A new series of advanced 3Cr-Mo-Ni steels for thick section pressure vessels in high temperature and pressure hydrogen service

A new series of 3Cr-Mo-Ni steels has been developed for use in thick section pressure vessels, specifically for coal conversion (high temperature and high pressure hydrogen) service. The new steels rely on minor alloy modifications to commercial 2.25Cr-1Mo (ASTM A387 Grade 22 Class 2) steel. Based on evaluations in relatively small heats (55 kg), the experimental alloys, which employ additions of Cr, Ni, Mo, and V, with Mn at 0.5 wt pct and C at 0.15 wt pct, display improved properties compared to commercial steels. Specifically, they show significantly improved hardenability (i.e., fully bainitic microstructures following normalizing of 400-mm (16-in.) plates), enhanced strength (i.e., yield strengths exceeding 600 MPa), far superior hydrogen attack resistance and better Charpy V-notch impact toughness, with comparable tensile ductility, creep rupture resistance and temper embrittlement resistance. The microstructural features contributing to these improved mechanical properties are briefly discussed.