Microstructure And Properties Of X80 Pipeline Steel Joint Welded By Ultrasonic Pulse TIG Under High Pressure

With the further laying of offshore oil and gas pipelines,the pressure and severity of pipelines are increasing.Environmental pressure and water are the most influential factors to the underwater welding process.The underwater dry welding method can be used to avoid the adverse effects of water on the welding process.The weld and heat affected zone of high strength pipeline steel are prone to the defects of uneven chemical composition,embrittlement,stress corrosion and toughness reduction.With high arc stiffness and concentrated energy,ultrasonic pulse TIG welding can bring strong stirring and vibration to the weld pool,enhance the fluidity of weld pool and refine weld grain,thereby improving the welding quality.The advantages of ultrasonic DC pulse TIG welding are used to overcome the adverse effects of high pressure environment.In this paper,the influence of main welding parameters(peak current,pulse frequency,duty cycle and welding speed)on weld formation under different pressure environments(atmospheric pressure,0.1MPa,0.3MPa and 0.5MPa)was studied.The plate surfacing experiment was carried out on a 10 mm thick X80 pipeline steel plate.The experimental results show that: when the pulse frequency changes between 10 k Hz and 35 k Hz.The increase of pulse frequency will make the weld width smaller,the weld depth deeper,the forming coefficient smaller,and the heat affected zone smaller.so the optimal pulse frequency is 35 k Hz;with the increase of welding peak current and duty cycle,the weld width and weld depth also increased,the forming coefficient is larger,and the residual current is smaller With the increase of welding speed,the weld width and penetration in the welding wire filling zone decrease,and the residual height decreases as a whole.According to the analysis of weld hardness test results obtained by different pulse frequencies under 0.3MPa and0.5MPa pressure,and the hardness value in the weld zone is the highest,followed by the fusion zone According to the experimental results,the best welding test parameters under different pressure environment are summarized: Under normal pressure,the base current is 100 A,peak current is 100 A,pulse frequency is 35 k Hz and duty cycle is 10%;under0.1MPa pressure,the base current is 100 A,peak current is 100 A,pulse frequency is 35 k Hz and duty cycle is 10%;under 0.3MPa pressure,the base current is 100 A,peak current is 120 A,pulse frequency is 35 k Hz and duty cycle is 10%;under 0.5MPa pressure,the base current is 120 A,peak current is 10%,the pulse frequency is 35 k Hz and the duty cycle is 10%.In conclusion,the V-groove welding of X80 pipeline steel was carried out according to the optimal welding parameters of ultrasonic DC pulse TIG welding under high pressure environment,and then the microstructure and mechanical properties of the welded joint were observed.Under different pressures,the microstructure of the weld zone is mainly composed of acicular ferrite with different grain sizes,which is interlocked with each other,which hinders the crack propagation and improves the strength and toughness of the weld zone..The tensile specimens of welded joints under different pressures are all broken at the base metal,and the tensile strength is about 700 MPa,which meets the requirements of tensile strength.Under the same environmental pressure,the average impact absorption energy of weld zone is higher than that of heat affected zone,but with the increase of environmental pressure,the average impact absorption energy of weld zone and heat affected zone decreases.The positive and back bending test results of the welded joint are not less than 140.It shows that the bending resistance of the joint is good.The results show that ultrasonic pulse TIG welding has the advantages of strong arc stability and energy concentration in high pressure environment.The excited arc ultrasonic will bring strong stirring and vibration effect to the weld pool which can refine the weld grain and significantly improve the performance of welded joint.It lays the foundation for the development of offshore oil and gas high strength steel pipeline laying and welding process,and also for the development of underwater X80 pipeline steel welding and maintenance of new technology to provide accumulation.