VOL. 12 July ISSUE YEAR 2011
Shot Peening in the Automotive Industry
in Vol. 12 - July Issue - Year 2011
About The Utility And The Cost Of Shot Peening Control
I have recently visited the plants of an Italian car components manufacturer that only recently introduced shot peening in its technological cycle. The reason why it introduced shot peening is quite typical: during this time with a global market where new customers from far-off countries are able to produce at low cost rates, if you want to remain competitive, you have to offer something more; you must be one step beyond. And shot peening is able to obtain an improved mechanical behaviour with an acceptable increase of the total cost of the product.
Indeed, this company made me responsible for a series of fatigue tests on real full-scale parts, being the aim to prove better fatigue behaviour after shot peening. The results were exciting and the fatigue strength at 10 million cycles resulted in twice that of the original one.
But to introduce a new treatment in an industrial process is not enough: you have to prove to be able to completely control the process and all the related parameters. You have to prove that you are able to provide a treatment that is able to give the same results on all the treated parts. To summarize, you have to offer a reliable treatment that satisfies all the quality requirements.
The question for the boss of the company, a small medium enterprise directed by the members of one family (this is not uncommon in Italy; sometimes this is a good thing, sometimes this is bad) was: "The results are ok, but how can I control the process? What are the ways to achieve a satisfactory and consistent process?". The answer is complex, and involves several considerations, from the set-up to the evaluation of the final result.
About the set-up, we all know that the Almen method is used to define the process parameters. But this is not free from uncertainties: each measurement contains more than one source of error. The Almen strip is a deformable element and it should be high quality and very accurate. Also the gauges, even if now they are generally accurate, cannot be perfect. Another aspect is the way we draw the Almen saturation curve, many times manually; this can also influence the set-up.
But there is another intrinsic (maybe stronger) limitation of the Almen method: the strip height is a global quantity; it is something like an average result of the plastic deformation induced by shot peening. It cannot control local damage induced by broken shots, or shots with uncorrected hardness. And we know that fatigue damage has a local initiation that is facilitated by the presence of small surface defects. Furthermore, the Almen intensity cannot be directly related to the residual stress trend. That is to say, that we can have parts with different residual stresses that were treated by considering the same Almen intensity, obtained by changing the single treatment parameters (shot diameter and hardness, shot flow velocity, …..).
But to improve the control of the result of shot peening and to be more confident of the reliability of the process, it is also necessary to introduce some method at the end of the process, to check the uniformity of the results, both on a statistical basis or by considering the whole production.
If we focus attention on the residual stress field, it is not easy to find a method that fits well in a production line without decreasing global productivity. X-ray diffraction requires time for each measurement and, if we are interested in the in-depth stress profile, we have to remove the surface layer of the material, thus making the part unusable. The hole-drilling method is faster but we need to drill the part and we cannot use it on severely notched components. The Barkhausen noise method is more attractive from this point of view, since it is fast and provides in-depth information without damaging the surface. It is a real non-destructive technique. But it is sensitive also to other factors and it can be used only on ferromagnetic materials.
In summarizing, my answer to the boss of the company was that I strongly recommend using some of these experimental techniques, to be defined on the basis of the exigency of the production plant, and to start to collect data aimed at relating the process parameter variability to the results of shot peening. This is the best way to reduce the spread of the peening results and to improve the reliability and the quality of the process. The boss answered me that it adds costs. Yes, but do not forget that a better process allows us to obtain overall cost savings and to improve our products, thereby remaining one step ahead of the competition.
Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
Author: Mario Guagliano