3D FIB-SEM and TEM characterization of an industrial 0.5-Mo low carbon steel subjected to high temperature hydrogen attack

This work focuses on the microstructural characterization of an industrial low carbon steel which was submitted to high temperature hydrogen attack for decades in petroleum refineries. Scanning electron microscopy (SEM) was used to make an initial assessment of the attack stage by identifying the defects. Cavities filled with methane are located at ferrite-ferrite or ferrite-pearlite grain boundaries as well as at the carbide/matrix interfaces. Cracks are also observed next to zones showing less advanced damage which reflects a heterogeneity of attack. In order to establish a link between the fine-scale microstructure, which may be responsible for this heterogeneity, and the formation of cavities, a 3D visualization of a zone displaying cavities was performed thanks to a 3D focused ion beam (FIB) -SEM analysis. The main result is that cavities mostly formed at the junction of a transgranular inclusion-enriched plane with a grain boundary or carbides in pearlite grain. These inclusion-enriched planes that pass through ferrite or pearlite grains may correspond to former austenitic grain boundaries that have been overtopped by subsequent ferrite and pearlite grain boundaries during the steel manufacturing. A transmission electron microscopy (TEM) observation of an attacked pearlite reveals that in most cavities located at carbide/matrix interface, an inclusion of AlN remains stuck. The potential catalytic effect of inclusions as AlN on the trigger of attack is not yet fully understood and will be the subject of further investigations on brand new C-steels.