Influence of Artificial Corrosion Pits and Hydrogen on the Fatigue Properties of Suspension Spring Steel After Shot Peening

Author:  Manabu Kubota, Daisuke Hirakami, Takahisa Suzuki and Kohsaku Ushioda
Source:  ICSP-13
Doc ID:  2017036
Year of Publication:  2017
Abstract:  
Introduction: To reduce the weight of automotive suspension springs, increasing the strength level of the spring steel is effective because the fatigue strength of the spring steel increases with the strength of the spring steel. However, it is considered that the corrosion fatigue phenomenon becomes pronounced with an increase in the strength level of the spring steel. Therefore, to increase the strength of suspension springs, improvement in the corrosion fatigue properties is one of the most important issues. Fatigue cracks in actual suspension springs initiate at the corrosion pit [1,2]. The actual corrosion pits of automotive suspension coil springs have been investigated in [1], wherein the average value of the maximum corrosion pit depth of all the investigated coil springs was approximately 150 μm and the maximum corrosion pit depth was 250 μm. These values are approximately equal to the depth of the compressive residual stress layer that is formed under typical shot peening conditions. Therefore, it is considered that the fatigue strength of suspension springs decreases because of the presence of corrosion pits. It has been demonstrated that compressive residual stress makes shallow artificial corrosion pits harmless [3,4]. In addition, since hydrogen is generated on the surface of steel because of the corrosion reaction,the effect of hydrogen on the corrosion fatigue properties should also be considered because hydrogen promotes the initiation and propagation of fatigue cracks [5-7].