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 degrees C, alloy design for a dispersion of fine and thermally stable precipitate particles has been investigated for 9Ci-3W-3Co-VNb steel and 9Cr-2W-VNbTi steel. Creep tests were carried out at 650 degrees C for up to about 10000h. 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 degrees 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-temperaturc normalizing at 1300 degrees C followed by tempering. This effectively decreases minimum creep rate and improves creep life. (c) 2006 Published by Elsevier Ltd.