| Coiled tubing is used in the oil industry, it is widely used in drilling, well completion, oil production, work over and other operations in various fields to solve a number of conventional operating techniques problem which are difficult resolved and effection is apparent. The coiled tubing in a cyclical fatigue loads to work, it is susceptible to fatigue damage caused by a continuous failure, result in shorten life of a coiled tubing. Therefore, how to design a reasonable composition and appropriate technology, to content the engineering needs of high-strength, high toughness, low-brittle transition temperature, good resistance to internal pressure bending properties of coiled tubing is the subject of science and technology workers are still researching.In this dissertation, A,B two types of components and microstracture of high-strength low-alloy steels were straight welded pipe, yield strength of A and B steel coiled tubing are respectively 677.5Mpa and 731.8Mpa. It is at 740℃,760℃,780℃,800℃,820℃,840℃heat treatment, and air-cooling to a certain temperature and then cooled. The microstructure and mechanical properties of the two types of steel with different temperature heat treatment were tested and analyzed by OM, SEM, EDS, TEM, universal testing machine and micro-hardness simulation cycles. The main conclusions were as follows:1. Two kinds of A,B different coiled tubing had the same heat treatment, with the temperature improving deformation of slender grains changed to equiaxed grains. With the heat and water colding temperature raising, pearlite and M-A organizations were increasing, and gradually increasing in size. The same heat treatment conditions, organization and the second phase particle size in B Steel is less than A Steel.2. As the heat temperature raising, mechanical properties of two kinds of A,B coiled tubing have the same tend, hardness and strength and yield ratio are decreased, the extension rate is increased. The hardness and strength of steel B is higher than A steel, and with the heat treatment temperature decreased slowly than the A steel. When heat treatment temperature higher than 800℃, the two steel yield strength are less than 551Mpa have not meet 80(551Mpa) requirement. The extension of B steel was lower than A steel, The gap was increasing with heat treatment temperature improved.3. Two kinds of A,B coiled tubing have been cycled in a certain pressure, cycles reached to the maximum at the yield strengthσsA=677.5Mpa,σsB=642.1 Mpa,extension ratio isδkA=14.32%,δkB=17.47%. Cycles of small organization B steel was larger than that of steel A steel, it was indicated that the coiled tubing has a high number of cycles in a certain intensity and small organization.4. After the simulation of fatigue failure, the compression area of cross-section had a significant deformation of grains, and the deformation of the neutral area of smaller deformation. The neutral section hardness after fatigue has a little changed is the cycle of steady-state. Hardness of the pressure section was increasing, produced cyclic hardening.5. Through the coiled tubing specimen fatigue damaged and recovered by intermediate tempering. At 400℃,90min tempered the highest number of cycles, so that has increased to approximately twice the cycle life. With the tempering temperature, grain boundary segregation of carbon gradually deffusing, Slender grains deformed by the extrusion changed to low-stress the rules of polygon grains; With the pre-tempering increasing, fatigue fracture on the development of secondary fatigue cracks stripes, deep dimples increasing, thoughness increased significantly after mid-tempering. |
PDF Full Text Request