Shot Peening,on Pelton Wheels: Methods of Control and.fi~i~Ullt~

Shot peening is normally used to increase fatigue resistance in rather small-sized mechanical parts (gears, transmission shafts, connecting rods). Residual stress induced by this treatment has been studied extensively in the past decade. SaIne automobile manufachlrers have recently introduced specifications concerning the depth of residual stress in gears. Most of the research done on this topic has made use of the X-ray diffraction method to de-termine residual stress. Because of the configuration of Pelton wheels, shot peened areas cannot be easily accessed with portable diffraction meters. For this reason, the authors have used the Barkhausen noise lnethod to measure residual stress and have then compared results before and after peening. The Barkhausen noise method was calibrated against Almen strips by X-ray diffraction and by hole drilling methods. 2 Shot Peening The purpose of shot peening is to increase fatigue resistance in mechanical parts. Essentially it is a process in which the surface of the part is bombarded with spherical particles made of the appropriate material and to the correct size. The impact of these spheres on the surface of the part causes a plastic deformation and creates "microcups " which are evident in the following SEMimage: This plastic deformation induces a state of compression which significantly increases resi-stance to fatigue. Obviously the performance of the part depends on the depth and on the value of compression. These variables (depth and value) are a function of shot peening procedure pa-ralneters, which are defined by intensity, shot size and coverage. Intensity is determined by measuring the effect of peening on an Almen strip, which is a har-dened C70 steel strip tempered to 44-45 HRC; strip dimensions are indicated in the followin