Study On The Influence Mechanism Of Velocity And Salinity On The Underwater Wet Welding Of X80 Steel

X80 pipeline steel is a low carbon micro-alloy high strength steel,which has high strength and toughness,and good weldability.It has been widely used in the field of oil and gas transportation.This paper studies the influence of flow velocity and water salinity of X80pipeline steel underwater wet welding with X groove,The X-groove butt joints under different water flow speeds and salinities are obtained through welding tests,Observe the microstructure of the joint and test its mechanical properties and corrosion resistance,and with the help of simulation calculation to study the influence of water flow on the temperature field and flow field of the welding process.In order to better carry out the welding test,this paper designed a gravity welding device.In order to debug the gravity welding device,an underwater wet welding experiment was carried out on the Q235 steel plate using TS208 underwater welding electrode.Observing the shape of the weld seam,it is found that if the angle of the slide rail and the electrode is too large or too small,there will be defects such as uneven weld morphology and poor formation.It is finally determined that the welding seam is better formed when the angles of the electrode and the slide rail are both 70°.The X-shaped groove underwater wet welding experiment was carried out on the 6 mm thick X80 pipeline steel plate,and the welded joints under different water flow speeds and different salinities were obtained.For the welded specimen,microstructure have been observed,microstructure of the weld metal is mainly consisted of grain boundary ferrite,polygonal ferrite,side plate ferrite,acicular ferrite and small amount of granular bainite,In the heat affected zone,the microstructures mainly consisted of lath martensite and bainite.As the flow velocity increased,the percentage of side plate ferrite and acicular ferrite in the weld zone and lath martensite in the heat affected zone increased.As the water flow velocity increased,the overall hardness of the welded joint increases,the tensile properties of welded joints deteriorate,the corrosion resistance first rises and then falls,when the water flow velocity is 0.2 m/s,the corrosion resistance is best.When the salinity of the water is 3.5%,the microstructure type of weld zone and heat-affected zone remains unchanged,However,the proportion of acicular ferrite and side lath ferrite in the weld zone and lath martensite in the heat-affected zone decreased compared with freshwater.The salinity of water increased to 3.5%,the hardness value of the welded joints decreased,the tensile properties of the joints decreased,and the corrosion resistance of the welded joints became worse.The simulation results show that the time required to reach the quasi-steady state of the molten pool changes from 4.69 s when the flow velocity is 0 m/s to 5.89 s when the flow velocity is 0.3 m/s.The required time increases with the increase of the flow velocity,and the pool size decreases in the quasi-steady state.The flow velocity has little effect on the heating stage,which is mainly reflected in the cooling stage.The value of t_8/3 decreased from 5.45 s at0 m/s to 3.52 s at 0.3 m/s,and the cooling rate increased significantly.The results of the flow of liquid metal in the molten pool show that the liquid metal on the upper surface of the molten pool is mainly affected by surface tension and flows from the center of the molten pool to the edge of the molten pool.The upper liquid metal of the longitudinal section of the molten pool flows to the front and back of the molten pool,forming two circulations at the front and rear of the molten pool,and the internal liquid metal forms a clockwise circulation.The liquid metal on the cross section of the molten pool flows to the left and right sides form two circulations,The liquid metal below the arc flows downwards directly,forming a clockwise circulation on the left and a counterclockwise circulation on the right.The increase in water flow speed did not have a major impact on the flow of liquid metal,only because of the increase in water flow speed,the heat dissipation rate increased,the size of the liquid molten pool was reduced,and the circulation range was reduced.