Effect of Residual Stresses on the Fatigue Behavior of Surface-Hardened Steel Wheels Under Rolling Contact

Abstract The present paper reports on the failure analysis of the steel wheel of a crane, where numerous cracks have been found. Since the residual stresses were likely to be one of the most important influencing factors, the distribution of the tangential residual stresses, which were introduced by inductive case-hardening, were measured to a depth of 30 mm by means of the Cut-Compliance (CC-) method. Unexpectedly, significant tensile residual stresses were found even in the hardened surface layer, where usually compressive stresses prevail. In fact, in an unused wheel that was investigated for comparison, the expected compressive stresses were present. This means that the original residual stresses were significantly altered by the service load of the damaged wheel. The phenomenon known as "ratchetting" is likely to be the cause for the unusual redistribution of the residual stresses. The region near the rolling surface is prone to ratchetting because of very high tensile residual stresses and the superimposed cyclic stresses due to rolling contact. By a rough engineering analysis based on fundamental relations of fracture mechanics, it could be shown that the present residual stresses are responsible for crack initiation as well as crack propagation, including the observed bifurcation in a depth of about 15 mm. It also could be shown that the cracks are unable to penetrate into the wheel deep enough to become unstable and to cause a catastrophic rupture of the wheel. In order to prevent such cracks, first of all the thickness of the hardened layer has to be increased.