Shot Peening Intensity Optimization to Increase the Fatigue Life of a Quenched and Tempered Structural Steel

A quenched and tempered medium-carbon alloyed steel was subjected to different shot peening treatments of varying intensities, from a low intensity 8A to a high intensity 21A, with 100% coverage. The surface roughness, subsurface hardening and residual stress profiles thus obtained were determined and compared. In addition, the fatigue lifes corresponding to the different shot peening treatments were evaluated on a rotating beam machine under alternative stresses of 45 and 50% of the tensile strength of the steel. Although all the shot peening treatments improved the cyclic behavior of the untreated specimens, the best fatigue behavior corresponded to the 10A treatment. High intensity shot peening treatments gives rise to worse fatigue behavior, in spite of an increase in surface hardening and deeper compressive residual stress fields, due to surface damage. This damage was not appreciated under the scanning electron microscope, but was indirectly detected by means of the relaxation of the surface residual stress.