Creep-rupture behavior of 3Cr–3W–V bainitic steels

A nominally Fe–3.0Cr–3.0W–0.25 V (3Cr–3WV) steel and this composition with 0.07% Ta (3Cr–3WVTa) were developed for elevated-temperature service in the power-generation and petrochemical industries. Creep-rupture strengths of the new steels to 600 °C exceeded those of the two advanced commercial 2.25Cr steels T23 (Fe–2.25Cr–1.6W–0.25V–0.05Nb–0.07C) and T24 (Fe–2.25Cr–1.0Mo–0.25V–0.07Ti–0.005B–0.07C). Moreover, the strength of 3Cr–3WVTa approached that of modified 9Cr–1Mo (T91) at 650 °C. Elevated-temperature strength in the new steels is obtained from a bainitic microstructure with a high number density of fine needle-like MX precipitates in the matrix. The presence of tantalum promotes a finer MX precipitate in the 3Cr–3WVTa than in the 3Cr–3WV, and it suppresses the coarsening of these fine precipitates during creep.