Abstract
An API X70 pipeline steel has been investigated with respect to hydrogen diffusion and fracture mechanics properties. Both base metal (BM) and weld simulated heat affected zone (HAZ) have been included. The electrochemical permeation technique was used to study hydrogen diffusion properties while fracture mechanics testing was performed in order to establish hydrogen influenced threshold stress intensity. Finite element simulations combining diffusion and fracture mechanics results have been performed to establish parameters for cohesive zone numerical simulation of fracture initiation in X70 pipeline steel. The average effective diffusion coefficient at room temperature was 7.60x10-11 m2/s for the base metal and 1.26x10-11 m2/s for the simulated HAZ. Both microstructures showed similar response to increased temperature. The sub-surface hydrogen concentration was not influenced by temperature and the simulated HAZ showed a higher level compared to the base metal. Hydrogen susceptibility was proved to be pronounced for the HAZ. 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 influence of hydrogen was confirmed by fracture surface inspection in Scanning Electron Microscope. The cohesive parameters which best fitted the experimental results were δc=0.3 mm and σc=1500 MPa (3.1•σy) for the BM and δc=0.3 mm and σc=2100 MPa (2.6•σy) for the simulated HAZ. Threshold stress intensities KIc,HE of 149 MPa√m and 142 MPa√m was found for BM and HAZ respectively.