develops a process in which a metal surface is detonated with particles with an alumina core and an outer layer of silica. The result is the fusion of silica to the surface. The process known as peen plating was developed by NASA. The fine powder of metal or non-metallic coated onto the metal surface using bead shot glass as a blast medium. The process has evolved to apply solid lubricants such as molybdenum disulfide to the surface. Biocompatible ceramics have been applied in this way to biomedical implants. Peen plating subject coating materials for high heat in collisions with shots and layers should also be available in powder form, limiting the range of usable materials. To overcome the heat problem, a process called mediated mediated-mediated collision coating (TM-CMC) has allowed the use of polymers and antibiotic materials as peened coatings. This layer is presented as aerosol which is directed to the surface at the same time as the particle flow of the shot. The TM-CMC process is still in the development stage of R & D.
Stress remaining compression
The substrate pressure residual stress profile was measured using techniques such as x-ray diffraction and hardness profile test. The X axis depth in mm or inches and the Y axis is the residual stress in the csi or MPa. The maximum residual stress profile may be affected by firing force factors, including: section geometry, part material, shot material, shooting quality, shot intensity, and firing range. For example, a peeling shot of a hardened steel section with a process and then using the same process for the other non-hardened part may lead to overpeening; causing a sharp drop in residual surface stress, but not affecting the voltage below the surface. This is very important because the maximum pressure is usually on the surface of the material. Mitigation of these lower surface tensions can be achieved by multi-stage post processing with varying shot diameters and other surface treatments that remove the low residual stress layers.
The residual stress of compression in the metal alloy is produced by the transfer of kinetic energy (K.E.) of the moving mass (particle of shot or peen ball) to the surface of the material with the capacity for plastic deformation. The residual stress profile also depends on the coverage density. The collision mechanism involves the nature of hardness, shape, and structure of the shot; as well as the nature of workpiece. Factors for process development and control for K.E. transfers for peening shot are: shot speed (wheel speed or air pressure/nozzle design), shot mass, chemical shots, impact angle and workpiece. Example: if you need very high residual stress, you may want to use large diameter cut wire shots, high intensity process, direct explosion to workpiece, and very hard work material material.
History and further developments
It is common practice for the blacksmith to hammer the sunken side of the leaf springs, which increases their lives, although the exact mechanism is unknown. The maximum tensile stress is located on the surface of the leaf spring part of the leaf; peening effectively offset the maximum tensile stress, also located on the surface, when the compressive stress is induced by peening with a peen ball hammer.
Peening was shot independently found in Germany and the United States in the late 1920s and early 1930s. The first commercial implementation was carried out in the United States on automotive valve springs.
As a further development of the peening shot process, other techniques that induce compression residual stress, such as low plasticity burnishing, Laser peening and High Frequency Impact Treatment are developed. This method creates deep compression residual stress to increase component life.
Treatment of surface mechanical destruction uses a process similar to peening shot, but with higher kinetic energy, to form a nanocrystalline layer on the metal surface.
See also
- Autofrettage, which produces compression residual pressure in pressure vessels.
- Hardening letters
- Differential hardening
- Abrasive steel
- Steel Belt Shot
- High frequency impact treatment after weld transition treatment
References
Further reading
- Momber, A.W.: Blast Cleaning Technology. Springer Publ., Heidelberg, 2008.
External links
- "ShotBlasting and shotpeening" (PDF) .
Source of the article : Wikipedia
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