Role Of Shot Peening In Fatigue

Life Improvement Of Metals

Overview

Ø Introduction

Ø Mechanical causes of fatigue failure

Ø Shot peening equipment

Ø Shot peening control

Ø Shot peening applications

Ø conclusion

IntroductionØ Cold working process.

Ø Used to produce a compressive residual stress layer.

Ø Modifies mechanical properties of metal like increasing strength & hardness.

Ø Done by bombarding shots.

Ø Prevents formation of cracks, thus increasing the fatigue life of component.

Mechanical causes of fatigue failure

Ø Residual Stresses from Surface

Finishing

Highly finished surfaces & fillets may lead to a false sense of security if the parts have highly residual stresses as the result of machining or straightening operations.

Ø Residual Stresses from Thermal Gradient

Internal stresses may occur in components where instantaneous temperature in a thin top surface layer is so great that its stressed by thermal expansion.

Ø Residual Stresses from Processing

Many parts such as crankshafts, camshafts require straightening. This is usually done at room temperatures & the parts are rarely stress relieved.

Ø Size & Distribution of Internal Defects

Ø Environment

Shot peening equipmentAirless Peening

MachinePneumatic Peening

MachineHigh cost of plants but low

cleaning costLow cost of plants

High maintenanceSimple to operate & negligible

maintenance

Lack of flexibility Extremely flexible

High production ratesLow & medium production

rates

Use of limited size of media Fine to very fine sizes of media can be used

Limitations: Distortion of material, Immobile

Unlimited scope

Mostly suitable for in-house operations

Suitable for in-house & outdoor operations

Shot peening controlØ Shot Material & Its

Hardness

Shots used are usually of

cast steel with hardness of

40-50 on Rockwell C scale.

Ø Shape & Size of Shots

The shots or beads should

be free from sharp edges

& deformed shapes.

Ø Shot Velocity & Impact Angle

Higher the shot velocity more will be the work done, & more will be the effect of shot peening process.

Ø Shot Peening Intensity

Shot peening applications

Some common fields of shot peening application are:

Ø Vehicles & agricultural machinery.

Ø Power drive & transmission system.

Ø Internal combustion engines.

Ø Steam & gas turbines.

Ø Aviation equipment.

Ø Chemical equipment.

Shot peening of gearsØ Excellent means of increasing beam strength of gears

with high load carrying capacity.

Ø Highest concentration of stress is in the tooth fillets at the gear roots.

Ø Subjected to bending fatigue failure.

Ø Shot peening treatment is generally applied to tooth fillets only.

Shot peening of welds

Ø Regular welds fail after 250,000 cycles.

Ø Shot peened welds fail after 2.5 million cycles, that too outside the weld area.

Ø Popular operation with aerospace equipment parts.

Ø Improves property performance by cancelling residual tensile stresses that develop in surfaces as welds cool.

Ø Fatigue tests on springs showed peened springs to have longer life than unpeened ones.

Ø Shot peening of springs is performed in batch type equipment where a centrifugal wheel is used to propel the shots.

Shot peening of springs

Ø Excellent corrosion resistance, austenitic 316L SS is used in wide range of environments.

Ø Presents relatively low strength & poor wear resistance.

Ø Thus its treated with shot peening process to increase its strength & wear resistance, without any loss of corrosion resistance & ductility.

Shot peening of Stainless steel

conclusion

Thus its concluded that shot peening results in increasing the fatigue life of components by inducing a layer of compressive stress on the surface of the metal.

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