Shot Peening – Notes for Designers

  • The manufacturing process introduces either tensile or compressive stresses to components; these stresses are neither predictable in their distribution nor repeatable from component to component.
  • Stresses introduced to components during manufacture are anisotropic; stresses induced by shot peening are compressive and predominantly isotropic
  • Typically turning can induce surface tensile peripheral stresses of +150Mpa and axial compressive stresses of -150Mpa. Whilst grinding or EDM is capable of producing up to +100 Mpa tensile stresses. ( Low alloy steels)
  • Components produced using wire erosion and EDM methods though efficient will require the removal of the recast layer prior to shot peening
  • Components need to be clean prior to shot peening, any surface oxide or contamination is detrimental to the process and will inhibit the formation of the compressive stress region
  • Shot peening changes the surface profile of components due to plastic impaction of the outer profile – how much depends on shot size, intensity, material type and condition
  • External diameters will increase during shot peening depending on material, shot size and intensity
  • Internal bores will reduce in diameter depending on material, shot size and intensity
  • Shot peening can be carried out prior to completion of the manufacturing process – typically parts can be final ground to avoid detail masking of areas – masking can be expensive
  • Any masking to resist shot peening will result in an abrupt stress transition –  if possible introduce a transition area to fade out shot peening wherever possible
  • Shot peening will not  penetrate into sharp corners – sharp corners are essentially stress concentrations and as such should be avoided in high stress areas
  • Shot diameter has to be at least half the size of the radius to achieve effective peening intensities and coverage
  • Sharp edges should be avoided- the shot peening process can burr edges resulting in localised stress concentrations - 0.25mm minimum break edge or radius is often sufficient.
  • Internal holes and shafts can be shot peened although the smaller the bore the more difficult it is to maintain an effective shot intensity due to angle of impact, shot size and media flow
  • Sheet and plate components can distort if shot peened on one face – it is often necessary to peen both faces in order to balance stresses

The application of shot peening is down to the science of the component and the environment in which it is designed to exist.
That is where our expert knowledge and experience is invaluable.