the hashimate university

Shot Peening Brief

Hamzah Mousa 1342475

Some mechanical parts get repaired by machining or welding this part, when doing these processes we generate a tensile stress surface on it, for example the heat from welding make the surface able to react with the gases near it, therefore it will get oxidized and become weaker, so it become less resistant to corrosion and more susceptible to crack.Residual stress after removing the source of the stress, the stresses in the various parts of the structure will not return to zero, it will remain in the various parts of the structure.

Residual stresses can occur through a variety of mechanisms including plastic

deformations, Heat from welding and structural changes.

Tensile Stressis the resistance of an object to a force tending to tear it apart or elongate the material in the axis of the applied load the tensile stress may be increased until the reach of tensile strength, the tensile strength of a material is the maximum amount of tensile stress that it can take before failure

Tensile stress causes an Initial surface crack (fig1+2) which is a small cracks develop on the external surface, in severe cases; it can end in structural failure fig3.

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Fig 1+2 Surface Crack

Fig 3 – fatigue

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Stresses that cause cracks arise from:

Residual cold work Welding Grinding Thermal treatment Or may be externally applied during service

Some mechanical parts cost thousands of dollars to avoid this lose we have an alternative method: “The Shot Peening”

What is shot peening?Shot peening is a cold working process, which utilizes metal shot, glass beads, or ceramic beads to beneficially condition a component surface. In effect, this action forms compressive residual stresses at the surface, increases the strength of metal, the ability to resist the corrosion, eliminate the effect of tensile stress, increase the surface hardness, it can increase the lifetime of some parts by up to 1000% or more

simply its done by shooting a stream of spherical beads at high velocity to dent the surface of the metal part.

Compressive residual surface:

When shots hit the surface it’s create a dimples (slight rounded surface) or a hardened layer (fig4) in the surface which considered as a plastic deformation The atoms just below the surface resist this displacement, creating compressive lateral stress that attempts to restore the surface to its original state. This stress hardens

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the surface and resists crack formation and propagation.

fig4

During each indentation we have the equivalent of a classic loading and unloading cycle creating plastic region containing residual compressive elastic strain following the diagram bellow (fig5-simplified stress-strain diagram for a single peening indentation)

Figure 5-simplified stress-strain diagram for a single peening indentation

O-A: at this point the impacting particle strikes the component’s surface the stress increases elastically to yield strength.A-B: the surface deforms plastically until the shot starts to rebound B-C: during the rebound the surface material relaxes elastically until stress is zero.

The point C indicates the residual strain which when multiplied by elastic modulus gives the level of localized compressive residual stress.

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The depth of the compressive layer (fig6) can be ranged from <0.05mm for light peening applications, up to > 0.75mm for high intensity peening of soft material, the following graph will show us that the compressive depth must not exceed this limit otherwise we will lose the benefits of the shot peening .

Fig6

Over peening can have the opposite effect, over-peening can happen either by using too high of an intensity, peening for a long time, or a combination of both. This can cause extremely localized high stress conditions (stress risers) and even create fatigue cracks that will dramatically reduce the fatigue life of a part, even to below the unpeened condition. Furthermore, excessive peening can lead to the erosion of the peened surface, permanently damaging the part. The fatigue strength decreases with increasing shot peening-time. The increase in roughness inherent during shot peening must always be considered as negative thing

The depth of the compressive layer depends on the intensity and the hardness of the reworked part.

shot Peening process:As we mentioned before we do the shot peening for machined or welded parts , so before we shot any part we do some tests on Almen strips-fig8- (a thin strip

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of SAE 1070 steel used to quantify the intensity of a shot peening process, there are 3 types of them A ,C and N (fig 9))* whether we know the intensity of shooting or not,(mostly this process to identify the intensity) in order not to damage this part , during process we shot the strips on a constant parameters four times and the last one is twofold time of the third one that’s mean in the 3rd time we shot for T¹ and the last time we shot for 2T¹ , after shooting the strips will get arched (check Fig10) ,so we measure the arch-height with a special tool called Almen Gauge (fig7) , the arch-height in the 4th shot must not exceed and not less 10% of the 3rd shot following the diagram of arch hight – time (saturation curve) (Fig11)

Fig7 - Almen Gauge

Fig8 Almen strips

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Fig9 types of Almen strips

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Fig10 – Almen strip before and after shooting

Fig11 Arch height-Exposure Time

If the ratio was right then we are able to shot the main part. Mostly in aviation the

intensity is known and given in a range, this curve is used to identify it.

The work done in this process can be measured using the kinetic energy formula:

0.5mv^2.

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Shot Peening machine:

Fig12

Fig13 Shooting chamber

Generally its work on air pressure, it can be increased and decreased , also there are other variable perimeters like the table speed (rpm) and the distance from the nozzle to the surface of the part, all these perimeters are to control the intensity of shooting , the beads get shot through tubes ends with nozzle(deflector nozzle shot at angle of 35 degree) , or in other words shooting the beads with high pressure,

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inside the shooting chamber there is a rotating table, under it the flour has holes and connected to a vibrator in order to make the used beads fall on a roller and move it to the beads box, an elevator with pockets move the beads to a box in the top of the machine , at the bottom of this box there is a tube , it suck the beads and send it to the shooting tubes.

As we said before the beads are made of metal, plastic and glass, there are types of media:

A. Cast Steel Shot: most common form of surface conditioning. It is used on parts which are not exposed to corrosive environments, used on parts requiring high intensity surface compression.

B. Conditioned carbon steel wire peening. (Cut wire shot): may be substituted for cast steel shot whenever cast steel is permitted.

C. Glass Bead Peening: usually used for low intensities because the fracture rate of glass beads makes this type of peening economically unfeasible.

D. Ceramic Bead Peening: Ceramic bead peening is an alternative to glass bead peening in the Almen N range. However, due to mass and size difference, ceramic media will peen differently when using glass bead peening parameters.

in every shot this media got damaged, the amount of shot that is damaged in a single blasting cycle can range from ~1% to 10% or more, when shooting a rough surface on high intensity, when shooting with a damaged beads it’s like you are doing nothing!, and that’s will lead us to waste time and money, to avoid this , the machine has a stage that make the beads pass through an air flow to aberuncate the damaged beads , and we can see that in the figure bellow (fig12)

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Fig14

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References:

-Ali Fatemi-University of Toledo

-COMCO INC.

-Mechanics of Material, Ferdinand P. Beer

-General Electric shoot peening manual

-Shot peener magazine; Residual stress in shoot peened components by Dr. David -Kirk vol.18 fall 2004

-MFN (metal finishing news) Vol.10 -2009

Reviewed by:

Eng. Mazen Majali- General Electric

Tech. Saad Abbadi shot peening expert –JALCO

Eng. Mohammed Mousa-RJ–engineering department

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