| Grade X80 pipeline steel,with high-strength and good toughness,has been widely used in the projects of West-to-East Natural Gas Transmission.Multi-wire spiral tandem Submerged Arc Welding（SAW）is the key technology for fabricating linepipes in industry.After the longitudinal spiral weld seams of Grade X80 steel linepipes have been welded by multi-wire tandem SAW,grains become coarsening in heat affected zone,and the weld joints get softening.In this study,the effect of welding parameters,fluid flow and heat input on the microstructure and mechanical properties of the weld joints are investigated by means of numerical simulation and welding experiments,so that the optimized welding procedure and technical measures for improving the performance of joints are put forward.Thus,it is of important theoretical significance and practical application value.Grade X80 pipeline steel with thickness 18.4 mm was selected as base metal to carry out the welding tests.X-type groove with root face was designed,and double-wire tandem spiral submerged arc welding process was used to join the inner weld seam and triple-wire tandem spiral submerged arc welding process was used to join the outer weld seam.For the welded specimen,macro-morphology and microstructure have been observed,and macro-hardness,tensile test and Charpy impact have been measured.The results show that the microstructure of the weld metal is mainly consisted of acicular ferrite and a certain amount of polygonal ferrite.The microstructure of the fusion zone is transformed from fine grain in the weld zone into coarse grain in the heat affected zone and the grain shape is highly irregular.Coarse grained heat affected zone primarily is composed of granular bainite and lath-shaped bainite,which are distributed on the ferrite matrix and have large grain size.Fine grained heat affected zone is mainly composed of ferrite and bainite which are fine and uniform.The hardness distribution of the joint exhibits the type of"W".The weld zone exhibits higher hardness,meanwhile the heat affected zone exhibits lower hardness with softening.The maximum tensile strength of the welded joint is from 710 MPa to 730 MPa.The average impact energy of weld zone is above 190 J.The tested tensile strength and toughness of the weld joint meet the requirement of the quality.Dynamic evolution of temperature field and molten pool morphology during the unsteady-state of spiral submerged arc welding with double and triple wire are analyzed.Due to the accumulation and superposition of energy,the temperature is rising and the melting depth and width are increasing.Double-wire spiral SAW and triple-wire spiral SAW achieved quasi-steady state at 2.4 s and 3.0 s,respectively.During the initial stage of continuous cooling,the temperature decreased rapidly and the morphology of molten pool changed slowly,and the cooling rate decreased gradually.2.2 s later,the pool for double-wire spiral SAW is disappeared,and 2.7 s later,the pool for triple-wire spiral SAW is disappeared too.Temperature field,flow field and molten pool morphology during the quasi-steady state were predicted and analyzed.The results show that,with the increase of the number of welding wires,the depth,width and length of the weld pool increase correspondingly,and the whole temperature field distribution and the weld pool morphology are elongated along the welding direction,however,the temperature gradient decreases.On the upper surface of the workpiece,the molten metal flow from the high temperature region to the low temperature region under the action of surface tension,and the velocity vector go from the center of the heat source to the rim of the weld pool.In the longitudinal section of the workpiece,doubler-wire spiral SAW and Triple-wire spiral SAW appear four and five vortices respectively.In the cross section of workpiece,both of them have two vortices.The curves of welding thermal cycle for a series of point along the weld centerline or perpendicular to the weld seam were analyzed.Based on the calculated curves,the cooling time t8/5 and the average cooling rate wc can be calculated.Compared with the diagram of SH-CCT for grade X80 pipeline steel,the microstructure and macro-hardness of the heat affected zone were predicted,and the predicted results were in good agreement with the experimental results. |
