Research of Fatigue Crack Growth Behavior of X70 Pipeline Steel in Corrosive Environment and Evaluation of Defect

Long distance pipeline transmission is considered to be the most economic and safest method to transport natural gas nowadays and pipeline safety problem is a focus of concern in the academic and industrial circles. Pressure fluctuation and corrosive environment can make the typical working condition of long distance pipeline become a low frequency corrosion fatigue condition, which leads to high risk. In this paper, mathematic model of fatigue crack growth rate (FCGR) considering frequency and stress ratio has been developed and the expressions of every parameter were fitted. Critical stress intensity factor (K_ISCC) in H₂S saturated environment (2300ppm) was measured. In view of the operating condition and applied stress of pipeline, low frequency, high stress ratio corrosion fatigue tests of X70 base metal were carried out in H₂S saturated and air environments respectively. According to the ASME BPVC Section XI, by using inverse inferring method, considering critical crack sizes of H₂S stress corrosion cracking, crack increments of corrosion fatigue and different service periods, allowable crack sizes of different service period were calculated and the curves between allowable crack depth and original crack length were obtained, which would supply more accurate data to determine the service period and to evaluate West-East natural gas transmission pipeline’s residual life.