Alloy design of creep resistant 9Cr steel using a dispersion of nano-sized carbonitrides

In order to improve creep strength of tempered martensitic 9Cr steel at 650 °C, alloy design for a dispersion of fine and thermally stable precipitate particles has been investigated for 9Cr–3W–3Co–VNb steel and 9Cr–2W–VNbTi steel. Creep tests were carried out at 650 °C for up to about 10 000 h. By reducing carbon concentration below 0.02%, a dispersion of nano-size MX nitrides of vanadium and niobium along boundaries as well as in the matrix is achieved in 9Cr–3W–3Co–VNb steel with 0.05% nitrogen. This gives rise to excellent creep strength at 650 °C, as shown by much higher creep rupture strength than conventional 9Cr steels P92 and T91. Excess addition of nitrogen, 0.07 and 0.1%, promotes the coarsening of MX nitrides and the formation of Z-phase during creep. A dispersion of nano-size TiC carbides is also achieved in 9Cr–2W–VNb steel containing 0.05% Ti by high-temperature normalizing at 1300 °C followed by tempering. This effectively decreases minimum creep rate and improves creep life.