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Delving Beneath the Surface of Shot Peening

Shot peening is a cold working process employing a centuries old principal in highly controllable and usually programmable modern machines, with great benefit. Mike Speak, general manager at Wheelabrator Group, reports on how this surface preparation technology is helping to enhance the fatigue resistance of highly stressed metallic components in the aerospace and automotive industries.

Shot peening involves propelling a stream of media (precisely manufactured round steel shot, glass or ceramic beads) at components, at high velocity under fully controlled conditions utilising a compressed air stream or, centrifugally, by means of a vaned wheel.

It helps to enhance fatigue life by introducing residual compressive stress in the surface of the component. This compressive stress can 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 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.

Components such as aeroengine compressor blades, aircraft structural parts and automotive transmission systems, as well as any metallic components subjected to cyclic stresses within elastic limits, can benefit from the shot peening process.

The Almen Test:
As peening is often used to improve the performance of safety-critical components, it is important to ensure 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 ie, 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 dependent on material composition and residual stresses. 

Reduced Friction, Less Corrosion, Wear Resistant:
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 making your 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.

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

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 have to be considered. 

Peen Forming:
Peen forming is a variation of shot peening. The forming of wing skins by pressing or rolling can introduce the 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.

The Next Step:
The latest peening technology to become available is Sonats’ patented ultrasonic Stressonic® range for which Wheelabrator is the UK’s only licensed distributor. This is suitable for shot-peening, straightening and peen-forming applications. It can be used on a range of aerospace components including engines, blades and bearings.

Capable of increasing efficiency and reducing costs further, Sonats’ process uses a different technique to traditional shot-peening methods. Instead of propelling the balls under pressure via nozzles, Stressonic® shot peening uses metallic elements to form an acoustic block, vibrating with an ultrasound frequency to maintain momentum of the spherical shot.

Shot peening has made a significant contribution to the aerospace and automotive industries, among others, and will continue to be one of the most advanced surface preparation technologies on the market for the foreseeable future.

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