A Novel Process For Assisting The GMAW Root Welding Using Flexible Pad

With the accelerating exploitation of the oil and gas in the South China Sea,the construction of deep-sea offshore platforms faced huge challenges,especially in the girth welding of risers and tension legs.Traditionally,an internal clamp with copper sliders is applied to achieve the efficient automatic MIG welding process of pipeline girth welds.However,the burning of copper sliders,the copper penetration of the weld bead and the poor formation of the root weld which is easy to form a step-shaped weld bead are occurred.These defects result in poor joint such that the risers and tension legs can’t meet the requirements.In this thesis,a flexible pad is placed between the work piece and the copper slider to prevent the copper slider from burning and improve the formation of the root weld.This research develops a useful exploration for improving the welding quality of the deep water offshore platform.Firstly,the FEM simulation of the GMAW root welding with a flexible pad was conducted to study the effect of the pad thickness and the thermal physical parameters on the welding temperature field.The results show that the flexible pad has significant effect on the welding temperature field.A flexible pad of typical insulation material with just 0.5mm thick can reduce the peak temperature on the surface of the copper slider from 700℃to below 80℃.Within the parameter range studied in this thesis,the variation of the thickness,density and specific heat capacity of the flexible pad have little effect on the welding temperature field,but the thermal conductivity can obviously affect the cooling speed and temperature gradient of the weld bead,which in turn affects the microstructure and performance of the weld bead.The flexible pad can increase the t_8/5of the root weld to about 10 times that without the flexible pad,which significantly reduced the cooling speed of the root weld and make the backside width wider and more uniform.Secondly,welding experiments were carried out on several typical pads.The results show that the backside width of the root weld becomes uniform if an appropriate flexible pad is applied,which is consistent with the simulation results.Metallographic analysis results show that using a flexible pad changes the growth direction of the columnar crystals of the root weld.From the perspective of the formation of the root weld,the flexible pads effectively reduces the transition angle of the welding toe and makes it smoother.Among the five tested typical materials of flexible pad,an aluminum silicate ceramic fiber papers exhibits better performance than the other flexible pads because it can obtain a smooth weld toe,a uniform backside melt width,and a good surface appearance.The preliminary results of this research show that using a thin layer of flexible pad can effectively avoid the burning of the copper slider and improve the backside formation of the root weld,but its impact on key performance indicators of welded joints such as fatigue performance still needs further analyze and research.