| With the excellent comprehensive performances and economical utilisation,dissimilar steel welded joints of stainless steel(SS)and low alloy steel(LAS)have very wide application prospects in the petrochemical plants,oil and gas exploitation and nuclear power plants pipe lines and so on.However,due to the significant discrepancies existing in the chemical composition,microstructure,physical properties and mechanical properties of the dissimilar materials,the welding metallurgical process of dissimilar steels is much more complicated than that of similar ones.To solve the problems such as carbon diffusion,dilution of the weld metal and high residual stresses during welding and surface failure behaviors such as wear and corrosion at service conditions of the dissimilar steel joints,the suitable filler materials for SS/LAS welding are self-developed and ultrasonic impact processing after welding is conducted in subsequent.Effects of the chemical composition of filler materials and ultrasonic impact on the microstructure and comprehensive performances of SS/LAS dissimilar steel welded joints are systematically studied.Three kinds of stainless steel flux-cored wires(FCWs)with different chromium equivalents(Creq)and nickel equivalents(Nieq)are self-developed as the filler metals for AISI304 SS/SA-508 Gr.3 LAS dissimilar steel joints prepared by manual gas tungsten arc welding(GTAW)process.The influence and reasons of Creq/Nieq variation(1.38,1.68 and 1.95,respectively)of the weld metals on the microstructure and comprehensive properties of the dissimilar steel joints have been systematically studied.Microstructure analysis shows that the weld metals with different Creq/Nieq are composed mainly of austenite and small amounts of ?-ferrite randomly distributed within the grain boundaries of the austenitic matrix.When Creq/Nieq increased from 1.38 to 1.68 and 1.95,the corresponding ?-ferrite content are about 7.6%,12.5%and 18.5%,respectively,and changed from the coarse dendritic and skeletal morphology to fine and uniform vermiform morphology.The tensile strength of the dissimilar steel joints reach 577 MPa,609 MPa and 627 MPa,and the corresponding elongation are 43.67%,41.75%and 37.12%with Creq/Nieq increasing.Under high speed,large load and oil lubrication condition,the wear rates of the dissimilar steel joints are 1.785×10-2 g/h,1.466×10-2 g/h and 1.323×10-2 g/h.and the wear mechanism changed from fatigue wear to adhesive wear and abrasive wear,respectively.The corrosion rates of the weld metals immersed in the mixed solution of 12%FeCl3+1.5M HCI+1.85M H2SO4 are increased with time when Creq/Nieq increases.During the electrochemical tests in 3.5%NaCl+1.85 M H2SO4 solution(pH=1.0),the weld metals present passivated features.The passivation potential Ecr are-0.121 V,-0.188 V and-0.160 V,and the corresponding passive current log(ip)are-6.669 A,-6.035 A and-5.062 A as Creq/Nieq increases.Results show that the composition of three stainless steel FCWs can effectively inhibit the carbon migration and the dilution of the weld metals.The contrast shows that the dissimilar steel joints exhibits favourable microstructure,comprehensive mechanical properties and corrosion resistance when Creq/Nieq is 1.68.Microstructure characterization of the dissimilar steel joints before and after ultrasonic impact processing is carried out by scanning electron microscopy(SEM),X-ray diffraction(XRD)and electron backscatter diffraction(EBSD).Effects of the impact parameters variations on the microstructure evolution of the dissimilar steel joints surface layer is studied systematically and the strengthening mechanism of surface nanocrystallization induced by UIT is discussed in detail.Results show that severe plastic deformation and gradient nanocrystalline structure accompanied by the deform-induced martensite ?' are formed on the dissimilar steel joint surface after UIT.When the impact times per unit area(mm2)N is increased from 40,000 times/(mm2)to 57,600 times/(mm2)and 75,000 times/(mm2),the thicknesses of the plastic deformation layers are about 151 ?m,220 ?m and 210 ?m,the depths of the nanocrystalline layer reach to 16.15 ?m,21.62 ?m and 23.37 ?m,the grain sizes of the smallest nanocrystals are below 120 nm,80 nm and 60 nm,and the corresponding contents(vol.%)of the deformation induced martensite ?' are 0.087%,0.136%and 0.172%,respectively.It indicates that the surface nanocrystallization of the welded joints is related to the refinement strengthening and phase transformation refinement induced by UIT.Surface nanocrystallization degree of the weld.joints gradually increases and tends to be stable with N increasing.The mechanical properties and corrosion behaviors of the dissimilar steel joints before and after UIT are studied.Results show that the comprehensive performances of the welded joints have been improved after UIT.The performance enhancements of the dissimilar steel joints can be attributed to the formation of nanocrystalline structure,compressive residual stress layer as well as the deformation-induced martensite ?'.Compared with the no UIT-treated joints,when N is increased from 40,000 times/(mm2)to 57600 times/(mm2)and 75,000 times/(mm2),the tensile strength is increased by 5.33%,8.83%and 15.82%and the reductions of wear rate reach 38.88%,71.83%and 85.13%,respectively.Distributions of elastic modulus(Er)and nano-hardness(Hr)of the welded joints surface layer present the gradient structure,and the maximum Er and Hr signifificantly decrease as the distance from the weld surface increases and gradually tend to the level of the coarse-grained matrix when the depth exceeds the thickness of the plastic deformation layer.Compressive residual stress is introduced and uniformly distributed over the dissimilar steel joints surface layer after UIT.The maximum and influencing depth of compressive residual stress are-77 MPa/147 ?m,-236 MPa/246 ?m and-309 MPa/278 ?m with N increasing,respectively.Compared with the no UIT-treated joints,the immersion corrosion rates of the weld metals decreased by 92.52%,88.78%and 82.55%under the same conditions.The passivation of the weld metals tends to weaken slightly after UIT.The passivation potential Ecr and currents log(ip)drop to-0.215 V/-7.160 A,-0.205 V/-6.975 A and-0.238 V/-6.857 A,respectively.By comparing the improvements in comprehensive performances of the welded joints after UIT with different N,it can be found that the more desirable improvements in mechanical properties and corrosion resistance of the dissimilar steel joints induced by UIT can be obtained when N is increased to 57,600 times/(mm2). |
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