Evolutionary Relationship Between Microstructure and Hydrogen Distribution During the Tensile of Pipeline Girth Welds

Hydrogen trapping is a key mechanism to mitigate hydrogen embrittlement (HE) in pipeline steels, yet how trapping behavior evolves during plastic deformation and which microstructural features are responsible remain poorly understood. In this work, interrupted slow strain rate tensile tests were performed on pipeline girth weld metal (WM) under 6.3 MPa H2 combined with thermal desorption spectroscopy (TDS), hydrogen microprint technique (HMT), and electron backscatter diffraction (EBSD). The results reveal that hydrogen trapping is highly strain-dependent. In the undeformed stage, only weak traps (Peak A at ~250 °C) and moderate traps (Peak B at ~500 °C) are present, as identified by TDS. Upon tensile straining to 15%, a new strong trap (Peak C at ~700 °C) emerges and intensifies, which is attributed to deformation-induced defects within acicular ferrite (AF). HMT shows that hydrogen distribution becomes homogeneous inside AF grains with increasing strain, while remaining localized at grain boundaries in proeutectoid ferrite (PF). EBSD indicates that AF undergoes more uniform plastic deformation and dislocation accumulation than PF. Most importantly, the WM with a higher AF content (74.5%) exhibits a significantly lower HE index (13.4%) than the WM with a lower AF content (53.9%, HE index 23.5%), confirming the beneficial role of AF. These findings demonstrate that AF is not merely a static hydrogen trap but a dynamic source of strain-induced strong traps, which effectively immobilize hydrogen and reduce embrittlement susceptibility under service-relevant deformation.