Research On X65Pipeline Steel GMAW Process Under High Pressure

Underwater welding is of great significance for the development of marineresources and facilities maintenance. It has the advantage of cost and permanentconnection. Dry hyperbaric gas metal arc welding (GMAW) can adapt to theconstruction in deep water. Dry hyperbaric welding is the most promising method ofunderwater welding. At present, lots of X65pipeline steel is used in submarine pipelinesystems in China, most of the pipeline has been in service for many years, they will facereplacement and maintenance problems. So it is necessary to research how thistechnology be used on X65pipeline steel. In this paper, welding experiment wasconducted in a high pressure chamber under the pressure of0.4-1.2MPa, using differentprocess parameters. The author analyzes the microstructure and mechanical propertiesof welded joints to study on the effects of various factors.At first, the author researched on GMAW process by the welding test under highpressure, studied on the effects of power polarity, pressure, process parameters andshielding gas on welded joints. When welding is connected by way of DCEN, the weldsurface forming is better than DCEP. The welding spatter generated easily whenwelding connected by DCEP. Pressures have influence on weld joints. When ambientpressure is low(0.8MPa or less), weld penetration and width increases as theenvironmental pressures increasing. But when pressure increases to1.2MPa, weldpenetration and width will decrease. The influence of process parameters and shieldinggas under high pressure is as same as atmospheric pressure. Then the author researchedon GMAW process on X65pipeline steel, based on the welding process which wasstudied before.Under high pressure, the author conducted welding tests by different processparameters and protective gas. In this paper, the author analysis the differences of thesurfaces、microstructure and mechanical properties of welding joint. The results showthat pressure does not change the appearance of weld joints very much, but processparameters and shielding gas have more influence. The microstructure of weld containsacicular ferrite, eutectoid ferrite and a small amount of upper bainite. When pressureincreases, the bainitic increases, and eutectoid ferrite change from bulk to the needle.The microstructure of CGHAZ contains a large number of martensite and bainite, whenpressure increases, bainite increases.In this paper, the author measured the hardness、tensile strength and Charpy impactenergy of the joints welded under different conditions. The results show that thedistribution of the hardness does not change a lot, but the hardness of the joints welded under protection of mixture is slightly reduced. Tensile strength of the weld metal isgreater than the base metal. Moreover, the impact toughness reduces when the weldingheat input increases or reactive gas is added in the shielding gas. According to theresults of the analysis, a optimization of GMAW welding process of X65pipeline steelunder high pressure environment was proposed. The author verified the feasibility of theoptimization program by analyzing the microstructure and mechanical properties of thewelded joints.