Peening

Those of you who have read previous articles of mine on surface treatments may have noted a point that I have continually stressed – that residual compressive stresses can have a very beneficial effect on a component’s endurance. One of the most popular processes for doing this is controlled shot-peening.

Many engineers are aware of the benefits of controlled shot-peening, but there has been a lack of in-depth understanding of where the benefits come from, and how the process can be altered to suit certain materials and applications. For many years, engineers have been used to specifying shot-peening on critical areas of a component, relying on the fact that the compressive stress is likely to bring a benefit. Very few however have looked into the process in enough depth to have known what is possible, or been in a position to ask for anything other than to have the surfaces peened. This passive attitude towards the application of shot-peening is beginning to be replaced though with a more proactive approach to the process.

The process of shot-peening puts the surface of a component into a state of compression, but the level of compressive stress occurring at the surface can be manipulated to an extent, as can the depth at which a certain level of compressive stress exists. Within limits, a shot-peening supplier may be able to help you achieve something very close to what you want in terms of a compressive stress field. Whatever your level of analytical capability, there is a certain amount you can do to predict the state of stress in a critical section of your component, whether this is via pencil and paper, spreadsheet or finite element analysis. You can also make some assumptions about the compressive stress field that can be achieved and, through superposition of the compressive stress field and the applied stress, arrive at a more realistic state of stress in the component.

So, the compressive stress requirements may not only depend on the material and its level of strength, but also on the loads it is expected to experience. For example, a component whose maximum stress occurs 1 mm below the surface is not going to benefit greatly from a compressive stress in the top 0.1 mm of the surface, but which has no residual compressive stress at the critical depth below the surface.

Some shot-peening companies have predictive tools that allow them to manipulate the compressive stress field to match that which the customer has requested. The desired stress field may require a complex multi-step peening process, with different media and peening intensities used to produce the desired result. In some cases the maximum compressive stress may be a small distance below the surface following initial peening, and a second step may be required to put the outermost surface layers into the greatest compression.

There are practical, though destructive, techniques to verify the magnitude and depth of the compressive stress field resulting from the peening process. Such verification tests can be carried out on relatively simple samples, however, so that expensive parts such as gears or con rods don’t need to be sacrificed.

Written by Wayne Ward