Microstructure And Properties Of High Grade Pipeline Steel Laser And Laser-Arc Welded Joints

With the increase of industrial demand for oil and natural gas and the progress of metallurgical technology,pipeline steel is developing towards for high grade pipeline steel.But the traditional multi-pss arc welding of high grade pipeline steel,the FZ(Fusion zone,FZ)and HAZ(Heat Affected Zone,HAZ)softening,grain coarsening and necklace M-A constituents formed near OAGB(Original Austenite Grain Boundary,OAGB)in IC-CGHAZ(Intercritically Coarse-Grained Heat-Affected Zone,IC-CGHAZ)lead to insufficient strength and toughness of welded joints.Therefore,exploring the new welding technology of welded joints is the primary task to promote the popularization and application of high grade pipeline steels.In this paper,The experiment of the high grade pipeline steel welded joint was carried out by welding thermal simulation technique,laser welding and laser-arc welding technology,and the evolution of microstructure and properties of welded joints was analyzed.The main conclusions are as follows:(1)The thermal cycle of CGHAZ(Coarse Grain Heat Affected Zone,CGHAZ)of high grade pipeline steel under different heat input was simulated by Gleeble 1500.The mechanical properties of the samples were analyzed.With heat input increased,the LB(Lath bainite,LB)was to GB(Granular Bainite,GB),and the OAG(Original Austenite Grain,OAG)was coarsen obviously.The volume fraction of M-A constituent and length of more than 1?m M-A constituent increases gradually.The impact toughness reduction can reach 88% in all the heat inputs due to the coarsening of M-A constituent and OAG,and the formation of necklace M-A constituent that segregated in OAGB.(2)The high grade pipeline steels were welded by IPG YLS 6000 fiber laser to study the influence of heat input on the microstructure and properties of welded joints.(1)The microstructures of the FZ,CGHAZ and the FGHAZ(Fine Grain Heat Affected Zone,FGHAZ)are LM(Lath Martensite,LM)under three heat inputs,the hardness of the three regions is higher than that of the BM(Base Metal,BM).The microstructure of the MGHAZ(Mix Grain Heat Affected Zone,MGHAZ)is composed of ferrite and bainite,its hardness is lower than the BM forming softening area.The strength of laser penetration welding joint reaches the base metal level,and the impact fracture modes of weld and heat affected zone are typical ductile fracture.(2)Laser welding of high grade pipeline steel in air and nitrogen environmen is feasible.The molten pool oxygen will be fixed by deoxidizing elements,formed 1?m harmless inclusions in rapid cooling,nitrogen can be dissolved in the weld,and the FZ hardness will be enhanced in the form of solid solution strengthening.The impact toughness of the weld under air environment is still better than that of the BM,which reaches 80% of the argon atmosphere.The cooling rate after welding is slow in the air environment,and the microstructure of the weld is mainly GB,which leads to a slight decrease in the impact toughness of the FZ.(3)Laser-MAG welding of high grade pipeline steel was carried out,and the influence of heat input on microstructure and properties was explored under the condition of low strength matching between the wire and the BM.(1)With heat input increased,the microstructure evolution of FZ and CGHAZ was LM to GB,and the hardness of FZ is gradually reduced,but it is still higher than BM.(2)Laser welding can obtain a large depth to width ratio.The FZ and CGHAZ of microstructure are LM and GB of laser-MAG welding,and laser welding of the FZ and CGHAZ are LM.The hardness and toughness of the laser-MAG welding welding joint are lower than those of laser welding.