The Use of Double Surface Treatments to Optimize the Fatigue Life of Components Made on Structural Steels

High intensity shot peening treatments have been combined with other different second treatments, such as low intensity shot peening, vibratory finishing and grit blasting with fine alumina powders, in order to optimize the fatigue life of a medium-alloyed, quenched and tempered, structural steel with a very high strength (2000 MPa). Single low intensity shot peening treatments using ceramic balls were also used with the same objective.Roughness and residual stress profiles were measured, while the fatigue behavior was analyzed using a rotating bending fatigue equipment. Fracture surfaces were also analyzed in a scanning electron microscope (SEM) and the location of the fatigue crack initiation was detected.Although the maximum stress in our fatigue tests is always applied on the surface of the specimen, the appropriate surface treatment combinations were able to optimize the fatigue response of the steel until the point where the fatigue crack initiated in the specimen interior (subsurface nucleation), due to the microstructural stress concentration provided by small alumina inclusions present in the microstructure of the steel. The amplitude of the stress intensity factor necessary to propagate fatigue cracks in the studied steel and the intrinsic stress intensity factor threshold, ΔKth, were finally obtained.