Shot-Peening / Controlled Peening

Controlled shot-peening is a treatment aimed at enhancing the fatigue resistance of case-hardened, nitrided, or quenched and tempered parts. This process can reach significant depths and is often used to delay or prevent failures caused by stress corrosion, fretting, pitting, and to mitigate the adverse effects of surface decarburization.

In the aerospace sector, it is also applied prior to electroplating to eliminate hydrogen embrittlement resulting from the coating process.

The improvements in fatigue resistance that can be achieved are often highly significant. These depend not only on the treatment parameters, but also on the surface condition of the part and its loading conditions.

Table 1 shows some of the most common results.

Table 1 – Achievable strength increases

If the applied load is below the fatigue limit, a considerable increase in reliability can be achieved. This effect is even more pronounced when there is a notch effect, a condition very common in mechanical components. It can be emphasized that controlled shot peening is most effective for all parts made critical by penalizing geometrical shapes, form factors, notch effects, and brittle materials. Table 2 shows the endurance increases that can be observed in some mechanical components.

Table 2 – Fatigue life increase of some mechanical components

It follows that, with respect to mechanical fatigue, the benefits derived from the treatment are truly remarkable.

Service life can be extended, load levels during operation increased, weight and overall dimensions reduced, and high-strength materials — normally considered too sensitive to notches — can be employed. In some cases, lubrication conditions can be improved, and costly redesigns avoided.

WHICH PARTS SHOULD BE SHOT PEENED?

The range of cases in which the treatment can be applied with excellent results is truly extraordinary. As already mentioned, controlled shot peening can be successfully used in the vast majority of tribological applications.

Springs are an excellent example. In these parts, the external load is almost always of the same sign, and the achievable benefits are significant.

Connecting rods are highly stressed mechanical parts whose geometry inevitably produces significant notch effects. Scientific studies have shown that controlled shot peening is particularly effective in the presence of such effects.

Shafts, axles, and crankshafts are often peened in grooves, fillets, and all areas that frequently act as crack initiation sites.

Welds are always critical points in mechanical components, often sources of defects and tensile residual stresses. The improvements achievable in these parts are remarkable and are mainly due to three effects:

• improved pitting resistance
• improved fatigue strength at the root of the tooth
• improved lubrication conditions: the generated roughness creates micro-reservoirs of lubricant that enhance adhesion to the surface

THE PROCESS

Controlled shot peening is a cold mechanical treatment. It consists of striking the surface of a metal part with a jet of small balls projected at high speed (80–120 m/s). The impact of the shot on the surface causes plastic deformation of the outer fibers; generally, this effect is limited to a few tenths of a millimeter in depth. The underlying metal remains unaffected by the plastic deformation. The outer elasticized fibers tend to stretch, while those below the plastically deformed layer tend to restore them to their original position. The resulting equilibrium produces residual compressive stresses in the surface layer. As already noted, this leads to improved fatigue life of the treated components.

In summary, controlled shot peening produces the following effects on the part:

• plastic deformation of the surface layers of the material
• introduction of residual compressive stresses
• work hardening of the plastically deformed layer
• alteration of surface roughness

To achieve maximum effectiveness, the process must be carried out under strictly controlled and repeatable conditions; otherwise, it would be impossible to predict the state of the parts after peening.