Research On Welding Softening Of Large Deformation Pipeline Steel

High-strain pipeline steel is a new kind of pipeline steel developed for strain-based design,which is mainly applied in abominable geological condition areas with earthquake or landslide et al.Polygonal ferrite-bainite(PF-B)dual phase pipeline steel has attracted much research interest because of its excellent strain capacity and standout microstructures controllability.According to the actual need,the strength of welded joint should be higher than the base metals.But after the heat cycles such as actual submerged arc weld,the grain coarsening and microstructure deterioration usually exist in the heat affected zone(HAZ).In the meanwhile,there also exist different degrees of softening phenomenons,which lead to serious inconsistency of the performance of HAZ with the pipeline steel.This phenomenon makes HAZ to be the weakest part of the whole welded joint and thus restricts the further application of high deformability pipeline,steel.In this paper,the HAZ softening phenomenon of high deformability pipeline steel during the welding thermal cycle has been studied in detail to reveal the mechanism.Firstly,the variation of the micro morphology and hardness of the HAZ welded joint of X80 grade pipeline steel with the base material of PF+B has been studied.The result indicated that the microstructure of coarse grain HAZ was composited of lower bainite and acicular ferrite which growned rapidly because of high temperature.The microstructure of fine grain HAZ was composited of acicular ferrite and fine MA component with uniform grain size.There were more PF,GB and MA components in the intercritical HAZ.The type of subcritical HAZ was consistent with the base metal.Meanwhile,the hardness of the whole HAZ was lower than base metal.The hardness of the intercritical HAZ and subcritical HAZ was the lowest in the whole HAZ.So the softening phenomenon in the HAZ was related to the tempering of the bainite in the base metal during the thermal cycling.To reveal the mechanism of softening zone in HAZ,the welding thermal cycle process of X80 high-strain pipeline steel was researched using welding thermal simulation.The characteristic of microstructure,crystallology and impact toughness of the softening zone of HAZ was analyzed by combining with metallurgical microscope,SEM,EBSD,TEM and impact testing.The result indicated that the softened region of heat affected zone presented at high temperature tempering zone with peak temperature from 600? to 700?.The microstructure transformed from ferrite plus bainite to the coarse ferrite plus tempering bainite with low hardness,and the recovery process was accelerated.Then the substructure decreased sharply and the dislocation density was reduced dramatically,which led to the formation of softened region.On the contrary,the impact toughness was better at the softened zone,but the M/A component assembled and coarsened at the intercritical zone of 800?.Meanwhile,the percentage of high angle grain boundary was also decreased resulting in the presence of the low toughness.The welding thermal cycle processes with different heat inputs were investigated with a Gleeble-3800 thermo-mechanical simulator.The characteristic of microstructure and hardness of the softening zone of HAZ under different heat inputs were analyzed.The result indicated that the softened region of HAZ presented at high temperature tempering zone with peak temperature of 600?-700? with the heat inputs of 23kJ/cm,17 kJ/cm and 10 kJ/cm.Reducing heat inputs not only can refine grain,but also can slow down the decrease of hardness of heat affected zone.