Effect of residual stresses on the fatigue behaviour of hardened steel wheels under rolling contact

Under rolling contact, the maximum shear stresses that eventually cause fatigue damage occur at a depth of about half of the width of the contact area. From there, the initiated sub-surface cracks grow predominantly in tangential direction. Their subsequent behaviour mainly depends on the residual stress profile.

In some inductively hardened steel wheels of cranes numerous cracks have been found after a few years in service. As expected, most of these cracks propagated more or less in circumferential direction, thus staying within the hardened layer, some eventually reaching the surface. However, a few of them grew in radial direction, forming gaping surface cracks that reach deep into the non-hardened zone of the wheel. At a depth of about 20 – 25 mm, these radially/axially oriented cracks changed the propagation direction to tangential/axial. They seem to come to rest after about 10 - 20 mm growth in circumferential direction.

This crack behaviour can be explained by the characteristics of contact forces in combination with the residual stress profile. The latter was measured by the Cut-Compliance method to a depth of about 30 mm. It is shown that not only the residual stresses in the hardened surface layer, but also those in the non-hardened zone below play important roles.