Abstract
An API X70 pipeline steel has been investigated with respect to hydrogen diffusion and fracture mechanics properties. A finite element cohesive element approach has been applied to simulate the onset of hydrogen induced fracture. Base metal, weld simulated heat affected zone and weld metal have been investigated. The electrochemical permeation technique was used to study hydrogen diffusion properties, while in situ fracture mechanics testing was performed in order to establish the hydrogen influenced threshold stress intensity. The average effective diffusion coefficient at room temperature was 7.60x10-11 m2/s for the base metal, 4.01x10-11 m2/s for weld metal and 1.26x10-11 m2/s for the weld simulated heat affected zone. Hydrogen susceptibility was proved to be pronounced for the heat affected zone samples. Fracture toughness samples failed at a net section stress level of 0.64 times the yield strength; whereas the base metal samples did not fail at net section stresses lower than the ultimate tensile strength. The initial cohesive parameters which best fitted the experimental results were σc=1500 MPa (3.1•σy) for the base metal, σc=1800 MPa (3.0•σy) for weld metal and σc=1840 MPa (2.3•σy) for heat affected zone. Threshold stress intensities KIc,HE were in the range 142 - 149 MPa√m.
