Impact of Environmental Hydrogen Pressure on Fracture Toughness and Fracture Behavior of X65 Pipeline Steel Welded Joints

Hydrogen embrittlement (HE) damages the fracture toughness, often promoting brittle fracture even at low stress levels. Since welded joints are sensitive to this kind of damage, there is a very high risk of failure. For this reason, this study systematically evaluates the fracture toughness and fracture behavior of X65 pipeline steel welded joints under a gaseous hydrogen environment. The crack tip opening displacement (CTOD) test and electron microscopy technology were adopted to carry out fracture toughness evaluation and microstructure analysis on the weld metal (WM) and heat-affected zone (HAZ) under hydrogen pressure conditions of 1.0 MPa, as well as 7.2 MPa. The results underscore that at the higher hydrogen pressure (7.2 MPa), the CTOD value decreased compared with that at 1.0 MPa; the reduction amplitudes of the CTOD values of the WM and HAZ were 79% and 84%, respectively. When the hydrogen pressure was 1.0 MPa, the WM and HAZ presented microvoid coalescence (MVC) fracture characteristics, the crack propagation path was tortuous, the plastic deformation at the crack tip was significant, and dislocations proliferated massively, leading to high fracture toughness. At 7.2 MPa, the fracture mechanism was into brittle fracture, the crack propagated along an almost straight path, plastic deformation and dislocation proliferation at the crack tip were significantly inhibited, and the fracture toughness decreased sharply.