| Grade X120 pipeline steel,which has ultra-high strength and high toughness,will be used to transport the oil and natural gas in the future.Pulsed Gas Metal Arc Welding(P-GMAW),as a new fusion welding technology,has the advantage of controlling the heat input.In this paper,Pulsed Gas Metal Arc Welding is adopted to weld the circumferential seam of Grade X120 steel pipelines,which is beneficial to minimize the sizes of grains in Heat Affected Zone(HAZ)and improve the quality of weldment.Both experiments and simulations are applied to test and analyze this welding process.Such as observing the macro and micro structures of weld joints,measuring the mechanical properties of weldments,analyzing temperature field and fluid flow in quasi steady state and non-quasi steady state,discussing the effects of electromagnetic force,surface tension,arc pressure and buoyancy on morphology of weld pool and fluid flow in the molten pool,and so on.It is significant for optimizing the welding parameters,improving the quality of weldment and laying foundation for the industry application of Grade X120 steel pipelines.The thickness of base metal is 16.0 mm.Compound double-V groove is fabricated and pipelines are fixed together with narrow gap,which is no more than 1.5 mm.Layer upon layer of root pass,hot pass,the 1st fill pass,the 2nd fill pass and cap pass are preheated with an oxyacetylene torch and joined with a single wire Pulsed Gas Metal Arc Welding system.Lower Bainite(B_L)and Martensite(M)are composed of the microscopic structure of the weld metal.Granular Bainite(GB),lath-shaped Bainite(LB)and lath-shaped Martensite-Austenite constituents(M-A)are found in the HAZ and the sizes of Bainite grains are greater than 25?m.The measured results of Vickers hardness for root pass,hot pass,the 1st fill pass,the 2nd fill pass and cap pass are shown in the Capital letter“W”.The hardness in the weld metal(WM)is greater than the base metal(BM),however,the hardness in the HAZ is less than the one in the BM.Therefore,it can be drawn a conclusion that the softening weld joint is caused by the coarsening grains.Temperature field and fluid flow for each pass in quasi steady state and non-quasi steady state are simulated by the commercial software Fluent.Taking the root pass as an example.After the welding arc had been ignited and lasted for 1.5 s,the quasi-steady stage has formed and the maximum temperature has risen up to 2 271 K.However,once the welding arc had been extinguished,the weld pool would be disappeared at 0.4 s.The length,width and depth of molten pool in quasi steady state are approximately 10.7 mm,5.0 mm and 4.1 mm.At the same time,there are 3 vortices along the longitudinal section and 2 vortices along the cross section.Effects of electromagnetic force,surface tension,arc pressure,buoyancy,welding current and heat input on morphology of weld metal and fluid flow in the molten pool have been discussed.Electromagnetic force and surface tension are very important driving forces.The depth of weld pool has been predominantly controlled by electromagnetic force and welding current,and the width of weld pool has been mainly affected by surface tension and arc voltage.Welding thermal cycle curves,for a series of points along the cross section or perpendicular to the weld metal,have been sketched.The nearer the distance is,the greater the influences of the heat source are.During the cooling stage,from 1 800 K to 1 600 K.a very minor cooling rate is emerged,which is caused by the latent heat of liquid-solid phase change process.Simulation results are in agreement with experimental ones very well.All these work have been done are reasonable and reliable,and all the results described in this paper are honesty and accuracy. |
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