AbstractA study has been made of the susceptibility to hydrogen attack of a newly developed 3Cr–1 Mo–1Ni pressure-vessel steel, intended for use in coal-conversion vessels, following long-term exposure to high-pressure (14-17 MPa) gaseous hydrogen at 550 and 600°C; the results are compared to the behaviour of 2·25Cr–1Mo steel. To simulate the condition of both surface and mid-section locations of thick-section (400 mm) plate during commercial normalizing, oil quenched and slowly cooled (8 K min−1) structures were examined after tempering at 650 and 700°C, with respect to their strength, ductility, and impact toughness properties. Compared to unexposed samples, structures exposed to hydrogen were observed to show some softening (up to 20% reduction in yield stress) and‘embrittlement’(up to 22% reduction in upper-shelf Charpy energies and increases in the transition temperatures by 65–100 K), although there were no visible signs of major microstructural damage. Moreover, the behaviour of the oil-quenched and slow-cooled structures was largely similar. In comparison to 2·25Cr–1 Mo steel, which can become highly susceptible to hydrogen damage in the very slowly cooled state, the present 3Cr–1Mo–1Ni steel was found to have far superior resistance to hydrogen attack. This was attributed primarily to two factors: namely, the increased hardenability, which resulted from the absence of pro-eutectoid ferrite in as-cooled microstructures, and the accelerated kinetics in the carbide precipitation sequence, which resulted in the more rapid replacement of M3C by more stable, higher-alloyed carbides, such as M23C6.MST/95