Study On Expansion Process On Forming Property Of Mono-diameter TWIP Steel Expandable Tubulars With A Thread Joint

With oil resources being increasingly scarce,deep well and ultra-deep well are becoming the main development fields of oil production.The mono-diameter expandable tubular technology can avoid diameter loss of the oil wells during the wells drilling,which is the ultimate goal of drilling.However,the structural integrity of the thread joint and the suiTable material selection limited the development of this technology.The thread joint undergoing the expansion process are prone to breakage and leakage,and the twinning induced transformation steel shows an increased tendency being the material for the expandable tubes due to its high strength,low ratio between the yield and tensile strength and superior plasticity capability.This article paid attention to the thread joint of the mono-diameter TWIP Steel expandable tubulars.The researches on the structural response of the thread joint during and after expansion process were performed with the methods of the combination of the analytical numerical and experimental solutions.The main research content and conclusions of the thesis are summarized as below:(1)The required expansion force in TWIP steel expandable tubular technology was investigated in this part.Based on the Mises flow rules together with the single curve hypothesis,the equations relating different factors,including the expansion ratio,friction coefficient and the cone angle,were derived using the rigid-perfectly plastic material and linear reinforcement rigid plastic material,respectively.The validity of the equation analytical results using linear reinforcement rigid plastic material was verified by the FEM results.The results revealed that the expansion force present a positive correlation trend with the expansion ratio,cone angle and the friction coefficient.The difference of the expansion force equation between the rigid-perfectly plastic material and linear reinforcement rigid plastic material increases with the expansion ratio increasing.Additionally,the expansion force equation of linear reinforcement rigid plastic material was proved to be reliable under the expansion ratio of 10%?15%?20% and 25% by the FEM results.(2)This chapter performed the research on the structural response of TWIP expandable tubular thread joint under different hardening constitutive relation,and the corresponding hardening parameters were obtained from the uniaxial tensile stress-strain curve.The research results showed that the high value residual stress was generated at the thread joint,and the stress concentration was found at the thread teeth.Compared with the kinematic hardening models,the less wall thickness reduction,more axial reduction,less rebound and high value residual stress were found in the isotropic hardening models.Because the isotropic hardening model takes the increase of the yield surface into account but neglects the translation of yield surface in stress space.The inconsistent deformation of box thread and pin thread would be generated after the expansion process,which led to the deterioration of contact condition between the pin thread and box thread.Besides,the deterioration of contact condition was more serious in the kinematic hardening models.As for the burst and collapse loads,the combined hardening models and kinematic hardening models,which considered the Bauschinger effect,revealed different variation trend as expansion ratio increases from 5% to 20% compared with the isotropic hardening models.(3)The expansion experiment of 20% expansion ratio of TWIP steel expandable tubular with thread joint was conducted in this chapter.The experiment results revealed that the expansion force had a fluctuation when the expansion cone was expanding the thread joint area.The box thread and pin thread have an inconsistent deformation undergoing the expansion process.The inner diameter of smooth tube increased,the wall of tube was thinned,the hardness and strength of the tube were increased significantly,and the plasticity and toughness are retained well.The metallographic structure of TWIP steel is austenite with annealing twins,and a vast amount of twin crystal was generated after the expansion process.The uniform deformation capacity and work hardening rate of TWIP steel decreased undergoing the expansion progress.