| Duplex stainless steel is a kind of stainless steel with the content of austenite and ferrite of about 50% respectively in the solid solution structure.Due to its excellent mechanical properties,physical properties and corrosion resistance,it has been widely used in nuclear power,chemical industry and coastal infrastructure and other corrosive environments.The traditional welding method is not only inefficient,but also has great welding deformation and stress,especially the roughening of the structure in the heat affected zone will seriously affect the mechanical properties and corrosion resistance of the joint.Narrow gap laser wire filling welding has many advantages,such as high welding efficiency,less filling materials,small post-welding deformation and narrow heat affected zone.In this paper,the influence of welding parameters on weld formation was studied by flat plate surfacing and laser wire filling in narrow groove of S32101 duplex stainless steel for nuclear power project.The 25 mm S32101 duplex stainless steel was successfully welded with optimized process parameters.The microstructure and the ratio of austenitic ferrite in different areas of the joint were analyzed,and the mechanical and corrosion properties of the joint were tested.On the basis of the research on the influence of laser wire filling welding process parameters on the welding seam formation of S32101 duplex stainless steel specimen surface,the single laser wire filling welding process optimization test in narrow groove was carried out.The results show that the laser power has little effect on weld forming.Too fast or too slow welding speed is unfavorable to weld formation.When the welding speed is 0.48m/min,the weld formation is good.The amount of defocusing has a significant impact on both the weld formation and the weld penetration.The amount of defocusing decreases and the weld penetration increases.The amount of defocusing of +10mm can ensure sufficient weld penetration and can be used for laying the weld bead.The defocusing amount of +20mm can obtain the well-formed weld.Excessive defocusing amount will easily cause the weld to have undercut defects.Wire feeding speed mainly affects the depth of fusion.When the wire feeding speed increases from 3m/min to 5.5m/min,the depth of weld increases first and then decreases.On the basis of the above experiments,the process optimization model is established.According to the process optimization model: as long as the laser power,wire feeding speed,welding speed of any of the three parameters,you can determine the other process parameters;The reasonable ratio between wire feeding speed and welding speed is between 7.5 and 12.5.Within the range of the test in this paper,the optimized process parameters of S32101 duplex stainless steel with a thickness of 25 mm are as follows: laser power 4500~5100W,wire feeding speed 2.3~4.5m/min,defocusing amount +10~+20mm,welding speed 0.48m/min,wire spacing 1mm.S32101 duplex stainless steel of 25 mm was successfully welded by the optimized welding process.The welded joint was composed of seven welds,and the welds were beautifully formed without defects such as incomplete fusion and porosity.Austenite in the center of the weld was mainly precipitated along the dendritic boundary,and a small amount was distributed in the grain.Due to the influence of secondary thermal cycle,the content of austenite increased and the grain was refined.The austenite in the nearseam zone of the welding heat affected zone is in a banded distribution.Due to the rapid cooling of welding,the amount of austenite precipitated is reduced but the grains are coarsened.Austenite and ferrite content in the parent metal ratio is about 51% : 49%,backing weld austenite and ferrite content on average about 40.5% : 59.5%,filling bead austenite and ferrite content average ratio of 41.5% : 58.6%,covering bead austenite and ferrite content average ratio of 45.2% : 54.8%,HAZ austenite and ferrite content average ratio of 39.4% : 60.6%,overlap weld austenite and ferrite content average ratio of 56.2% : 43.8%,conform to the requirements.The results of hardness distribution test in different areas of welded joints show that the hardness of the backing pass is not different from that of the base metal,and the hardness is the lowest in the fusion zone.The hardness value of the filled bead at the center of the weld is up to 309.9HV.The highest value of the hardness of overlapped pass appears in the fusion zone.The hardness of the weld on the cover surface is similar to that of the base metal and is uniform.The average tensile strength of the welded joints is 705 MPa,greater than that of the base metal.In different areas of the weld center-40? impact test results show that the impact energy is greater than 79 J.Uniform corrosion test results show that the corrosion resistance of weld metal is close to that of base metal.Due to the influence of secondary heat cycle,the proportion of austenite in the overlapping area of weld bead increases and the corrosion resistance is higher than other areas.In the heat affected zone,the austenite ratio in the near-seam zone decreases,resulting in poor corrosion resistance.Weld area in 3.5% NaCl solution to form passivation film with the parent metal passivation film density,uniformity were similar,but in the cover surface weld position contains sample passivation membrane resistance of heat affected zone,the youngest,poor corrosion resistance,heat affected zone is weak link of corrosion,it may accelerate the precipitation of secondary phase passivation membrane rupture,reducing the corrosion resistance.Through the analysis of M-S curve,the carrier concentration of base metal and filling weld is lower,the impedance is higher,and the corrosion resistance is the best. |
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