Abstract
One of the reasons that ferritic/martenstic steels have been considered as candidate materials for nuclear power reactors is their superior creep resistance at elevated temperature. The creep rupture strength of 9% chromium steel could be improved by a fine dispersion of secondary precipitate phase. The precipitate phases in extra-low carbon 9% chromium steel with tempered conditions were investigated by transmission electron microscope and energy-dispersive X-ray analysis. The steel specimens were normalized and then tempered at different temperatures. Niobium-rich MN nitrides (Nb 0.6V 0.3Cr 0.1)N, and two kinds of vanadium nitrides, (V 0.6Nb 0.2Cr 0.2)N and (V 0.45Nb 0.45Cr 0.1)N having a f.c.c. crystal structure, were identified in the steel specimens tempered at 600–780 °C, and 750 or 780 °C respectively. Hexagonal chromium-rich M 2N precipitate phases with different lattice parameters, a = 2.80 Å/ c = 4.45 Å and a = 7.76 Å/ c = 4.438 Å, were determined in the tempered steel specimens. The M 2N phase showed a decrease/an increase in its chromium/vanadium content as the tempering temperature was increased. The influence of precipitates and heat treatment conditions on the high temperature properties of 9% Cr steel was discussed.
Published Version
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