Creep behaviour and structural changes of Cr2.25Mo1 steel after long-term service in hydrogen

Creep properties and structural changes of the Cr2.25Mol steel from a pipeline system exposed to 135000 service hours at 530 degrees C with hydrogen at 3.5 MPa as the operating medium were evaluated. No effect of hydrogen on creep properties degradation of the Cr2.25Mol steel was detected after this exposition. However, additional annealing of the exposed material in hydrogen at pressures 5 and 20 MPa at 600 degrees C for 500 and 1000 hours proved significant effect of the annealing pressure and time on creep properties. Time to fracture and creep rate are determined by the density of fine precipitates inside grains and fracture occurred by the transcrystalline damage mechanism.