Thermal Simulation Study On Microstructure And Properties Of X80 Pipeline Steel Automatic Welding Heat Affected Zone

X80 pipeline steel has excellent mechanical properties and good welding performance,and is widely used in high-pressure large-diameter long-distance natural gas pipeline projects.Automatic welding has the significant advantages of high welding efficiency,good forming quality and stable weld performance.It is the development direction of pipeline girth welding process.However,there is a problem of large toughness fluctuation in the heat affected zone of girth welding.It is an urgent problem to select reasonable welding heat input to ensure the good microstructure and excellent mechanical properties of the heat affected zone of automatic welding.In this paper,X80 pipeline steel is taken as the research object.The thermal simulation method is used to study the evolution of microstructure and properties of the welding heat affected zone under different heat input(t_8/5).OM and SEM characterization methods are used to carry out Charpy impact,hardness and CTOD tests to obtain the best automatic welding heat input range,which provides technical guidance for optimizing the automatic welding process.Therefore,this research has important scientific value and engineering significance.In this paper,X80 pipeline steel is taken as the research object,and the welding thermal cycle of heat affected zone under different heat input is simulated by using Gleeble thermal simulation method to adjust t_8/5.The influence of heat input on the microstructure and properties of heat affected zone of automatic welding is analyzed by means of OM,SEM,impact,hardness and three-point bending.The continuous cooling transformation curve of X80 pipeline steel welding heat affected zone was measured by Gleeble-3500.The results show that when t_8/5 is 4s～6s,the microstructure is bainite ferrite,lath martensite and a small amount of granular bainite.When t_8/5 increases to 7.5s～15s,the microstructure transforms into bainite ferrite and granular bainite.When t_8/5 further increases to 20s～30s,the microstructure transforms into bainite ferrite,granular bainite,polygonal ferrite and pearlite.With the increase of t_8/5,the hardness decreases gradually,and when t_8/5 is 15s～30s,the microstructure softens.Adjusting t_8/5 to study the change rule of low temperature toughness in single pass coarse grain zone.The results show that when t_8/5 is 4s～6s,the low temperature impact toughness is very excellent above 300 J,but the CTOD is very poor below 0.2mm.When t_8/5 is 10s,the CTOD is the largest.With the increase of t_8/5,the low-temperature impact toughness gradually decreases to 190 J,and the CTOD first increases and then decreases.The microstructure and mechanical properties of double-pass reheated coarse grain zone（UCGHAZ,SCCGHAZ,ICCGHAZ,SCGHAZ）were studied by adjusting the second pass t_8/5 and peak temperature.There is a chain distribution of M/A at the grain boundary of ICCGHAZ,and the low temperature impact toughness and CTOD are greatly reduced.With the increase of heat input,the low temperature impact toughness and CTOD decreased first and then increased,the hardness of UCGHAZ,ICCGHAZ and SCGHAZ decreased gradually,and the hardness of SCCGHAZ increased first and then decreased.