Degradation of nanostructured bainitic steel under rolling contact fatigue

The consequences of rolling contact fatigue on a carbide-free nanostructured bainitic steel intended for bearing applications are presented for the first time. Tests performed at various intervals followed by mechanical, microscopical, and crystallographic characterization lead to the conclusion that the degradation mechanism is ductile void formation at interfaces, followed by growth and coalescence into larger voids that lead to fracture along the direction of the softer phase. This is different from the conventional damage mechanism that involves crack initiation at inclusions and propagation, for example in typical bearings steels such as 52100. The huge density of interfaces in the nanostructure allows the formation of a large dispersion of voids, and ultimately cracks, at depths consistent with the maximum orthogonal shear stress below the contact surface. This study should prove useful for the eventual usage of nanostructured bainitic steels in rolling bearings.