Welding Process And Microstructure And Properties Of X80 Pipeline Steel Cold Metal Transfer Welded Joints With Magnetic Field

At present,although the main processes of pipeline steel welding(submerged arc welding,manual arc welding,gas shielded welding,etc.)can obtain relatively ideal welded joints,the problems of softening and embrittlement of welded joints caused by high heat input conditions hindering the further development of pipeline steel.Cold metal transfer(CMT)technology has the characteristics of low welding heat input and good welding stability.The magnetron welding technology can improve the wetting of the melting pool metal,reduce the defects of welding joints while refining the weld grain,so that welding joints get good mechanical properties.The introduction of CMT technology and magnetron welding technology to X80 pipeline steel welding is expected to achieve good mechanical properties of welded joints while reducing welding heat input.In this paper,three CMT welding processes for X80 pipeline steel are designed to study the influence law of root weld heat input on the structure and performance of weld and its heat affected zone.At the same time,the worst performing area in the heat affected zone is analyzed with the help of welding thermal simulation technology.Inter-Critically Reheated Affected Zone(ICCGHAZ)organization for inspection and analysis,to explore the influence of root welding heat input on its microstructure and mechanical properties;innovative introduction of magnetron welding technology to X80 pipeline steel CMT In the welding process,the welding process is optimized by using an external longitudinal alternating magnetic field,and the influence of the excitation current on the structural properties of the welded joint and its heat affected zone is obtained.The main work and research results are as follows:(1)A thermal cycle test was performed using welding thermal simulation technology to analyze the formation mechanism of the critical coarse grain region and study the influence of root welding heat input on the impact toughness of the critical coarse grain region.The analysis indicates that with the increase of root welding heat input(4.3kJ/cm～12kJ/cm),the matrix structure changes from tempered martensite to a mixed structure of tempered martensite and granular bainite,the average martensite lath width increased from 1.36μm to 2.07 μm,and the proportion of granular bainite is gradually increasing.At the same time,with the increase of root welding heat input,the M-A components distributed along the original austenite grain boundary changed from an approximately continuous distributed block structure to a chainshaped elongated structure,the proportion of M-A components in the structure was decreased from 9.7%(4.3kJ/cm)to 6.7%(12kJ/cm).The heat input in the critical coarse grain region within 4.3kJ/cm～6.4kJ/cm has higher toughness than the base metal,the highest value(4.3kJ/cm)32.6J exceeds the base metal 109.8%.(2)Welding X80 pipeline steel by CMT process,designing three welding processes for 9mm thick X80 pipeline steel for pipeline engineering,to study the influence of root welding heat input on the microstructure and properties of welded joints.According to the analysis,under low heat input(2.9kJ/cm),the structure of the root weld is mainly tempered martensite.As the heat input increases,the distance between the martensite laths increases.The heat input of 6.4kJ/cm can eliminate the effect of porosity defects on toughness.Its punching and tensile strength matches the base metal and the impact toughness reaches 75%of the base metal.(3)Based on the CMT process of X80 pipeline steel,an external longitudinal magnetic field is introduced,and the influence mechanism of the magnetic field strength on the microstructure and properties of the welded joint by changing the excitation current.According to the analysis,the longitudinal alternating magnetic field makes the arc rotate and expand and contract through the Lorentz force,which stir the melting pool and molten metal,and the molten metal is affected by the Lorenz force will lead to the melting pool convection enhancement,which improves the melting pool’s cladding performance and inhibits the production of weld defects.The applied magnetic field can control the production of pore defects within a certain range while refining the solder seam grain and eliminating the large column crystal slot in the weld tissue,improving the mechanical performance of the weld.Excitation current for 10A under weld tensile strength(687MPa)to reach the level of the base metal,impact toughness(81.9J)to reach 82.9%of the master material,to meet the mechanical performance requirements of welding.