When we think about shot peening application in the automotive industry, first of all we think of gears, sprigs, crankshafts and other mechanical parts connected to the engine or to the power transmission system. This is due to the fact that these are the components that are more strongly subjected to fatigue loads and require shot peening to avoid over sizing them. More than this, these parts are manufactured using high-strength steels that allow for a major advantage by applying shot peening.

However, it is true that shot peening is also used to improve the mechanical behaviour of welded details of cars. In this case the material used is generally a steel with low yield and ultimate tensile stress. Due to the fact that low-strength steels are not the best materials for the more effective application of shot peening, in these cases the results of shot peening are of primary importance. But what is the effect of shot peening on welded parts?

To understand this effect it is necessary to know the peculiar fatigue behaviour of welded parts.
If you compare the S-N curves of smooth or notched specimens of a given material with the ones of a welded detail made with the same material, you will note two main differences. The first one is that the slope of the curve is different, being stronger for the welded detail and the fatigue limit is much lower than the one of the smooth specimens, the difference cannot be justified only by considering the macroscopic notch effect induced by the weld profile. The second is that if you perform fatigue tests by changing the fatigue load ratio R (equal to the ratio between the minimum and the maximum stress of the cycle), the S-N curves will be not significantly affected by R.
The first effect is due to the fact that weldings have a number of microscopic defects that can be considered as pre-existent defects, thus reducing the fatigue strength of a welded part and its ability to resist fatigue crack propagation: the presence of a pre-crack renders useless the definition of a fatigue crack initiation phase for weldings. The second is due to the presence of tensile residual stresses associated with the welding process. These make the effective fatigue cycle in the most stressed zones of a weld, independent in respect to the fatigue load ratio R.
The final result of this peculiar behaviour of weldings is that international standards for fatigue life and strength assessment of welded steel parts based on the determination of the so called nominal stress do not consider either the static strength of the steel or the value of R.

Another consequence is that relaxation of tensile residual stresses by means of heat treatments is effective only if the fatigue cycle has a proportion of compressive stress, that is to say that R<0. What can shot peening do to improve the fatigue strength of welded parts and to remove the previously mentioned limitations?

In these cases shot peening can be really effective. First of all, the mechanical action of the impact of the shots against the welded surface could remove or reduce the presence of the defects. In fact the plastic deformation induced by the shots impact can model the surface removing the defects or creating a more favourable geometry in view of a fatigue application. But, more than this, shot peening is able not only to reduce or relax tensile welding residual stresses (like annealing) but, as all readers know, to induce compressive residual stresses: this positively affects the fatigue behaviour of welding.

If applied in combination with some other treatment to modify the weld profile (like TIG-dressing, disc grinding or burr grinding) its effectiveness can be increased, since shot peening will act on a surface where defects and imperfections have already been removed and the weld profile has been geometrically improved. But these very good results can be achieved only after an accurate investigation in the case of interest, that is to say that the choice of shot peening parameters should be chosen bearing in mind the material, the type of welded detail, the analysis of the stress state under the in-service load: like in many other cases, we can take the best advantage only by applying a “design by analysis” approach.
In this case the fatigue behaviour of welded parts will be similar to the one of monolithic notched parts, and the fatigue limit much higher, making welding more convenient in  respect to other joining techniques.

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