Influence of local differences in microstructure and hardness on the fatigue behaviour of a slewing bearing steel

Abstract

For a more accurate prediction of the service life of slewing bearings under oscillating mode mechanical customized fatigue test strategies and suitable calculation methods have to be developed. In this study large-diameter slewing bearings of onshore wind turbines made from induction hardened and tempered steel 42CrMo4 are investigated. Test samples were cut directly out of the bearing ring (core condition). The orientation of the test cross-section was chosen to corresponding with the presumed most critical orientation of non-metallic inclusions. After sampling three heat treatments were carried out to mimic the different heat treatment conditions of the bearing (raceway, transition region, core). Results of static tests (fracture toughness, tensile strength) and cyclic testing (crack propagation, S-N-curve) were obtained for the three different heat-treated sample batches. The results where correlated to hardness, cleanliness and local residual stresses to develop a deep understanding of crack initiation and crack propagation mechanisms and making a first step towards a more accurate prediction of fatigue life. In order to be able to transfer the analytic results to the real parts, a simulation model will be developed later on.