Influence of shot peening on bending tooth fatigue limit of case hardened gears

The effect of different surface treatments on the fatigue behaviour at the tooth root of spur gears is investigated. Case hardening and case hardening followed by shot peening were considered for 16MnCr5 steel gears. Pulsating tests (R=0.1) were carried out on gear teeth to determine the fatigue endurance at 6 × 10⁶ cycles. Residual stress profiles were measured at the tooth root by means of the XRD technique and carefully analysed in order to establish the effect of different treatment parameters on the residual stress field and to find a correlation with the measured fatigue properties. The XRD technique was also adopted for measuring the retained austenite content. The effective stress field at the gear tooth during the fatigue test was reproduced by finite element modelling to check - using a multiaxial fatigue criterion - whether the fatigue crack initiation can be considered as the precondition for failure. A satisfactory agreement between experimental and predicted fatigue limits was found for unpeened gears, whereas for peened specimens a significant underestimation of fatigue strength was found. This is discussed by considering the micro-structural improvement and the importance of compressive residual stress peaks in the early stages of fatigue crack propagation.