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. (c) The highest fatigue strength can be obtained by the double shot-peening with higher and finer shots, to the higher hardness spring. (2) Analysis of fatigue phenomenon for shot-peened spring (a) Compressive residual stress around the surface is the most effective for improving fatigue strength. (b) Stage I type crack first initiates and propagates, and then can transfers into Stage II type crack. Size of Stage I crack seems to be almost equal to the depth of compressive residual stress.
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