The Effect Of Laser-MAG Hybrid Welding Parameters On The Microstructure And Properties Of X90 Pipeline Steel Joints

X90 pipeline steel has become an ideal choice of next generation pipeline steel due to its high strength,excellent toughness.Welding is the key technology of pipeline steel's production.As an advanced welding technology,hybrid laser-arc welding has the advantages of superior welding stability,deeper welding penetration and low production cost.Those merits are aspects of significant interest in the welding of medium thickness metal materials.The effects of different welding parameters on microstructures,mechanical properties and corrosion performance of X90 pipeline steel of the joints were investigated in this paper.We expect that valuable theoretical references and technical guidance in the application of actual production can be provided in this paper.X90 steel Overlaying on the plate test of laser welding(laser power PL:1.5-3.5 kW),MAG welding(wire feeding speed VM:6.7-9.0 mm/s)shown that depth of fusion,weld width,and excess weld metal presented a positive correlation with PL and VM.Applying hybrid laser-arc welding overlaying on the plate test(PL:2.5-3.6 kW,VM:6.7-9.0 mm/s)on this condition.When VM was 6.7 mm/s and PL was 3.6 kW,the fusion zone had superior apperance,meanwhile,plates of 6mm was completely penetrated.The weld of X90 plate hybrid laser-arc butt welding with different welding parameters PL(1.0-2.5 kW),VM(8.5-9.5 mm/s)had favourable apperance.The joints of laser-arc welding presented a macroscopic feature of "wine cup like",chemical elements kept smooth in the nearby zone of fusion line,and the phase structure of weld was a-Fe.The top weld zone mainly included fine massive ferrite(MF),acicular ferrite(AF),granular bainite(GB)and M-A;The microstructure of the middle fusion zone mainly included MF,AF and a small amount of bainite was generated.There was residual M-A in the internal of MF;In the bottom of fusion zone emerged massive MF,residual M-A was found in the internal of grains,at the same time,lath bainite(LB),GB and a small number of pearlite(P)was discovered.In coarse-grained zone,there had AF,LB,GB,a small number of P and M-A was distributed in internal of massive austenite grain;The microstructure of fine-grained zone mainly included MF,GB,and a small amount of P and M-A;mixed-grained zone mainly included the original organization of LB and ferrite,among them distributed fine ferrite and a small quantity of P.The width of heat-affected zone of weld joints shown a positive correlation with PL and VM,the welding parameters had much lower impaction to phase type of different locations in weld joints.An increase of PL always causes a decrease of AF in fusion zone,but the grains of coarse-grained zone had a tendency of growing up.The bigger VM,the more massive gains of weld and coarse-grained zone were obtained.Weld joints all presented ductile fracture,but compared with basic metal,percentage elongation after fracture and tensile strength appeared a decrease to a certain extent.Tensile strength,yield strength and percentage elongation after fracture presented an inverse relation with PL and VM.Sample 1(PL:1.0 kW,VM:8.5 mm/s)had approximate tensile strength and yield strength with basic metal,of the order of 816.6 MPa and 741.6 MPa,respectively.But its percentage elongation after fracture only amount to 41.6%of basic metal.Sample 4(PL:1.0 kW,VM:9.0 mm/s)had minimum tensile strength and yield strength,of the order of 763.3 MPa and 693.3 MPa,respectively.Sample 5(PL:1.0 kW,VM:9.5 mm/s)had minimum percentage elongation after fracture,only 6.13%.In top and bottom edge of weld joint,hardness which range from the center of fusion zone to basic metal presented the tendency of descending first and then ascending.The lowest hardness zone was heat-affected zone,the hardness from top to bottom of central zone of fusion zone presented the tendency of increasing first and then decreasing and then increasing.Among 5 group samples,the softening of sample 4 heat-affected zone was the most serious,reduced to 249 HV.The results of impact test indicated that impact toughness of weld joints which were welded under 5 group parameters all lower than basic metal,and presented an inverse proportion relationship with PL and VM.The impact absorbing energy of sample 1 was the highest,of the order of 239.10 J,reached 81.0%of basic metal.The impact absorbing energy of Sample 5 was the lowest,only reached 58.9%of basic metal.Fracture mode of weld joint fracture was micropore aggregation,there was a small amount of sulfide of Mn precipitated in base of basic metal dimples and a small quantity of carbide or oxide of Ti precipitated in base of joints dimples.With the increase of PL and VM,the size of dimples decreased,depth became shallower,but there was no brittle fracture region appeared.There was no obvious passivation behavior for basic metal and fusion zone.Icorr of basic metal was lower than weld;Rct of basic metal was higher than weld.With the increase of PL,Icorr of weld presented the tendency of increasing first and then decreasing,sample 3(PL:2.5 kW,VM:8.5 mm/s)<sample 1<sample 2(PL:1.5 kW,VM 8.5 mm/s);Rct of weld presented the tendency of decreasing first and then increasing,sample 3>sample 1>sample 2.With the increase of VM,Icorr,of weld was gradually increased,sample 1<sample 4<sample 5;Rct of weld was gradually decreased,sample 1>sample 4>sample 5.In summary,the sample 1(PL:1.0 kW,VM:8.5 mm/s)had the best comprehensive mechanical properties,and the corrosion resistance of sample 3(PL:2.5 kW,VM:8.5 mm/s)was the best in the process parameters of the 5 groups.