, , , , Yaoshan Chen, Ming Liu, Yong-Yi Wang
Pipeline steels suffer significant degradation of their mechanical properties in high-pressure gaseous hydrogen, including their fatigue cracking resistances to cyclic loading. Fatigue crack growth experiments were performed on X52 and X70 steels in both air and hydrogen gas pressurized to 5.5 MPa and 34 MPa. Experimental results indicate that the presence of gaseous hydrogen increases the FCGR of all materials tested. At lower values of ΔK (ΔK values between 7 MPam1/2 and 9 MPam1/2), the increase in FCGR may be modest over that of air; whereas the FCGR is vastly increased over that of air for higher values of ΔK. The rate of FCG approaches that of air for ΔK values below about 7 MPam1/2 and above about 15 MPam1/2. Hydrogen-assisted FCGR appears to increase with increasing hydrogen pressures at ΔK values below approximately 15 MPam1/2. However, the FCGR response for all materials in all hydrogen pressures tested appears to fall within a relatively narrow band. Furthermore, decreasing the test frequency has an effect similar to increasing the hydrogen test pressure. FCGR does not appear to be affected by the strength of the steel, as it is in monotonic tensile testing. FCGRs of X70 were not significantly different, meaning that results did not fall outside of the experimental uncertainty, than those for X52.
Final Report to the DOT
fatigue crack growth rate, hydrogen embrittlement, hydrogen gas, low frequency, pipelines, steel, X52, X70