Shot peening is a cold working process that uses small spherical media called shot to impact a metal surface and impart compressive residual stresses. As overlapping dimples form from countless impacts, a uniform layer of compressive stress develops just below the surface. This compressive layer increases the resistance of metals to fatigue failure and cracking. Key factors that affect the shot peening process include shot size, velocity and impact angle, as well as exposure time which can be measured using an Almen strip test to determine the desired intensity level.

1. Technical Presentation on
Shot Peening

2. Contents
1. Introduction
2. Why & Where shot peening used ?
3. How shot peening works ?
4. Introduction to the Machine.
5. Controlling the Process.
6. Steel shots specification.
7. Factors affecting shot peening.
8. Benefits of shot peening.

3. Introduction
 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.
 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.

4. Why shot peening ??
The atoms on the surface of manufactured metal will be mostly under
tensile stresses left over from grinding, welds, heat treatments & machining.
Cracks propagate easily in areas of tensile stress because the tensile stresses
are already working to pull the atoms of the metal apart. By shot peening the
material you introduce a layer of compressive stress by compacting the
material.
As the shot peening is performed, the atoms on the surface of the metal
become crowded and try to restore the metal's original shape by pushing
outward.

5. Where shot peening used ?
Shot peening is used on gear parts, cams and camshafts,
clutch springs, coil springs, connecting rods, crankshafts,
gearwheels, leaf and suspension springs, rock drills, and
turbine blade.

6. How shot peening works ?
 The shot peening process impacts the metal surface with small
pellets (round metallic, glass or ceramic particles) with a high enough
force to deform the surface plastically.
 A compressive layer is generated when the impact of all the
particle shots produce small indentations in the top surface layer.
 The layer beneath the top surface is then compressed, generating
a compressively stressed layer underneath the shot peened area.
 This layer helps to prevent the stressed area to crack as a crack
cannot propagate in a compressive environment.
Compressed Material
Multiple Shots
Compressed layer
Multiple shots
Shots

7. Controlling the process
1. Fluorescent test (Coverage test)
 The job is painted with fluorescent paint and shot peened with
same parameters that we do with Almen strip test and if the paint
is completely washed away then the parameters are said to be ok.
 This inspection of test is done using magnifying lens to check the
coverage of the peening.
Before After

8. Size & material of the spherical shot is
important, as its velocity ,rate and angle
at which the blast pattern sweeps across
the surface. The relative work done to
the surface is called the ‘Peening
intensity’.
If a flat strip of metal is shot peened
on one side only it will slightly curl away
from the side which has be treated and
produce a convex surface. If a standard
strip is used, the degree of curvature is a
measure of the peening intensity.
2.Almen strip test (Intensity test)
Types of almen strips
It is made from spring steel of carefully controlled quality to a size within
close tolerances. It is used in three thickness called C, A and N. The C strip is
thickest and N strip the thinnest.
The curvature or are height, of the strip is measured with the aid of a dial
gauge Fig.1 after the strip is placed and retained magnetically against two
pairs of ball contacts a fixed distance apart. The gauge is zeroed with the
unpeened strip in position.

9.  After peening the strip is replaced against the contacts with the
unpeened side towards the dial gauge stem and the Almen are
height is read directly in thousandsth of an inch or millimeters.
 When peening intensity is measured it is important to subject
one side of the Almen strip to exactly the same blast conditions
as the object to be peened.
 1 Thou = 0.0254 mm = 25.4 microns

10. Steel shots specification

11. Factors affecting shot peening
1. Peening exposure time
2. Shot Size & Type
3. Impact angle
4. Shot velocity
5. Distance between nozzle & surface

12. Saturation Curve
 The saturation curve is generated by exposing the Almen
strip to the shot stream for varying amounts of time. When
arc height is plotted against time.
 The (first) point on the curve for which doubling the peening
time increases the arc height by 10%.
1.Peening exposure time

13. 2.Shot Size & Type
 The physical and mechanical properties of the shot
determine the quality of the impact on the surface. If
the shot is softer than the surface it is intended to
compress, then the shot will be deformed and the full
intended peening effect will not be achieved.
 The method is careful to prevent shot fracture in the
conditioning stage. Other factors, such as roundness,
broken or fractured particles and miscellaneous
contamination.
 The depth of deformed layer decreases with increasing
hardness of the material treated.
 Peening intensity increases as the shot size increased.

14. 3.Impact angle
The angle of impact of the shot with the target should be as
close to normal (i.e 90 ) , with attention paid to effect of
rebounding shot interfering with the impinging shot
. Below 45° is considered to be detrimental practice for surface
treatment for two reasons. First, the energy imparted is less
than optimum and will not impart a deep compressive layer.
The velocity of the shot as it impacts the surface is largely
determined by three factors: nozzle size, air pressure and distance
to the work-piece.
4.Shot Velocity

15. Effect of Peening on cycles of failure.
• These compressive stresses
improved the surface fatigue
life, of the gears by 1.6 times
compared to those that were
not shot peened.