Surface nanocrystallization and its effect on fatigue performance of high-strength materials treated by ultrasonic rolling process

Surface nanocrystallization plays a great role in the enhancement of global behavior of products. In this paper, a theoretical modal for grain refinement of ultrasonic thread root rolling process (UTRR) is explored, and its effect on surface nanocrystallization is investigated by conducting comparative experiments of UTRR and conventional thread root rolling process (CTRR). Various surface analytical techniques, such as transmission electron microscopy, metalloscope, X-ray diffraction, and Vickers hardness tester are then adopted to quantitatively study surface nanocrystallization of different treated specimens. The fatigue tests are last performed to evaluate effect of grain refinement on fatigue performance. Results show that nanocrystalline is successfully synthesized on thread surface by UTRR, with grain size ranging from 15 to 20 nm, while hardness in top surface is remarkably improved from 580 to 700 HV due to surface nanocrystallization. By comparison, only subgrains are generated by CTRR confirming that the ultrasonic technique plays a key role on surface nanocrystallization. The result also shows that the increased relative motion distance between roller and surface greatly contributes to nanocrystallizaion for UTRR. Fatigue tests show that specimens treated by UTRR achieve the longest fatigue lives, which confirms that nanocrystallization of root surface greatly contributes to the improvement of fatigue performance.