Effect of hydrogen on impact fracture of X80 steel weld: Various heat inputs and coarse grain heat-affected zone

In this work, the impact fracture of coarse grain heat-affected zone (CGHAZ) of an X80 steel weld with various heat inputs was investigated in the absence and presence of hydrogen (H)-charging by metallographic characterization, microhardness and impact testing, electrochemical H permeation measurements. The results show that, as the heat input (HI) during welding decreases, the microstructure of the CGHAZ changes from granular bainite (GB) to lath bainite (LB). When the LB content exceeds 70%, the microhardness of the material increases remarkably. For H-free specimens, the fracture surface transforms from ductile to brittle fracture at the HI of 29.2 kJ/cm. Upon H-charging, the fracture surfaces show brittle transgranular cleavage fracture when the HI is 18.1 kJ/cm. The H atoms decrease the threshold HI where a brittle fracture occurs. Compared with the GB microphase, the LB-containing microstructure requires a higher H concentration to cause transgranular impact brittle fracture.