Coiled tubing all-position automatic TIG welding process and its microstructures properties of welded joint

In order to solve the issue of unstable welding quality for manual TIG welding of coiled tubing,a kind of all-position automatic TIG welding process was designed. By using this welding technology to conduct butt welding of QT900 coiled tubing with Ф50. 8 mm×4. 11 mm in dimensions,and the joint appearance,microstructures and mechanical properties of the welded joint were analyzed. The results showed that for the coiled tubing pipe-pipe butt joint welded by this welding technology,the welds are well formed and the microstructures of weld,fusion zone and phase transformation zone are mainly consisted of equiaxed ferrite,granular bainite,granular pearlite and lath-like bainite under the effect of phase transformation recrystallization. At the same time,the grain sizes in the above zones become thinner compared with that of the base metal. The hardness of joint obviously increases,its tensile strength is improved about 2. 6% and fracture occurs at base metal. Fine-grain strength and phase transformation strength are the main factor for the increase of hardness and tensile strength.