Effect Of Ultrasonic Impact Treatment On Microstructure And Mechanical Properties In X80 Pipeline Steel Joints

The distribution of oil and gas resources is uneven in China,and the natural gas of cities in the east of China is in short supply.Therefore,it is important to transport the oil and gas by pipelines safely.Increasing the grade of pipeline steel can save steel,so X80 grade pipeline steel has been used recently.The X80 grade pipeline steel used in the West-to-East natural gas pipeline project is in more complex conditions and worse natural environment.Therefore,it is of great significance to improve the mechanical properties of joints of X80 grade pipeline steel.Because the ultrasonic impact technology can control the quality of welding joints from inside and outside,the X80 grade pipeline steel joints was treated by three kinds of ultrasonic impact processes in this paper: ?1 Ultrasonic impact treatment was carried out on the weld surface after removing weld height,and the strengthening mechanism was studied;?2 Ultrasonic impact treatment was carried out on the weld toe,and the fatigue fracture mechanism of X80 grade pipeline steel joints was studied;?3 X80 grade pipeline steel was subjected to layered ultrasonic impact treatment during the welding process,and the effects of layered ultrasonic impact treatment on the internal structure and mechanical properties of welds were studied.(1)Surface strengthening mechanism of the weld after ultrasonic impact treatment was studied in detail.The results showed that a plastic deformation layer appeared on the weld surface layer after ultrasonic impact treatment.The microstructure in the deformation layer was refined,and the microhardness was increased;The plastic deformation mechanism of the weld surface after ultrasonic impact treatment was dislocation slip;Dislocation entanglements,dislocation walls,subgrain boundaries and grain boundaries were formed by dislocation slipping.(2)After ultrasonic impact treatment on the weld toe,the weld toe presentedsmooth transition and the stress concentration decreased;The surface of welding toe was converted from tensile stress to compressive stress;Fatigue test results showed that the fatigue life of X80 grade pipeline steel joints after ultrasonic impact treatment was three times that of untreated joints.(3)X80 grade pipeline steel was subjected to layered ultrasonic impact treatment,namely,each welding layer was subjected to ultrasonic impact treatment.Subsequent weld beads were grown on the ultrasonic impact treatment layer.The grain refinement mechanism of layered ultrasonic impact treated welds was studied.The result shows that the number of columnar crystals decreased and the number of fine equiaxed crystals increased in X80 pipeline steel joints after layered ultrasonic impact treatment.The nanocrystals and oxides produced by the ultrasonic impact treatment promoted the nucleation of subsequent weld beads,inhibited the growth of columnar crystals,and refined the grains inside the weld.(4)Charpy impact toughness test and fatigue crack growth rate test were carried out on untreated weld and layered ultrasonic impact weld respectively.The results showed that the fatigue crack growth rate of layered ultrasonic impact treated welds was reduced and the impact toughness was improved.