| Nowadays, U75 V steel is the main material of high-speed train rail commonly jointed by means of flash butt welding procedure. Considering the fact that fatigue failure is the main failure mode of railways and gaining an adequate service life of railways, this work mainly investigated the fatigue properties of flash butt welded joints of U75 V steel by analyzing their microstructure, hardness, S-N curve and fatigue fracture morphology under three welding procedures. Ultrasonic peening treatment(UPT) was used to improve the fatigue performance of this welded joint, fatigue fracture mechanism of the welded joint before and after UPT were analyzed in detail. Research on corrosion fatigue of welded joint before and after UPT based on the complex service environment of high-speed train rail,exploring fatigue fracture mechanism under the dual function between internal and external defects. Meanwhile, the feasibility that UPT improve the corrosion fatigue strength also was studied.Research shows that the upset pressure influences the mechanical properties of flash welded joint significantly. The high level of upset pressure is beneficial to the improvement of fatigue strength of welded joint. Fatigue life of welded joint decreased a lot after artificial seawater corrosion. Nanocrystals and residual compressive stress formed on the surface of the welded joint after UPT. Fatigue life, wear resistance and corrosion resistance of the welded joint were improved for the surface nano-enhancing caused by UPT. Fatigue strength of the flash welded joints increases by 23% after UPT. Fatigue strength of the flash welded joints treated by UPT after corrosion only decreases by 8% with that before corrosion while the corrosion-fatigue strength of flash welded joints decreased by 13.5%comparing with conventional-fatigue strength before UPT. The wear resistance of the flash welded joints after UPT improved by 29%.Analyzing the fatigue fracture mechanism based on the different welded joints: thecharacteristic of brittle fracture is obvious presentation for the welded joints fractured at welded joint of three kinds of welding process; the characteristic of ductile fracture is presentation for the weled joints fractured at base metal. The fatigue fracture based on welding process(I) only contains two regions(without obvious instantaneous fracture region), while the fatigue fracture of other welded joints contain the typical three regions. |
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