Development of a New 9Cr-1Mo Steel for Thin Plate by Controlled Addition of V, Nb and N

Abstract

A new 9Cr-1Mo steel, which is applicable to such low normalizing temperature as 930 to 950°C, was designed using the experimentally determined solution products of [V] [N] and [Nb] [N]. Although the creep rupture strength (CRS) at 600°C for 10⁵ hours of the new steel is about 10MPa lower than that of Mod. 9Cr-1Mo steel which is normalized at higher temperature than 1040°C, it is about 20MPa higher than the Mod. 9Cr-1Mo steel which is normalized at 930 to 950°C. The new steel exhibits almost the same CRS as Mod. 9Cr-1Mo steel in the weakest heat affected zone (H-AZ) of welding. The CRS ratio of the weakest HAZ to base metal is higher in the new steel than that in Mod. 9Cr-1Mo steel. The welding heat cycle range where the CRS is degraded is more restricted in this steel than in Mod. 9Cr-1 Mo steel. Moreover, this steel has the potential CRS as high as Mod. 9Cr-1Mo under the treatment of TMCP. By the experimental results, the chemical composition can be optimized as 0.1% C-9 %Cr-1 % Mo-0.1% V-0.02% Nb-0.025 % N. This steel is expected to be used in various installation of ultra-super critical boilers.