2009 年 2009 巻 54 号 p. 7-12
Torsional fatigue tests were conducted for a spring steel, SUP7, with Vickers hardness of 480. The effect of shot peening on the torsional fatigue strength of smooth and artificially notched specimens in the corrosive environment and air was studied. Following results were obtained. The increase of the fatigue strength of smooth specimens in air by shot peening was relatively small, and the surface roughness due to peening did not reduced the fatigue strength. On the other hand, the fatigue strength of notched specimens increased greatly by shot peening and the notch sensitivity was reduced. In corrosion fatigue, the fatigue strength increased by shot peening in the medium life range, around 106 cycles. The strength of shot-peened specimens was reduced in the long life range around 107 cycles and was approached to that of not-peened specimens. A similar improvement was observed for notched specimens in the medium life range, and the notch sensitivity was reduced by shot peening. The compressive residual stress layer made by shot peening extended about 0.3 mm below the surface, and relaxed by fatigue loading. The residual stress distribution was used to interpret the effect of shot peening on the fatigue strength of spring steel by modified Goodman diagram.