What is shot peening and how does it work?

Shot peening is used to prevent or delay fatigue failure and to prevent stress corrosion. By using compressive stress in the surface layers, shot peening can reduce the risk of failure and maintenance costs. Shot peening can also be used to shape components in the correct position. In that case, the goal is not to use compressive stress, but to simply reform the component.

Not shot peened vs. Shot peened

How does shot peening 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.

Shot peening uses small spherical particles, called media or “shot”, usually made of steel, ceramic or glass. Hard, spherical particles hit the surface at a relatively high speed during shot peening. Typically this is 30 to 100 meters per second leading to 108 to 360 kilometers per hour. The dents or dimples cause plastic deformation of the surface and create a residual pressure layer. Multiplying and overlapping the dimples compresses and protects the entire surface with shot peening.

Shot peening process

Metal fatigue

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.

The crack will continue to grow until it reaches a critical size, which happens when the crack’s stress strength factor exceeds the material’s fracture toughness, causing rapid propagation and, in most cases, complete structural fracture. Both high and low cycle fatigue failures follow the same basic steps process of crack initiation, stage II crack growth, and finally ultimate failure.

Shot peening with compressed air

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.

Precise targeting

Selected coverage (1)

Selected coverage (2)

Difficult geometries

Simultaneous peening

Shot peening with Wheel Turbines

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.

Straight blades

Curved blades

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.

Shot peening by means of flap-peening

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.

The Flapper Peening method can also be applied to thin aerostructures for straightening. Flap peening employs a flap of balls trapped in a Kevlar matrix at its extremities. A pneumatic or electric grinding wheel rotates the flap, which is fixed on a shaft. After that, the slap is applied to the peened or formed portion, and the end balls strike it to treat it. The process is controlled by the grinding wheel’s rotational speed controller in conjunction with the flap’s size.

Flap peening

The applications & industries

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.

The growth of the automotive industry in the first half of the twentieth century resulted in highly cyclically stressed parts. For both safety and cost reasons, a way to protect these components from failure was required. One of the first pieces to be shot peened were valve springs. A large range of car parts, including engine and power transmission components, chassis, and suspension structures, are shot peened these days.

Shot peening in the automotive industry

The development of the aviation industry in the mid-twentieth century resulted in products being used beyond their efficiency limits, as well as a greater incentive to avoid failures. Now, a large proportion of aircraft components are shot peened to protect against fatigue or stress corrosion.

Shot peening in the aerospace industry

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.

Typical life cycle enhancement for car components

Shot peening and surface treatment

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.

Although some rise in temperature is produced, shot peening is the cold working process of the surface that causes plastic deformation in the surface layers. With a thick component, the core material does not expand, which limits the surface expansion and generates compressive residual stresses. With a thin component, balancing internal residual stresses leads to deformation.

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