| Summary: | Fatigue life tests and fatigue crack growth rate (FCGR) tests in the air and water environment were conducted on X80 pipeline steel welded joints (welded by CO<sub>2</sub> arc welding). Scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) were utilized to investigate the internal influential mechanisms of the water environment during fatigue crack initiation and propagation stages, respectively. Results show that a great many oxide particles induced by the water environment gradually formed the fatigue crack initiation site and decreased fatigue life of welded joints. Meanwhile, the preferred grain orientation of <001>//ND and CSL boundaries of Σ3, Σ11, Σ13c, Σ17b, Σ25a, and Σ25b are both prone to fatigue propagation when loading in the water environment. In addition, a coalescence of the stress intensity factor (SIF) range and water environment accelerated FCGR by motivating secondary slip systems of {112}<111> and {123}<111> in bcc crystalline structures.
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