| In this paper,the effect of ultrasonic impact on the very high cycle fatigue performance of P355NL1 steel welded joint was investigated.The specimens were divided into 5 groups were cylindrical base metal,cylindrical as-welded joint,plate base metal,plate as-welded joint and treated specimen.Five groups'specimens were subjected to ultrasonic fatigue test,and fatigue test data of five groups and S-N curve were obtained.Compared with the fatigue test data of five groups,UIT on the P355NL1 steel welded joint's very high cycle fatigue influence was obtained.The fatigue failure modes of the five groups were observed by scanning electron microscopy?SEM?.By means of metallographic microscope and residual stress tester,the mechanism of very high cycle fatigue influence of UIT on welded joint is explored under the action of stress concentration,residual stress and surface grain refinement of weld root.By means of transmission electron microscopy?TEM?,the nanoscale mechanism of the weld root surface under UIT was revealed.The following conclusions are obtained through the above experiments.1.The curve equation is obtained by fitting the S-N curve from the experimental data.Plate base metal specimen was lgN+34.72lg??=90.654×105?N?4.54×108;plate as-welded specimen was lgN+10.64lg??=30.954×105?N?4.40×108;cylindrical base metal specimen was lgN+14.29lg??=44.044×105?N?3.09×108;cylindrical as-welded specimen was lgN+14.93lg??=44.794×105?N?5.79×108;treated specimen was lgN+13.55lg??=39.344×105?N?4.30×107,lgN+76.22lg??=186.22 4.30×107?N?5.42×108?2.Under the same fatigue life condition,the fatigue strength of the cylindrical specimen is much higher than the plate specimen.Under the 1.0×106 cycles,the fatigue strength of the cylinder base metal is 1.68 times that the plate base metal and the fatigue strength of the cylindrical as-welded specimen is 1.80 times than the plate as-welded specimen.Compared with the plate as-welded specimen,the fatigue properties of the treated specimens were greatly improved.Under the 1.0×108 cycles,the fatigue strength of the treated specimen is 217MPa,while the plate as-welded specimen is 143MPa,and the fatigue strength is increased by 51.7%.It can be found that the specimen will occur when fatigue failure in very high cycle range?more than107 cycles?,and the fatigue limit does not exist.3.Under the stress load of 220MPa,the fatigue life of treated specimen is 3.98×107 cycles,while the plate as-welded specimen is 1.06×106 cycles,and the fatigue life is increased 37 times.Therefore,UIT can greatly prolong the fatigue life of welded joint.4.The crack propagation path of the plate as-welded specimen is straight,while the treated specimen is tortuous.In the process of crack propagation presenting the trans-granular fracture and accompanied by a trace of plastic deformation.5.For cylindrical base metal specimen and as-welded specimen crack source are located in the surface machining micro scars and some located in subsurface inclusions and no found the fatigue crack source located in internal specimen,no typical"fish eye"morphology.The fracture surface of the extended area is flat and the river pattern and tearing edge can be observed with secondary cracks and cleavage steps.The transient fracture zone shows the dimple morphology,which indicates that the fatigue fracture mechanism of the cylindrical base metal specimen and as-welded specimen are quasi cleavage fracture.6.For plate as-welded specimens and treated specimens there is little difference in fracture mechanism.Most cracks are initiation in welded root,the difference that the as-welded specimen crack initiation in the surface of welded root and the treated specimen crack initiation in the subsurface of welded root.The crack propagation area has tearing edges and secondary crack morphology.The transient fracture zone shows the dimple morphology and the second phase particles are in the bottom.It shows that the fatigue failure of the treated specimen and the plate as-welded specimen is a quasi cleavage fracture.It shows that UIT does not change the mechanism of very high cycle fatigue failure of welding joint.7.UIT can increase the arc transition radius between the base metal and the weld.The stress concentration coefficient of the as-welded specimen is Kt1=1.82,the stress concentration coefficient of the treated specimen is Kt2=1.35,and the stress concentration coefficient decreases by 25.8%.Therefore,the UIT can obviously improve the stress concentration of the welded joint.8.The residual tensile stress of X direction in the welded root reaches 11.1MPa and the residual tensile stress in the direction of Y is 20.2MPa.After ultrasonic impact treatment,the residual tensile stress at the welding root was replaced by the residual compressive stress.The residual compressive stress of the X direction in the treated specimen reaches-40.5MPa,while the residual compressive stress of the Y direction is-23.6MPa.UIT will transform the residual tensile stress at the root to residual compressive stress,which will increase the critical stress factor?Kth value at the crack tip and prolong the fatigue life of the specimen.9.Under the conditions of 5min/1.5A,10min/1.5A and 20min/1.5A,the average grain size of the treated specimen is about 400nm,300nm,200nm,respectively.The crystallographic orientation between the grains is random which indicating that it has a high grain boundary deviation.The thickness of the impact deformation layer of the three groups is about 300?m,460?m and 600?m,respectively.When the impact current is certain the longer the impact time and the deeper the plastic deformation layer.10.The process of UIT and surface grain refinement probably was the followings.Firstly,the UIT produces high frequency energy on the surface of welding joint root,the surface layer was subjected to severe plastic deformation and a large number of dislocations were generated inside the original grain.Secondly,with the increase of the impact time,the dislocation can interact with each other and form dislocation tangles and dislocation walls.Thirdly,with the time of impact increasing the strain gradually increased.In the distance between the dislocations decreases constantly,dislocation walls and dislocation tangles will be gradually transformed into small angle dislocation subboundary.Fourthly,high density dislocations in the sub grain boundaries continue to interact,and the orientation on both sides of the grain boundary is getting bigger and bigger,and small angle subgrain boundaries gradually transformed into large angle grain subgrain formation. |
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