| Summary: | To analyze the causes and mechanisms affecting the fracture toughness of X80 pipeline steel welded joints against H<sub>2</sub>S, the fracture toughness of different zones of X80 pipeline steel welded joints in both air and saturated H<sub>2</sub>S solution was investigated. The fracture toughness of welded joints degraded significantly in the saturated H<sub>2</sub>S solution, where the crack tip opening displacement (CTOD) characteristic value in the coarse grain heat-affected zone (CGHAZ) and weld metal (WM) was only 8% and 12% of that in air, respectively. However, the sub-critical grain heat-affected zone (SCHAZ) showed better resistance to H<sub>2</sub>S corrosion, with the CTOD characteristic value reaching 42% of that in air. The resistance of the welded joint to H<sub>2</sub>S corrosion was sensitive to microstructures. The grain boundary ferrite (GBF) presented in WM, and the angle of grain boundary orientation in CGHAZ was not conducive to hindering crack propagation. Moreover, the formation of the resultant hydrogen cracks owing to the H<sub>2</sub>S corrosion also reduced the fracture toughness of the welded joint.
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