Abstract
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 CO2 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.
Highlights
Oil and gas pipelines have been experiencing severe security challenges brought about by complex environments and different stress states during their service periods
The corrosion failure and fatigue failure are the most serious and prominent failure forms among those of pipelines [1,2,3]. These two failure modes often occur at the same time, which refers to corrosion fatigue (CF), further shorting service life of pipelines
Instead of sufficient studies [2,4,9] showing that the transgranular fracture surface was observed in CF as stress corrosion cracking (SCC) did in the near-neutral pH environment, indicating that CF and SCC may share similar cracking mechanisms [10], abundant essential questions pointing to the possibility, severity, and rate of CF propagation are still of necessity to further investigations, especially the problem of CF mechanism in the near-neutral pH environment
Summary
Oil and gas pipelines have been experiencing severe security challenges brought about by complex environments and different stress states during their service periods. The periods for crack initiation are significantly compressed under the influence of corrosive environments and the byproducts produced in this process could further accelerate the fatigue propagation It is widely accepted [1,5,6] that CF and stress corrosion cracking (SCC) are the dominant failure modes of environment-assisted cracking (EAC) depending on the loading scheme. Despite extensive and considerable research conducted on the inner mechanisms of SCC in pipeline steels [7,8,9], there are few papers focused on CF, especially for the less corrosive or critical environments The former investigates the crack behavior in corrosive environments under constant loads and several classic features and modes have been characterized according to the crack morphologies and critical situations. SEM and EBSD analyses were necessarily operated to characterize the fracture surfaces and deformation structure of fatigue life testing specimens and fatigue crack growth rate testing specimens, respectively, so that the effect of the water environment on both fatigue crack initiation and fatigue crack propagation of X80 welded joints can be further understood
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