Uniaxial fatigue properties of closed die hot forged 42CrMo4 steel: Effect of flash and mechanical surface treatments

Uniaxial fatigue properties of 42CrMo4 steel, produced by closed die hot forging and then heat treated by conventional quenching and tempering treatment, are studied in the present work. The steel specimens underwent sand blasting. Successively, a set of samples was subjected to shot peening treatment, using 2 different intensities. Surface morphology and chemical composition as well as microstructure were investigated by scanning electron microscopy and stylus profilometry. Residual stresses were evaluated by X-ray Diffraction. The mechanical properties were studied by tensile tests and Vickers micro-hardness profiles. The fatigue limit was calculated with the Dixon Mood. In order to assess the causes of crack initiation, a simple model based on the local fatigue strength concept was adopted, considering the influence of residual stresses. The experimental findings showed that the surface treatments affect the fatigue properties. The crack nucleation site for the sand blasted specimens is close to the surface, while for the shot peened specimens the crack nucleation is linked to the activation of surface defects or to the presence of inclusions locally reducing the fatigue limit. The most probable crack nucleation region is in proximity of the flash.