Shot peening is a process of compressing and strengthening metal surfaces using mechanical stress. It has been used since prehistoric times and is now done by machine using metal shot. Shot peening is used to improve metal parts that experience repeated stress and deformation, such as gears and crankshafts. Laser peening is a specialized area that uses high-energy pulses to hammer metal surfaces to a depth four times deeper than shot peening.
Shot peening is a process of introducing mechanical stresses into the surface layer of a sheet metal or metal part to compress and strengthen it against future fracture and wear. The process of hand peening metal can be traced back to prehistoric times with the use of a ball hammer, which has a hemispherical ball-shaped end on one side so that metal structures can be pounded by hand. Hammers are considered to be among the earliest tools invented by mankind, with a legacy of their use dating back approximately 2,500,000 years. Modern shot peening is often done by machine, however, using spherical metal shot in a process known as shot peening or cold working the metal to shape and strengthen it.
While metal cold forming often involves small ball or pen shaped pellets made of metals such as iron or steel, the material of the pen can also be composed of glass, plastic or high strength ceramic depending on the type of metal being worked and the amount of stress the part will experience in the future. The compression of the surface through repeated impacts by the pen material also adds other beneficial characteristics to the surface. These include making the metal surface resistant to corrosion, as well as sealing any microscopic pores or holes in the surface to give it a smoother, more uniform appearance. Some carefully controlled shot peening work is also used to add unique textures to metal surfaces where the metal will serve some sort of aesthetic function. Another related industry that uses a similar process is surface sandblasting, where a coarser pen material is used to clean the metal surface of rust, paint, or other coatings.
The primary area where shot peening is used is the improvement of metal parts that experience repeated mechanical stress and deformation over time. These include components such as gears, gears and crankshafts which are common and vital components of many types of machinery. Uniformly shaped parts such as gears are often hammered in a chamber where the process is precisely controlled. One of the industries making widespread use of commercial shot peening is the automotive manufacturing industry which has used the process since it was introduced in Germany in 1929.
Metal surfaces that are irregularly shaped and are under stress, such as joints where metal has been welded together and shrinks as it cools, are usually hammered in by hand using ball hammers. A specialized arena where shot peening is used to machine unusually shaped parts includes the shaping of metal surfaces for aircraft structures such as wings. This has been practiced since the WWII era on aircraft surfaces and gives them ideal aerodynamic qualities where the fatigue strength of the metal can be increased by a factor of 1000%.
Laser peening is another specialized area of the field that developed with the invention and construction of the first ruby laser in 1960. Peening lasers can fire a high-energy pulse at metal surfaces with a pressure level up to to 1,000,000 pounds per square inch (70.307 kilograms per square centimeter). This creates a shock wave effect in the metal surface which hammers the surface to a depth that is approximately four times deeper than that produced by shot peening. The process uses a neodymium glass laser and robotic machinery to control the metalworking effect and is currently being used in countries such as the US and UK.
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