| As a high strength,SMA490 BW steel has been used in the train steering framework.The stress concentration and residual stress at the welding toe of the bogie welding frame lead to fatigue cracks on the welding joints of the bogie.As the continuous expansion of these cracks leads to the failure of the welded frame,the fatigue life of the train bogie is greatly reduced.Therefore,the welding toe is the weak link of the bearing capacity of the welder.Ultrasonic impact treatment is a surface strengthening technology that refining metal surface grain,increase the degree of surface hardening,reduce material weaknesses in and residual compressive stress.The technology use ultrasound as the power source,the metal surface is impacted by its high frequency and high energy characteristics,the surface layer produces a strong plastic deformation,grain refining to nanometer scale,thus improving the performance of the metal surface.In this paper,the shock treatment of SMA490 BW weather-resistant steel cross welded joints is investigated by ultrasonic impact gun,and the fatigue properties of the cross welded joints of this kind of steel are studied.With German Carl ZEISS production of ZEISS ?IGMA type thermal field emission scanning electron microscope fatigue fracture morphology.The microstructure of welding toe surface was observed by transmission electron microscopy,and the hardness and thickness of the welding toe surface were analyzed.The fatigue test results show that the position of the fatigue fracture of the sample before and after the ultrasonic shock is all at the welding toe.The welding toe of the cross welded joint is still the weakest point in the whole sample whether the impact or not;although shock treatment cannot change the position of fatigue fracture of cross welded joints,but the fatigue life can be improved greatly after ultrasonic shock treatment.The condition fatigue limit of the welded joint was characterized by the stress of 2×106 cycles,and the condition fatigue limit of the welding state specimen was 165.96 MPa.The condition fatigue limit of the impact state sample was 219.28 MPa,and the fatigue strength of the SMA490 BW steel cross welded joint was increased by 32.13% after ultrasonic shock treatment.The effect of ultrasonic impact treatment on the stress concentration coefficient of welding cross joint specimen is significant.After the impact of 10 min,the stress concentration coefficient Kt decreased from 3.09 to 1.86,and then remained around 1.85.Although the impact treatment can reduce the stress concentration factor Kt,it cannot change the position of stress concentration,and the maximum stress is at the welding toe still.The effect of ultrasonic shock treatment on metal surface morphology is great also,The depth of sunken and the radius of the transition arc increased with the increase of the impact time,but the change rate of the surface morphology of the welding toe gradually decreased.when the sunken depth h increases from 0.5mm to 1.2mm,Kt changes between 1.8 and 2.0;when the arc radius r isincreased from 2.0mm to 4.5mm,Kt is changed from 2.2 to 1.6.Compared with the depth h of sunken,the former has a greater influence on the stress concentration factor Kt of the specimen of welding cross joint.The initiation of fatigue cracks occurs in the resident slip zone(PSB),grain boundary or twin boundary,surface inclusions or the second phase boundary usually.Improper operation or excessive ultrasonic impact treatment may be formed weld toe "Spiral Defect" in the surface,leading to the original crack formation,greatly shortens the crack formation and extension of the first phase of the cycle.The crack of the welding sample was generated from the surface of the material,and there were more than one crack source.There are many crack sources in the treatment,which are mainly generated by the subsurface of the sample or the defects caused by ultrasonic shock treatment.In the crack expansion area,the welding sample and the impact state sample formed obvious tear ductility and secondary cracks.On the fracture of some specimens,the fatigue striation and cleavage steps can be found,and the form of crack propagation is the form of crystal cleavage.At the end of the crack propagation,there is a plastic fracture,and there is a large number of fracture dimples on the fracture surface,and the dimple is mainly in the dimple.Ultrasonic shock treatment has great influence on the surface morphology of samples,Severe plastic deformation occurred on the surface after treatment,The surface of some sample will form a rotary tissue,and fatigue cracks are usually produced.In 15 min,the thickness of the plastic deformation layer was larger.The maximum thickness of the deformation layer at 25 min was about 320?m.However,the influence of impact time on the thickness of the deformation layer is weakened.Ultrasonic shock treatment can refine the grain on the surface of the material,Some materials can even be refined to the nanoscale.the reason of grain refinement is related to the formation,annihilation and rearrangement of high density dislocation wall in the impact process.The hardness of material surface can be improved greatly after ultrasonic shock treatment,when the impact time is10 min,the hardness is about 220 HV,it is higher than the 170 HV of the parent material,when the ultrasonic shock time reaches 25 min,the hardness value is 340 HV about,with the increase of the impact time,the hardness of the heat affected zone on the toe surface increases,but the increase gradually decreases. |
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