Pulse TIG Root Welding Method And Process For Medium And Thick Plates

In many industries with high quality requirements,such as petroleum,chemical,electric power and nuclear industry,the special vessels and pipelines made of medium and thick plates are used.Involveing the root welding problem of sealing weld,and the back cannot be welded due to the special structure,limited welding position and other factors.There may be hidden dangers of stress concentration and corrosion in the traditional root welding technology with backing plate,which will lead to degradation of weld properties.Single-side welding and double-side free forming of welds is required for high quality root welding,which is very difficult in technology.Manual TIG and SMAW used for root welding is very demanding for welders,although automatic root welding may improve welding efficiency,there are some shortcomings in welding medium and thick plates,such as limited arc energy density,low penetration capacity and efficiency.Double-sided forming of 3~6mm thick steel plate without groove and gap can be easily complated by the plasma welding,while the groove is needed in the root welding of 20 mm thick plate.Compared with the common TIG welding torch,the plasma welding torch is larger in size and less accessible,a large amount of metal will be removed from the groove,causing a great burden on the filling welding.This paper focuses on improving the energy density of the traditional TIG arc and optimizing the TIG root welding process,and proposes the superimposed root welding method of constant and high frequency pulse current.This method realises deep penetration,good accessibility and stable process,which will greatly improve the quality and efficiency of the root welding of medium and thick plate structure parts,and provide reference for the high-efficiency automatic root welding technology.In order to explore the influence of TIG arc dynamic on the forming and stability of the root welding,based on the current pulse TIG welding methods,the welding process experiments were carried out on the flat plate,and the characteristics of the pulse arc with different frequencies were analyzed and demonstrated.The thermal,mechanical and electromagnetic characteristics of pulsed TIG arc during its dynamic change are analyzed through the arc morphology characteristics acquired by highspeed camera,and the current and voltage waveforms acquired by system for acquiring signal.The high frequency shrinkage effect of pulsed TIG arc is verified,and the welding parameters that make the arc shrink most remarkably are found.When the effective current remains the same,the shrinkage effect of superimposed arc is better than single high frequency pulse arc welding,and the penetration ability is stronger.Then the superimposed TIG method was applied to the butt welding of 16 MnR medium and thick steel plate,the single-side welding and double-side forming with high efficiency and stability was obtained.Using high-speed camera system,the changes of arc morphology during root welding were recorded and analyzed,and the idea of predicting weld formation by observing the changes of arc characteristics was proposed.At the same time,the shape of arc passing through weld and the flow direction of liquid metal in weld pool were observed.By analyzing the stress state and observing the morphology of the weld pool after solidification,the morphology of the poroid weld pool in the process of root welding is constructed,which provides a foundmental for controling the singleside welding and double-side free formation.Through many experiments,the parameters of low-frequency pulse TIG,highfrequency pulse TIG,constant and high frequency pulse superposition TIG and plasma welding which can form steadily under wire-filling and without wire-filling conditions are obtained,respectively.The advantages and disadvantages of different welding method are compared,and the low-temperature impact properties,microstructure and Mn element content of the weld in each welding parameter are compared and analyzed.Reasons affecting low-temperature toughness of welds are discussed,and a feasible scheme for optimizing the constant and high frequency current superimposed root welding process is proposed.