Effect of environment-assisted cracking on the premature fatigue failure of high-strength valve springs

In this work, the failure mechanism of four high-strength valve springs were investigated. It shows that the premature fatigue failure of valve spring can be attributed to the presence of an intergranular surface crack formed by the environment-assisted cracking (EAC) mechanism. The surface cracks significantly reduces the fatigue strength to a level of 320-350 MPa, which is much lower than the designed stress (720 MPa). Detailed analysis reveal that the reduced shot peening effect caused by the soft and brittle PbO adhesion contributes to the occurrence of EAC. Furthermore, it suggests that the nucleation and propagation of EAC crack controls the failure process.