Shot Peening in the Automotive Industry
in Vol. 13 - July Issue - Year 2012
Shot Peening, a pre-heat-treatment process
The ability of shot peening to improve the mechanical behaviour of materials is well known. In particular, shot peening is widely used in the automotive field for beneficial effects with respect to fatigue life and strength. Today, it is used as a standard treatment to guarantee expected performances in terms of strength and in-service life. These beneficial properties are generally related to the induced compressive residual stresses on the surface layer of material and to the surface work hardening of the same layer.
For this reason, shot peening most of the times is considered as the final treatment, since every modification of the surface state generated by shot peening could be detrimental, and could be the cause of the loss of the properties obtained by applying this treatment. For instance, a mechanical operation like grinding could partially or totally remove the surface layer of material concerned with the action of shot peening, and a heat treatment should relax both the residual stresses and the surface work hardening just mentioned.
Indeed, recent technological advances have evidenced the ability of shot peening as a technological method able to cause grain refinement, down to a nanometer scale. From this point of view, it can be seen as one of the so-called "severe plastic deformation" treatments; able to cause grain fragmentation thanks to the increment of the dislocation density and to other related microstructural processes activated by the large amount of the accumulated plastic deformation. This means that the treatment should be set with parameters able to induce an extremely high plastic strain. Thinking about shot peening, it means that we should perform a treatment with unusual parameters. Since the accumulated plastic deformation is a function of total energy of impact, we should modify both the intensity and the time of the treatment, and we should define a useful combination mainly aimed at grain refinement and not to the generation of a residual stress state.
But what is the real advantage of this severe shot peening treatment? One of the most interesting properties of this kind of treatment is that nanosized grains improve the diffusion of selected elements at high temperature. This could lead to a remarkable improvement in some very popular thermo-chemical treatments, like nitriding or low-pressure carbonitriding.
This peculiar property can be used in different ways. The first is to leave unaltered the original treatment parameters, thus obtaining a deeper penetration and hopefully increasing, at least in some in-service situations, the mechanical properties (fatigue, wear, …).
Another rather unexplored way to use severe shot peening as a pre-heat (or thermochemical)-treatment process is to modify the heat treatment parameters; that is to say, to modify the temperature and/or the time required for obtained a desired result. For instance, nitriding could be modified, by decreasing both the treatment time and by using the same treatment temperature (about 500°-550°C). This would be a great technological and economic advantage, since we know that nitriding and related processes generally act as a sort of bottleneck of the entire production process, especially for large production volumes typical of the automotive field.
On the other side, you could decrease the treatment temperature, thus decreasing the energy effort to perform the treatment. In addition, in this case, the economic advantage is clear. However, we know that it is less appealing for the treatment performers.
Indeed, this is still a research topic and we know that before applying any modification in a technological cycle, a deep and exhaustive knowledge is required together with an experimental evidence of the induced improvement. We are just at the first steps, but I believe we will hear about it in the near future.
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