in Vol. 17 - November Issue - Year 2016
New Treatments On The Horizon?
As everyone knows, shot peening was introduced into industrial practice before World War II, and the automotive industry has been one of the most important fields of application of this treatment from the beginning. Indeed, this industry is not the only important industrial sector; many others are currently of interest and it is easy to find new contributions in the technical and scientific literature, along with new materials and new practical applications for shot peening.
But, let me say, the automotive industry is still one of the sectors where the characteristics of shot peening can be better exploited. This is due to the large volume of production of cars, which makes the treatment flexible, versatile as well as effective very attractive both for the cost and the performance of the treated parts, with the environmental impact being much lower with respect to other surface treatments that could be used with similar aims.
Some criticisms with respect of shot peening often come up with respect to possible side-effects: the surface roughness is modified by the repeated impacts of the shots and this could somehow decrease the benefit of the treatment in terms of fatigue strength, the depth of the compressive residual stress state being not so much, and this makes possible (or easier) the relaxation of the induced residual stress in the surface and sub-surface layers of the material, where the fatigue damage generally originates. In addition, I could mention other possible issues, but this is not the aim of my present column.
This is just to underline that shot peening is not perfect and that the continuous demand of improved performances, reliability and safety in many industrial sectors is pushing toward new treatments, derived from shot peening, that are able to guarantee a better final result and to avoid the drawbacks of shot peening.
The principle of these treatments is the same as shot peening: an impact force is applied to the target surface and induces a non-uniform plastic deformation, thus causing material hardening and compressive residual stresses. What is different in these processes with respect to shot peening are the media and the plants: no more small hard shots and pneumatic or centrifugal machines but different media and different technological plants.
For instance, in cavitation peening, cavitation impacts are used in the same way as shot peening to improve fatigue strength and/or to introduce compressive residual stress.
In the so-called Surface Mechanical Attrition Treatment (SMAT), steel shots are used, but they are in a closed box where the sample to treat is fixed and the energy needed for the multiple impacts is given by a piezoelectric tool and by a mechanical device able to move the box and everything inside it. This treatment provides random directions of impact, making the refinement of the grains easier and the generation of an ultrafine surface layer with grain dimension down to less than 100 nm. Moreover, the surface roughness after this treatment is generally lower than after shot peening.
No need to talk about laser peening; that is already used in many industrial applications and is able to guarantee a much deeper layer of compressive residual stresses with a considerable amount of published results showing the greater effectiveness in terms of fatigue strength and life (indeed, I read in some papers also about a welded detail used in automotive where shot peening was able to get a better fatigue behaviour).
And I could mention other shot peening-inspired treatments currently under investigation, but I will stop here.
Once underlined that the mentioned treatments have already proven worthy of being used in other sectors, I wonder if automotive shot peening can be replaced in the near future by any of them.
Well, it is not easy to say, but if we consider the large production volumes that characterize the automotive industry, the request of saving time and money, the need of flexible processes and not only the final performance of the treated part, it is expected that shot peening will be largely used in this industry for a long time but for race cars and luxury cars, of course.
Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy