CONTROLLED SHOT PEENING

Controlled shot peening is the bombardment of a surface with small high quality spherical media called shot in a technically defined and controlled way.

The shot can be steel, stainless steel, glass or ceramic.

Each piece of shot striking the metal acts as a tiny peening hammer imparting a small indentation or dimple into the surface. The action of impinging the surface yields the material in tension, further movement is restrained by the core and a surface residual compressive stress results. The magnitude of the compressive stress is directly related to the yield strength of the base material and is approximately equal to 80% of that value in compression.

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Parameter selection - the choice of shot peening parameters is dependant on a variety of conditions:

• knowledge of the application of the component
• component geometry
• manufacturing method
• mechanical properties of the base material
• strain sensitivity of the base material
• environment
• service conditions, loads and cycles
• cost sensitivity

All the above must be considered when deciding on parameter selection and, equally important, maintained throughout the life of the product in a repeatable and consistent manner.

Depth of compressive layer - this is the depth of the compressive layer resisting crack initiation and propagation. The layer depth can be increased by increasing the peening impact energy, but section thickness has to be considered. A deeper layer is generally desired for crack growth resistance and severe environmental condition.

Surface stress - this magnitude is usually less than the maximum compressive stress, which is subsurface but can be tailored to suit the application.

Controlling the process - To ensure reliability and repeatability the variable parameters of the shot peening

process have to be accurately and repeatedly controlled.

Controlled shot peening is different from most manufacturing processes in that there is no non-destructive method to confirm that it has been performed to the proper specification. Techniques such as x-ray diffraction require the part be sacrificed to generate a full compressive depth profile analysis.

To ensure peening specifications are being met for production batches, the following process controls must be maintained: media intensity, coverage, shot direction and repetition. Equipment should be mechanised to ensure the motion of component to shot flow is consistent - hence the term controlled shot peening.

Media control - Controlling shot shape and size will result in a residual compressively stressed layer on the surface of uniform magnitude and depth:

Poor shot shape and size will result in an irregular residual stress profile, excessive surface disruption and potential stress raisers:

Intensity control - Shot peening intensity is the measure of the energy of the shot stream. It is one of the essential means of ensuring process repeatability. The energy of the shot stream is directly related to the compressive stress that is imparted into a component. Intensity can be increased by using larger media and/or increasing the velocity of the shot stream.

Other variables to consider are the impingement angle and peening media. Intensity is measured using Almen test strips and must be conducted initially during set up and repeated at approved intervals.

Coverage control - Complete coverage of a shot peened surface is crucial in performing high quality shot peening. Coverage is the measure of original surface area that has been obliterated by shot peening dimples. Coverage should never be less than 100% as fatigue and stress corrosion cracks can develop in any non peened area that is not encased in residual compressive stress. Some strain sensitive materials perform better with levels of coverage in excess of 100%.

Go to: The shot peening process

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