Effect Of Ultrasonic Impact On The Surface Microstructure And Very High Cycle Fatigue Properties Of Welded Cross Joint For Train Bogie

The conventional fatigue design codes were formulated based on the data from test stress cycles less than 107 cycles. Most of them could not be applied for welded structures serviced in the ultra-long life region so the new recommendations were discussed. Characterization of very high cycle fatigue(VHCF) behavior is of significant issue for ensuring long-term durability and reliability of the bogie of high speed train due to the rapid development of high-speed railway. The life time of bogie has to endure up to 109~1010 cycles. The bogie is manufactured by welding method, welded joints were considered to be the weak links in the structure. It is known that ultrasonic impact treatment(UIT) is a remarkable post-weld technique applying mechanical impacts in combination with ultrasound into the welded joints and can improve the fatigue strength. This investigation was carried out on metal inert gas(MIG) welded samples out of SMA490 BW steel applied to high-speed train bogie.In this paper UIT was applied to the welded samples and its effect in the VHCF was investigated. Ultrasonic testing method was used to study very high cycle fatigue behavior. Since ultrasonic tests are conducted at a very high loading frequency, they are particularly convenient for fatigue tests in the very high cycle regime. Fracture surface was observed and analyzed with JSM-6360 LA type SEM(Scanning Electron Microscope). The microstructure of treated surface of weld toe was observed with JEM-2100 type TEM(Transmission Electron Microscope).Compared to the un-treated specimens, the welded joints of SMA490 BW steel after UIT showed a remarkable improvement in fatigue strength. The fatigue strength increased up to 25% in the regime of VHCF. Whether high stress amplitude or low stress amplitude is applied, there are several crack initiation locations on the fracture surface for the as-welded specimens, whereas, there is only one crack source for all the treated specimens. On as-welded samples the fatigue crack initiated on the free surface, probably in correspondence of a large stress concentration due to small weld toe radius. After UIT, the crack initiation point is also on the free surface, it is assumed that after surface strengthening the fatigue crack initiation is shifted toward the subsurface layers, but the microcracks created by peeling off layers of material due the severe plastic deformation created a stress concentration point where the fatigue crack initiated.The improved strength mainly resulted from modifying weld geometry by changing weld toe radius and weld toe angel, the generation of nanocrystalline structure, the enhanced surface hardness and compressive residual stress. Weld toe angel was reduced by about 17 percent and weld toe radius was increased by about 430 percent. It led to significant reduction in stress concentration in weld toe, therefore, reduction in potential of fatigue cracking achieved. UIT causes to decrease in residual stresses by about 185.9 percent in the surface of specimen. Decreasing in endured loaded due to exerting of compressive residual stresses. UIT introduced plastic deformation on surface of specimen. It led to increase in dislocation density and interaction between them. These interactions increased surface hardness by about 47.3 percent which shows increasing in plastic deformation. Increasing static strength on surface led to prevent crack propagating under fatigue loadings. A surface layer of nanocrystalline microstructure could be produced by implementing the UIT due to the effect of severe plastic deformation exerted on the topmost surface layer. The mean grain size is about 30 nm for the UIT parameter of 10min/1.5A. The weld toe surface of SMA490 BW steel nanometer mechanism has the relation to the high density dislocation response, high density dislocation tangle and dislocation walls. The repeated processing exacerbates the plastic deformation in surface, resulting in finer grains. As the initial processing already produces dislocation walls and dislocation tangle, which become a disadvantage for further plastic deformation, the top surface absorbs most of the energy in the following process. Hence, grains in surface layer relatively far from the top surface bear insufficient plastic deformation, thus unable to be refined.