Effect of Warm Shot Peening on Fatigue Strength of Spring

Abstract

Warm Shot Peening is well-known as the shot peening process within warm temperature range. however, the effect of warm shot peening to fatigue strength was not certain and no substantial study has been made.The requirements of coil spring with higher fatigue strength and sag resistance have been increasing to obtain the mass saving of vehicle. While several new spring materials have been developed in recent years, the essence of those developments were to make the strength of spring higher, so-called high strengthed spring. As for high strengthened spring, the shot peening process becomes further essential to relieve the increased notch sensitivity due to higher strength, However, for the shot peening process to high strengthened spring, the shot with higher hardness is also required. This can decrease the life of both shot and shot peening equipments.Warm shot peening under the tempering temperature may become one solution to those problems, because the hardness of spring can be temporarily reduced only during the shot peeningprocess. In this paper, the following experimental results concerned with the effect of warm shot peening, are summarized.1) The effect of warm shot peening on fatigue life is larger for high strengthened spring. It is especially more effective when the spring hardness is over the shot hardness.2) The appropriate temperature of warm shot peening is from 473 to 598k.3) The main reasons why the warm shot peening is effective to the improvement of fatigue life, are both the increase of compressive residual stress distribution and higher coverage, which can be caused by more effective deformation under warm temperature. The increase of surface hardness by warm shot peening could be an another reason for the improvement of fatigue life.