Table of Contents

Introduction to Shot Peening

Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the metal acts as a tiny peening hammer, imparting a small indentation or dimple on the surface. In order for the dimple to be created, the surface layer of the metal must yield in tension. Below the surface, the compressed grains try to restore the surface to its original shape, producing a hemisphere of cold-worked metal highly stressed in compression . Overlapping Dimples develop a uniform layer of residual compressive stress.

Peening Action

When individual particles of shot in a high-velocity stream contact a metal surface, they produce slight, rounded depressions in the surface, stretching it radially and causing plastic flow of surface metal at the instant of contact. In the stress distribution that results, the surface metal has induced or residual compressive stress parallel to the surface, while metal beneath has reaction-induced tensile stress. Peening action improves the distribution of stresses in surfaces that have been disturbed by grinding, machining, or heat treating.

Equipment Used for Shot Peening

The principal components of shot peening equipment are a shot-propelling device, shot recycling and classification arrangements, and a work handling conveyor.

The air blast method introduces the shot, either by gravity or by direct pressure, into a stream of compressed air directed through a nozzle onto the work to be peened. Air blast method can develop higher intensities with small shot sizes, permits the peening of deep holes and cavities by using a long nozzle, consumes less shot in peening small areas on intricate parts, and has lower initial cost.

Methods Used For Shot Peening

Conventional (Mechanical) Shot Peening –
Conventional shot peening is done by two methods. Method one involves accelerating shot material with compressed air. Shot is introduced into a high velocity air stream that accelerates the shot to speeds of up to 250 ft/s. The second method involves accelerating the shot with a wheel.

Laser-shot Peening –

Laser-shot peening utilizes shock waves to induce residual compressive stress. The primary benefit of the process is a very deep compressive layer with minimal cold working. Layer depths up to 0.40” on carburized steel and 0.100” on aluminum alloys have been achieved. Mechanical peening methods can only produce 35% of these depths.

Dual Peening
Dual peening further enhances the fatigue performance from a single shot peen operation by re-peening the same surface a second time with smaller shot and lower intensity. Peening the surface a second time drives the peaks into the valleys, further increasing the compressive stress at the surface.

Strain Peening
Where dual peening increases the compressive stress on the outer surface of the compressive layer, strain peening develops a greater amount of compressive stress throughout the entire compressive layer. This additional stress is generated by preloading the part within its elastic limit prior to shot peening. When the peening media impacts the surface, the surface layer is yielded further in tension because of the preloading. The additional yielding results in additional compressive stress when the metal’s surface attempts to restore itself.

Parameter Selection

The choice of shot peening parameters is dependent 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

Following process controls are used and critical to maintaining process integrity:

• Media

• Intensity

• Coverage

• Equipment & Process Integrity

Application of Shot Peening

After Some Manufacturing Processes

Processes like Grinding , Electro Discharge machining (EDM), Plating , Electro-Chemical Machining (ECM), Anodizing, Plasma, Decarburization

SERVICE EFFECTS REMEDIED BY SHOT PEENING

Cavitation Damage, Fretting and Fretting Fatigue, Galling, Pitting, Crack Arrest, Stress Corrosion Cracking , Figure shows the concept of compressive crack arrest.

PRODUCTS BENEFITTING FROM SHOT PEENING

Jet Engine Blades
Connecting Rods
Crankshafts
Gears
Shafts and Axles
Springs
Torsion Bars

Benefits Of Shot Peening

Enhances fatigue strength.
Improves ultimate strength.
Prevents cracking due to wear.
Prevents corrosion.
Prevents fretting.
Possible to increase the fatigue strength of damaged parts extending the wear.
Increases lubricity by creating small pores in which lubricants can accumulate.
Leaves a uniformly textured, finished surface ready for immediate use or paint and coatings.
Can be used to curve metal or straighten shafts without creating tensile stress in a Peen forming process.

Shot Lining

In this method the thin foil can be joined to the surface of the substrate bringing about large plastic deformation and the pressure generated by the hit of many shots are utilized for the joining.

The lining process using shot peening is suitable for the joining of thin and dissimilar foils required for the improvement of surface properties. By means of peening with many shots, the foil is successfully joined over the surface of the substrate.