| X80 pipeline steel is a low-carbon microalloyed high-strength steel with high strength and excellent impact resistance,and is currently the mainstream oil and gas pipeline steel in the international community.The welding seam connection area of pipeline steel is a weak link and a frequent location for oil and gas pipeline failure accidents.At present,the welding joints of pipeline steel mostly use traditional fusion welding methods,which often produce solidification defects such as cold cracks and hot cracks.At the same time,the problem of grain coarsening in the joints is also very serious.Friction stir welding is a new type of solid state welding technology,which has obvious advantages over traditional fusion welding.However,there are few relevant research literatures and industrial applications in steel materials,and it is still unclear how to change the microstructure and properties brought about by changing the relevant welding parameters.In particular,the short life of stirring tools and the high peak temperature in the welding process are the research difficulties of friction stir welding at present.In this paper,friction stir welding of 7mm thick X80 pipeline steel plate was carried out.By changing the rotation speed of the stirring tool(300rpm,375 rpm,475rpm,600rpm),different auxiliary preheating(ordinary temperature,150 ℃,270 ℃)was carried out on the steel plate before welding,and different cooling media(air,high-speed gas,water)were added during welding to systematically study the microstructure evolution and mechanical properties of the weld joint.The main conclusions are as follows:(1)The main heat input of friction stir welding is the friction heat generated by the stirring tool.The heat input of welding with increasing the rotational speed of the stirring tool will increase significantly.The welding temperature at the backward side is lower than that at the forward side.When the auxiliary heat temperature is added,the lower auxiliary heat temperature will reduce the peak temperature,and the higher auxiliary heat temperature will increase the peak temperature.(2)At low rotational speed,the mixed zone of the joint is composed of granular bainite,Flat noodles bainite and ferrite;At medium speed,the stirring zone has completed complete austenite transformation.After cooling,the final structure is granular bainite due to the higher critical cooling rate of Flat noodles bainite;At high rotational speed,the peak welding temperature is higher,and the microstructure is mainly composed of granular bainite and Flat noodles bainite.(3)The bainite transformation in the mixing zone of the joint is difficult to avoid the generation of the second phase M-A component.The increase of rotational speed will significantly increase the number and volume of M-A components,and the additional cooling can significantly reduce the generation of M-A components.(4)Under different welding parameters,the hardness of the mixing zone of the joint is the highest point,and the hardness of the outer Heat-affected zone is the lowest point and lower than the base metal.The hardness of the Heat-affected zone gradually increases with the proximity to the mixing zone,and the hardness of the mixing zone decreases with the increase of the rotational speed.The tensile strength of the joint shows a decreasing trend as the rotational speed increases.At 375 rpm,the tensile strength reaches 96.4% of the base metal,while other welding parameters fracture in the stirring zone.A lower auxiliary heating preheating temperature can improve the tensile strength of the joint,and under water-cooled conditions,the tensile strength reaches 100.8% of the base metal.(5)The impact toughness of the joint shows a decreasing trend with the increase of rotational speed.A lower auxiliary heating temperature can enhance the impact toughness,and additional cooling conditions can significantly increase the impact toughness of the joint. |
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