Shot peening is a well known process that is used widely as a reaction to fatigue failures and as a design tool to avoid them in the first instance. The choice of whether to use peening as a ‘curative’ or a ‘preventative’ treatment depends on how well understood the components are that the process is applied to, although peening processes are applied at some of the highest levels of motorsport as a curative.
The magical powers of peening are due to the residual compressive stresses it imparts on the workpiece. At the risk of banging on endlessly on the same point, it is always worth setting the new reader/novice engineer on the path of discovering what help residual compressive can give us.
Shot peening has a number of parameters that can be controlled in order to change the magnitude and distribution of the compressive stresses in the surface of a given component. Good examples here are the size and material of the peening media, and peening intensity.
By introducing further variables, however, we can ‘play tunes’ with the shot-peening process in order to increase the compressive stresses beyond that achievable with what is a highly controlled blasting process. Readers should refer to issue 74 of Race Engine Technology and the article on Surface Treatments for more detail on the processes described briefly below.
Warm peening or hot peening is, as the name suggests, a process undertaken at temperatures higher than ambient. The control of temperature is important, as the magnitude and depth of compressive stress are very sensitive to peening temperature. The mechanism by which the process differs from ambient peening lies in the fact that the yield strength of the workpiece decreases with temperature. There is nothing to be gained by peening steel at 100 C for example; the peening temperature depends on the alloy being peened.
Stress peening places the workpiece under load during the process. Defining and achieving the optimum load in practice for most components would be prohibitively time-consuming and expensive, but it is used to very good effect on racing valve springs, which can be loaded simply and accurately to produce a predictable result. Research, reported in the above Race Engine Technology article, found it is possible to increase the compressive stress due to peening by around 50% in spring steels. Beyond the case of helical and torsion springs, there are few components we can place into realistic states of stress with sufficient ease to make it commercially viable.
Combining elevated temperature and stress while peening can improve on both of the above variations on shot peening, but the complexity and expense of achieving the correct parameters means this is almost certainly going to be only of academic interest as far as motorsport is concerned; such complex processes are likely to be used only in industries such as defence and aerospace. Again, if the process is to be used in race engines, the component which is likely to derive the most benefit and which is going to be the easiest and most cost-effective to treat will be valve springs.
Written by Wayne Ward