X80 High Strength Pipeline Steel Weled By Ultrasonic Pulse TIG Welding

The harsh service environment of oil and gas pipelines drives the development of pipeline steel in the direction of toughness and high strength.Compared with the rapid development of pipeline steel,the development of welding technology is relatively lagging.Under the existing welding technology,the welded joints of high-strength pipeline steel are prone to HAZ coarsening and softening,HAZ partial embrittlement,porosity and other welding defects,which limit the wide application of high-strength pipeline steel.The ultrasonic pulse TIG welding arc has high stiffness and strong penetration force,and the Lorentz force excited by arc ultrasonic has the effect of stirring and vibrating on the welded melting pool,which can improve the forming quality of weld seam,the welding speed and refine the weld grain.Therefore,this paper has carried out a research which works on the ultrasonic pulse TIG welding process for X80 high strength pipeline steel.The influence of pulse current frequency on current waveform at 20kHz and 40 kHz is analyzed by using electric signal acquisition.It is found that the current waveform under two pulse frequencies is rectangular square wave,the arc can burn stably,and the change rate of pulse current is no less greater than 50A/us.It shows that the self-developed ultrasonic pulse TIG weld power supply can realize the demand of high-current welding.The high-speed camera acquisition system is used,and the influence of pulse current frequency on the shape of ultrasonic pulse TIG welding arc is analyzed by image processing technology.With the increase of the pulse DC frequency,the arc undergoes radial contraction,and the arc radial contraction is about0.65mm compared with 20 kHz and 40 kHz.Taking 10mm thick X80 high strength pipeline steel as the research object,the plate surfacing test is carried out to analyze the influence law of peak current,base value current,welding speed,duty gap and pulse frequency on weld forming.The results show that the increase of peak current,base current,duty cycle and pulse frequency will lead to the increase of weld penetration width;as the welding speed increases,the weld penetration decreases.The V-groove of 10mm thick X80 pipeline steel is under-welded.The results show that when the peak current I_p=220A,the base current I_b=120A,the duty ratio?=60%,the pulse frequency f=50kHz,the speed V=0.2 m/min and the wire feed speed Vs=2500 mm/min,the front and back sides of the weld are stably formed.The V-groove of 10mm thick X80 high-strength pipeline steel plate is welded in turn by bottom welding,manual filling welding and manual cover welding.The welded workpiece is machined and the quality of welded joint is tested.The results show that the microstructure of the ultrasonic pulse TIG welding weld zone is composed of needle-like ferrite and granular bainite,and its grain size is thin;The coarse-grained zone of the heat-affected zone is composed of bainitic ferrite and granular bainite.Compared with the parent metal,the grain is coarse,but the M-A component is reduced.The mechanical properties of the welded joint are tested.The hardness of the weld zone is 350 HV,the hardness of the heat affected zone is 173 HV,and the average tensile strength is 721 MPa.The impact absorption of the weld zone and the heat affected zone are 156.3 J and 174.6 J,which the welded joint has good strength and toughness.The research work shows that the Lorentz force induced by the arc of ultrasonic pulse TIG welding has the effect of stirring vibration on the weld pool,which can refine the weld grain and improve the welding quality.The ultrasonic pulse TIG welding is a new,high quality and efficient welding process that can be applied to X80 pipeline steel welding.