Mechanical surface treatments to optimize the fatigue behavior of quenched and tempered high strength steels

The aim of this work is to maximize the fatigue life of an AISI 4340 steel (quenched and tempered at 200 °C) with a tensile strength of 2000 MPa. To do so, different shot peening treatments were studied. Some of these treatments used traditional steel shots made of cut wire, and others used ceramic shots made of zirconia (Zirshot Y 300), which had not been used in previous works and had to be completely characterized. Additionally, secondary peening treatments and grit blasting were applied with the objective of decreasing the surface damage induced by the previous treatment. To characterize both primary and secondary shot peening treatments, the surface topography of the samples was analyzed using a roughness tester and scanning electron microscopy (SEM). The compressive residual stress field induced by these treatments was measured with X-ray diffraction (XRD). Afterwards, fatigue behavior was studied by means of four-point rotating bending tests. Fracture surfaces were analyzed using SEM. The results showed that an optimum combination of surface treatments (primary + secondary) avoided the initiation of cracks in the surface of the specimens (typical characteristic of fatigue failure under bending loads). In these cases, crack initiation always took place beneath the compressive stress field induced by the treatments, in alumina inclusions that acted as stress concentrators. Furthermore, the size and distribution of these alumina inclusions (analyzed using SEM) allowed determining the steel threshold amplitude of the stress intensity factor, ∆ K th .