Shot peening is a cold working process used to enhance the fatigue resistance of highly stressed metallic components such as compressor blades, aircraft structural parts and transmission systems. Any metallic component that is subjected to cyclic stresses within its elastic limit can benefit from the shot peening process.
The process is applied using highly controllable, usually programmable machines. It is achieved by propelling a stream of media at components, at high velocity under fully controlled conditions utilizing a compressed air stream or, centrifugally, by vaned wheel.
COMPRESSIVE STRESS HELPS PREVENT CRACK FORMATION
Shot peening works by introducing residual compressive stress in the component surface. The compressive stress helps to prevent crack initiation, as cracks cannot propagate in the compressive environment generated by peening.
Compressive stresses are generated when the impact of each particle of shot produces a small indentation. It follows that if the surface has been dented then the material beneath the dent has been compressed. Peening generates not just one dent but many thousands over the surface. Eventually the component becomes encased in a compressively stressed layer.
REDUCED FRICTION, LESS CORROSION, WEAR RESISTANT
Fretting and galling – components which are moving in relation to each other, for example as bolted or riveted assemblies, or within bearings as sliding or rolling members, can wear and fail as a result of microscopic transfer of material from one surface to the other. The surface finish produced by peening provides pockets for lubricant retention; it also reduces the surface area in contact under rolling or sliding conditions, thus reducing friction. Peening also has a surface hardening effect and helps the ‘skin’ of the material to resist wear. These characteristics provide excellent anti-galling properties making components more resistant, harder working and providing increased efficiency.
Corrosion – intergranular and stress corrosion cracking can be inhibited by shot peening which modifies the properties of the metal at a metallurgical level. The process advantageously alters the granular structure at and near the surface, producing a condition less prone to corrosion.
Equipment and process selection, whether to employ a compressed air system with blast nozzles or a wheel blasting technique, are of paramount importance when developing solutions for new components. The selection of the correct equipment and process parameters is complex. Component variables such as throughput, size, shape, material, hardness, application and operating environment all must be considered. The topography of the component is another important factor. For instance, if a radius is smaller than that of the shot, then a non-peened area results. Sharp edges, blind holes or hidden areas need special attention to ensure good coverage and avoid damage.
Peen forming is a variation of shot peening. The forming of wing skins by pressing or rolling can introduce the tensile stresses responsible for the initiation and propagation of cracks. Because peening introduces compressive stresses, it is possible to form thin components like wing skins into shape. The results are a component of the correct form but with inherent compressive stresses present on both sides, thus preventing crack initiation.
Correcting distortion without compromising quality or efficiency
Since it is possible to form parts with peening, it follows that the process can also be used to correct distorted components, without including undesirable tensile stresses. Components machined from solid material and of thin section (large airframe parts, body segments or wing ribs) are especially prone to distortion, no matter how good the machining or stable the original billet. Correction, or flattening, is achieved by introducing compressive stresses to counteract the existing tensile stresses that are causing the part to distort.