Abstract
In order to reduce the weight of suspension coil springs used automobiles, high strength spring steel has been developed. The suspension coil springs are subject to torsional stress in a corrosive environment. So, torsional fatigue tests were conducted for spring steels whose Vickers hardness values were 430HV, 480 HV, 550 HV and 620HV. The effects of the corrosion environment and surface artificial pits on the fatigue strength were studied. It was found that the fatigue strength of 550HV smooth specimens was higher than those of other specimens in air, but in the case of corrosion fatigue the fatigue strength of 480HV specimens were the highest of all specimens. It is important to note that the explanation for this is that the fatigue was caused by the intergranular fracture. The fatigue fracture of smooth specimens grew from shear cracks in the case of fatigue in air and from pits in corrosion fatigue. The stress intensity factor range for crack initiation form corrosion pits was nearly the same for all materials. The fatigue strength reduction rate was constant under fatigue loading in air, but it decreased with increasing fatigue life and hardness in the case of corrosion fatigue.
