Reliability evaluation for shot-peening conditions affecting durability life of automotive suspension coil springs

Abstract

Shot peening (SP) is a cold working method used to improve the fatigue life and corrosion resistance of metallic materials subjected to repeated loading. However, optimal SP conditions can only be found through experiments with actual operating parts. In addition, studies on the effective SP conditions and ensuring safety for improving the durability of parts are lacking. In this study, SP of various conditions were applied to automobile suspension coil springs (ASCS). Consequently, the process factors, damage assessment, and microstructure were analyzed to determine the factors that exerted a dominant effect on durability life. The durability life of springs applied with stress SP (SSP) was evaluated to be approximately 3.8–4.8 times higher compared to that of springs applied with other SP conditions. In addition, by analyzing the process factors affecting the durability life, the absence of a direct effect of the arc height and Vickers hardness was observed. Moreover, the effect of surface roughness was observed to be small. However, the durability life in response to the change in maximum compressive residual stress (MCRS) exhibited a tendency to change linearly, and this relationship was expressed in an equation through regression analysis. The fracture surface of spring with a high durability life tended to have a large number of spiral shapes, and that of spring with a low durability life tended to have a large number of flat shapes. Before conducting the durability test, a circumferential metal flow was observed in the work-hardened layer below the spring surface upon shot peening. After conducting the durability test, partial crack initiation and propagation and delamination were observed in these areas. Despite the varying SP conditions, the surface microstructure of each spring was observed to be similar before and after the durability test. In this study, it was confirmed that SSP is the most effective method to improve the durability life of ASCS, and it was possible to understand through damage evaluation and microstructure analysis that MCRS affects the durability life by delaying the initiation of cracks in the spring. The analysis method and results of this study are expected to be useful for improving the reliability of not only ASCS but also SP-applied components in the future.