Study On Microstructure And Property Of High-strain Pipeline Steel Welding Heat Affected Zone

The use of high strength pipeline steel is the inexorable trend for long distance petroleum or gas transmission pipeline, which offer major cost benefit. The cost benefits arise from a combination of reduced materials and construction costs and the ability to operate the pipeline at higher operating pressure thereby reducing the pipeline diameter required for a given throughput. To prevent the failure of pipeline, high-strain pipeline steel base on stain design are studied. To meet the requirement of high pressure, large capacity transmission, high-strain pipeline steel has the ability to prevent the buckling, destabilization. Welding is the key process for the pipeline construction. There may have the softening and coarse grain problems in the welding coarse grain zone. These problems may result in the mismatch between the welded joints and pipeline steel.The microstructure, micro hardness and toughness in coarse grain zone of high-strain pipeline steel X80were studied by means of welding thermal simulation. Simulated heat affected zone continuous cooling transformation curve of X80pipeline steel was measured. Influence of cooling rates after welding on the microstructure and properties in coarse grain zone were investigated. The results illustrated that there were mainly four types of microstructure in coarse grain zone of high-strain pipeline steel X80:polygonal ferrite(PF) and a small quantity of pearlite, granular bainite, bath bainite and bath martensite. coarse polygonal ferrite(PF) and a small quantity of pearlite were obtained in coarse grain zone when cooling rates were in the range of0.05℃/s～0.5℃/s。While t8/5was during1℃/s and5℃/s, mainly granular bainite(GB) was obtained in coarse grain zone of X80. When t8/5was about10℃/s and30℃/s, bath bainite was more likely to generate. While t8/5was higher than30℃/s, bath martensitic microstructure was appeared in coarse grain zone. he softening in coarse grain zone is ubiquitous problem for high-strain pipeline steel X80. While cooling rate after welding was between15℃/s and30℃/s, mainly bath bainite, which had the better microhardness, charpy toughness in room temperature and-10℃, was obtained in coarse grain zone of high-strain pipeline steel X80. The best hardness and toughness were obtained while the cooling rate after welding was20℃/s.