The Research On GMAW Weldability And Welding Procedure Of X80Steel

Based on the JU-2000E drilling platform which building by Shanghai WaigaoqiaoShipbuilding Co., Ltd. The specification of X80pipeline steel is Ф323.85mm×28.58mm.The welding procedure and joint’s mechanical properties is studied in this paper.The carbon equivalent is used to assess the cold cracking tendency of X80steel, thecalculation results show that the value of X80steel’s Pcm is0.2128%, the steel has tendencyof cold cracking. So the steel need preheating before welding. According to the Pcm value,the theoretical preheat temperature was24℃. THEY-Slit Type Cracking Test is used toevaluate the weldability of X80steel, the results indicated that there is no crack found in testspecimen under four kind of preheating temperature.The crack-resistant property of X80steel is good.LB-52U electrode is used in welding root and DUAL SHIELD II101H4M is used infiling and covering. Different kinds of preheating temperature and post weld heat treatmentare used in Welding procedure test. according to the API5L standards to evaluate themechanical properties of X80steel, the results show that under the same condition of heattreatment after welding,with the increasing of preheating temperature, the tensile strengthirregular change in same scope, there is no different on the impact toughness of weldingjoint when the preheat temperature was50℃and100℃, the impact toughness of weldingjoint was decreased obviously when preheating150℃, preheat temperature has nosignificant effect on the joint hardness distribution; Under the same condition of preheatingtemperature, The welding procedure of keep warm2hours after welding can improve theimpact toughness of welding joint, also can make the hardness distribution more smoothlyand make the mechanical property more homogeneity. Combined with offshore damp coldworking environment, Choosing the welding procedure of preheat100℃and keep warm2hours after welding as the best welding parameters.Analysis the impact toughness of different parts of welded joint, the root of weld andweld fusion line are lower than others, Scanning Electron Microscope(SEM) has been usedto observe the fracture morphology. the root of weld has quasi-cleavage fracture, the weldfusion line has max fracture with quasi-cleavage fracture and ductile fracture, localembrittlement is found in both of them. Optical microscope has been used to observe themicrostructure, the results showed that the reason of root weld embrittlement is higher heat input cause coarse grain of root weld. Pro-eutectoid ferrite distribute along the grainboundary as mesh structure. mesh structure will accelerate the crack propagation,the sideplate ferrite and lath bainite in grain will also increased the brittleness of the material; thereason of Heat affected zone embrittlement was grain coarsening, lath-shaped structurecoarsening and along the grain boundary resulting chain M-A constituent.