The mutual effect of hydrogen and cyclic plastic deformation on ductility degradation of X65 reeled-pipeline welded joint

Abstract

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 (IUE) 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.