Electrochemical hydrogen charging to simulate hydrogen flaking in pressure vessel steels

Abstract

The current study investigates hydrogen flaking in large forgings. Two industrially forged pressure vessel materials, exhibiting different segregation behavior are compared for this purpose. Electrochemical hydrogen charging is used to simulate the flakes, present in real-life applications, on a small scale. As such, the sensitivity of the materials to hydrogen flaking is assessed by varying the charging conditions in terms of time and applied current density. The resulting cracks are subsequently evaluated by optical and scanning electron microscopy. Additionally, X-ray micro-computed tomography scans are performed to generate non-destructive data on the interior performance of the tested samples. MnS inclusions are found to act as crack initiation sites. An increased number of inclusions results into a larger number of small hydrogen induced cracks, whereas less inclusions lead to a smaller number of larger hydrogen induced cracks for the same applied hydrogen charging parameters. The artificial hydrogen flakes, as introduced by electrochemical charging on lab-scale, are induced at the same locations as real-life flakes, i.e. in segregated areas along the MnS inclusions. However, differences in the exact geometry and size of the flakes are observed.