| The pipeline safety is directly related to the safety of the whole project. The pipeline’s leakage or damage will cause incalculable economic losses and casualties. The pipeline is connected through welding. Welding joint properties of the pipeline steel has a decisive role in the safety of pipeline. Whereas the welding joint of pipeline steel will experience a series of complicate, non-equilibrium physical and chemical process, this will result in the chemical composition segregation and microstructure defects in the weld seam and the heat-affected zone. Therefore, the welding joint often possesses a lower mechanical properties and the lower corrosion resistance. In the same time, the underwater pipeline works in a lower temperature, low-temperature impact toughness is requires for the underwater pipeline. Therefore, the outer wall of CTOD process on the oil and gas pipeline steel and composite pipe welding is of great importance.By reading literatures review, the mechanical composite tube (outer carbon steel X65and stainless steel316L) were analyzed by CTOD test, and the difficulties of the welding machine composite pipe, developed a new welding process, then the different welding process of compound pipe welded joints of welding procedure qualification test, obtained a more ideal results.(1) The average CTOD of the X65pipe line steel at-20℃is1.007mm, and meets the technical requirements. The test conditions and test procedures is accorded to the test requirements. The test result is reliable.(2) The mixed shielded gas of Ar+20%He used in welding process can improve the arc temperature and the movement of the melted metal in welding pool. This prohibited the formation of the large gas porous in the weld seam. This can improve the surface energy and avoid the formation of the defects.(3) The microstructure of the welding seam is typical columnar crystal. Disperse small gas porous is distributed in the columnar crystal. The size and the amount of the gas porous defects meet the technical requirements.(4) The chemical composition in the center of the welding seam is similar to that of the weld wire. In a narrow region nearby the welding fusion line, the chemical composition has a great difference with the weld wire for the melted base metal.(5) The hardness in the center of the welding seam is higher compared with the base metal and the HAZ. The hardness range of the weld joint meets the technical requirements.(6) The impact energy of the HAZ is fluctuate largely and lower than he base metal and the weld seam. The tensile property, bending property and the low-temperature impact property of the weld joint meet the technical requirements. |
