Result of Two-Stage Shot Peening Performed on Engine Valve Springs

Two-stage shot peening was performed on engine valve spring prototypes in order to increase their fatigue strength, thereby decreasing the percentage of fatigue failures originating on valve spring surfaces. The two-stage shot peening comprises a high-arc-height shot peening as the first stage and a low arc height shot peening as the second one. Two types of prototypes were prepared, those processed by ordinary one-stage shot peening and those processed by the two-stage shot peening. The residual stress distribution of each of the prototypes was measured by the X-ray inspection, the fatigue testing was carried out on each of them. It is confirmed that an effective residual stress distribution provided by the two-stage shot peening improves the fatigue strength of engine valve springs. The performance of the two-stage shot peening allows high residual compression stress distribution not only on the surfaces but also on the insides. The effect of residual stresses in such distribution provides fatigue strength higher than that obtained by the use of the ordinary one-stage shot peening, which prevents the failures from originating on the surfaces, thereby prolonging the lives.