| Author(s) |
Zhenzhen Yu, Lydia Hines, Jackson McCloskey, Chen Ni, Matthew Connolly, May Ling Martin, Gregory Ebel, Joe Bundy, Andres Acuna, Ravi Menon, Anoop Samant, Jorge Penso |
| Abstract Scope |
A lack of understanding persists in literature regarding how individual subzone microstructures within complex high-strength low-alloy (HSLA) welds influence hydrogen susceptibility, largely due to the difficulty of isolating these narrow regions for mechanical testing. To address this gap, a robust framework was established for the systematic study of X-series pipeline multipass welds by separating evaluations into the heat-affected zone (HAZ) and weld metal (WM). A comprehensive methodology utilized Gleeble thermomechanical simulation, finite element modeling (FEM), and metallurgical characterization to design specialized single-edge bend (SE(B)) specimens, successfully isolating bulk coarse-grained, fine-grained, intercritical, and reheated HAZ subregions (CGHAZ, FGHAZ, ICHAZ) from X65M and X70M pipes. Fracture toughness tests in 21 MPa gaseous hydrogen revealed that the final thermal cycle dictates toughness, with the final CGHAZ cycle averaging the lowest toughness among all conditions. Complementing the HAZ framework, a systematic WM testing matrix evaluated variations in welding technologies, heat inputs, and filler chemistries. SE(B) specimens from X65 and X70 plates were notched parallel to the welding (WD) and normal (ND) directions to assess microstructural anisotropy impacts. The hydrogen fracture toughness of most WM conditions was comparable to the base metals and consistently exceeded the CGHAZ subzones, with WD notches yielding higher toughness than ND notches. To support both tracks, a robust multi-scale metallurgical characterization procedure implemented targeted etching, optical microscopy, and electron backscatter diffraction. This framework definitively identified that coarser martensite-austenite (M-A) constituents drive toughness degradation in both the WM and HAZ, while the influence of grain boundary ferrite and acicular ferrite depended on notch orientation. Ultimately, this systematic framework provides a foundational methodology for optimizing weld designs tailored to in-service crack paths in hydrogen infrastructure. |