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The hydrogen embrittlement (HE) susceptibility and hydrogen permeation behavior of reeling-pipeline welded joint with/without cyclic plastic deformation (CPD) were studied using the electrochemical hydrogen charging technique. Results indicated that the surface of welded joint emerged hydrogen-induced damage containing cracks and blisters. The degree of hydrogen-induced damage increased with the increase of hydrogen charging time and current density. When the hydrogen charging current density and time was 50 mA/cm2 and 4 h, respectively, the area ratio of hydrogen-induced damage of overall welded joint with CPD process was reduced from 6.61% to 2.28%, and the damage ratio of different sub-zones in welded joint was also decreased. The oxidized inclusions enriching Al–Mg–Ca elements acted as the initiation sites for hydrogen-induced damages. The effective diffusion coefficient of as-welded joint was 2.63 × 10−6 cm2/s, while that of welded joint with CPD showed a smaller value of 1.36 × 10−6 cm2/s. The welded joint with CPD process presented better resistance to HE, which was attributed to the increased density of hydrogen traps and the formation of dislocation cells to disperse hydrogen uniformly and reduce the possibility of local accumulation and recombination of diffusible hydrogen. Sub-zones in welded joint without CPD process were considerably more sensitive to hydrogen-induced damage, which indicated the important role of microstructure and dislocation density in HE mechanisms. The order of HE susceptibility from low to high was weld metal, base metal and heat affected zone. • Cyclic plastic deformation (CPD) reduced the hydrogen embrittlement susceptibility. • CPD reduced the diffusion coefficient of hydrogen in welded joint and its sub-zones. • The cellular dislocation configuration restricted the diffusibility of hydrogen. • The order of increasing susceptibility to hydrogen embrittlement was WM, BM and HAZ. [ABSTRACT FROM AUTHOR]

API X60 steel used for reeled pipeline was subjected to different amounts of cyclic plastic deformation (CPD) by multiple tension–compression tests in order to study the deformation behavior and microstructural evolution during each stage of reel-lay installation process. It was found that the yield strength was significantly reduced by about 110 MPa during the first CPD cycle due to the Bauschinger effect. However, the yield strength was little affected by the subsequent CPD cycles, and the yield strength difference between tension and compression tended to reach a steady-state value of 23 MPa after multiple CPD cycles. With the increase in CPD cycles, the proportion of low-angle grain boundaries declined from 50.9 to 35.7%, and the CPD contributed to the formation of immovable dislocation walls and cells due to the dislocation rearrangement, causing the stable yield strength. The CPD process with strain level of 3% had no prominent effect on the tensile strength and elongation, because the change in microstructures was not enough to alter the work hardening completely after multiple CPD cycles. [ABSTRACT FROM AUTHOR]

The safe service of reeled-pipeline welded joint subjected to multiple cyclic plastic deformation (CPD) is challenged by hydrogen embrittlement (HE) in the process of gas and petroleum transportation. Therefore, this work aims to study the interrelation of hydrogen and the ductility of X65 reeled-pipeline welded joint with CPD by using electrochemical hydrogen charging. The results showed that hydrogen could lead to the ductility degradation, regardless of whether the CPD process was performed. But the hydrogen-induced ductility degradation for the welded joint with CPD was retarded. When the hydrogen-charging current density was 50 mA/cm2, the hydrogen embrittlement index (I UE) based on the uniform elongation for welded joint with CPD was 0.33, while that of as-welded joint was 0.53, which indicated that CPD process reduced the sensitivity to HE. The morphology of tensile fracture indicated that brittle cracks initiated at the composite oxide inclusion enriched Al–Mg–Ca–S elements, and propagated radially by quasi-cleavage fracture pattern. The CPD process led to the increase of dislocations acting as reversible hydrogen traps, which significantly enhanced the hydrogen content of welded joint from 1.001 ppm to 2.516 ppm. However, the dislocation walls and cells related to the CPD process hindered the diffusion of hydrogen, and homogeneous distribution of dislocations after CPD dispersed trap sites for capturing hydrogen, which played important roles in delaying hydrogen-induced fracture. [ABSTRACT FROM AUTHOR]

Abstract Reel-lay process is an efficient and economic method for installing offshore pipelines. During reel-lay process, cyclic plastic deformation (CPD) is introduced into the pipeline and it will modify the mechanical properties of the pipeline steels. In this research, cyclic tension-compression plastic deformation was conducted on the weld metals of X60 pipeline to simulate the strain experienced in reel-lay process. Then the dislocation configurations and mechanical properties of the weld metals were investigated after CPD. The total dislocation density was increased by the CPD process. The dislocation configurations evolved from dislocation lines and tangles into dislocation walls under 2% CPD and dislocation cells upon 5% CPD, respectively. The formation of dislocation walls and cells was a dynamic process, in which dislocation multiplication and intersection competed with dislocation annihilation. The yield platform of the weld metals disappeared and the yield strength decreased after CPD, but the tensile strength had no obvious change with CPD. The work hardening ability and uniform elongation rate reduced with the CPD level. The reduction of work hardening ability was mainly associated with the disintegration of dislocation wall or cell structure during tensile tests. [ABSTRACT FROM AUTHOR]