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What is Shot Peening - INTRODUCTION

  • SHOT PEENING is a cold working controlled process used to produce a compressive residual stress layer and modify the mechanical properties of variety of different metals. It entails impacting a surface by bombardment of shot (metallic balls, glass bead, SCCW or ceramic particles) with a force sufficient to create plastic deformation thus changes surface mechanical properties.

     

    Shot peening delays or prevent crack generation in a highly tensile stressed alloy components or components subjected to fatigue/reversal loading, by introducing/enhancing residual compressive stresses in the surface. The tensile stresses on the surface of components are developed by manufacturing process like, milling, turning, drilling, welding, severe grinding, bending, forming, etc which reduces the component operational life.

  • What is Shot Peening ?
 

HOW IT WORKS:

  • Shot peening is performed by accelerating shots towards the surface of a part to be treated. In shot peening process, compressive stresses are generated when the impact of each particle of shot on the component produces a small indentation thus stretching the surface of part, then the material beneath the dent resists and creates an area of compressive stress. When group of shots/beads hit the surface, generates multiple indentation on the surface and the component becomes encased in a compressively stressed layer. This replaces the tensile stress on the surface with a compressive layer.

     

  • Shot Peening

HOW IS PEENING EFFECT MEASURED:

  • The use of Hardened and flat strips called Almen strips of type N, A & C to measure the arc height generated on strip during peening operation. The Almen strip is securely attached to a metal plate fixture with four screw and is subjected to the uniform controlled shot stream for predetermined periods of time. The degree of curvature generated on strip is measured using an Almen gauge. The arc height measured is a result of the layer of compressive stress developed.

     

  • Peening Effect

SATURATION CURVE:

  • A saturation curve plots Almen strip arc height vs exposure time on a graph. 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 intensity of the shot stream can be determined by evaluating the first point on the best fit line where, when the exposure time is doubled, there is a 10% increase in arc height.

  • Saturation Curve

Factor Affecting Shot Peening:

  • Shot Size & Type
  • Shot Velocity
  • Component surface hardness
  • Peening Exposure time
  • Distance between nozzle & surface

APPLICATION
  • Turbine Disks
  • Compressor airfoil and roots
  • Aircraft wheels
  • Landing Gears
  • Rings and pinion gears
  • Crankshafts
  • Coil & Leaf spring
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