Welding X65 for Sour Service: Microstructural Evolution and Mechanical Degradation of Pulsed GMAW Joints in H2S Environments

This study investigates pulsed gas metal arc welding (pGMAW) of API 5L X65 pipeline steel for sour service applications where H2S exposure is anticipated. Mechanized pGMAW in the 5G downhill position was employed to fabricate girth welds using ER70S-6 filler wire with Ar-20%CO2 shielding. Comprehensive characterization, including optical microscopy, tensile testing, fractography, EBSD, and fracture toughness evaluation via SENT specimens, was conducted on specimens tested in both air and H2S-precharged sour conditions. Microstructural analysis revealed ferritic–pearlitic base metal, weld metal with acicular ferrite and bainitic constituents, and a transformed HAZ gradient. Tensile testing demonstrated severe hydrogen embrittlement in sour conditions, with elongation dropping from 22% in air to 4% after H2S exposure, accompanied by a transition from ductile cup–cone fracture to quasi-cleavage morphology. EBSD showed texture sharpening toward ⟨101⟩ fiber post-deformation, with a broader orientation spread under sour conditions, indicating heterogeneous strain localization. Fracture toughness testing revealed approximately a 50% reduction in CTOD values under sour exposure, with the weld centerline exhibiting greater degradation (0.50 mm to 0.27 mm) compared to the HAZ (0.92 mm to 0.47 mm). Fractography confirmed hydrogen-assisted cracking features, including shallow dimples, cleavage facets, and secondary cracking. These findings establish critical baseline data for engineering a critical assessment of pGMAW-welded X65 pipelines in sour service.