Peening is a cold working process in which the surface of the component is deliberately deformed, in the basic method, by hammering.
During peening, the surface layer attempts to expand laterally but is prevented from doing so by the elastic nature of the sub-surface, bulk material. This results in the development of beneficial, compressive residual stresses in the surface layer, which are balanced by tensile residual stresses elsewhere.
The layer of compressed surface material produced by peening resists the development and propagation of cracks and increases resistance to fatigue failure, corrosion fatigue, stress corrosion and cavitation erosion. The main peening techniques are:
- roto (flail)
This method involves bombardment of the metal surface by spherical shot. The shot consists of rounded particles of cast iron, cast steel or stainless steel in various mesh sizes.
In practice, a high velocity stream of air is used to propel the shot. Both mechanised and manual systems are used, depending on the particular component and area to be treated.
Roto (flail) peening
In roto or flail peening, the surfaces are impacted by two or more shot-coated flaps which extend radially from a spindle. Each flap consists of shot (cast steel or tungsten carbide) embedded into a wire mesh or fibre. The tool is inserted into the chuck of a drill (typical rotation speed 3000rpm) and the rotating flaps brought into contact with the surface. The shot energy is determined by the rotational speed.
This technique uses what is essentially a de-scaling gun but with needles having radiussed ends. The cluster of metal needles is usually made from hardened tool steel. In operation, the surface being treated receives repeated and multiple impacts.
Hammer peening is similar to needle peening except that a single metal rod, rather than a cluster of needles, is used and applied in a similar way. The basic method, using the ball end of a hammer, is often used as a means of applying stress relief between weld passes when making highly stressed welds.
Where peening techniques are used to increase service performance, strict and positive process controls are necessary, even to the inclusion of computer control, to provide reliable and consistent results for compliance with company and national specifications.
Peening is also employed during the welding of cast iron and, in certain cases, where post-weld heat treatment is not possible.
Methods of fatigue life improvement for welded joints in medium and high strength steels. This PDF file is available only to TWI Industrial Members.