Search

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]