Prestressing and surface treatments

To improve the service life of a metal part subjected to overall mechanical stress, particularly cyclic stress, or to local contact stress, possibly associated with a hostile environment, several approaches are possible.

The first is to change the operating conditions by modifying the loading and/or the environment. This approach leads to the design of a new part or assembly, with the risk of not being able to eliminate all the problems at the root of its poor longevity. Such an approach may not be economically viable.

The second is to look for a material better suited to the stresses and environment. In this case, there's no need to redesign the part, so costs are reduced, but new problems may arise. For example, by improving the endurance limit, other properties such as impact resistance or deformability may be degraded.

The third approach, which appears to be the wisest and ultimately the least costly, consists in improving the material's local properties by applying compressive prestressing to the most stressed zones, i.e. most often, the part's surface layers. What's more, in many cases, the introduction of surface residual compressive stresses is associated with an increase in the material's hardness and endurance limit, which further justifies such an approach.

The problem lies in choosing the most suitable and effective long-term process.

Improving the properties of the surface zones of parts is therefore a curative means of guarding against the harmful effects of overloading in the broadest sense of the term.

Gradually, as surface treatments become more widely mastered and more cost-effective, a new trend is gaining ground in industrial practice: the integration of surface treatments into part design.

Indeed, from both a mechanical and an economic point of view, these treatments often make it possible to replace a noble or difficult-to-make material with an alloy that is less rich in alloying elements, less costly or easier to obtain.