| In oil and gas transportation,the pipeline transportation is most convenient and efficient transportation method.With the substantial increase in oil and gas demand,the demand for high-strength and toughness pipeline steels,especially high-grade pipeline steels with excellent low-temperature toughness is very urgent.However,as the strength of pipeline steel increases,the addition of a large amount of alloying elements is often accompanied,which is generally detrimental to toughness.X65 pipeline steel is a pipeline steel with good strength and toughness matching.Its cost is low and it is mainly used for oil and gas transportation in China.In pipeline laying,welding is the most important link.At present,traditional fusion welding is mainly used for girth seam connection.However,due to the large heat input of fusion welding,problems such as hydrogen embrittlement,porosity,and severe coarsening of grains are easy to occur in the joint,which seriously deteriorates the mechanical properties of the joint.Therefore,a welding method with lower heat input needs to be found.Friction Stir Welding(FSW)is a low heat input solid phase connection technology with simple process and stable quality.In this paper,by changing the rotating speed of the stirring tool(300-600 rpm)and different cooling media(air,water,CO2),friction stir welding of 6 mm thick hot-rolled X65 pipeline steel plates and the evolution of the microstructure of the welded joint and mechanical properties is studied.The main work and results are as follows:(1)The thermal cycle history of the nugget zone under different rotating speed is measured.The results show that as the rotating speed increases,the heat input and peak temperature gradually increase,and the cooling rate after welding gradually decreases,above all,the peak temperature at 300 rpm is the lowest,which is 823℃,and it also has the fastest cooling rate,which is 39℃/s.(2)The formation laws of bainite and M-A islands and the changes of high angle grain boundaries in the nugget zone at different rotating speed were studied.The results show that as the rotating speed increases,the grain size becomes larger,the bainite lath bundle becomes wider,the M-A islands become larger and the distribution is more dispersed,and the proportion of high-angle grain boundaries decreases with increasing rotating speed.(3)By analyzing the hardness distribution of welds with different rotating speed,it is found that the nugget zone at 400 rpm has the highest hardness due to the large amount of lath bainite.The research on the tensile and impact properties of the nugget zone at different rotating speed shows that the transverse tensile test specimens are all fractured at the base metal.The mechanical properties of the weld at 400 rpm are the best.(4)The formation laws of bainite and M-A islands and the change of high-angle grain boundaries in the nugget zone under different cooling conditions are studied.The research shows that as the cooling rate increases,the grain size decreases and the bainite lath became narrower with the increasing dislocation density,the M-A islands became smaller and disperser,and the proportion of high-angle grain boundaries increased.(5)The hardness test results of welds under different cooling conditions show that the hardness of the weld nugget zone is the lowest under CO2 cooling conditions.The tensile and impact test results of the nugget zone under different cooling conditions show that the nugget zone under water cooling has the highest strength.The impact properties of the nugget zone under three cooling conditions are good.Therefore,the mechanical properties of the weld under water cooling conditions are the best. |
