Study on Optimizing a Shot-Peening Process

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. (1)
   Research on suitable shot-peening conditions for high strength spring:
   1. (a)
      In case of single shot-peening, higher surface compressive residual stress and smaller surface roughness can be effective to high fatigue strength.
   2. (b)
      In case of double shot-peening, harder and finer shot shows higher fatigue strength.
   3. (c)
      The highest fatigue strength can be obtained by the double shot-peening with higher and finer shots, to the higher hardness spring.
2. (2)
   Analysis of fatigue phenomenon for shot-peened spring
   1. (a)
      Compressive residual stress around the surface is the most effective for improving fatigue strength.
   2. (b)
      StageI type crack first initiates and propagates, and then can transfers into StageII type crack. Size of StageI crack seems to be almost equal to the depth of compressive residual stress.