Shot peening – an overview of the process

Citation

Brickwood, R. (2003), "Shot peening – an overview of the process", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 4. https://doi.org/10.1108/aeat.2003.12775daf.002

Shot peening – an overview of the process

Shot peening – an overview of the process

Keywords: Peening, Fatigue, Shot blasting

Shot peening is used widely to enhance the fatigue resistance of highly stressed metallic components: fan blades, discs and other aeroengine components, aircraft structural parts and both aviation and automotive gearboxes and transmission systems. Any metallic component that is subjected to cyclic stresses within its elastic limit can benefit from the shot peening process (Plate 1).

Plate 1

Although the process is applied using highly controllable, usually programmable, machines, the basic peening principle has been around for centuries. Medieval blacksmiths discovered that shaping metal by hammering significantly improved the strength of armour and swords. "Peened" armour offered increased resistance to penetration and sword blades survived longer before fracturing. This is the origin of the "ball peen" hammer.

Nowadays, the hammer has been replaced by precisely manufactured round steel shot, glass or ceramic beads, which are propelled at the component in a controlled manner utilising a compressed air stream, or centrifugally, by means of a vaned wheel.

The shot peening process works by introducing residual compressive stress in the surface of the component. The compressive stress helps to prevent crack limitation, as cracks cannot propagate in the compressive environment generated by peening.

Compressive stresses are generated when the impact of each particle of shot on the component 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.

The benefits of the process have been well proven – both with components operating in a highly-stressed, but relatively short-lived environment, such as in motor racing, and, of course, for those critical parts with a much longer and predictable operating life in aeroengines and aircraft structures.

Measuring the peening effect

Because the process is often used to improve the performance of safety-critical components, it is important to ensure that the correct intensity of stress is being created. This is achieved with the proven "Almen Strip" testing procedure. The "Almen Strip" – manufactured from spring steel to strict tolerance of hardness, size and flatness – is peened on one side only. The effect of the induced compressive stress on the strip results in bowing or curving. The extent of the curve is proportional to the energy imparted by the shot and is measured on an "Almen Gauge". The Almen Strip arc height varies according to both the velocity and mass of the shot i.e. the amount of energy imparted by the stream of shot and absorbed by the strip. X-ray diffraction techniques also provide an accurate method of measuring the actual stresses within the component and quantifying the actual effect of the peening process. Results are achieved by measuring the angle of reflection in relation to the angle of incidence that varies depending on the material composition and residual stresses (Figure 1).

Figure 1

Other benefits of peening

In addition to preventing premature failure caused by fatigue, there are other important benefits from the process.

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 the peening process 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, when combined, provide excellent anti-galling properties.

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.

Process selection

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, and 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 have to be considered. The topography of the component is also an important factor. For instance, if a radius is smaller than that of the shot, then a non- peened area will result. Sharp edges, blind holes or hidden areas also need special attention to ensure good coverage and to avoid damage.

Peen forming

Peen forming is a variation of shot peening. The forming of wing skins by pressing or rolling can introduce tensile stresses that are responsible for the initiation and propagation of cracks. Because shot peening introduces compressive stresses, it is possible to form thin components like wing skins into shape, simply by using the peening technique. The results are a component of the correct form, but with inherent compressive stresses present on both sides, thus preventing crack initiation.

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 (certain large airframe parts like body segments or wing ribs, for example) 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.

Other applications

In addition to the mainstream aerospace and automotive applications, peening is regularly used in many other areas. Here are a few typical examples:

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  artificial limbs,

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  percussion tools,

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  high-speed mechanisms,

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  textile machines,

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  ski lifts,

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  extrusion equipment,

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  heat exchangers,

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  agricultural equipment,

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  mechanical seals,

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  stream and water turbines,

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  tunnelling equipment and

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  welded steel structures.

All of these and many other applications are well known to the shot peening equipment and service providers in USF Surface Preparation Group, who are always delighted to discuss new applications for the process.

Details available from: USF Vacu-Blast International. Tel: +44 01753, 526511; Fax: +44 01753 538093; E-mail: brickwood@vacu-blast.co.uk

Roger BrickwoodUSF Vacu-Blast International