Research On Thermal Accumulation Due To Butt Welding Of Coiled Tubing With Thin Wall Thickness And Large Diameter

Working as oil and gas production string, coiled tubing has characteristics of long length and should sustain lower radius bending, very harsh environment and complicated loading. The material of coiled tubign should have great mechanical properties. Welded joints are usually fractured earlier under complicated loading condition. The previous research results show that welding thermal will accumulated in heat affected zone (HAZ) because of smaller diameter and thiner wall thickness, which will increase the size of HAZ, decrease the properties of HAZ and introduce softening in HAZ even with lower heat input. The softening zone will become the greatest threatening to security and bearing capability. So the research about thermal accumulation effect of butt welding of the coiled tubing is very meaningful.This research is focused on the welding temperature field and thermal cycle of coiled tubing through experimental and finite element methods.Through investigating the thermal cycle of coiled tubing butt weld joint, thermal accumulation evaluating indicators are presented, including sencondery peak temperature (?), secondary cooling time (?) and secondary remaining time above600℃(?) during secondary heating procedure.The research objects are CT80grade coiled tubing with dimensions050.8X4.44mm and038.1x2.7mm. Welding temperature field and thermal cycle, microstructure and properties of HAZ are studied through experimental method and finite element method(FEM). The analysis results about thermal cycles obtained through experiment and simulation show that the developed FEM model can reveal the actual situation of TIG butt welding.Thermal accumulation is obvious in340°-0°-100°zone along the whole circurmferencial welded joint, and the most server thermal accumulation appears at the location of arc start point (named as0°location), where the accumulation indexes are(?=1278℃,(?))=20s,(?)=30s. At this location, coarse grain zone, recrystallization zone, partially recrystallization zone and temperd zone are distributed in zones of4mm-6mm,6mm-8mm,8mm-9mm,9mm-10mm respectively along longitudinal direction. The diameter and wall thickness influence thermal accumulation remarkably, secondary peak temperature will appear earlier with small diameter, and the thermal accumulation indexes beacome higher with lower ratio ofdiameter to thickness. Results of micro structure and hardness in HAZ show obvious sotfeningin HAZ induced by thermal accumulation.