Study On The Fatigue Property For Ultrasonic Impact 16MnR Welded Joint

Welding has become the main connection methods. Fatigue fracture is about 80 to 90% in fracture failure mode. In recent years, with train speed increasing in our country, the risks of fatigue fracture were improved. Therefore, the study of fatigue properties of welding joints has become more and more important.The weld toe area of 16MnR butt joints and cruciform joints were treated by ultrasonic impact. The fatigue properties of welding joints before and after ultrasonic impact were study on. The S-N curve equation of welded butt joints was lgN = 44.97?16.01lgΔσ; The S-N curve equation of butt joints with UIT was lgN = 58.21?20.66lgΔσ; The S-N curve equation of welded cruciform joints was lgN = 26.76?9.29lgΔσ; The S-N curve equation of cruciform joints with UIT was lgN = 32.54?11.05lgΔσ.At the N=2×106 cycles, the conditional fatigue limit of 16MnR welded butt joints was 265MPa, the conditions fatigue limit of the UIT butt joints was 325MPa, the conditions fatigue limit of welded cruciform joints was 157MPa, the conditions fatigue limit of UIT cruciform joints was 235MPa, the conditions fatigue limit were increased by 22.6% and 49.7%.The hardness of weld toe area of butt joints before and after ultrasonic impact were measured as HV185 and HV243, the hardness was increased by 31%. The weld toe area being bring a beneficial residual compressive stress(σx=-241MPa,σy=-263MPa) by UIT, the surface defects of welded joints were reduced, and the transition radius of weld toe were increased. All these factors were benefited for the improvement of the fatigue properties of welded joints.It was found that the fracture characteristics of welding samples were similar to the samples of UIT by SEM. That crack source was always produced on the surface. The characteristics of the beach tricks were shown in the process of crack propagation, which indicated that quasi-cleavage fracture was occurred and some plasticity was happened.Grain reached the nanometer scale and distributed randomly after UIT by TEM. The crack source was reduced and crack propagation was hindered due to grain refinement, so the fatigue property of welding joints was enhanced.