Effect Of Parameters On Anticorrosion Performance Of Ultrasonic DC Pulse TIG Welding Under High Pressure Environment

As the length of submarine oil and Gas Pipelines Increases,the severe marine service environment and the trend of extending to the deep sea force the development of pipeline steel to the direction of large diameter,high strength and toughness.Underwater welding technology plays a vital role in the construction and maintenance of offshore platforms and the laying and maintenance of submarine pipelines,but the welding technology of pipeline steel in our country lags behind the development and laying requirement of pipeline steel.The quality of high pressure dry underwater welding is better than wet underwater welding,the utility model can avoid the influence of unfavorable factors,such as water,has wider application and better prospects.Current commonly used welding processes and technical conditions,the welding ARC will be unstable in high pressure welding,defects such as Blowhole,cold crack,local embrittlement of HAZ,softening of HAZ and coarse grain are easily produced in welded joint of pipeline steel.Seawater has strong electrolytic property,which greatly affects the corrosion performance of welded joints and the service life of seawater environment.The magnetic field generated by ultrasonic TIG welding can make the arc energy more concentrated,more rigid,more penetrating,the stirring effect of the welded joint pool is enhanced during the welding process,which accelerates the flow of the welded joint pool during the welding process,refines the grain of the welded joint,to improve the forming quality of the welded joints and the welding efficiency.In this dissertation,the effect of gas pressure on the corrosion resistance of X80 ultrasonic TIG welding head is studied by electrochemical tests and microstructure.Research shows:Under0.1MPa～0.7MPa gas pressure,as the gas pressure increases,the denseness and uniformity of the weld zone organization decreases,the corrosion resistance decreases,the corrosion tendency increases;The microstructure of the weld area is mainly composed of needle-like ferrite and polygonal ferrite,with a small amount of granular bainite,the organization of the weld area is fine and uniform.With the increase of gas pressure,the content of needle-like ferrite in the weld zone tissue decreases and the grains become more coarse,while the polygonal ferrite gradually increases.In this dissertation,the effect of corrosion performance of X80ultrasonic TIG welding head weld zone is studied by electrochemical tests at 0.1MPa and 0.7MPa gas pressure through four welding process parameters:pulse frequency,duty cycle,welding speed and peak current.Research shows:When the pulse frequency,duty cycle and peak current are increased,the open circuit potential and impedance modulus of the weld zone increase,the self-corrosion current density decreases,i.e.,the corrosion resistance of the weld zone is enhanced;When the welding speed is increased,the open circuit potential and impedance modulus of the weld zone decrease,the self-corrosion current density increases,i.e.,the corrosion resistance decreases.In this dissertation,orthogonal tests were designed at 0.1MPa and0.7MPa gas pressure using L₁₆（4⁵）for four parameters such as pulse frequency,analysis of the weight of the effect of the four parameters on the corrosion resistance of the weld by conducting polar and variance on the corrosion rate data,resulting in pulse frequency as the main influence factor,duty cycle,welding speed and peak current as secondary influencing factors;When frequency is 35k Hz,duty cycle is 40%,welding speed is 30cm/min and peak current is 180A,the theoretical corrosion rate is the minimum,which are the best parameters for welding.This dissertation systematically investigates the corrosion performance of each partition of the ultrasonic DC pulse TIG welding head for X80 pipeline steel with optimal parameters.Research shows:The corrosion tendency in the weld zone of the best parameter joints under0.1MPa and 0.7MPa gas pressure is lower than that in the heat affected zone and the base material zone;The open-circuit potential,self-corrosion potential and impedance modulus values of the joints corresponding to each zone under different gas pressures do not differ significantly,ultrasonic TIG welding has good resistance to the negative effects brought about by the increase in ambient pressure;The microstructure of the weld zone of X80 high-strength wireline steel ultrasonic TIG welding head is mainly needle-like ferrite and polygonal ferrite,as well as a small amount of granular bainite,the organization of the weld zone is fine and uniform,the meshing between the tissues is closer,which not only enables the weld zone of X80 wireline steel to have good strength,toughness and the ability to resist crack extension,but also enables the weld zone to have good corrosion resistance and other mechanical properties;The organization of the heat-affected zone compared to the weld zone,the organization of the needle ferrite content is reduced,polygonal ferrite content increased,its needle ferrite grains are coarser than the needle ferrite grains in the weld zone,the uniformity of the tissue distribution is reduced,so the X80 ultrasonic TIG welding head weld zone has the best corrosion resistance,the heat-affected zone is the second,the base material zone is the worst;Using the percentage of each element and its mass in the EDS experimental test results,it is inferred that the tissue product of X80 super-audio TIG joints is mainly Fe CO₃,with a slight amount of Fe（CH₃COO）₂;The pitting pits in the weld zone are very few and small,the heat-affected zone and the base material zone have more pitting pits and a greater depth,the pitting pits are mainly circular,no obvious directionality,the degree of immersion corrosion in the weld zone is significantly lower than the heat-affected zone and the base material zone,while the difference in the degree of immersion corrosion between the heat-affected zone and the base material zone is not significant,i.e.,the X80 ultrasonic TIG welding head has the best corrosion resistance in the weld zone.