Shot peening is often considered a mature treatment, that is to say that many people consider the knowledge and the known data about it sufficient to define at best the parameters for treating a particular element or a structural detail.
This is only partially true, and it is sufficient to take a look at references in magazines and scientific journals to understand that many aspects are related to how shot peening influences the mechanical behaviour of materials. Indeed, in the past, the greatest effort of scientists, researchers and engineers was devoted to relate the residual stress field induced by shot peening to the treatment parameters and to define predictive models, especially for the assessment of the fatigue life, that were able to consider the induced residual stresses. In recent years, the attention has also been focussed on the surface hardening and on its role in modifying the behaviour of peened elements and some of the latest models for the evaluation of the performances of shot peened parts also includes this factor, even if only for fatigue life assessment (at far as I know).
But shot peening introduces other important surface modifications. One of these is surface roughness, which represents an unwanted, detrimental and not negligible effect, and not only in regard to fatigue. This is in particular true when not surface-hardened materials are peened. In fact, if carburized or nitrided parts are peened, the roughness will be modified but a proper choice of the treatment parameter will avoid a strong increasing of the final value. Its influence on the mechanical behaviour of the peened parts (typically gears, shafts…) will remain limited (and it is also true that if we focus the attention on gears, the shot peening-induced gears cause an improvement of the lubrication conditions, thus increasing the resistance to pitting).
However, the case of materials that were not hardened before shot peening is different and roughness can cause a strong limitation of the benefits that can be achieved with shot peening.
For instance, a quenched and tempered part will suffer for increased roughness. The definition of the optimal parameters is not trivial and requires the development of tests, measurements and calculations to determine the best compromise of roughness, residual stresses and surface hardening and the consequent peening parameters that make obtaining this optimal mix possible.
This was one of the topics discussed at the last international conference on shot peening (ICSP11) held in South Bend (Indiana), a nice and typically American city.
Indeed most of the papers presented did not discuss any possible approach to treat and solve the problem but were limited in only recognizing that the problem exists and would permit explanation of some unexpected results in terms of fatigue life and strength.
It is interesting that most of these contributions were from automotive or automotive-related companies. This could be due to two reasons: the first is that, in order to save money, there is an attempt to reduce the number of parts subjected to thermo or thermo-chemical treatments and to substitute these treatments with shot peening or some other mechanical treatment (for instance, crankshafts are more commonly deep-rolled than shot-peened). The second reason is that roughness has a negative effect especially in the field of high cycle and very high cycle fatigue (in this latter case at least if the crack initiation remains on the surface, but this is a topic that was analysed in the previous issue and will be recalled in some future column). That is to say, that this is a life span typical of a car, where it is not possible to define a maintenance plan to control and substitute parts at regular intervals, as happens in other industrial fields like aeronautics. In this case, structural parts subjected to cyclic loads are designed by considering the presence of an initial defect and by defining periodic and regular non-destructive control to verify the eventual presence of cracks in critical zones and to estimate their residual life (the so called damage-tolerant design approach).
Indeed the solution is not easy and requires a great effort by all the subjects involved: car manufacturers, shot peening companies, media suppliers, research centres and universities, each one with its specific role.
The first step for sure is to find the relation between the induced roughness and the shot peening parameters and the definition of formulas (even if approximated) able to describe the process. Then it will be possible to study also the role of the induced roughness on the fatigue (and not only fatigue) behaviour and to define some parameter able to quantitatively estimate the role played by roughness, residual stresses and surface work hardening for fatigue strength evaluation.
It is not easy and fast but the results could open new applications for this mature treatment.

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