Hydrogen embrittlement in structural steel welds – influence of varying the stress concentration

Hydrogen Embrittlement (HE) has been a known phenomenon for several decades. Despite that, it still represents a critical problem for Carbon Manganese steel used in structural applications. For the particular case of subsea pipelines and their weldments, atomic hydrogen from the cathodic protection system and/or present in the welding consumables tend to further embrittle the susceptible microstructure in the heat affected zone. Thus, residual stress and stress concentrators inherent to welded joints can results in unexpected and catastrophic failures. This paper describes a combined experimental and modelling work aimed to establish the dependency of stress concentrators on the hydrogen susceptibility of a weld simulated Coarse Grained Heat Affected Zone in X70 structural steel. A notch tensile, constant load stepwise procedure was applied. Five different geometries were tested both in air and in in-situ electrochemically hydrogen-charging conditions: smooth, U-notched, V-notched, spark eroded and fatigue pre-cracked specimens. The results show a clear influence of stress concentrators on the material resistance toward HE: almost no influence is recorded for the smooth and the U-notched specimens while the fracture stress reduction becomes increasingly consistent with an increasing stress concentration. We also present results from post-mortem fracture surface investigations in order to confirm the different degrees of hydrogen influence in the failure process. Finally, the results serve as a verification of a first principle based, hydrogen influenced cohesive zone FE modelling framework for the prediction of hydrogen enhanced cracking in structural steel weldments.