What is Shot Peening?

Shot peening is a process that can increase the fatigue resistance, durability and surface hardness of a component by plastically deforming and therefore stress relieving the material surface using small beads of shot media fired at the material surface at high speed.

The process uses compressed air or a centrifugal turbine to fire beads of shot media, typically steel, ceramic or glass beads, at high velocity towards the surface of the material. The impact of the media on the material surface cold works and plasticly deforms the material improving the mechanical properties.

Stress Relief

When metals are forged or formed, as the material cools and contracts, residual tensile stresses begin to grow at the grain boundary. As these boundaries begin to pull apart micro cracks can form on the material’s surface.

All cracks start with a micro crack

Shot peening helps to alleviate these micro cracks by inducing compressional stresses into the material surface to balance out the forces. As the shot media impacts the surface of the material, the tiny indentations cold work and plastically deform the material much like a blacksmith with a tiny hammer giving it millions of taps all over.

Components which undergo cyclic loading such as springs, gears, blades and aerodynamic stays can greatly benefit from shot peening. By reducing the residual stresses, the tensile loads at the surface of the material are reduced so as you begin to load the component in bending, torsion or any cyclic loading the overall tensile loads seen at the surface of the component are reduced too!

Advantages and Disadvantages of Shot Peening

Advantages

• Improves fatigue life, strength and durability of the material
• Reduces fretting
• Fast, reliable and inexpensive process

Disadvantages

• Multiplicity of parameters make the precise control and repeatability of shot peening very problematic
• Shot media can shatter on impact damaging the workpiece
• Can distort high tolerance parts, although this can be managed with very fine shot particles

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Shot Peening vs Shot Blasting

Both these processes use very similar methods of firing shot material at high velocities towards a components surface, however, they do have different purposes!

Shot peening is used to improve the mechanical properties of a material, whereas shot blasting is used to modify a components surface finish in preparation for further surface treatments such as painting.

If required a component can undergo both processes during manufacturing if required!

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Shot Peening Process

The important factors to consider during the application of shot peening are: coverage, exposure time, intensity, application angle and media. The quality and repeatability of shot peening can be quantified using a Almen Test.

Media Types

There are various forms of shot media used in the shot peening process. Each media type can be categorised by size, shape and hardness. Each of these play a part in defining the amount of energy/force applied to the workpiece.

Cast Steel Shot (40-50 HRC)

Suitable for most applications. Typically spherical in shape, the desired hardness of this media can be easily customised through heat treatments. A downside to this media is the cast shot can shatter on impact causing damage to the workpiece

Ceramic Shot (57-63 HRC)

Ceramic shot has a much higher hardness compared to steel shot making it optimal for increasing surface hardness

Glass Beads (>40 HRC)

Primarily used for softer materials such as aluminium and magnesium. Peening with glass can be used as a secondary process to improve the components surface waviness/smoothness.

Cut Wire Shot (40-60 HRC)

Cut wire shot can be produced from a number of materials. Cut from stock wire, this shot media has the highest consistency in shape, low contamination and low dust generated as the shot often deforms into a spherical shape opposed to shattering on impact. Due to manufacturing limitations, cut wire shot is can be ruled out when small sized shot is required.