Chapter V Shot Peening

History of Shot Peening

Discovered centuries ago by sword smith’s & black smith’s.

They found that peening the surface of sword or wagon spring greatly increases its resistance to breaking.

Round knob of the “ball peen” hammer was the smith’s tool for cold working/peening.

What is Shot Peening

Shot peening is a cold work process.

Used to finish machine components to improve fatigue and stress corrosion failures.

Small spherical shots bombards the surface.

Dimpling the surface and develops compressive stress.

Shot peening can be done without changing part design.

• Media used for the shot peening include: steel, ceramics and glass.

Shot Peening Mechanism

When a round part (steel ball ) strikes a part of surface at high velocity the contact area is a point.

This concentrates the impact energy in a very small area.

causing a radial plastic flow at the impact point.

This plastic flow or movement of metal leaves compressive stresses in the part.

Complete coverage of the with overhauling ball impacts leaves a

thin permanent compressive stress layer in the part surface.

Why Residual Stress?

Metals fail under tension & not under compressive loads.

The failure crack usually initiate at the part surface where tension stresses are highest and a stress raiser exists.

when part is shot peened, the failure producing tensile stresses are thus reduced by the amount of compressive stresses (residual).

The lowering of the effective tensile strain will then allow the part to accept higher loading.

Typical Shot Peening Curves

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Shot Peening

(a) Mechanism for formation of residual compressive stresses in surface by cold plastic deformation (shot peening). (b) Hardness increased in surface due to shot peening.

Applications of Shot Peening

Gear Parts

Cams and Camshafts

Clutch Springs

Connecting Rods

Crankshafts

Gearwheels

Leaf and Suspension Springs

Rock Drills

Turbine Blades

Shot Peening vs Shot Blasting

Shot Peening (S.P.): Shot peening is a cold working process, which

uses the mass and velocity of a shot stream to produce residual

compressive stress at the surface of the part. precisely controlled

process relying on careful selection and control of media, intensity,

coverage and equipment.

Shot Blasting (S.B.): Shot blasting or blast cleaning is a process in

which an abrasive material is accelerated through a pressurized nozzle

or centrifugal wheel and directed at the surface of a part to clean or

otherwise prepare the part surface for further treatment.

Media Used in S.P. & S.B.

Shot Blasting: Blasting media includes sand; steel shot, cut wire shot, garnet, a sharp hard abrasive glass beads .

Shot Peening: Peening media must remain predominately round and uniform in diameter to avoid surface damage upon impact and to maintain a uniform compressive stress layer.

Applications S.P. & S.B.

Shot Blasting: Shot blasting can be used on castings, forgings, and stampings to produce a uniform surface texture and for descaling, deburring, and deflashing. Shot blasting is used in a wide variety of industries including automotive, marine, mining, and medical applications.

Shot Peening: connecting rods, crankshafts, compression springs, torsion and anti-sway bars and metal implants.