Effect Of Ultrasonic Impact Treatment On Very High Cycle Corrosion Fatigue Properties Of P355NL1 Steel Welded Joint

In this paper,P355NL1 steel has been taken as the research object.Through ultrasonic fatigue life measurement,residual stress measurement,electrochemical corrosion rate measurement and ultrasonic impact joint surface grain refinement mechanism analysis,the influence on ultra-high cycle corrosion fatigue performance of P355NL1 steel welded joint is studied.In ultrasonic fatigue test,the samples were divided into three groups,namely,welded samples,welded samples(6%,9%,12%NaCl corrosive liquid)and impinged samples(6%,9%,12%NaCl corrosive liquid).Ultra-high cycle fatigue test was conducted on these groups of samples to obtain their fatigue results and S-N curve.The effect of ultrasonic shock treatment on corrosion fatigue property of P355NL1 steel was compared and analyzed.The samples were divided into two groups in electrochemical corrosion test.The first group was 6%NaCl corrosive solution.The ultrasonic impact parameters were set at 1.5A/5min,1.5a/10min and1.5a/20min,respectively.In the second group,the ultrasonic impact parameters in 9%NaCl corrosion solution were set as 1.5A/5min,1.5A/10min and 1.5A/20min respectively.The effects of different ultrasonic impact parameters on the corrosion rate of P355NL1 steel welded joints under different corrosion solution concentrations were compared and analyzed.After that,the fatigue fracture morphology of each sample was analyzed by scanning electron microscopy(SEM),including the crack source,crack propagation zone and transient fracture zone,and the fracture mechanism was analyzed.Optical microscope,hardness tester and residual stress tester were used to explore the distribution of stress field of the joint sample,grain refinement of residual stress,and the influence mechanism of ultrasonic impact on ultra-high cycle corrosion fatigue performance of the joint.The surface microstructure of P355NL1 steel welded joint was observed and analyzed by high resolution transmission electron microscope to further understand the mechanism of microstructure refinement.The research results show that:1.Origin was used for fitting analysis of fatigue test data to obtain the S-N fatigue life curve.Through comparative analysis,it was found that the fatigue strength of the impact test sample was significantly higher than that of the welding test.Each set of sample curve equation,respectively:As welded:lgN+10.57lg??=30.78;As welded(6%,9%,12%NaCl):lgN+8.33lg??=25.64 lgN+8.58lg??=26.00 lgN+8.80lg??=26.36;UIT(6%,9%,12%NaCl):lgN+16.97lg??=48.06 lgN+19.28lg??=53.36 lgN+18.98lg??=52.49.The calculation of S-N curve equation shows that the impact fatigue strength of corrosion solutions in 6%NaCl,9%NaCl and 12%NaCl is 1.32,1.33 and 1.37 times higher than that in the welding state,respectively,under the cycle times of 1.0×10⁶.In corrosion solutions of 6%NaCl,9%NaCl,and 12%NaCl,the impact fatigue strength was 1.52,1.54,and 1.58 times that of the welding state,respectively,in a cycle of 1.0×10⁷.In corrosion solutions of 6%NaCl,9%NaCl and 12%NaCl at a 1.0×10⁸ cycle,the fatigue strength of impact state was 1.75,1.8 and 1.83times that of the welding state,respectively.Under the same cycle of 1.0×10⁸,the fatigue strength of impact state was increased by 83.3%compared to that of the welding state.2.For the fracture locations of welded joint samples,most of the crack sources are corrosion pits.In NaCl corrosion solution of different concentrations,the higher the concentration is,the more serious the surface corrosion of the sample will be,and the more pitting pits there are,the more the crack sources will be.For the impact state sample,the surface grain refinement is obvious,the surface erosion is weakened by the corrosive liquid,pitting pits are significantly reduced,and some fatigue cracks are also generated from the surface of the sample to the secondary surface.At the crack propagation stage,the sample morphology shows fluvial pattern and tearing edge,fatigue speckles can be seen,and there are a few secondary cracks.The instantaneous fracture zone shows obvious dimple morphology,and the second phase particle exists at the bottom of the dimple.Both the welded and impinged samples are quasi cleavage fracture.This indicates that the fracture failure mechanism of the specimens subjected to ultrasonic impact treatment under different corrosion solution concentrations will not change,but will be significantly improved.3.When the ultrasonic impact parameters were 1.5A/5min,1.5A/10min and 1.5A/20min,the depth of plastic layer on the surface of P355NL1 steel welded joint was 350?m,400?m and500?m,respectively.It is shown that within a certain range,the duration of ultrasonic impact is proportional to the depth of the plastic deformation layer.4.different samples at different concentration of corrosion electrochemical corrosion test shows that under the same concentration of corrosion environment,after ultrasonic impact treatment sample corrosion rate significantly decreased,and the different ultrasonic impact parameters on its influence degree,with ultrasonic impact parameters from 1.5 A/5 min to 1.5A/10 min,and 1.5 A/20 min,the polarization curve has the obvious change,passivation range decreases,and the corrosion current density decreases,and the corrosion potential is to move,to slow down the corrosion rate.5.After ultrasonic impact treatment,obvious plastic deformation occurred on the surface of the samples,and the microhardness also changed significantly.When the impact parameters were 1.5A/5min,1.5A/10min and 1.5A/20min,the corresponding maximum microhardness values on the surface of the samples were 221HV?208HV?199HV,respectively.6.The residual stress at the weld root of the welded specimen before ultrasonic impact treatment is positive,that is,the residual stress is all tensile residual stress,and the residual tensile stress is released after wire cutting.After ultrasonic impact treatment,the tensile residual stress at the root of the sample is eliminated by the introduced impact energy,and the residual compressive stress is introduced at the same time.Ultrasonic impact treatment transforms the residual tensile stress into compressive stress,the value of?K_th increases,and the life of the welded joint is greatly improved.7.Ultrasonic impact refines the surface grain structure of the welding heel as follows:firstly,ultrasonic impact generates high-frequency energy on the welding root surface of the welding joint,resulting in violent plastic deformation of the surface structure and a large number of dislocation in the original grain;secondly,with the increase of impact time,the dislocation will interact with each other and entangle to form the dislocation entanglement and dislocation wall.Thirdly,with the increase of impact time,the dependent variable increases gradually,the distance between the dislocation decreases continuously,and the dislocation wall and the dislocation entanglement will gradually change into the small Angle sub-grain boundary.Fourthly,High-density dislocations continue to interact with each other at the subgrain boundary,making both sides of the grain boundary more and more oriented,and gradually transforming the small-angle subgrain boundary into the large-angle grain boundary to form the subcrystal.