One of my first columns on MFN was about a new way to intend shot peening. In that column I remarked that shot peening was a possible candidate to become a successful severe plastic deformation process to obtain not only an important and useful residual stress field but also a grain refinement able to decrease the grain size up to 100 nm.
It is an important property of shot peening, because it is known that materials with grains with dimensions in that range have superior mechanical properties (wear, fretting, fatigue….) and that nanostructured surfaces are able to increase the diffusion of elements during thermo-chemical treatments (i.e.: nitriding). And the tests performed show that the nanostructure is stable at high temperatures, thus making the treatment appealing also for high-temperature applications.
Indeed, there are other processes that have been used and are under investigation to obtain the same effect, but what is interesting about the application of shot peening for obtaining nanostructured surfaces is its ability to be used in many industrial fields (and the automotive industry is one of these) with good production rates.
Now, some years later, we know something more about this particular way to perform shot peening, which can be considered, due to the very particular treatment parameters, another process with respect to "traditional" shot peening.
The key to obtain grain refinement is to increase the kinetic energy of the impact, this can be achieved either by increasing the shot diameter or its velocity. In any case, we need to increase the coverage and the time required to terminate the treatment; in this way the accumulated plastic strain reaches values suitable for increasing the dislocation density and the consequent rearrangement and refinement of the grains. This latter could appear too high for the industrial applications but, as I wrote before, this is another treatment and the expected results are much different with respect to shot peening.
For instance, it was proven that by using severe peening parameters to generate a nanocrystalline surface in notched specimens made of a low alloy steel, it is possible to increase the fatigue strength much more than twice with respect to the untreated material; much more than what you can expect by using conventional shot peening parameters.
And there are other data about the effect of this particular treatment on wear properties of aluminium alloys that show a beneficial result both in dry and lubricated contact conditions.
I want to cite just another paper that I recently read where the results of ultra high-cycle fatigue (UHCF, up to 10 to 9 cycles) tests of a steel 50CrMo4 used in the automotive industry for car wheel hubs were shown.
This is a very particular fatigue case, and the damage mechanisms leading to fatigue failure after so many cycles are quite different with respect to high- cycle fatigue. That is to say that fatigue crack initiation often starts from an internal inclusion and not from the surface. This can limit the application of shot peening for alleviating UHCF. However, after having obtained a nanocrystalline surface by shot peening performed with severe parameters, the fatigue strength after 10 to 9 cycles increased to more than 20%. And if you consider that the specimens were smooth (hourglass shaped) and the test was an axial fatigue one (the worst combination of specimen and fatigue cycle to appreciate the positive effect of shot peening), the result was more than promising and interesting.
I could cite many other results and cases of the successful application of severe shot peening but I stop here and, on the basis of the results obtained in the last few years, I conclude the present column by remarking, as I did some years ago, that it is worthwhile increasing and deepening investigations in this field with appropriate collaborative research projects that would open new fields for shot peening.

Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
E-mail: mario@mfn.li

Author: Mario Guagliano