A comparative study of hydrogen embrittlement susceptibility of X60 pipeline base metal and its weld joints made by rotary friction welding

The coarse-grained heat-affected zone (CGHAZ) of conventional fusion weld joints such as submerged arc weld is particularly prone to cracking in hydrogen environments. The rotary friction welding (RFW) joint of X60 pipeline steel consists of a weld zone (WZ) and a thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ). RFW process can effectively induces recrystallization, forming fine equiaxed grains with grain size consistently smaller than those in the base metal. The average grain size is 5.2 μm in the base metal whereas it is 4.1 μm in the HAZ, 2.3 μm in the TMAZ and 1.6 μm in the weld zone. The consumption of dislocations during recrystallization of the WZ and TMAZ zones reduces the number of reversible hydrogen traps in these subzones, while the increase fraction of high-angle grain boundaries (HAGBs) in this weld joint can effectively impedes the crack propagation, collectively improving hydrogen embrittlement (HE) resistance. Slow strain rate tensile (SSRT) tests in a 6.3 MPa hydrogen environment showed that the RFW joint exhibits significantly higher HE resistance than the base metal. The ratio of elongation-to-failure in high-pressure hydrogen gas to that of sample tested in air is 0.97 for the RFW joint, which is considerably better than a value of 0.78 for the base metal.