Automotive Coil Spring is always required to increase fatigue strength for the weight saving. There are mainly two methods to increase fatigue strength. (a) to increase vickers hardness, b to increase compressive residual stress. For the method(a, since the present vickers hardness, HV is very high around 600HV, it is difficult to increase the hardness further. On the other hand, if the HV is increased further, spring will become too sensitive for corrosion fatigue and hydrogen embrittlement. For the method (b), shot peening is a very popular technique to apply compressive residual stress. Based on the above background, we studied on the following subjects, (1) Stress intensity factor of surface cracks by compressive residual stress are calculated. By using the calculated results, it was considered what is the most suitable compressive stress to increase the fatigue life of coil spring. (2)To apply the most suitable residual stress in coil spring, following four shotpeening techniques were studied, systematically, (2.1) Double shot-peening, (2. 2)Stress shot-peening,(2 3) Warm shot peening, and (2.4) Combined shot-peening above. (3) Fatigue tests were carried out on coil springs which were processed under the above shot peening techniques. It was found, that (a) the compressive residual stress has significant effect on fatigue strength and optimum stress distribution can be predicted, (b) proposed combined shot-peening an excellent effect on increasing the fatigue strength of coil spring.
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