| In this paper,the SMA490 BW steel of high-speed train bogie was used as experimental material.The welded joints of SMA490 BW steel were treated by ultrasonic impact treatment(UIT).The fatigue properties of the specimens were measured by ultrasonic fatigue system.The effects of stress concentration,grain refinement and residual stress on the very high cycle fatigue of SMA490 BW steel welded joints of bogies were systematically analyzed by fatigue comparison test,and the dominant factor was obtained.The fatigue failure modes of the specimens were analyzed by scanning electron microscope.Before and after UIT,the change of plastic deformation and residual stress of the welded joints were investigated by means of metallographic microscope and residual stress analyzer.Finally,The mechanism of circulating cooling water on erosion corrosion of specimens was investigated.The main conclusions are as follows:(1)Effect of residual stressBefore and after low temperature heat treatment,the average fatigue life of the welded samples was 0.26×107 cycles and 0.21×107 cycles,respectively.The effect of residual tensile stress is not obvious.Besides,the average fatigue life of the specimens treated by ultrasonic impacting was 7.86×107cycles and 6.84×107 cycles respectively.(2)Effect of grain refinementBefore and after the surface grain refinement of the weld toe,the average fatigue life was1.68 × 107 cycles and 2.35×107 cycles,and the fatigue life was increased by 42.4%.The weld reinforcement of specimens were polished,and ultrasonic impact at different time.The results show that the average fatigue life was 0.63 × 107 cycles and 1.53 × 107 cycles,under different impact time(15min and 20min).The fatigue life is increased by 142.9%.(3)Effect of stress concentrationThe full coverage impact and local impact were used to treat the samples,and the average fatigue life of the specimens was 2.61 × 107 cycles and 2.92 × 107 cycles,respectively.The results show that the local impact method is better than the full coverage impact.The average fatigue life of the original welding samples was only 0.26 × 107 cycles.The average fatigue life was increased to 1.71 × 107 cycles after reducing the stress concentration factor.The average fatigue life was up to 22.46 × 107 cycles which grinding the weld reinforcement,but still lower than the fatigue life of base metal specimens.(4)The contribution ratio of stress concentration,grain refinement and residualcompressive stress on the life extension of welded joints is about 58%,29% and 13%,respectively.(5)The fatigue failure of welded joints mostly comes from the surface of weld toe.The fatigue crack initiation position may change from the material surface to the internal defect after UIT.Fatigue fracture morphology is quasi cleavage fracture mechanism.Ultrasonic impact treatment can’t change the mechanism of high cycle or very high circle fatigue failure of welded joints.(6)The microhardness of the surface layer was 180 HV without ultrasonic impact treatment.When the impact parameters were 1.5A/5min,1.5A/10 min,1.5A/15 min and1.5A/20 min,the corresponding thickness of plastic deformation layer were 150μm,260μm,320μm,360μm,respectively.The corresponding microhardness of the surface layer were275 HV,283HV,290 HV,310HV,respectively.With the increase of impact time,the residual compressive stress increases first and then decreases,while the transverse residual compressive stress of weld toe surface decreases monotonically.After low temperature heat treatment(200℃/2h),the residual compressive stress on the surface of the impact specimen decreased obviously,but the magnitude of the decrease was different.(7)The base metal is relatively fast moving with the fluid under the loading condition.The high frequency shock between them disturbs the original laminar flow state,which results in the generation of turbulence.Erosion corrosion is the result of interaction between electrochemical corrosion and fluid erosion wear in turbulent flow environment. |
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