Experimental study of microstructural pathways for hydrogen-induced damage in X80 line pipe steel weld

• Hydrogen damage in X80 pipe weld via electrochemical and low-temperature Charpy impact tests were studied. • ALF and AF interface, and TM and BF were more susceptible to hydrogen-induced damage. • Non-uniform kernel average misorientation increased susceptibility to hydrogen-induced damage. • Al 2 O 3 and MnS inclusions with (Nb, Ti) (C, N) precipitates were observed in crack pathway. • Steels with lower low-temperature toughness were more susceptible to hydrogen-induced degradation. Microstructural features responsible for hydrogen-induced damage in X80 linepipe weld were systemically investigated through quantitative microstructural analysis, electrochemical hydrogen charging, hydrogen microprint and permeation techniques, and corrosion testing. Additionally, Charpy V-notch impact tests were conducted over a temperature range from 0 °C to −65 °C. The results showed that the fusion zone and fusion line, especially in the weld middle were the most susceptible to hydrogen-induced cracking, blistering, and micro pitting. These damages primarily occured at allotriomorphic ferrite and acicular ferrite interfaces in the fusion zone, and tempered martensite and bainitic ferrite in the fusion line, where increased hydrogen accumulation was observed. Significant differences in kernel average misorientation (KAM) between allotriomorphic ferrite and acicular ferrite, along with elevated KAM value in bainitic ferrite and tempered martensite, contributed to enhanced hydrogen trapping density in the weld metal. The morphology of cementite was also detrimental, as shown by tempered martensite with spheroidal cementite trapping comparatively less hydrogen than bainitic ferrite containing lamellar cementite. Non-metallic inclusions (Al 2 O 3 , MnS) and (Nb, Ti) (C, N) carbides were also present along the crack path. Compared to the base metal, the weld metal, i.e. fusion zone and fusion line, exhibited higher density of hydrogen-trapping sites (3.44 × 10 +21 vs. 1.50 × 10 +21 cm −3 ) and greater hydrogen solubility (2.22 × 10 −4 vs. 1.18 × 10 −4 mol. cm −3 ), along with accelerated corrosion rate (0.8 vs. 0.09 mm/year). Additionally, the weld metal showed poorer low-temperature toughness in the non-hydrogen-charged condition. This reduced toughness was associated with a higher susceptibility to hydrogen-related degradation, highlighting the influence of microstructure on weld performance under hydrogen exposure.