Effects of aluminum content on the mechanical properties of a 9Cr-0.5Mo-1.8W steel

The effects of aluminum content on mechanical properties of a 9Cr-0.5Mo-1.8W steel have been investigated. It was found that aluminum addition had a beneficial effect on toughness, but significantly reduced the creep resistance of the steel, especially on the long-term side. Examination of the microstructure and precipitation characteristics revealed that almost all of the aluminum added existed as AIN-type nitrides after normalizing and tempering. The undissolved AIN in high aluminum steels resulted in a dramatic refinement of prior austenite grains, which contributed to the improvement of toughness and was also partially responsible for the decreased creep rupture strength. The formation of AIN suppressed the precipitation of VN-type nitride; AIN also provided formation sites for Nb(C, N) and M₂₃C₆ type, which had an equivalent effect to the coalescence of these precipitates on AIN, resulting in the reduction of precipitate density and, therefore, decreased creep resistance.