Corrosion fatigue behavior of X65 pipeline steel welded joints under different stress ranges

The corrosion fatigue behavior and mechanism of X65 pipeline steel welded joints under different stress ranges in H₂S environment were studied through corrosion fatigue experiment. The microstructure, fracture and crack propagation path of the specimens were observed. The results show that the microstructure of X65 pipeline steel weld is mainly composed of proeutectoid ferrite, granular bainite and M/A component, and the M/A component increases the brittleness of the weld. The coarse grain heat affected zone is mainly composed of lath bainite and granular bainite. The hardness of weld and coarse grain heat affected zone is high, and the toughness is low. The corrosion fatigue mechanism of X65 pipeline steel welded joints under different stress ranges is mixed of anodic dissolution and hydrogen embrittlement, but damages caused by corrosion is more significant under low stress ranges. With the increase of stress amplitude, the corrosion fatigue life of the specimens decreases significantly, and the rate of crack propagation grows rapidly. In addition, the secondary cracks mainly spread along the grain boundary of the bainite strip, and the crack tip passivates at the acicular ferrite and the proeutectoid ferrite, which have exhibited excellent resistance to hydrogen embrittlement.