VOL. 12 January ISSUE YEAR 2011
Shot Peening in the Automotive Industry
in Vol. 12 - January Issue - Year 2011
Nitriding Plus Shot Peening or Viceversa? A Synergic Action to be Better Investigated
Nitriding and shot peening are often seen as alternative types of surface treatment to improve the mechanical behaviour of machine elements. Indeed, the effect of nitriding and shot peening in terms of surface characteristics modification is really different, but many times they are used with the same aim, that is to say to improve the fatigue strength of a notched part. But nitriding mainly tends to improve the fatigue behaviour by generating hard compounds, while the main effect of shot peening is the generation of compressive residual stress in the surface layer of material. Anyway, both methods have an additional positive outcome: shot peening also increases the surface hardness and nitriding introduces compressive residual stresses in a thin surface layer of material.
But, if it is common to shot peen parts (first of all gears) previously carburized (another thermo-chemical treatment able to increase hardness and to introduce residual stresses), the combined use of nitriding and shot peening is not so common. This is due to the fact that the surface hardness of nitrided parts is so high that the application of shot peening would not be useful to further improve fatigue behaviour appreciably. But, is this true? Is there enough scientific knowledge to verify this statement? To the best of the author’s knowledge, there are only a few studies on the combination of these two treatments aimed at verifying if there is a synergic action of the induced effects. The best results were obtained on severely notched components, like crankshafts, that are frequently nitrided for high-speed internal combustion engines. But the problem is the proper selection of the shot peening parameters, able to optimize the results in terms of fatigue strength. First of all, due to the high hardness of nitrided surfaces, very hard shots are needed, that is to say that conditioned cut-wire shots should be used, but their dimension has to be checked as a function of the geometry of the crankshaft. And also the intensity, strictly related to the process parameters (air pressure, size of the hose, nozzle size, shot flow..), has to be properly selected, as well as coverage (even if this is often chosen equal to 100%). A well-established approach to guide the correct choice of process parameters is still lacking but some guidelines can be drawn on the basis of published results.
First of all it is clear that, if shot peening is applied on nitrided elements characterized by a moderate notch effect, no appreciable improvement can be noted, that is to say that it is not advantageous to apply shot peening. In fact, due to the low in-depth gradient of the applied stress, both in the case of tensile loads or bending moments, residual stresses are only able to shift the crack initiation point from the surface to the inner part of the component, generally in correspondence of a non metallic inclusion. But if the element is severely notched, like in the case of the fillet between web and the crankpin, the stress gradient is severe and good results are possible with a proper selection of peening parameters.
Regarding the shot, it should be chosen with a diameter of about one half of the notch diameter, thus allowing the fillet surface to be completely hit by the shot. On this basis one could think to use even smaller shot, but in this case it would be too light, preventing an appreciable plastic deformation of the surface, thus preventing the generation of the desired compressive residual stress state in the sub-surface material. Concerning the intensity, unclear data, sometimes contradictory, can be found, but it is true that a relatively high Almen intensity should be used, ranging from 12A to 16A, in relation to the material used and the hardness obtained with previous nitriding. Experimental results indicate that, with a correct selection of the treatment parameters, the fatigue limit can be increased by up to 20-30%, which is considerable, if we bear in mind that the typical application of combined nitriding and shot peening is found in race car engines, where 1/1000 of a second less on one lap can decide the order of arrival of the competition.
Other studies were performed on nitrided and shot peened parts, investigating different aspects and peculiarities of this compound treatment, and, maybe, they will be discussed in future columns. Nevertheless there is one topic that has not been adequately addressed (at least as far as I know): what happens if shot peening is applied before nitriding by using parameters able to generate ultrafine grains? The global result should be better due to the increased nitrogen diffusion in the peened layers or, at least, this compound treatment should lead to a reduction of nitriding temperature/time, thus reducing the costs. Anyway, more investigation is needed.
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