Numerical Modeling Of Coupling Electricalthermal Fields And Thermal-stress Fields And Study On Joint Performances Of Flash Butt Welding Of X65 Pipeline Steel

The strong-coupling triple fields of electricity,temperature and stress during the flash butt welding of X65 pipeline steel pipes,as well as the mechanism of defects forming in joints,are analyzed by using experimental study and numerical simulation,which provides basic data and theoretical instruction for the application of flash butt welding technology in joining of high grade pipeline steel pipes.A multi-coupling numerical model of electricity,temperature and stress on flash butt welding is established to investigate the electric,thermal and stress behaviors in flash butt welding of X65 pipeline steel pipes.While taking temperature-depended physical properties and phase transformation into consideration,and using the birth-death element method and the element entity layered loading technology to simulate the flash burning and heat production,the temperature distribution in the flashing stage and temperature variation of welded joints at different flashing speed conditions are obtained in the electro-thermal model for the flash butt welding of X65 pipeline steel pipes.In the thermal-stress model,the deformation of welded joint at the upset stage is simulated based on the rigid-viscoplastic contact analysis.Therefore the stress-strain curves of welded joints and the displacement variation of particles in the end section at cases of different forging allowance is predicted.Influences of welding parameters on thermal and stress behaviors of welded joints are studied by using these models,and then optimized welding parameters of flash butt welding of X65 pipes are achieved ultimately.Analysis of mechanical properties,microstructure and welding defects of flash butt welded joints of X65 pipeline steel were carried out.Results of metallographic study indicate that coarsening of grains in welding zone(WZ)and partial heat affected zone(HAZ)takes place as a consequence of strong heat input from flashing,thus widmanstatten structure forms in these area,which reduce the impact toughness of joints.Mechanical tests show reasons for the decrease of plasticity and toughness of welded joints come from the transformation of microstructures in joints.However,the tensile strength of joints increase because the forging pressure causes a higher dislocation density in joints.Improper welding parameters could lead to the formation of over burning,slag inclusion and falt spot defects in welding zone.Intensive cracks distributed in and at the edge of gray spot defects in fracture surfaces result in a brittle cleavage fracture of welded joints.