Rotary bending fatigue behavior of maraging steel with an anisotropic gradient nanostructured surface layer

Surface modifications are imperative approaches to promote the fatigue performance of maraging steels, which typically possess high surface notch sensitivity. In this study, a gradient microstructured surface layer with nanolaminates and anisotropic mechanical properties was manufactured on an 18% Ni maraging steel by surface mechanical grinding treatment (SMGT). Rotary bending fatigue tests demonstrated that the fatigue performance of SMGT samples is significantly promoted, with an increase of ∼31% in the fatigue limit relative to as-received samples. Investigations of fatigue mechanisms indicated that the promoted fatigue performance is first related to the enhanced strength and hardness in the gradient nanostructured surface layer. Meanwhile, the anisotropic microstructure and mechanical properties of nanolaminates also play decisive roles in significantly enhancing the fatigue performance under high stress amplitudes. This is because the nanolaminates might not only suppress the growth of surface defects by hindering dislocation transmission along the normal direction, but also depress the damage accumulation in microstructure by accommodating dislocation slide along the rolling and transverse directions during fatigue.