Shot Peening in the Automotive Industry
in Vol. 10 - January Issue - Year 2009
Process Controls in Automotive Industries
Michele Bandini (Ph.D.), Contributing Editor MFN
The great production volumes in the automotive field require an extraordinary level of production process reliability. Failures in this field have such a high economic impact that they require a specific analysis and control method of the production processes, in particular of the ones that have a primary impact on the working and resistance characteristics of the components.
In his first article, Prof. Mario Guagliano pointed out the well-known Almen intensity control process method. It was developed in the 1930’s and ‘40’s and it is still largely utilized and universally appreciated. We will now try to understand the reasons why, its merits and its limitations. First of all we have to state that Almen intensity is an indirect measurement of shot stream kinetic energy. Given the difficulty of measuring shot stream velocity, we measure the elastic release of a metallic strip whose surface has been bombarded and deformed by the shot. The higher the kinetic energy of the shot stream, the greater the strip deformation. Really a very clear and understandable concept.
Probably in the early years the method consisted of measuring only the arc height of one or two peened strips. It might sound unbelievable, but in some cases I have actually seen this still being done. Anyway, the method was later completely revised up to the current definition of the saturation curve by four points as prescribed by the major shot peening specifications. The method has been sharpened and very well standardized but it is still a simple and economic control method.
In addition, it has the great advantage of being one of the ways in which we can set up a shot peening machine to have standardized results, independent of the type of peening machine. Its effectiveness allows us to use it as a form of production control.
Nevertheless we can’t think of summarizing the entire shot peening process with only a description of the Almen intensity method. First of all we have to keep in mind that, like all methods of measurement, this one has its own chain of errors, that is to say it is not perfect. Modern digital gages and very high precision class Almen strips decrease the error but they don’t eliminate it.
In addition we mustn’t forget that the shot is the tool by which we introduce the desired residual compressive stress, so our attention has to be addressed also in this direction. The dimples generated by incorrectly shaped shot are potential crack nucleation points. We must absolutely avoid this type of dimples. Similar considerations must be made regarding shot hardness. The most stringent specifications give an acceptable hardness range of seven Rockwell C. According to my personal experience I can state that in some special applications, this range is really too wide.
Last but not least comes coverage control. Despite someone thinking that saturation time and cycle time are strictly related to intensity, actually they are related only by the uniformity and not by the magnitude nor by the depth of the treatment. Anyway we have to ensure the best possible uniformity. We can use fluorescent tracers or special inks but it is still a form of visual control, so the importance of having very skilled personnel is clear.
If we want to achieve better performance and greater reliability of the shot peening process, we have to apply the best possible form of control, we have also to design the best shot peening process for each single application. Too many times I see either poor design or poor process control; in both cases we will have an inadequate performance or an unreliable process.
Shot Peening in the Automotive Industry
by Michele Bandini (Ph.D.)
Contributing Editor MFN
Teaches at Mechanical Engineering University of Bologna & Milan