Comparison of gaseous hydrogen effects in 1200 MPa high strength martensitic and pearlitic steels

This study compares gaseous hydrogen effects in a high strength martensitic and pearlitic steel (ultimate tensile strength of 1200-1300 MPa). When tensile tested in 10 MPa gaseous hydrogen at room temperature, the relative reduction of area (RRA) is 14 % for the martensitic steel while the pearlitic steel shows much improved behaviour with an RRA of 75 %. The microstructures of both steels are characterized in detail pre- and post-mortem. Fractography reveals that hydrogen assisted fracture occurs predominantly along prior austenite grain boundaries for the martensitic steel while tearing topography surface characteristics are observed for the pearlitic steel. Thermal desorption spectroscopy measurements show similar total hydrogen contents of about 2 wppm in both microstructures. In the martensitic microstructure, hydrogen is weakly trapped, whereas in the pearlitic microstructure, about 30 % of the hydrogen is strongly trapped. This seems to be one important reason for the higher RRA of the pearlitic microstructure in 10 MPa gaseous hydrogen.