Influence of Microstructural Inhomogeneity and Residual Stress on Very High Cycle Fatigue Property of Clean Spring Steel

The present study investigated the very high cycle fatigue (VHCF) properties of a spring steel SUP7-T386 under the conditions of surface grinding and electro-polishing by performing the axial loading test at a stress ratio of −1. The influence of the microstructural inhomogeneity (MI) generated in the process of heat treatment and the residual stress induced by surface grinding on the VHCF properties was discussed. This steel with surface grinding exhibits the continuously descending S-N characteristics, corresponding to the surface flaw-induced failure at high stress level and the interior flaw-induced failure at low stress level. Otherwise, with surface electro-polishing, it exhibits continuously descending S-N characteristics with lower fatigue strength, but only corresponding to the surface flaw-induced failure even at low stress level. Compared with the evaluated maximum inclusion size of about 11.5 μm, the larger MI size and the compressive residual stress play a key role in determining fatigue failure mechanism of this steel under axial loading in the VHCF regime. From the viewpoint of fracture mechanics, MI-induced crack growth behavior belongs to the category of small crack growth, and threshold stress intensity factors controlling surface and interior crack growth are evaluated to be 2.85 and 2.51 MPa m1/2, respectively. The predicted maximum MI size of about 27.6 μm can be well used to evaluate surface and interior fatigue limit of this steel under axial loading in the VHCF regime, combined with the correction of residual stress.