Shot peening was discovered centuries ago by blacksmiths who found it increased the strength of swords and wagon springs. It is a cold working process where small spherical shots are projected at high velocity onto a metal surface, dimpling and inducing compressive stresses that increase fatigue life. While shot blasting uses abrasives to clean surfaces, shot peening uses precisely controlled round media to impart a compressive layer that reduces tensile stresses and cracking. It is commonly used on automotive and machinery parts like gears, camshafts, and turbine blades.

1. Chapter V
Shot Peening

2. 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.

3. 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.

4. 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.

5. 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.

7. Typical Shot Peening Curves
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8. 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.

9. Applications of Shot Peening
 Gear Parts
 Cams and Camshafts
 Clutch Springs
 Connecting Rods
 Crankshafts
 Gearwheels
 Leaf and Suspension Springs
 Rock Drills
 Turbine Blades

10. 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.

11. 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.

12. 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.