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The torsion bar must be changed periodically due to its lack of durability and a phenomenon related to stress relaxation. Therefore technical development regarding the torsion bar's durability is urgently needed. In order to improve the fatigue life and durability of the dynamic components, new surface treatment technology such as ultrasonic shot peening, deep rolling, laser shock peening, etc. are developing widely. In this study, Ultrasonic Nanocrystal Surface Modification (UNSM) technology is applied as an advanced one to replace the presetting method (PM). UNSM and PM technology also induced the compressive residual stress on the surface layer of the torsion bar, which is the main improvement factor of fatigue life. DIN17221 material as a new MIL specification of torsion bar and SCM440 (as an alternative one to a old MIL-DTL-62567C) were processed with the UNSM technology to obtain the basic data and compare it between two, and then torsion fatigue tests of two materials were carried out to obtain the characteristics of torsion fatigue in this study.

At first, the specimens of SKD-61 are prepared and tested to verify the effects of ultrasonic nano-crystal surface modification (UNSM) technology on the variation of mechanical properties under static load variation. 20 kHz frequency was applied to the ball tip, and the applied static forces were changed three kinds of load level 40, 60, and 80 N, respectively. The grain size of SKD-61 surface treated by UNSM becomes very fine to nano-scale crystal and structure is observed till certain depth. The compressive residual stress becomes -810, -1200 and -1400 MPa to a 150 µm depth after the UNSM process according to three kinds of load level 40, 60, and 80 N, respectively. Fatigue limits of 10⁷ cycles are increased by 8.3, 11.2, and 17.9% after UNSM at the smooth specimen according to three kinds of load level, respectively. Interior-originating fracture, fish-eye crack, occurs after UNSM because of a nano structured modification by a surface plastic deformation and compressive residual stress in the case of the smooth specimen. UNSM improves the mechanical properties effectively and is becoming a practical method to improve the service life of the trimming knives. Productivity and reliability of a cold rolling process have improved more than 2 times by the application of the UNSM trimming knives.

UNSM(Ultrasonic Nanocrystal Surface Modification) technology has been applied to test specimens of bearing rings and rollers made of SAE52100. Mechanical properties, especially rolling contact fatigue characteristics, are analyzed using two-roller test and 6-ball test before and after UNSM treatment. The main effects to improve rolling contact fatigue characteristics by UNSM treatment are explained in the view points of residual stress, hardness and surface topology.