Development of ultrasonic thread root rolling technology for prolonging the fatigue performance of high strength thread

Fatigue failure commonly occurs at mechanical products, especially the threaded joints with high stress concentration. Even though various high strength alloys are adopted, accidence due to fatigue failure of thread still happens. As a novel method to improve the thread fatigue life, ultrasonic thread root rolling (UTRR) process is developed in this paper. The configuration of the UTRR apparatus together with the principle of the UTRR is first introduced and described. Rolling experiments are then conducted on AerMer100 thread specimens by use of the UTRR process and conventional thread root rolling (CTRR) process respectively. Mechanical characteristics and fatigue properties for these treated specimens and untreated specimens are tested. Compared with CTRR, surface finish of thread root is farther improved from Ra 0.106 to Ra 0.051, and higher residual stress is achieved for UTRR. The depth of microstructure-refined zone induced in UTRR is about 20 μm, while that of CTRR is approximately 12 μm. These further enhanced mechanical properties greatly contribute to the improvement of fatigue performance, and it is well proved by fatigue results that an increase of about 7 times for fatigue life of AerMet100 specimens is achieved after UTRR, while only 3 times is achieved for CTRR. Striations from fracture surface explicitly confirm this result.