| Duplex stainless steel because of its excellent corrosion resistance is widely used in many fields such as offshore engineering,nuclear power,petrochemical industry and other harsh working environment,the corrosion resistance requirements are demanding.Up to now,many domestic and foreign enterprises have mainly adopted gas shielded welding and electrode arc welding as their main welding methods in the welding process of duplex stainless steel.Traditional welding methods generally have disadvantages such as low welding efficiency,large heat-affected zone,severe welding deformation,and imbalance of the two phases after welding,which reduces the comprehensive mechanical properties and corrosion resistance of duplex stainless steel.As one of the new fusion welding methods,narrow gap laser welding technology can make up for the shortcomings of traditional welding methods.It has high energy density,small weld and heat-affected zone,small heat input,small welding deformation,less residual stress,less filling material,material consumption and other advantages.It can effectively control the weld structure and improve the welding quality.In this thesis,the test method of controlled variable method is adopted,and the laser wire filler welding is carried out under different joint forms such as the surface of the test plate,the narrow gap groove,the thick-walled pipe butt.The influence law and effect of the process parameters on the weld formation are studied,the welding process parameters were optimized,and the narrow gap welding of S32205 duplex stainless steel pipe joints with a wall thickness of 25mm was finally realized.After welding,the microstructure and two phases structure of the joints were observed and analyzed.The characteristics of the phase ratio were tested,the mechanical properties and corrosion resistance of the joint were tested,and the internal connection between the weld structure morphology,the two phases ratio cases and the joint performance was established.First,the laser filler wire welding test was carried out on the surface of the test plate and the narrow gap groove.The welding seam surface formation and the cross-sectional macro morphology of the welded joint were analyzed,the factors affecting the welding quality were clarified,and the welding process was optimized.The research results show that:the laser power(P)mainly has a significant effect on the weld penetration size.The defocus(f)mainly determines the size of the spot.When f=-10mm,it is suitable for the welding of the backing bead,and when f=-20mm,it is suitable for the welding of the filling bead.The welding speed(V_w)is closely related to the wire feeding speed(V_f).When V_w=0.5m/min is combined with V_f=0.5m/min for welding,the filling effect and weld formation are better.The upper surface is"concave",which is conducive to the spreading of the next filling metal in the narrow gap multi-pass welding.Within the research scope of this thesis,the welding pass of S32205duplex stainless steel pipe joint with 25mm wall thickness is composed of 7 passes,and the optimal process parameters after optimization are as follows:P=4.4-5.0k W,f=-10～-20mm,V_W=0.5m/min,V_f=3.0-4.5m/min,Q=20L/min.At this time,the welding seam is well formed,and there are no welding problems such as pores,side walls and bottom not being fused,the filler metal is spread evenly,the welding seam is full and the transition is smooth.The microstructure analysis of the welded joint shows that the austenite in the weld can be divided into three kinds of morphology,and has obvious growth direction.As the weld cools gradually,austenite precipitates preferentially at the boundary of ferrite grains,forming dendritic and crisscrossed grain boundary austenite,and then gradually diffusing into the interior of ferrite grains to form the widmannstatten structure and sparry,smaller size of intracrystalline austenite.The proportion of ferrite to austenite in the root pass is about58.4%:41.6%,the proportion of ferrite to austenite in the build-up pass is about 57.2%:42.8%,the proportion of ferrite to austenite in the cover pass is about 54.3%:45.7%,the proportion of ferrite to austenite in the overlap area is about 42.7%:57.3%,and the proportion of ferrite to austenite in the heat affected zone of welding joint is about 61%.The average two-phase ratio of the joint with the wall thickness of 25mm is 53.5%:46.5%,which tends to be 1:1compared with the traditional fusion welding,indicating that the joint has good corrosion performance.The comprehensive mechanical properties study showed that the average hardness value of the joint was 262.9HV,and the peak value was 276.1HV located in the overlap area of the filler bead and the HAZ area of the welded joint,which was less than 290HV.It met the relevant standards and the performance reached the standard.The ferrite content in the microstructure of the welded joint can affect the comprehensive mechanical properties of the joint to a certain extent,especially the hardness of the joint is more obvious.The precipitation of harmful phases will accumulate in the weld area,which will also increase the hardness of the part.The tensile specimens were all fractured at the base metal position,indicating that the tensile strength of the weld was higher than that of the base metal.Low temperature impact toughness has reached the standard,meet the performance requirements,comprehensive mechanical properties are good.The corrosion resistance research results show that the corrosion rate and the degree of self-corrosion in the weld area(root pass,build-up pass,and cover pass)are similar to those of the base metal.Both the cover weld and build-up weld have a slight passivation effect,and their compactness is similar to the passivation film of the base material,and the corrosion resistance is good.When the cover weld bead is in preferential contact with the corrosive environment,the corrosion resistance of the joint is guaranteed. |
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