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In recent years, hardness of springs has been targeted to be higher for improving fatigue strength. Shot-peening technology has been improved, and new shot-media and multi-shot-peening technologies have become to be employed in productions. In this study, the experimental shot-peening conditions were widely chosen in the possible range of manufacturing processes. Single shot-peening and double shot-peening, and hardness of spring were studied. The effects of each shot-peening process on fatigue strength were considered, in relation to surface compressive residual stress and surface roughness. The results are summarized as follows. (1) Research on suitable shot-peening conditions for high strength spring: (a) In case of single shot-peening, higher surface compressive residual stress and smaller surface roughness can be effective to high fatigue strength. (b) In case of double shot-peening, harder and finer shot shows higher fatigue strength.

Warm Shot Peening is a 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(hardness) of spring higher, so-called high strengthened 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.