Strength Control Mechanism And Method Of Butt Welded Joint Of CT90 Coiled Tubing

In recent years,coiled tubing has been widely used in various petroleum engineering operations due to its advantages of low cost,safety and reliability,and the welding of coiled tubing is often an essential link of the use.The heat affected zone of welded joint is affected by the heat circulation of welding,which makes the structure and performance uneven,resulting in the service life of coiled tubing greatly reduced.In this paper,the microstructure,properties and welding technology of CT90 welding heat affected zone are systematically studied,which provides theoretical basis and reference for the future coiled tubing welding operation.In this paper,Glebble-3500 welding thermal simulation technology is used to simulate the welding heat affected zone of CT90 coiled tubing,and the obtained samples are analyzed and studied.It is found that the hardness and strength of the coarse-grained zone in the heat affected zone increase,but it can meet the actual demand.In the tempering zone and incomplete recrystallization zone of the heat-affected zone,the incomplete recrystallization zone softens the most,and the yield strength and tensile strength of the incomplete recrystallization zone decrease.The existence of softening zone lets the overall uniformity of the welded joint decrease.In order to clarify the softening mechanism of HAZ,EBSD and TEM were used to analyze and study the incomplete recrystallization zone.The results show that:(1)there is {110} < 001 > texture in the incomplete recrystallization region,which reduces the toughness and yield strength;(2)the grain coarsens under high heat input,according to Hall-Petch formula,the grain coarsens lead to the decrease of properties;(3)the content of 20°~47° angle grain boundary increases under high heat input,which reduces the content of granular bainite and leads to the material properties It can decrease;(4)the density of dislocations in the region decreases,the cementite clusters diffuse to the grain boundary,and the decrease of the percentage content of M-A component leads to the decrease of hardness.It can be seen from the above results that the performance of HAZ can be effectively improved and the uniformity of welded joint can be guaranteed by reducing the heat input as much as possible under the condition of process permission.But in the process of welding,too low heat input will lead to welding defects,so the softening of HAZ can be further improved by changing the cooling rate.The water cooling regulating device and control system are designed and completed.The influence of different water cooling conditions on welding heat softening is analyzed.The results show that softening occurs in the heat affected zone without forced cooling.The softening phenomenon can be basically eliminated by forced cooling in low heat input.Although the softening phenomenon can not be completely eliminated in high heat input,the width and degree of softening can be reduced by increasing the forced cooling parameters.