| As the most economic and efficient way of transportation, pipeline steel for petroleum and natural gas has been in using today. With the transformation of mining area, the complicated geological environment and various climate such as ocean,desert, permafrost areas become a region of pipeline laying. Under the special environment,the pipelines not only need high hardness and high tenacity, but also need to have great deformation, welding performance and excellent corrosion resistance to satisfy increasingly rigorous demand of surroundings. At the same time we can’t ignor the performance of welded joint, because, as the weakest of pipeline steel which is extremely easily damaged in the process of using. As a special environment with pipeline steel, submarine pipeline steel and large strain of pipeline steel need have excellent ability of large deformation, crushing resistance, and welding performance in common, but, the difference is that submarine pipeline steel needs to consider the effect of corrosion of sea water, for having long time under low temperature, high pressure, strong corrosion of the bottom of the sea. The paper is concerned with the base metal and welded joint of X65 submarine pipeline steel and large strain thick specifications of X80 pipeline steel. By means of tensile test, charpy impact test, DWTT and hardness tests, and combined with the analysis results of SEM, EDS, optical microscope, strength and toughness and corrosion resistance of X65 submarine pipeline steel and large strain thick specifications of X80 pipeline steel have studied. The results of the study are as follows:Microstructure and mechanical properties of X65 submarine pipeline steel and weld joint were characterized using tensile tests. The results prove that longitudinal and transverse of base metal has showed higher elongation, low yield ratio, it has a better deformation performance. At-20° temperature, base metal, weld, fusion line and fusion line + 2 mm have been tested for Charpy V-notch,and the results indicated that the welded joint has the excellent toughness. Lastly, using scanning electron microscopy(SEM) observe microstructure of base metal, weld and heat affected zone.The results of experiment indicated that base metal combined with ferrite plusbainite, which has great mechanical properties; Microstructure of weld formed by intracrystalline nucleation ferrite which induced by inclusion, has higher strength and toughness. Affected by the thermal cycle of welding, the heat affected zone of coarse crystal area primarily consisted of granular bainite. Due to the decomposition of M-A island, the hardness of base metal closed to the heat affected zone has reduced.In this work, the corrosion resistance of the welded joint and base metal of submarine X65 pipeline steel with ferrite plus bainite dual-phase microstructure was investigated by electrochemical and immersion tests. The corrosion product films were characterized using X-ray Diffraction(XRD), Scanning Electron Microscopy(SEM) and Optical microscopy. To research the development of pitting corrosion in each zone, pitting diameter statistics were finished under optical microscopy and the forming mechanism of pitting induced by inclusion was analyzed. The experimental results proved that the corrosion resistance of the welded joint was better than that of the base metal, both from the general corrosion and pitting corrosion standpoint. In the welded joint, the weld metal has the best corrosion resistance, and then was the heat affected zone(HAZ). However, there was little difference between parent metal(near HAZ) and base metal(far HAZ) on the corrosion resistance. XRD and EDS analysis showed that the composition of the outer rust layer was mainly composed of iron oxide(Fe2O3), magnetite(Fe3O4), ferric hydroxide(Fe OOH) and iron hydroxide(Fe(OH)3), and the inner layer was consisted of iron oxide(Fe2O3), magnetite(Fe3O4)and a little ferric hydroxide(Fe OOH). Combined with the microstructure of the each zones, it can be concluded that the best corrosion resistance of the weld metal attributes to the refined grain size. Due to a creation of a galvanic effect between the ferrite and bainite, the dual-phase microstructure of the base metal and base metal closed to the heat affected zone displayed weak corrosion resistance.The mechanical properties and microstructure of the welded joint of large strain thick specifications of X80 pipeline steel using the straight seam submerged arc welding technology were studied. The experimental results show that the large strain thick specifications of X80 pipeline steel has the uniform microstructure and great mechanical performance, which can completely meet the requirements of related engineering standards; In addition, weld metal and heat affected zone havenormal microstructure and possess good strength and toughness after welding. Due to the second thermal cycle influence, secondary intracrystalline ferrite has generated in inside weld,and the strong toughness of welded keeps great. As far away from the outside weld, the microstructure of heat affected zone coarse grain zone of the inside weld have changed, which contains the appearance of a chain distribution organization of M- A block that near the inside weld area, in addition,this organization will weaken the toughness of heat affected zone. |
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