Study On Microstructure And Properties Of Pure Nickel Welded Pipe Joints Using Different Welding Methods

Nickel and nickel alloy with high strength,plasticity and malleability have good mechanical properties in cold or hot state,so it can be made into sheets,foils,wire rods and pipe products by cold or hot processing.Nickel and nickel alloy tubes are often used in alkali,chlor-alkali chemicals,organic chlorides and food processing industries,especially in alkali resistant equipments.Plasma arc welding(PAW)of high energy density and strong penetration,can achieve one side welding both sides formation of nickel and nickel alloy under the thickness of 6mm,so it has advantages in nickel and nickel alloy pipe welding.Tungsten inert gas welding(TIG)with high reliability,easy operation,stable arc and good covering ability,has extensive application in welding nickel and its alloys with good weld appearance and performance.Two different welding methods,PAW and PAW + TIG welding,were used to weld the ?112mm × 3.5mm × 6500 mm pure nickel straight seam pipes without filler material and the microstructures and properties of welded joints were analyzed and studied,respectively.The macro-morphologies of joints show that two welding methods are able to achieve single-sided welding double-sided molding for the thickness of 3.5mm pure nickel straight seam pipe.At the same time,PAW+TIG can effectively avoid the joint defects such as undercutting,depression and stomata of PAW,can obtain the good weld appearance and increase the production of straight seam pipe.The microstructure observation shows that the base material is a homogeneous and small austenite grain with the average grain size of 47.32?m;the heat affected zone of PAW joint is equiaxed crystal structure with the average grain size of 96.43?m;and the columnar crystal structure is formed in the weld metal with an average grain size of 463.57?m.Due to the remelting effect of TIG arc,the microstructure size of PAW+TIG joint is larger than PAW joint;the heat affected zone is equiaxed grain structure with an average grain size of 103.75?m;and the coarse columnar crystal structure is formed in the weld metal with an average grain size of 547.32?m.The XRD analysis results show that the phase composition of two joints is the same as that of the base metal,and the matrix is austenite.The mechanical properties show that the tensile strength of the PAW+TIG joint reaches 85.60% of the base metal and the impact energy reaches 61.58% of the base metal,which is lower than that of the PAW joint.But the value can meet the mechanical requirements of nickel pipe.The residual stress of PAW and PAW+TIG joint was measured by blind hole method and the results show that the effect of TIG arc on the weld joint was the effect of remelting and heat treatment.When compared with PAW welding,PAW+TIG welding can significantly reduce the axial residual stress and circumferential residual stress.The results of room temperature immersion and electrochemical tests show that the corrosion resistance of base metal,PAW joint and PAW+TIG joint in different corrosion media have the same trend with the range of corrosion resistance: base metal,PAW+TIG joint,PAW joint.It can be concluded that the PAW+TIG welding can meet the requirements of the mechanical properties of the nickel pipe joint,effectively avoid the defects(undercut and pore),and obtain the good weld appearance with increase the production of straight seam pure nickel pipe.What's more,the residual stress of PAW+TIG joint is obviously reduced with the better corrosion resistance.So PAW+TIG welding is an ideal method for pure nickel N6 straight seam welding.