Peening is a process of introducing mechanical stress into the surface layer of a metal sheet or part to compress and strengthen it against future fractures and wear. The process of manual peening of metal can be traced back to prehistoric times with the use of a ball peen hammer, which has a hemispherical, ball-shaped end on one side so that metal structures can be beat manually. Hammers are considered among the earliest of tools invented by humanity, with a legacy of their use stretching back about 2,500,000 years. Modern-day peening is often done by machine, however, using spherical metal shot pellets in a process known as shot peening or cold working of metal to shape and strengthen it.

While the cold forming of metal often involves small ball-shaped or peen shot made of metals like iron or steel, the peen material can also be composed of glass, plastic, or high-strength ceramics depending on the type of metal being worked and the amount of stress that the part will undergo in the future. Compressing the surface through repeated impacts by the peen material also adds other beneficial features to the surface. These include making the metal surface corrosion-resistant, as well as sealing any microscopic pores or holes in the surface to give it a more uniform, smooth appearance. Some carefully controlled shot peen 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 utilizes a similar process is that of surface blasting, where a more roughly-shaped peen material is used to clean the metal surface of rust, paint, or other coatings.
 

The main arena in which peening is used is for the improvement of metal parts that undergo repeated mechanical stress and strain over time. These include components like cogs, gears, and crankshafts that are common and vital components of many types of machinery. Uniformly-shaped parts such as gears are often peened in a chamber where the process is precisely controlled. One of the industries that makes widespread use of commercial peening is the automotive manufacturing industry that has been using the process since it was pioneered in Germany in 1929.
 

Metal surfaces that have an irregular shape and are under stress, such as the joints where metal has been welded together and shrinks as it cools, are usually peened manually using ball peen hammers. A specialized arena where shot peening is used to work unusually shaped parts includes the shaping of metallic surfaces for aircraft structures like wings. This has been practiced since the era of World War II on aircraft surfaces, and gives them ideal aerodynamic qualities where resistance to metal fatigue can be increased by a factor of 1000%.
 

Laser peening is another specialized area of the field which has developed with the invention and building of the first ruby laser in 1960. Peening lasers can fire a high energy pulse at metal surfaces with a pressure level of up to 1,000,000 pounds per square inch (70,307 kilograms per square centimeter). This creates a shock wave effect in the metal surface that peens the surface to a depth that is about four times deeper than what is produced via shot peening. The process uses a neodymium glass laser and robotic machinery to control the metalworking effect, and is currently employed in countries such as the US and UK.