| As the rapid development of economy, the demand for petroleumand natural gas is increasing quickly, and some geological hazards regions such aspolar region, deep ocean, earthquake region, where were out of our reach before,began to come into our sight. Especially, the demand for performance of pipeline steelbecomes more rigor as the development of high-H2S oil field. As a result, recent studyof pipeline steels is mainly focused on four trends: high strength and high toughness,high deformability, thick-wall and high anti-H2S property. Furthermore the productionof pipeline steels which have high strength and high toughness as well as highanti-H2S property represents the highest level of a state for steels production. Up tonow, most work of the anti-H2S property for pipeline steels, however, was focused onthe lower level grade such as X52/65, while the study on the high strength and hightoughness pipeline steel is lack. So, under the financial supports by a National“Twelfth Five-Year” Plan for Science&Technology Support Project, and a project“Development of high strength and corrosion resistance pipeline steel used in oil andnatural gas transportation”, the anti-H2S corrosion property of high performancepipeline steels was studied in this thesis.In the present work, the experimental pipeline steels with different strengthgrades and microstructures were adapted, and the microstructural characteristics andmechanical properties were analyzed firstly. Based on the analysis results, the HICexperiment was carried out on series of pipeline steels according to the standard ofNACE TM0284. The experimental results showed that the HIC cracks had variouscracking path where always incubated and grew along the interface between twophases and the hard microstructure such as the large size M/A islands and the BF withstraight lath boundary for pipeline steels with different strength and microstructures.The more severe HIC experiment was also conducted on series of pipeline steels bylowering the pH value and increasing the immersion time. The results showed that theHIC parameters including cracking length ratio (CLR), cracking thickness ratio (CTR)and cracking sensitivity ratio (CSR) were increased in various degrees, and the CTRpossessed the greatest increment. Furthermore, the density of hydrogen inducedblistering on the steel surface was increased with the lower pH value.HIC resistances of acicular ferrite (AF), polygonal ferrite+bainitic ferrite(PF+BF) and polygonal ferrite+martensite (PF+M) with same chemical compositionfor high performance pipeline steels were compared. The experimental results showedthat the AF microstructure behaved excellent anti-HIC property even after immersionin NACE solution A for288hours at pH=1.9. It was analyzed that the uniquecharacteristics of AF microstructure, such as the more even and finer grain sizes, fine dispersed carbonitrde precipitations in matrix, high density tangled and pinneddislocations in AF and irregular grain boundry, contributed to the reason for the above.It was found that it was a complex problem during the discussion of the anti-HICproperties for high performance pipeline steels with different strength or hardnesslevels, because it was believed that the strength level of pipeline steel mainly dependson its microstructure.SSC sensitivities of two acicular ferrite steels were compared according to thestandard of NACE TM0177, finally. It was analyzed that the large size M/A islandsplayed an important role in the anti-SSC property. Hydrogen atoms permeated fromsurface of steel preferring the accumulation within the interface between M/A islandand the matrix, which made SSC easier to happen under the applied stress. The crackswould incubate and expand along between the interface of the large size M/A islandand the matrix as soon as the hydrogen concentration got to the critical value. Theresults implied that minimizing size of the M/A island is an effective way to developthe high performance and anti-H2S pipeline steel. |
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