What is shot peening and how does it work?

Fluctuating tensile stresses increase or cause a surface or underground crack. Tensile stresses concentrate around the crack. The crack grows quickly until the material fails. The purpose of shot peening is to prevent cracking, with permanent compressive stresses. Each shot particle acts like a hammer and the core material acts like an anvil. By indenting, or “pitting” the surface, the surface expands, but the core material resists this expansion. To make the dimples, the shot must be as hard or harder than the target surface.

Fatigue is characterised as the initiation and propagation of cracks in a material as a result of cyclic loading in materials science. If a fatigue crack has begun to develop, it usually produces striations on certain sections of the fracture surface with each loading period.

Air peening works well for applications that require precise targeting or coverage only in selected areas. Moreover it is often applied to difficult-to-access geometries or when different intensities on the same part are needed. Furthermore, it is used for applying simultaneous intensities on two surfaces and for low intensities (N scale – glass and ceramics). Shot peening with compressed air offers multiple shot delivery systems: Suction, Gravity Suction and Direct Pressure. Suction can use many nozzles, from 1 to 24 or more. It offers lower intensities and has higher air consumption and/or the lowest efficiency. Moreover, suction requires lower initial costs. With gravity suction, the hopper is located above the guns. The intensities are slightly higher than they are with normal suction. Moreover, gravity suction uses a suction type nozzle with larger air jets and the guns must point downward. With direct pressure, the shot media is stores in a pressurized vessel. It provides the highest intensities and uses air efficiently. Direct pressure does require the highest initial cost; however, it is also the most flexible.

To start, the principle of wheel peening will be explained. At first, the wheel is moved by gravity, across the funnel in the center of the wheel. Then, the impeller boosts shot throughout the cage window in charge of directing he shot toward the work piece. Lastly, shot is taken in by the blades that will speed it up and eject it from the blade tip to compose the blast stream. Some wheels are reversible to improve coverage on parts and provide versatility and a longer lifetime of spare parts. Blades are available in different material. The curved blades consume less energy and media.

The principles regarding media recovery and recycling apply to both air and wheel-type peening machines. Due to the much larger quantity of shot flowing through a wheel vs. air nozzle, some limitations exist. For example, a spiral separator can accept a small part of the full flow of a turbine. Several separators can be used in series, e.g. vibratory screen and a spiral separator. With a vibratory separator, the top screen removes oversize debris, the bottom screen removes undersize and broken shot and the middle screen has reusable shot. Multiple decks can be used to separate different shot size used in the same machine. A spiral separator removes non spherical shapes: this shot is accelerated and ejected outside. Non spherical particles are not accelerated and kept in the middle. Typical shot for wheel blasting machines are: cast steel shot. Conditioned cut wire shot, stainless steel shot and ceramic shot.

Flap Peening (also called Roto-Peening or Flapper Peening or Rotary Flap Peening) is a portable shot peening process, suitable for the localized treatment of small and difficult-to-access areas. Flap peening is mostly used by aircraft and helicopter maintenance and repair practitioners because it enables a shot peening treatment to be applied directly to the area to be handled without disassembling the component.

Blacksmiths and armourers have been hammering metals to increase their hardness and toughness since the dawn of time. Metals were used more extensively during the Industrial Revolution, as were the innovations that improved their performance.

Shot peening is used to fight tensile residual stresses after the manufacturing process: machining, grinding, welding, casting and rolling. Moreover, it is used to counteract tensile stresses from external loading, for instance: bending, axial loads, torsional loads, heat/cooling cycles, pressure fluctuation and stress corrosion cracking. Overall, shot peening applications enhance the typical lifecycle of components.

Shot peening is a surface cold working process. Shot peening is not a surface preparation like blasting. It improves the condition of the surface to provide the required benefits.