Damage evolution of a hydrogen charged grade X56 pipeline steel evaluated using X-ray micro-CT

In the light of the energy transition, part of the European natural gas pipeline grid will be converted to hydrogen gas pipelines. One of the challenges of this conversion is the well-acknowledged reduction in mechanical properties of steel in the presence of hydrogen, commonly known as ‘hydrogen embrittlement’. The steels in-use are of variable characteristics (with respect to chemical, microstructural and mechanical properties), resulting in a variety of plasticity and fracture behaviors. The present work investigates the effect of hydrogen on the fracture behavior of a relatively old grade API 5L X56 pipeline steel produced by normalized rolling which resulted in a banded microstructure. Tensile tests were performed on smooth and notched round bar specimens that were hydrogen pre-charged electrochemically (ex-situ), and compared to tests on uncharged specimens as a reference. The fractured specimens were scanned using high resolution X-ray computed tomography (X-ray micro-CT) to visualize and quantify the damage underneath the fracture surface. Statistics regarding the void size distribution and void shapes are provided. The fracture process in the absence of hydrogen is characterised by significant void development. The presence of hydrogen accelerates the fracture mechanisms without fundamentally altering them. This is in contrast with previous results obtained on a different pipeline steel grade, demonstrating the sensitivity of hydrogen embrittlement susceptibility to material characteristics.