Mechanical Properties of X70 Welded Joint in High-Pressure Natural Gas/Hydrogen Mixtures

The effect of hydrogen on the mechanical properties of X70 welded joint was investigated in simulated natural gas/hydrogen mixtures at 10 MPa. The hydrogen volume fraction was set as 0, 5.0 and 10.0 vol.%. The slow strain rate tensile test showed that the reduction in the area of the welded zone (WZ) metal was reduced more than that of the base metal. The variation in fatigue crack growth rate (FCGR) from high to low followed the order: heat-affected zone (HAZ) metal, base metal and WZ metal. In addition, the difference became more obvious with increasing hydrogen volume fraction. For the HAZ metal, the FCGR in 10.0 vol.% hydrogen mixtures was approximately 22 times of that in nitrogen. Furthermore, based on FCGR and fracture mechanics, the predicted fatigue life of the X70 pipeline with an initial flaw depth of 0.5 mm dropped sharply from 34,302 cycles to 3457 cycles even though 5.0 vol.% hydrogen was added in the simulated natural gas.