Abstract
• A gradient layer about 300 μm is formed on the surface after USRP. • Tensile properties and fatigue resistance are improved after USRP. • USRP reduces the ratcheting effect, prolongs fatigue life. • Transition of crack initiation mode is a reason for fatigue strengthening. Surface gradient structures with small to large grain sizes were obtained on 18CrNiMo7-6 steel by ultrasonic surface rolling processing (USRP). Both tensile properties and fatigue resistance have been enhanced. Based on the gradient layer generated after USRP, the fatigue strengthening mechanism was discussed. After USRP, the reduction of the ratcheting strain reduced the fatigue damage of the material under the same stress level and prolonged the fatigue life. Meanwhile, USRP changed the grain orientation of the surface layer, the initiation mode of the fatigue crack was transformed from the surface to the interior at a low stress level.
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