High fatigue strength of coil springs for automobiles is intensely required for better fuel economy and weight reduction. For this subject, springs have been developed in terms of both material and processing sides. The coil springs developed with combination of the designed material, the advanced nitriding and shot peening processes achieved distinction in fatigue strength. In the present paper, following topics for this development are introduced:
1) Spring steels with high resistance to temper softening at nitriding temperature were designed.
2) A shot peening technique was developed to increase residual stress on the nitrided surface of springs.
3) It was verified that the developed springs had similar notch sensitivity to a conventional spring discussing fatigue strength of the steels using both test wires and spring with artificial defects.
4) Fatigue strength of the developed springs was improved approximately 35% as compared with that of the conventional spring.
5) Improvement of the fatigue strength was considered to be caused by the nitriding and shot peening which shifted the initial point of fracture to inner side of the material.
