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The effect of hydrogen on the tensile properties of AISI 304 K-TIG weld metal is investigated in this study. Fractographic analysis demonstrates a transition from quasi-cleavage/cleavage fracture to intergranular fracture in the brittle zone as the hydrogen-charging current density increases, which is attributed to the interplay of crystalline defect, hydrogen atoms, and α′ martensite transformation sites. It is believed that at low current density, α′ martensite nucleates and grows homogeneously in the interior of the grain. At high current density, the stress-induced α′ martensite forms preferentially at the grain boundary, which undergoes severe strain localisation and hydrogen atom segregation. [ABSTRACT FROM AUTHOR]